Types of basic methods and technology of electrical measurements. Types and methods of electrical measurements

ELECTRONIC TUTORIAL

IN THE DISCIPLINE "ELECTRICAL ENGINEERING"

MEASUREMENTS"

Performed:

teacher of CST Arkhipova N.A.

Kstovo 2015

Reviewed at the PCC

electrical engineering disciplines

"___"_________20___

Protocol No._________

Chairman of the PCCN.I. Fomochkina

Approved

on methodological

council

"___"_________20___

Chairman of the Methodological CouncilE.A. Kostina

Tutorial intended for students studying in the specialty 220703 Automation of technological processes and production (by industry) full-time.

CONTENT

INTRODUCTION 4

Section 1. State system ensuring uniformity of measurements 5

Topic 1.1 Main types and methods of measurements, their classification 5

Topic 1.2.Metrological indicators of measuring instruments 7

Section 2 Instruments and methods of electrical measurements 9

Topic 2.1 Mechanisms and measuring circuits of electromechanical

devices 9

Topic 2.2 Instruments and methods for measuring current 14

Topic 2.3 Instruments and methods for measuring voltage 18

Topic 2.4 Instruments and methods for measuring power and energy 21

Topic 2.5 Instruments and methods for measuring parameters of electrical circuits 24

devices 28

Section 3 Waveform Study 31

Topic 3.1 Oscilloscopes 31

Topic 3.2 Instruments and methods for measuring frequency and time interval 32

Topic 3.3 Instruments and methods for measuring phase shift 35

INTRODUCTION

The purpose and objectives of the academic discipline. Brief information from the history of the development of electrical measurements. The connection of this academic discipline with other disciplines.

Carrying out measurements is one of the main means of obtaining objective knowledge about the world, and the accumulated experimental material isa basis for generalizations and establishing the laws of its existence anddevelopment. At the same time, carrying out measurements has an unconditional practicalvalue is largely based on measurement results and technicaldevelopment, and interaction between individual economic entitiesactivities. Among all measurements, electrical measurements occupy a special place due to the universality of electrical signals and availablepossibilities for their processing and storage, often when measuring magnetic andnon-electrical quantities, the output signal of the converter isnamely an electrical signal.

Section 1. State system for ensuring unity

measurements

Topic 1.1 Basic types and methods of measurements, their

classification

The role and significance of electrical measuring equipment. Definition of the concept "measurement". Units physical quantities. Classification of measurement methods and their brief characteristics. Direct and indirect methods. Methods of direct assessment and comparison methods (differential, zero, substitution). The concept of measuring instruments: measures of basic electrical quantities, electrical measuring instruments, electrical measuring installations, measuring transducers, Information Systems. Classification and marking of electrical measuring instruments.

To the number technical means measurements include measures, measuring transducers, measuring instruments and measuring systems. A measuring transducer is a device designed to convert a measured parameter into a signal convenient for further transmission over a distance or into a control device circuit.

Converters are divided into primary (sensors), intermediate, transmitting and scale. The measured quantity is called the input, and the result of the transformation is called the output signal.

Primary converters are designed to convert physical quantities into signals, and transmitting and intermediate converters generate signals that are convenient for transmission over a distance and registration.

Scale converters include converters with the help of which the measured quantity changes a given number of times, that is, they do not convert one physical quantity into another.

A measuring instrument is a device designed to generate measurement information in a form accessible to direct perception by an observer (operator). Measuring instruments are divided into two groups.

The first group includes analog instruments whose readings are a continuous function of the parameter being measured.

The second group includes digital devices. They produce discrete signals of measured information in digital form.

The measuring system combines measuring transducers and instruments, providing parameter measurements without human intervention.

The state standard establishes the application of the International System of Units (SI) in all fields of science and technology.

The SI consists of seven basic units, two supplementary units, and twenty-seven major derived units. The basic units include: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd).

SI supplementary units include the radian and steradian, and all other units are derived. For example, the unit of force, newton (N), imparts an acceleration of 1 m/s2 to a body weighing 1 kg; The unit of pressure is the pascal (Pa), the unit of pressure is such a uniformly distributed pressure at which a force equal to 1 N acts on 1 m2 normal to the surface.

All measurements are divided into direct and indirect. For direct measurements numeric value The measured parameter is determined directly by a measuring device: for example, measuring temperature with a thermometer or the linear dimensions of a part with a measuring tool.

Indirect measurements involve determining the desired parameter based on direct measurement auxiliary quantity associated with the measured parameter of a certain functional dependence. For example, determining the volume of a body by its length, width and height, or measuring temperature by changing the electrical conductivity of a resistance thermometer.

Self-test questions

What is measurement?

What is the classification of types of measurements?

What is the difference between exemplary measuring instruments and working ones?

How are electrical and radio measuring instruments classified and designated?

Topic 1.2. Metrological indicators of measuring instruments

Types of errors, their classification according to the form of the numerical expression, according to the pattern of occurrence, according to the probability of implementation.

Systematic errors, their assignment and assessment. Random errors, sources of their occurrence. Laws of error distribution. Characteristics of normal distribution. Identifying mistakes.

Errors as characteristics of measuring instruments. Types of errors and the main reasons for their occurrence. Determination of instrument error based on the accuracy class of the device. Limit, division value, sensitivity of an electrical measuring device. Typical methodology for testing electrical measuring instruments. General information on processing measurement results.

Any measurement must take place according to a system: planning, carrying out measurements, mathematical processing of measurement results. When processing, pay attention to identifying mistakes. It is very important to learn how to calculate the resulting error, to know how systematic and random errors are summed up, and how the resulting error is determined with a given level of probability.

Depending on the reasons, errors are divided into five groups: errors of the measurement method, instrumental, settings of the device and its interaction with the measurement object, dynamic and subjective errors.

Errors in a measurement method are the result of a chosen measurement design that does not eliminate sources of known errors.

Instrumental errors depend on the imperfections of the measuring devices, i.e., on the manufacturing errors of the parts of the measuring device.

The adjustment errors of measuring instruments are determined by operating conditions. Errors may arise during the interaction of the device with the object being measured; for example, errors that are caused by the influence of the measuring force on the deformation of the part being measured.

Dynamic errors arise when converting the measured quantity. Dynamic errors appear as a result of the inertia of changes in the measured parameter.

Subjective errors appear due to the limited physical capabilities of the operator.

Depending on the operating conditions, two types of errors are distinguished: basic and additional.

The main errors occur under normal operating conditions of the measuring device, when the influence of external factors is minimal.

Additional errors are caused by external factors that disrupt the normal operating conditions of the device, for example, changes in ambient temperature or pressure.

If the absolute error value attributed to the true value A0 of the measured parameter, we obtain the relative error , i.e.

= / A0.

Absolute error ratio to the instrument scale rangeNis called the reduced relative error.

Self-test questions

By what criteria are errors classified?

How does the relative error differ from the given one?

What indicators are used to characterize random error?

How can you identify a “miss” in a number of obtained measurement results?

What is the difference between equal-precision and unequal-precision measurements?

What is the methodology for processing the results of indirect measurements?

How calculate the resulting error?

OPTION #1

Questions

1. What is the absolute error?

difference between measured and actual values ​​of a quantity

2 . What is the sensitivity of the device?

attitude change

this is the number of units of the measured value per one division of the instrument scale

3 . The reading range is

range of scale values, limited by the final and initial values ​​of the scale

which normalizes the permissible errors of the measuring instrument

4 . What is SI calibration?

a set of operations performed to determine the actual values ​​of metrological characteristics

a set of operations and types of work aimed at ensuring the uniformity of measurements.

5 . Reduced error

the ratio of the absolute error to the actual value, expressed as a percentage

the ratio of the absolute error to the standard value, expressed as a percentage

difference between measured and actual value of a quantity

OPTION #2

Questions

1 . What is the relative error?

the ratio of the absolute error to the standard value, expressed as a percentage

difference between measured and actual value of a quantity

the ratio of the absolute error to the actual value, expressed as a percentage

2.What is the division price of the device?

the number of units of the measured value per one division of the instrument scale

attitude change

output signal to the change in the measured value that caused it

range of scale values, limited by the final and initial values ​​of the scale

3 . The variation in instrument readings is

difference between measured and actual value of a quantity

the largest difference in readings for the same value of the measured quantity

4 . The measuring range is

range of values ​​of the measured value, forwhich normalizes the permissible errors of the device

difference between measured and actual value of a quantity

range of instrument scale values, limited by the final and initial scale values

5 . What is SI verification?

a set of operations performed to determine the actual values ​​of MX.

a set of operations and types of work aimed at ensuring the uniformity of measurements

a set of operations performed to confirm the compliance of measuring instruments with metrological requirements

Section 2 Instruments and methods of electrical measurements

Topic 2.1 Mechanisms and measuring circuits

electromechanical devices

Measuring mechanisms of magnetoelectric, electromagnetic, electrodynamic, ferrodynamic, electrostatic, induction systems. General principle for creating various electrical measuring mechanisms. The principle of operation of electromechanical devices. The concept of measuring circuits. Measuring circuit of electrical measuring instruments: voltmeters, ammeters, wattmeters. Symbols applied to devices.

The main functional part of a magnetoelectric device is the measuring mechanism. StructurallymagnetoelectricmechanismperformedorWithmobilecoil (frame),either withmobilemagnet. Greater Application has the first of these groups.

The principle of operation of the magnetoelectric mechanism is based on the interaction of the magnetic fields of a permanent magnet and a coil (frame) through which current flows. The counteracting moment can be created mechanically and electromagnetically.

Magnetoelectric devices are used as: 1) ammeters and voltmeters for measuring currents and voltages in DC circuits (for these purposes, devices of other groups are used in rare cases); 2) ohmmeters; 3) direct current galvanometers, used as zero indicators, as well as for measuring small currents and voltages; 4) ballistic galvanometers used for measuring small amounts of electricity; 5) instruments for measuring in circuits alternating current: a) oscillographic galvanometers used for observing and recording fast processes; b) vibration galvanometers, used mainly as zero indicators of alternating current; c) rectifier, thermoelectric and electronic devices containing an AC-to-DC converter.

Advantages magnetoelectric devices are: 1) high sensitivity; 2) high accuracy; 3) low own power consumption; 4) uniform scale; 5) low influence of external magnetic fields.

TO shortcomings magnetoelectric devices include: 1) low overload capacity; 2) comparatively complex design; 3) application, in the absence of converters, only in DC circuits.

The main part of an electromagnetic device is the electromagnetic IM. PrincipleThe action of the electromagnetic measuring mechanism is based on the interaction of the magnetic field, created by the conductor with current, and a ferromagnetic core.

Currently applied big number various types of electromagnetic devices, which differ in purpose, design of the IM, shape of coils and cores, etc.

Depending on the inertia of the moving part or the frequency of its natural oscillations, all electromagnetic devices are divided into two groups: resonant and non-resonant. Resonant ones operate only on alternating current.In non-resonant devices, the moment of inertia of the moving part is significant, and the displacement of the moving part is proportional to the square of the effective current value.

Both groups of devices are divided into two subgroups: polarized and non-polarized. In polarized devices, in addition to the magnetizing coil, there is a permanent magnet. Polarized non-resonant devices do not have high accuracy. Of the resonant devices, reed hertzmeters are mainly used.

Depending on the nature of the magnetic circuit, non-resonant devices are divided into devices with a magnetic circuit, conventionally called closed, and without a magnetic circuit. Devices with a magnetic core have lower inherent power consumption, but at the same time significant errors due to losses in the magnetic core from eddy currents and hysteresis.Devices without a magnetic core have a small intrinsic magnetic field and a greater dependence of readings on the influence of external magnetic fields andallow you to create devices high precision for operation on direct and alternating current. These devices are divided into repulsive and retractive devices. In devices of the first type, ferromagnetic cores located inside the coil with current are magnetized in the same way and repel each other

Electrodynamic MIcomprisessystems of fixed and moving coils (frames), stands, elastic elements, damper, reading device, magnetic protection means. The coils are made round or rectangular. Round coils provide, compared to rectangular,increasesensitivity by 15-20%. Devices with rectangular coilshave smallervertical dimensions of the device.

Ferrodynamic devices are based on a ferrodynamic measuring mechanism. The operating principle of the ferrodynamic measuring mechanism isininteractionmagneticfields of two systems of conductors with currents, and is essentially a type of electrodynamic mechanism. Differenceisin that, to increase sensitivity, the MI contains a magnetic core made of soft magnetic material.Availabilitymagnetic circuitmuchincreasesmagneticfield in the working gap and at the same time the torque increases.

Electrostatic instruments are built on the basis of an electrostatic measuring mechanism, which representsis a system of mobileAndstationaryelectrodes.Underactionvoltage applied to the electrodes,movable electrodes deviate relative to the stationary ones. In electrostatic IMs, the deviation of the moving part is associated with a change in capacitance.

Electrostatic devices are characterized by: 1) very smallown power consumption at direct current and low frequencies. This is explained by the fact that it is caused only by a short-term charging current and the flow of very small leakage currents through the insulation. On alternating current, power consumption is also low due to the small capacitance of the IM and small dielectriclossesVisolation;2) widefrequencyrange(from 20 Hz to 35 MHz); 3) low dependence of readings on changes in the shape of the measured voltage curve; 4) the possibility of using them in direct and alternating current circuits for direct measurement of high voltages (up to 300 kV) without the use of measuring voltage transformers. Along with this, electrostatic devices also have disadvantages: they are susceptible to strong influence external electrostatic fields, have low sensitivity to voltage, have an uneven scale, which must be leveled by choosing the shape of the electrodes, etc.

The accuracy of electrostatic devices can be achieved high through the use of special design and technological measures to reduce errors. Currently, portable devices of accuracy classes 0.2 have been developed; 0.1 and 0.05.

Structurally induction measuring mechanismconsists of one or more stationary electromagnets and a moving part, which is usually made in the form of an aluminum disk mounted on an axis. Variable magnetic fluxes directedperpendicular to the plane of the disk, piercing the latter,induce eddy currents in it. The interaction of flows with currents in the disk causes movement of the moving part.

According to the number of magnetic fluxes,crossing the moving part, they can be single-threaded or multi-threaded. Single-flow induction mechanisms are not currently used in measuring technology.

When studying devices of electromagnetic, electrodynamic and ferrodynamic systems, it is necessary to pay attention to the fact that, according to the principle of operation, these devices are suitable for measurements in both direct and alternating current circuits.

Self-test questions

1. Write and explain the condition of static equilibrium of the moving part of the indicating device and the equation of its scale.

2. How are counteracting moments created in indicating instruments?

3. What is the device’s own energy consumption, what impact can it have on the measurement results?

4. What are the operating principles and design of the magnetoelectric system device?

5. What are the principles of operation and design of devices of electromagnetic, electrodynamic and electrostatic systems?

6. How are logometers of a magnetoelectric system constructed and what is the operating principle?

7. What methods are used to expand the measurement limits of instruments? various systems?

Topic 2.2 Instruments and methods for measuring current

Current measurement methods. Device, principle of operation, specifications, varieties, scope of application of the main types of ammeters, current clamps. Extending measurement limits using current transformers and shunts. Application of combined instruments for current measurement. Selecting a device for measuring current, connecting it to a circuit, measuring, processing the measurement result.

Before measuring current, you need to have an idea of ​​its frequency, shape, expected value, required measurement accuracy and the resistance of the circuit in which the measurement is being made. This preliminary information will allow

select the most suitable measurement method and measuring device. To measure current and voltage, the direct assessment method and the comparison method are used. To measure current in any circuit, an ammeter is connected in series to the circuit.

Ammeter was designed so thatinternal resistance was as small as possible. Therefore, if you turn it on not in series, but in parallel with the load, the circumstances may be unpredictable.It is precisely because of the low resistance inside that a large current will flow through the ammeter, which will lead to the device burning out or the wires burning out.

Ammeter– a measuring device for determining the strength of direct and alternating current in an electrical circuit. The readings of the ammeter depend entirely on the amount of current flowing through it, and therefore the resistance of the ammeter, compared to the load resistance, should be as small as possible. According to their design features, ammeters are divided into magnetoelectric, electromagnetic, thermoelectric, electrodynamic, ferrodynamic and rectifier.

Magnetoelectric ammeters are used to measure low current in DC circuits. They consist of a magnetoelectric measuring mechanism and a scale with marked divisions corresponding to different values ​​of the measured current.

Electromagneticammetersare designed to measure the strength of flowing current in DC and AC circuits. Most often used to measure force in AC circuits of industrial frequency (50 Hz). They consist of a measuring mechanism, the scale of which is marked in units of current flowing through the coil of the device. To make a coil, you can use a large cross-section of wire and, therefore, measure a large current (over 200 A).

Thermoelectricammetersused for measurements in high frequency alternating current circuits. They consist of a magnetoelectric device with a contact or non-contact transducer, which is a conductor (heater) to which a thermocouple is welded (it may be located at some distance from the heater and not have direct contact with it). The current passing through the heater causes its heating (due to active losses), which is recorded by a thermocouple. The resulting thermal radiation affects the frame of the magnetoelectric current meter, which deviates by an angle proportional to the current strength in the circuit.

Electrodynamic ammeters are used to measure current in DC and AC circuits of high frequencies (up to 200 Hz). The devices are very sensitive to overloads and external magnetic fields. They are used as control devices for checking working current meters. They consist of an electrodynamic measuring mechanism, the coils of which, depending on the value of the maximum measured current, are connected in series or parallel, and a graduated scale. When measuring currents low strength the coils are connected in series, and the large one in parallel.

Ferrodynamic ammeters are durable and reliable in design, and are insensitive to external magnetic fields. They consist of a ferrodynamic measuring apparatus and are mainly used in automatic controller systems as recording ammeters.

Each ammeter calculated for a certain maximum value of the measured value. But situations often arise when it is necessary to measure a certain quantity, the value of which is greater than the measurement limits of the device. However, it is always possible to expand the measurement limits of this device. To do this, a conductor is connected parallel to the ammeter, through which part of the measured current passes. The resistance value of this conductor is calculated so that the current passing through the ammeter does not exceed its maximum permissible value. This resistance is called shunt resistance. The result of such actions will be that if an ammeter designed, for example, for a current of up to 1 A, it is necessary to measure a current 10 times greater, then the shunt resistance should be 9 times less than the resistance of the ammeter. Of course, in this case the cost of calibration increases by 10 times, and the accuracy decreases by the same amount.

To expand the measurement limit of the ammeter (ink times) in DC circuits, shunt resistors are used, connected in parallel with the ammeter.

Ammeter scales are usually calibrated directly in units of current:

amperes, milliamps or microamps. Often in laboratory practice uses multi-range ammeters. Several different shunts are placed inside the housing of such devices, which are connected in parallel to the indicator using a measurement limit switch. On the front panel of multi-limit instruments, the maximum current values ​​that can be measured at a particular position of the measurement limit switch are indicated. The scale division price (if the device has a single scale) will be different for each measurement range. Often, multi-range instruments have several scales, each of which corresponds to a specific measurement limit.

Self-test questions

How to measure current?

What is an ammeter?

Main types of ammeters

How is the ammeter connected?

Purpose of shunts

Solving problems on the topic “Instruments and methods for measuring current”

OPTION 1

Task 1.

An ammeter with an internal resistance of 0.28 ohms has a scale of 50 divisions. with division price 0.01 A / division. Determine the division price and the maximum value of the measured current when connecting a shunt with a resistance of 0.02 Ohm.

Task 2.

The MI scale with a resistance of 5 Ohm is divided into 100 divisions. Value of division

0.2 mA/div. From this mechanism it is necessary to make a 10A ammeter. How to do it? What current in the circuit will the ammeter measure if the needle deviates by 35 divisions?

Task 3.

Determine the value of the shunt resistance required to expand the measurement limit of an ammeter with an internal resistance of 5 Ohms, from its nominal value of 4 mA to a value of 15 A.

OPTION 2

Task 1.

The MI scale with an internal resistance of 2 Ohm is divided into 150 divisions. The division value is 0.2 mA/div. From this mechanism it is necessary to make a 15A ammeter. How to do it?

What current will the ammeter measure if the needle deviates by 20 divisions?

Task 2.

Determine the value of the shunt resistance to expand the measurement limit of an ammeter with an internal resistance of 0.58 Ohm, from a nominal value of 5A to a value of 150A.

Task 3.

To an ammeter rated at 5A with an internal resistance of 0.6 Ohm and a scale of 10 divisions. a shunt with a resistance of 0.025 Ohm is connected. When measuring the current, the needle deviated by 8 divisions. Determine the current in the circuit measured with an ammeter.

Topic 2.3 Instruments and methods for measuring voltage

Voltage measurement methods. Device, principle of operation, technical characteristics, varieties, scope of application: electromechanical voltmeters, electronic voltmeters, digital voltmeters, compensators. Application of combined instruments for measuring voltage. Selecting a device for measuring voltage, connecting it to a circuit, measuring, processing the measurement result.

Voltmeters are used to measure voltage. Voltmeters are connected in parallel to the section of the circuit where the voltage needs to be measured. To ensure that the device does not consume high current and does not affect the voltage of the circuit, its winding must have a high resistance. The greater the internal resistance of the voltmeter, the more accurately it will measure the voltage. For this purpose, the voltmeter winding is made of a large number of turns of thin wire. To expand the measurement limits of voltmeters, additional resistances are used, connected in series with the voltmeters. In this case, the network voltage is distributed between the voltmeter and the additional resistance. The amount of additional resistance must be selected in such a way that in a circuit with increased voltage the same current passed through the voltmeter winding as at the rated voltage.

Most of the stationary measuring devices currently used are classic analog electromechanical devices. Their operational and metrological characteristics can be considered sufficient to solve the main problems of technical measurements. The accuracy classes of these devices range from 0.1 to 4%.

Operating principleelectromechanical measuring instrumentsbased on transformation electrical energy input signal into the mechanical energy of the angular motion of the moving part of the reading device. In addition, electromechanical devices, in addition to standalone use, can also be used as output devices for other electronic analog devices.

INelectromechanical devices implement different physical principles, allowing you to convert the value of the measured characteristic into a proportional deviation of the pointer. The design of an electromechanical device of any type can be represented as a series connection input circuit, measuring device and reading device.

Of the variety of systems, designs and circuits of electromechanical measuring instruments, the following main classes can be noted: magnetoelectric, rectifier, thermoelectric, electromagnetic, electrodynamic, electrostatic, induction.

Electronic voltmeters are a combination of an electronic converterand measuring instrument. Unlike voltmeters of the electromechanical group, electronic voltmeters of direct and alternating currents have high input resistance and sensitivity, wide measurement limits and frequency range (from 20 Hz to 1000 MHz), and low current consumption from the measuring circuit.

Electronic voltmeters are classified according to a number of characteristics:

by purpose - voltmeters of direct, alternating and pulse voltages; universal, phase-sensitive, selective;

by measurement method - direct assessment devices and comparison devices;

by the nature of the measured voltage value - amplitude (peak), average quadratic value average rectified value;

by frequency range - low-frequency, high-frequency, ultra-high-frequency.

In addition, all electronic devices can be divided into two large groups: analog electronic devices with a dial readout and discrete type devices with a digital readout.

Voltage meters, regardless of their purpose, must not disrupt the operating mode of the circuit of the measured object when turned on; ensure a small measurement error, while eliminating the influence of external factors on the operation of the device, high measurement sensitivity at the optimal limit, quick readiness for operation and high reliability.

The choice of instruments that perform voltage measurements is determined by a combination of many factors, the most important of which are: the type of voltage being measured; approximate frequency range of the measured quantity and amplitude range; shape of the measured voltage curve; power of the circuit in which the measurement is carried out; power consumption of the device; possible measurement error.

In low-power DC and AC circuits, digital and analog electronic voltmeters are usually used to measure voltage. If it is necessary to measure voltages with higher accuracy, you should use instruments whose operation is based on comparison methods, in particular the contrast method.

Modern digital voltmeters contain microprocessor units and are equipped with a keyboard, which allows you to automate the measurement process, carry it out in accordance with a given program, carry out the required processing of measurement results, and expand the functionality of the device. Turn it into a multimeter that allows you to measure not only DC voltage, but also many other quantities: AC voltage, resistance, capacitance, frequency, etc.

Self-test questions

How can you measure voltage?

How are electronic voltmeters classified?

List the main blocks of digital voltmeters

How are voltage measuring instruments selected?

What are the values ​​of the crest and shape coefficients for a sinusoidal voltage?

Draw circuit diagrams of voltmeters with linear, peak and square-law detectors.

What are the types of block diagrams of digital voltmeters?

Topic 2.4 Instruments and methods for measuring power and energy

Methods for measuring power and electricity. Device, principle of operation, technical characteristics, types, scope of application: wattmeters and electricity meters. Selection of instruments for measuring power and electricity, connecting them to the circuit, measurement, processing of measurement results. Expansion of measurement limits.

From the expression for DC power P =IUIt can be seen that power can be measured using an ammeter and voltmeter by an indirect method. However, in this case it is necessary to carry out simultaneous readings from two instruments and calculations, which complicate the measurements and reduce its accuracy.

To measure power in direct and single-phase alternating current circuits, instruments called wattmeters are used, for which electrodynamic and ferrodynamic measuring mechanisms are used.

Power in electrical circuits is measured by direct and indirect methods. For direct measurement, wattmeters are used, for indirect measurement, ammeters and voltmeters are used.

Electrical measuring instruments are used in power supply systems. The most applicable are ammeters, voltmeters, power meters (wattmeters and varmeters), active and reactive energy meters. When choosing instruments for measuring electrical quantities, one should take into account the type of current - direct or alternating.

Wattmeters are used to measure active power. Wattmeters have two measuring coils, current and voltage. The torque produced by these coils is proportional to the currents flowing through them.

To measure electricity consumption, single-phase or three-phase electricity meters are used. These devices have induction measuring mechanisms.

Wattmeter– a measuring device whose purpose is to determine the work done by electric current per unit time for the passage of current through any conductor (determining the power of an electric current or an electromagnetic signal).

A wattmeter can determine the number of watts required to produce a certain amount of electric light in each second of time or determine the amount of work performed per unit of time by some electrical device. The work performed by an electrical device per unit of time (its power) is determined in watts and is the product of the number of amperes (current strength) consumed by a given type of electrical consumer by the potential difference (+ -) of the ends of this part of the circuit, measured in volts.

To determine the power of electric current and are usedwattmeters, which are nothing more than an electrodynamometer. The passing current is distributed into two parts, one of which is, in fact, control, and the second is experiment, changing the resistance on the experimental part and measuring the potential difference at the output and the power of the electric current is determined.

By purpose and frequency rangewattmeters can be divided into three main categories:
– low-frequency (and direct current);
– radio frequency;
– optical.

Radio wattmeters are divided into two types according to their intended purpose: transmitted power, connected to the break of the transmission line, and absorbed power, connected to the end of the line as a matched load. Depending on the method of functional transformation of measurement information and its output to the user, wattmeters can be analog (displaying and recording) and digital.

Low frequency wattmeters used primarily in industrial frequency power supply networks to measure power consumption; they can be single-phase or three-phase. A separate subgroup consists of varmeters - reactive power meters. Digital instruments usually combine the ability to measure active and reactive power.

Radio frequency wattmeters form a very large and widely used subgroup of radio wattmeters. The division of this subgroup is mainly associated with the use of various types of primary converters. Manufactured wattmeters use converters based on a thermistor, thermocouple or peak detector; Much less frequently, sensors based on other principles are used. When working with absorbed power wattmeters, it should be remembered that due to the mismatch between the input impedance of the receiving sensors and the characteristic impedance of the line, part of the energy is reflected and in reality the wattmeter does not measure the real power of the line, but the absorbed power, which differs from the actual one.

The operating principle of a thermistor converter is based on the dependence of the resistance of the thermistor on its heating temperature, which, in turn, depends on the dissipated power of the signal supplied to it. The measurement is carried out by comparing the power of the measured signal, dissipated in the thermistor and heating it, with the power of a low frequency current, causing the same heating of the thermistor. The disadvantages of thermistor wattmeters include their small recording range - a few milliwatts.

Extension of the measurement limits on direct current voltage is carried out using additional resistances - shunts. When measuring on alternating current, the limits are expanded using current and voltage transformers. In this case, it is necessary to ensure that the generator terminals of the wattmeter are connected correctly.
Power measurement in three-phase three-wire networks is carried out using two single-phase wattmeters connected in two phases.
Extension of measurement limits is carried out using current and voltage transformers. In these same networks, a three-phase wattmeter is used to measure power.
In three-phase four-wire networks, active power is measured using three single-phase wattmeters or one three-element wattmeter.
Reactive power in single-phase networks is measured using one wattmeter connected according to the circuit, and in three-phase networks - using three wattmeters.

Self-test questions

Give definitions and analytical expressions for active and reactive power.

What are the methods for measuring active power in DC and single-phase AC circuits?

Draw a diagram of a reactive power meter.

What methods are used to measure assets?
new power and energy in three-phase circuits?

Topic 2.5 Instruments and methods for measuring parameters of electrical circuits.

Resistance measurement. Ohmmeters. Voltmeter and ammeter method: connection circuits, their advantages and disadvantages. Errors of the method. Bridge circuits. Single DC Bridge Theory. Double bridge.

Measurement of parameters of capacitors and inductances. Bridge circuits. Resonant circuits. Measurements by substitution method. Measurement errors.

Used to measure resistance various methods depending on the nature of the objects and measurement conditions (for example, solid and liquid conductors, grounding conductors, electrical insulation); on requirements for accuracy and speed of measurement; on the value of the measured resistances. When studying the theory of bridges, it is necessary to understand the reasons that prevent the use of a single DC bridge for measuring low resistances. Consider the double bridge theory. In the theory of interconnected current bridges, it is necessary to consider equilibrium conditions that differ from the equilibrium conditions of direct current bridges.

Methods for measuring small resistances differ significantly from methodsmeasurements of high resistances, since in the first case it is necessary to take measures to eliminate the influence of the resistance of connecting wires and transition contacts on the measurement results.

The main methods for measuring DC resistance are: indirect method; direct estimation method and bridge method. The choice of measurement method depends on the expected value of the resistance being measured and the required accuracy. The most universal of the indirect methods is the ammeter-voltmeter method.

Ammeter-voltmeter method - obased on measuring the current flowing through the measured resistance and the voltage drop across it. Two measurement schemes are used: measurement of large resistances and measurement of small resistances. Based on the results of measuring current and voltage, the desired resistance is determined.

Direct assessment method - pinvolves measuring DC resistance using an ohmmeter. Measurements with an ohmmeter give significant inaccuracies. For this reason this method used for approximate preliminary resistance measurements and for testing switching circuits.

Bridge method - pTwo measurement schemes are used - a single bridge scheme and a double bridge scheme.A single DC bridge consists of three reference resistors (usually adjustable) that are placed in series with the measured resistance Rx in the bridge circuit. To measure resistances below 1 ohm, used war Thomson Bridge.

Consider possible methods measurements of inductances and capacitances. Advantages and disadvantages of resonant measurement circuits. Sources of errors. Equivalent circuits, understand what their advantage is over other measurement methods. Devices for direct assessment and comparison - to measuring devices for directestimates of the value of the measured capacitance refermicrofaradmeters, the action of which is based on the dependence of the current or voltage in the alternating current circuit on the value included in it . The capacitance value is determined using the dial meter scale.

Wider to measure and inductances are usedAC balanced bridges, allowing to obtain a small measurement error (up to 1%). The bridge is powered by generators operating at a fixed frequency of 400-1000 Hz. Rectifier or electronic millivoltmeters, as well as oscilloscope indicators, are used as indicators.

Self-test questions

How can you measure resistance in AC and DC networks?

How is the insulation resistance of wires measured?

What is the block diagram of a device for measuring non-electrical quantities?

Consider the principle of operation, structure and basic theory individual types converters.

What options exist for connecting ammeters and voltmeters for measuring resistance?

Draw a diagram of a single bridge and indicate the elements that are the source of errors when measuring small resistances.

What electrical quantities can be measured using an AC bridge?

What are the sources of errors in resonant measurement circuits?

What are the advantages of measuring equivalent circuits?

Topic 2.6 Universal and special electrical measuring instruments

devices

Basic parameters and types of universal and special electrical measuring instruments, brief technical characteristics. Multimeters, voltammeters, combined instruments. Diagram of the measuring circuits of the combined instrument.Digital multimeters, block diagram, switches for the type of measurements and measurement limits. Units of measurement. Multimeter input resistance. Measurement of resistances, currents, voltages, electrical capacitances, parameters of semiconductor devices.

There are a large number of measuring instruments used to perform strictly defined work: maintenance, testing cable lines, measuring power network parameters. Each of them is ideal for performing a specific set of measurements, but nothing more. Therefore, repair or adjustment of various devices is impossible without conventional measuring instruments: multimeters, oscilloscopes, universal and special generators, frequency meters, RLC meters, logic analyzers.WITHToday, most of these devices are available in desktop, portable and wearable versions. Therefore, such a device can always be selected in accordance with any intended operating conditions: from laboratory to field, powered by AC mains, on-board power supply or batteries. And the fundamental differences between devices of various designs concern, perhaps, only two points: the accuracy class and the possibility of integration into measuring systems. Typically, wearable modifications have worse accuracy and a simpler set of service functions, but the introduction of digital signal processing changes this situation.The scope of application of computer-controlled measuring systems is limited, as a rule, scientific experiments and various serial tests. Exactly there important has automation of the process of collecting and processing measurement results . Multimeters and oscilloscopes are some of the most common instruments. Every day the number of basic and additional functions integrated into them is growing. Moreover, in terms of their capabilities, these devices are getting closer. The oscilloscope may have a built-in multimeter, and the multimeter may have the ability to display the measured signal.Multimeter(from multimeter , tester- from test - trial,avometer- from AmpereVoltOhmmeter) - combined , which combines several functions. In the minimum set it is , And . Exist And multimeters.

A multimeter can be either a lightweight, portable device used for basic and troubleshooting, as well as a complex stationary device with many capabilities.

The simplest digital multimeters have 2.5 digital digits ( usually about 10%). The most common devices are with a bit resolution of 3.5 (the accuracy is usually about 1.0%). Slightly more expensive devices with a bit resolution of 4.5 (accuracy is usually about 0.1%) and significantly more expensive devices with a bit resolution of 5 and higher are also produced. The accuracy of the latter strongly depends on the measurement range and the type of measured value, therefore it is discussed separately for each subrange. In general, the accuracy of such devices can exceed 0.01%, despite their portable design.

The digit capacity of a digital measuring device, for example, “3.5” means that the device display shows 3 full digits, with a range from 0 to 9, and 1 digit with a limited range. Thus, a device of the “3.5 digit” type can, for example, give readings ranging from0,000 before1,999 , when the measured value goes beyond these limits, switching to another range (manual or automatic) is required.

The number of digits does not determine the accuracy of the device. The accuracy of measurements depends on the accuracy , on the accuracy, thermal and temporal stability of the radioelements used, on the quality of protection from external interference, on the quality of the .

An analog multimeter consists of a pointer magnetoelectric measuring device, a set of additional for measuring voltage and dialing for measuring current. Resistance measurements are performed using a built-in or external source. In an analog multimeter, the measurement results are observed by the movement of the hand (like on a watch) along a measuring scale on which the following values ​​are labeled: voltage, current, resistance. The popularity of analog multimeters is explained by their availability and price, and the main disadvantage is some error in the measurement results. For more precise adjustment, analog multimeters have a special tuning resistor, by manipulating which you can achieve a little greater accuracy. However, in cases where more precise measurements are desired, using a digital multimeter is best.
The main difference between digital and analog is that the measurement results are displayed on a special screen. In addition, digital multimeters have higher accuracy and are easy to use, since you do not have to understand all the intricacies of calibration measuring scale, as in arrow variants.

Self-test questions

What device is called a multimeter?

Types of multimeters

Characteristics of an analog maltimeter

Digital Multimeter Specifications

Section 3 Waveform Study

Topic 3.1 Oscilloscopes

General information and classification of electron ray oscilloscopes. Device, principle of operation, purpose, technical characteristics, block diagram of a cathode-ray oscilloscope. Using a cathode ray oscilloscope to observe an electrical signal, to measure the amplitude, frequency and period of a periodic signal.Types of oscilloscopes. Block diagram of an electronic oscilloscope. Preparation, calibration and measurement of various signals. Features of preparation, calibration and measurements with two-beam, oscilloscopes-multimeters and oscilloscopes with information storage. Features of measuring non-electrical quantities with electronic oscilloscopesAnalog oscilloscopes, digital storage oscilloscopes, digital phosphor oscilloscopes, digital sampling oscilloscopes, virtual oscilloscopes, handheld oscilloscopes

Electromechanical oscilloscopes are widely used to observe and record quantities that rapidly change over time. What is an oscilloscope? This is a device that is designed to study all kinds of electrical signals by visually observing a special signal recorded on a photographic tape or on a graph screen, as well as to measure the amplitude and time parameters of the signal according to the shape of the graph.

All cathode ray oscilloscopes have screens that display graphs of the input signals. Special markings are applied to the screen in the form of a grid. If applicable , then his images in the form of a finished picture are displayed on a display, which can be monochrome or color. Analog oscilloscopes use a cathode ray tube as a screen with so-called electrostatic deflection.

All oscilloscopes used today differ in their purpose, as well as in the method of outputting measurement information and, of course, in the method of processing the input signal used.

Oscilloscopes for observing waveforms with periodic sweeps on the screen. The screen can be either electron beam or liquid crystal. Continuous scan oscilloscopes for recording curves on photographic tape. They are also called loop oscilloscopes. There are also digital and analog oscilloscopes

When studying them, it is necessary to understand the reasons why electromechanical oscilloscopes are used only for studying processes with a frequency not exceeding several thousand hertz.

Self-test questions

Application areas of electromechanical oscilloscopes?

How is the sweep of the test voltage curve achieved in an electronic oscilloscope?

What determine the amplitude and phase errors of electronic and electromechanical oscilloscopes?

Topic 3.2 Instruments and methods for measuring frequency and time interval

Methods for measuring frequency and time interval. Design, principle of operation, technical characteristics, types, scope of application of frequency meters. Measuring time intervals.Measuring generators. Block diagram. GeneratorsR- C, L- C, on beats, noise, standard signals, pulsed. Characteristics of signals. Rules for setting up and connecting. Matching devices. Safety regulations.

Direct frequency measurement is carried outfrequency counters, which are based on various measurement methods depending on the range of measured frequencies and the required measurement accuracy. The most common frequency measurement methods are:capacitor recharge method, resonant method, discrete counting method , a method of comparing the measured frequency with a reference one.Frequency counters are used infrequently. For the most part, the frequency meter built into the multimeter is sufficient. But in cases where an accurate result or external control is needed, a special device is indispensable. Such frequency meters can measure the frequency, period and duty cycle of periodic signals, determine the duration of intervals, and carry out reference timing. Complex models provide the possibility of computational processing of the results of a set of measurements and several channels for implementation complex algorithms starting a count, processing signals with different parameters, or performing relative measurements.

Generators are used much less frequently and mainly when debugging and testing various devices. Generators are divided into low-frequency, high-frequency and functional. The former generate a sinusoidal signal or meander with a frequency from several hertz to hundreds of kilohertz, the latter - with frequencies up to hundreds of megahertz with the ability to modulate the signal according to a given law by an external or internal signal. Functional generators generate signals of complex shapes (sine, rectangle, triangle, saw, trapezoid) in the frequency range up to tens of megahertz with a given duty cycle, as well as digital signals with TTL and CMOS levels. Some models can work as sweeping frequency generators (according to a given law) or generate a simple amplitude- or frequency-modulated signal.

Method of recharging a capacitor for each period of the measured frequency - sThe average value of the recharge current is proportional to frequency and is measured by a magnetoelectric ammeter, the scale of which is calibrated in frequency units. They produce capacitor frequency meters with a measurement limit of 10 Hz - 1 MHz and a measurement error of ±2%.

Resonance method, based on the phenomenon of electrical resonance in a circuit with tuned elements in resonance with the measured frequency. The measured frequency is determined by the scale of the adjustment mechanism. The method is applied at frequencies above 50 kHz. The measurement error can be reduced to hundredths of a percent.

Discrete counting methodlies at the heart of the workelectronic counting digital frequency meters. It is based on counting pulses of the measured frequency over a known period of time. Provides high measurement accuracy in any frequency range.

Method for comparing the measured frequency with the reference - electrical vibrations the unknown and reference frequencies are mixed in such a way that beats of a certain frequency appear. When the beat frequency is zero, the measured frequency is equal to the reference frequency. Frequency mixing is carried out using the heterodyne method (zero beat method) or oscillographic method.

The solution to many radio engineering problems involves measuring time intervals. Usually it is necessary to measure both very small (units of picoseconds) and very large (hundreds of seconds) time intervals. Time intervals can also be not only repeated, but also single.

There are two main ways to measure time intervals: oscillographic and digital.

Measuring time intervals using an oscilloscope is carried out using an oscillogram of the voltage being tested using a “linear” sweep. Due to the nonlinearity of the scan, as well as large errors in reading the beginning and end of the interval, the total measurement error is a few percent. In recent years, time intervals have mostly been measured using digital methods.

Measuring time intervals using a digital frequency counter - measurement of the time interval Tx by the digital method is based on filling it with pulses following with a standard period T0, and counting the numberMxof these impulses during the time Tx.

Questions For self-tests

What are the most common methods for measuring time intervals?

Draw block diagram digital time interval meter.

What methods are there to reduce the error?

What frequency measurement methods do you know?

Draw a functional diagram of an oscilloscope frequency meter.

Topic 3.3 Instruments and methods for measuring phase shift

Methods for measuring phase shift. Design, principle of operation, technical characteristics, types, scope of application of phase meters.

The solution of many radio engineering problems is impossible without measuring, along with the amplitude and frequency, the phase shift (PS) of signals. Phase measurement methods make it possible to solve many problems related to measuring range, coordinates, noise-resistant information transmission, etc.

For example, phase radio engineering systems for short-range navigation provide measurement of range and coordinates with an error of 0.1–1 m, satellite systems Global navigation allows you to determine distance with an accuracy of several millimeters, angular position with an accuracy of several minutes of arc. Devices based on phase methods using laser technology can measure small distances with an accuracy of 10 -9 m or less.

The concept of phase shift is introduced only for harmonic signals with the same frequency:
U 1 = U m 1 sin ( w t + j 1 ) y = w t + j 0 – oscillation phase
U 2 = U m 2 sin ( w t + j 2 ) j 0 – initial phase
j = y 1 - y 2 =( w t + j 1 )- ( w t + j 2 )= ê j 1 - j 2 ê
Phase shift – modulus of the difference between the initial phases.
Knowing the phase shift allows you to identify the causes of signal distortion.
The condition for undistorted transmission is that the phase response must be linear.
To measure the phase shift, use following methods: oscillographic, compensation, conversion of phase shift into current pulses, discrete counting method, etc. Measuring the phase shift using the oscillographic method can be realized using linear, sinusoidal and circular scanning methods. To measure the phase shift using the compensation method with oscillographic indication, a measuring setup is assembled, consisting of a single-beam oscilloscope, a standardφ arr. and auxiliaryφ V phase shifters.

The measurement of the phase shift by the discrete counting method is based on a formula into which one should substitute ∆ for the time intervalsTand T the corresponding number of pulses with a constant repetition rate. Direct-indicating phase meters of this type are called electronic-counting, or digital, phase meters. There are several schemes of digital phase meters, but integrating phase meters, in which the measurement result is the average value of the phase shift over a large number of periods of the measured voltage, are most widespread. Such phase meters provide good noise immunity.

Microprocessor phase meter - a significant expansion of functionality, increased reliability and some other characteristics of phase meters are ensured when they are built on the basis of a microprocessor working in conjunction with measuring transducers. Such phase meters make it possible to measure the phase shift between two periodic signals for any selected period, observe fluctuations of such shifts and evaluate their statistical characteristics: mathematical expectation, dispersion, standard deviation. It is also possible, as in the digital phase meters discussed above, made according to circuits with strict operating logic, to measure the average value of the phase shift.

The phase shift between two harmonic signals of the same frequency can be measured with a phase detector.

A phase shifter is a device with which a known and adjustable phase shift is introduced into an electrical circuit. The design of the phase shifter depends on the operating frequency range for which it is intended.

Questions For self-tests

1. What is the meaning of the concept of “phase” of a signal?

2. What is the phase shift between two signals?

3. List the main methods for measuring phase shift.

4. What is the linear sweep method for measuring phase shift?

5. On what principle do compensation phase meters work?

6. How does a microprocessor-based digital phase meter work?

1Option

The magnetoelectric milliammeter has an upper measurement limit of 100 mA. A change in the measured current by 12 mA corresponds to the movement of the arrow by 6 divisions.Determine the number of divisions, the division price and the sensitivity of the scale.

After repairing an ammeter with an accuracy class of 1.5 and a measurement limit of 5 A, it was calibrated. The largest absolute error was 0.07 A. Did the ammeter retain its accuracy class after repair?

A voltmeter with an internal resistance of 5 kOhm is connected with an additional resistor having a resistance of 45 kOhm. Determine how many times the measuring limit of the voltmeter has increased. Draw a circuit diagram for connecting a voltmeter with an additional resistor.

Test on the discipline "Electrical measurements"

Option 2

A voltmeter with an upper measurement limit of 600 V has a sensitivity of 0.25 div/V. When measuring voltage, the voltmeter needle deviated by 50 divisions. Determine the number of scale divisions, the division value and the voltage measured by the voltmeter.

An ammeter with an internal resistance of 1.2 ohms is connected with a shunt having a resistance of 0.3 ohms. Determine how many times the ammeter's measurement limit has increased. Draw a circuit diagram for connecting an ammeter with a shunt.

An ammeter with an accuracy class of 2.5 and an upper measurement limit of 20A showed a current value of 11.5 A. Determine within what limits the actual current value lies.

When measuring the current in the circuit, the magnetoelectric milliammeter pointer moved 10 divisions from the 10 mA mark to the 20 mA mark. The milliammeter scale has 100 divisions. Determine the upper limit of measurement of the device, the division value and the sensitivity of the scale.

Test on the discipline "Electrical measurements"

3Option

An ammeter with a scale of 10 divisions and an upper limit of measurement of 20 A showed a current in the circuit of 15 A. Determine the value of the division, the sensitivity of the scale and the number of divisions by which the arrow deviated when measuring the current.

When calibrating a voltmeter that has an upper measurement limit

50V, the largest absolute error was 1.1 V. What accuracy class is assigned to the voltmeter?

A voltmeter having an internal resistance of 200 ohms and an upper limit of measurement of 50 V must be used to measure voltages up to 450 V. How can this be done? Draw a diagram and perform the necessary calculations.

The actual value of the current in the circuit is 5.23 A. An ammeter with an upper measurement limit of 10 A showed a current of 5.3 A. Determine the absolute, relative and reduced measurement errors.

Test on the discipline "Electrical measurements"

4Option

The milliammeter is designed for a current of 200 mA and has a current sensitivity of 0.5 div/mA. The milliammeter needle deviated by 30 divisions. Determine the number of scale divisions, division value and measured current.

The accuracy classes of the two voltmeters are the same and equal to 1. The upper measurement limit of the first voltmeter is 50 V, and the second voltmeter is 10 V. Determine the ratio of the largest permissible absolute errors of voltmeters.

A magnetoelectric ammeter has an internal resistance of 0.05 Ohm and an upper measurement limit of 5 A. How can the ammeter's measurement limit be expanded to 125 A?Draw a diagram and make the necessary calculations.

An actual current of 2.4 A passes through a resistor with a resistance of 8 Ohms. When measuring the voltage across this resistor, the voltmeter showed a voltage of 19.3 V. Determine the absolute and relative error voltage measurements.

ELECTRICAL
MEASUREMENTS IN
SYSTEMS
ELECTRIC SUPPLY
Teacher: Ph.D., Associate Professor, Department of EPP
Buyakova Natalya Vasilievna

Electrical measurements are
a set of electrical and electronic measurements,
which can be considered as one of the sections
metrology. The name "metrology" is derived from two
Greek words: metron - measure and logos - word, doctrine;
literally: the doctrine of measure.
IN modern understanding metrology is a science
about measurements, methods and means of ensuring them
unity and ways to achieve the required accuracy.
IN real life metrology is not only a science, but also
area of ​​practical activity related to
the study of physical quantities.
Subject
metrology
is
receiving
quantitative information about the properties of objects and
processes, i.e. measuring the properties of objects and processes with
required accuracy and reliability.

Measurements are one of the most important ways of knowing
nature by man.
They provide a quantitative description of the environment
of the world, revealing to man the actions in nature
patterns.
Measurement is understood as a set of operations,
performed using special technical
a means of storing a unit of measured quantity,
allowing you to compare the measured value with its
unit and get the value of this quantity.
The result of measuring the value X is written in the form
X=A[X],
where A is a dimensionless number called numerical
the value of a physical quantity; [X] − unit
physical quantity.

ELECTRICAL MEASUREMENTS

Measurement of electrical quantities such as voltage,
resistance, current, power are produced with
using various means - measuring instruments,
circuits and special devices.
The type of measuring device depends on the type and size
(range of values) of the measured value, as well as from
required measurement accuracy.
Electrical measurements use basic
SI units: volt (V), ohm (Ohm), farad (F),
henry (H), ampere (A) and second (s).

STANDARDS OF UNITS OF ELECTRICAL QUANTITIES

Electrical
measurement
This
finding
(experimental methods) values ​​of physical
quantities expressed in appropriate units
(eg 3A, 4V).
The values ​​of units of electrical quantities are determined
international agreement in accordance with laws
physics and units of mechanical quantities.
Since the "maintenance" of units of electrical quantities,
determined
international
agreements,
associated
With
difficulties,
their
present
"practical"
standards
units
electrical
quantities
Such
standards
supported
state
metrological laboratories of different countries.

All common electrical and magnetic units
measurements are based on the metric system.
IN
consent
With
modern
definitions
electrical and magnetic units they are all
derived units derived from certain
physical formulas from metric units of length,
mass and time.
Since most electrical and magnetic
quantities
Not
so that
Just
to measure,
taking advantage
mentioned standards, it was considered more convenient
install
by
relevant
experiments
derived standards for some of these
quantities, and measure others using such standards.

SI units

Ampere, a unit of electric current, is one of the
six basic SI units.
Ampere (A) - the strength of a constant current, which at
passing along two parallel straight lines
conductors of infinite length with negligibly small
circular cross-sectional area,
located in vacuum at a distance of 1 m one from
another, would call a conductor at each section
1 m long, the interaction force is equal to 2 ∗ 10−7 N.
Volt, unit of potential difference and electromotive
strength.
Volt (V) - electrical voltage in the area
electrical circuit with a direct current of 1 A at
consumed power 1 W.

Coulomb, a unit of electricity
(electric charge).
Coulomb (C) - the amount of electricity passing
through cross section conductor at
direct current of 1 A for a time of 1 s.
Farad, a unit of electrical capacitance.
Farad (F) - capacitance of the capacitor, on the plates
which, when charged at 1 C, produces an electrical
voltage 1 V.
Henry, unit of inductance.
Henry is equal to the inductance of the circuit in which
a self-induction emf of 1 V occurs with uniform
change in current strength in this circuit by 1 A in 1 s.

Weber unit of magnetic flux.
Weber (Wb) - magnetic flux, decreasing
which reaches zero in the circuit connected to it,
having a resistance of 1 ohm, leaks
electric charge equal to 1 C.
Tesla, a unit of magnetic induction.
Tesla (T) - magnetic induction of a homogeneous
magnetic field in which the magnetic flux
through a flat area of ​​1 m2,
perpendicular to the induction lines, equal to 1 Wb.

10. MEASURING INSTRUMENTS

Electrical measuring instruments most often measure
instantaneous values either electrical quantities, or
non-electric converted to electrical.
All devices are divided into analog and digital.
The first ones usually show the value of the measured
quantities by means of an arrow moving along
graduated scale.
The latter are equipped with a digital display, which
shows the measured value of a quantity as a number.
Digital instruments are more
are preferred because they are more accurate and more convenient
when taking readings and, in general, are more universal.

11.

Digital universal meters
(“multimeters”) and digital voltmeters are used
for medium to high precision measurements
DC resistance, as well as voltage and
AC power.
Analog
devices
gradually
are being forced out
digital, although they still find application where
low cost is important and high accuracy is not needed.
For the most accurate resistance and impedance measurements
resistance (impedance) there are measuring
bridges and other specialized meters.
To record the progress of changes in the measured value
recording instruments such as strip recorders and electronic oscilloscopes are used in time;
analog and digital.

12. DIGITAL INSTRUMENTS

In all digital measuring instruments (except
protozoa) amplifiers and other electronic
blocks for converting input signal into signal
voltage, which is then converted into digital form
analog-to-digital converter (ADC).
The number expressing the measured value is displayed on
light-emitting diode (LED), vacuum fluorescent or
liquid crystal (LCD) indicator (display).
The device usually operates under the control of a built-in
microprocessor, and in simple devices the microprocessor
combined with the ADC on one integrated circuit.
Digital instruments are well suited to work with
connection to an external computer. In some types
measurements, such a computer switches measuring
functions of the device and gives data transfer commands for their
processing.

13. Analog-to-digital converters (ADC)

There are three main types of ADCs: integrating,
successive approximation and parallel.
The integrating ADC averages the input signal over
time. Of the three types listed, this is the most accurate,
although it is the slowest one. Conversion time
integrating ADC ranges from 0.001 to 50 s and
Moreover, the error is 0.1-0.0003%.
Successive approximation ADC error
slightly more (0.4-0.002%), but time
conversion - from 10 µs to 1 ms.
Parallel ADCs are the fastest, but also
least accurate: their conversion time is on the order of 0.25
ns, error - from 0.4 to 2%.

14.

15. Sampling methods

The signal is sampled in time by quickly
measuring it at separate points in time and
holding (saving) measured values ​​for a while
converting them into digital form.
Sequence of obtained discrete values
can be displayed in the form of a curve having
signal shape; squaring these values ​​and
summing up, one can calculate the root mean square
signal value; they can also be used for
calculations
time
growth,
maximum
value, time average, frequency spectrum, etc.
Time sampling can be done either in
one signal period (“real time”), or (with
sequential or random sampling) per row
repeating periods.

16. Digital voltmeters and multimeters

Digital
voltmeters
And
multimeters
measure
quasi-static value of a quantity and indicate it in
digital form.
Voltmeters directly measure only voltage,
usually DC current, and multimeters can measure
DC and AC voltage, current strength,
DC resistance and sometimes temperature.
These are the most common instrumentation
general-purpose instruments with a measurement error of 0.2
up to 0.001% can have a 3.5 or 4.5 digit digital display.
"Half-integer" sign (digit) is a conditional indication that
the display may show numbers outside the limits
nominal number of characters. For example, a 3.5-digit (3.5-digit) display in the 1-2 V range may show
voltage up to 1.999 V.

17.

18. Impedance meters

These are specialized instruments that measure and display
capacitor capacitance, resistor value, inductance
inductors or impedance
connecting a capacitor or inductor to a resistor.
There are devices of this type for measuring capacitance from 0.00001 pF
up to 99.999 uF, resistance from 0.00001 Ohm to 99.999 kOhm and
inductance from 0.0001 mH to 99.999 G.
Measurements can be made at frequencies from 5 Hz to 100 MHz, although neither
one device does not cover the entire frequency range. At frequencies
close to 1 kHz, the error can be only 0.02%, but
accuracy decreases near the boundaries of frequency ranges and measured
values.
Most instruments can also display derivatives
quantities such as coil quality factor or loss factor
capacitor, calculated from the main measured values.

19.

20. ANALOG DEVICES

For measuring voltage, current and resistance on
permanent
current
apply
analog
magnetoelectric devices with permanent magnet And
multi-turn moving part.
Such pointer-type devices are characterized
error from 0.5 to 5%.
They are simple and inexpensive (for example, automobile
instruments indicating current and temperature), but not
are used where any amount is required
significant accuracy.

21. Magnetoelectric devices

Such devices use the interaction force
magnetic field with current in the turns of the moving winding
part trying to turn the latter.
The moment of this force is balanced by the moment
created by the counter spring, so that
each current value corresponds to a certain
pointer position on the scale. The moving part has
the shape of a multi-turn wire frame with dimensions from
3-5 to 25-35 mm and made as light as possible.
Movable
Part,
installed
on
stone
bearings or suspended on a metal
ribbon, placed between the poles of a strong
permanent magnet.

22.

Two spiral springs that balance the torque
moment, also serve as current conductors for the moving winding
parts.
Magnetoelectric
device
reacts
on
current,
passing along the winding of its moving part, and therefore
is
yourself
ammeter
or,
more precisely,
milliammeter (since the upper limit of the range
measurements does not exceed approximately 50 mA).
It can be adapted to measure currents larger
force, connecting parallel to the winding of the moving part
low resistance shunt resistor to
only a small fraction was branched off the winding of the moving part
total measured current.
Such a device is suitable for currents measured
many thousands of amperes. If in series with
connect an additional resistor to the winding, then the device
will turn into a voltmeter.

23.

The voltage drop across such a series
connection
equals
work
resistance
resistor to the current shown by the device, so that it
The scale can be graduated in volts.
To
do
from
magnetoelectric
milliammeter ohmmeter, you need to connect it to
sequentially measured resistors and apply to
This
sequential
compound
permanent
voltage, for example from a battery.
The current in such a circuit will not be proportional
resistance, and therefore a special scale is needed,
corrective nonlinearity. Then it will be possible
make a direct reading of resistance on the scale, although
and with not very high accuracy.

24. Galvanometers

TO
magnetoelectric
devices
relate
And
galvanometers - highly sensitive instruments for
measurements of extremely low currents.
Galvanometers do not have bearings; their moving part is
suspended on a thin ribbon or thread, used
stronger magnetic field and the arrow is replaced
a mirror glued to the suspension thread (Fig. 1).
The mirror rotates along with the moving part, and
corner
his
turning
is assessed
By
displacement
the light blip it casts on the scale,
installed at a distance of about 1 m.
The most sensitive galvanometers are capable of giving
scale deviation equal to 1 mm when changing current
by only 0.00001 µA.

25.

Figure 1. MIRROR GALVANOMETER measures the current
passing through the winding of its moving part placed in
magnetic field, according to the deflection of the light spot.
1 - suspension;
2 - mirror;
3 - gap;
4 - permanent
magnet;
5 - winding
moving part;
6 - spring
suspension.

26. RECORDING DEVICES

Recording devices record a “history” of change
values ​​of the measured quantity.
The most common types of such devices include
strip recorders that record a change curve with a pen
values ​​on chart paper tape, analog
electronic oscilloscopes unfolding the process curve
on
screen
electron beam
tubes,
And
digital
oscilloscopes that store single or infrequent
repeating signals.
The main difference between these devices is speed
records.
Tape
recorders
With
their
moving
mechanical parts most suitable for registration
signals changing in seconds, minutes and even slower.
Electronic oscilloscopes are capable of recording
signals varying over time from parts per million
seconds to several seconds.

27. MEASURING BRIDGES

Measuring
bridge
This
usually
four-armed
electric
chain,
compiled
from
resistors,
capacitors and inductors designed for
determining the ratio of the parameters of these components.
Connects to one pair of opposite poles of the circuit
power source, and to the other - a null detector.
Measuring bridges are used only in cases where
The highest measurement accuracy is required. (For measurements with
average
accuracy
better
enjoy
digital
devices because they are easier to use.)
The best
transformer
measuring
bridges
AC current are characterized by an error (measurement
ratio) of the order of 0.0000001%.
The simplest bridge for measuring resistance is named
its inventor Charles Wheatstone

28. Double DC measuring bridge

Figure 2. DOUBLE MEASURING BRIDGE (Thomson bridge) a more accurate version of the Wheatstone bridge, suitable for measurement
resistance of four-pole reference resistors in the area
microohm

29.

It is difficult to connect copper wires to a resistor without introducing
in this case, the contact resistance is of the order of 0.0001 Ohm or more.
In the case of a resistance of 1 Ohm, such a current lead introduces an error
about only 0.01%, but for a resistance of 0.001 Ohm
the error will be 10%.
Double measuring bridge (Thomson bridge), the diagram of which
shown in Fig. 2, designed to measure
resistance of small-value reference resistors.
The resistance of such four-pole reference resistors is
defined as the ratio of voltage to their potential
terminals (p1, p2 of resistor Rs and p3, p4 of resistor Rx in Fig. 2) to
current through their current terminals (c1, c2 and c3, c4).
With this technique, the resistance of the connecting connections
wires does not introduce errors into the measurement result of the desired
resistance.
Two additional arms m and n eliminate the influence
connecting wire 1 between terminals c2 and c3.
The resistances m and n of these arms are selected so that
the equality M/m = N/n was fulfilled. Then, changing
resistance Rs, reduce the imbalance to zero and find Rx =
Rs(N/M).

30. AC measuring bridges

The most common measuring bridges
AC current are designed to measure either
network frequency 50-60 Hz, or at audio frequencies
(usually around 1000 Hz); specialized
measuring bridges operate at frequencies up to 100 MHz.
Typically in AC measuring bridges
instead of two shoulders that precisely define the relationship
voltage, a transformer is used. To the exceptions
This rule applies to the measuring bridge
Maxwell - Guilt.

31. Maxwell - Wien measuring bridge

Figure 3. MAXWELL MEASURING BRIDGE - VINE for
comparison of the parameters of reference inductors (L) and
capacitors (C).

32.

Such a measuring bridge allows you to compare standards
inductance (L) with capacitance standards on an unknown
exactly the operating frequency.
Capacitance standards are used in high-voltage measurements
accuracy,
because the
They
constructively
easier
precision inductance standards, more compact,
they are easier to shield and create virtually no
external electromagnetic fields.
The equilibrium conditions for this measuring bridge are:
Lx = R2*R3*C1 and Rx = (R2*R3) / R1 (Fig. 3).
The bridge is balanced even in the case of "unclean"
power supply (i.e. a signal source containing
harmonics of the fundamental frequency), if the Lx value is not
depends on frequency.

33. Transformer measuring bridge

Figure 4. TRANSFORMER MEASURING BRIDGE
alternating current for comparison of similar full
resistance

34.

One of the advantages of AC measuring bridges
- ease of setting the exact voltage ratio using
transformer.
Unlike voltage dividers built from
resistors, capacitors or inductors,
transformers maintain for a long time
the established voltage ratio is constant and rarely
require recalibration.
On
rice.
4
presented
scheme
transformer
measuring bridge for comparing two similar complete
resistance.
The disadvantages of a transformer measuring bridge
Can
attribute
That,
What
attitude,
given
transformer, to some extent depends on the frequency
signal.
This
leads
To
necessity
design
transformer
measuring
bridges
only
For
limited frequency ranges in which it is guaranteed
passport accuracy.

35. MEASUREMENT OF AC SIGNALS

In case of time-varying AC signals
usually it is necessary to measure some of their characteristics,
associated with instantaneous signal values.
More often
Total
preferably
know
root mean square
(effective) values ​​of electrical quantities of alternating
current, since the heating power at a voltage of 1V
DC corresponds to the heating power at
voltage 1 V AC.
Along with this, other quantities may be of interest,
for example the maximum or average absolute value.
RMS (effective) voltage value
(or alternating current) is defined as the root
square of time-averaged voltage squared
(or current):

36.

where T is the period of the signal Y(t).
The maximum value Ymax is the largest instantaneous value
signal, and the average absolute value YAA is the absolute value,
averaged over time.
With a sinusoidal oscillation, Yeff = 0.707Ymax and
YAA = 0.637Ymax.

37. AC voltage and current measurement

Almost all voltage and force measuring instruments
AC current show the value that
proposed to be considered as the effective value
input signal.
However, in cheap devices it is often actually
the absolute average or maximum is measured
signal value, and the scale is calibrated so that
indication
corresponded
equivalent
effective value under the assumption that the input
the signal has a sinusoidal shape.
It should not be overlooked that the accuracy of such instruments
extremely low if the signal is non-sinusoidal.

38.

Instruments capable of measuring true effective
meaning of AC signals may be
based on one of three principles: electronic
multiplication, signal sampling or thermal
transformations.
Devices based on the first two principles, like
usually respond to voltage, and thermal
electrical measuring instruments - for current.
When using additional and shunt resistors
All devices can measure both current and
voltage.

39. Thermal electrical measuring instruments

Highest measurement accuracy of effective values
voltage
And
current
provide
thermal
electrical measuring instruments. They use
thermal current converter in the form of a small
evacuated glass cartridge with heating
wire (0.5-1 cm long), to the middle part of which
A tiny bead is attached to the hot junction of the thermocouple.
The bead provides thermal contact and at the same time
electrical insulation.
With an increase in temperature directly related to
effective
meaning
current
V
heating
wire, a thermo-EMF occurs at the output of the thermocouple
(DC voltage).
Such transducers are suitable for measuring force
alternating current with a frequency from 20 Hz to 10 MHz.

40.

In Fig. 5 shows a schematic diagram of thermal
electrical measuring instrument with two selected
according to the parameters of thermal current converters.
When AC voltage is applied to the circuit input
Vaс at the output of the thermocouple of the converter TC1 appears
DC voltage, amplifier A creates
constant
current
V
heating
delay
converter TC2, in which the thermocouple of the latter
gives the same DC voltage, and normal
A DC instrument measures the output current.

41.

Figure 5. THERMAL ELECTRICAL MEASURING DEVICE for
measuring the effective values ​​of voltage and alternating power
current
Using an additional resistor, the described current meter can be
turn into a voltmeter. Since thermal electrical measuring
devices directly measure currents only from 2 to 500 mA, for
For measuring higher currents, resistor shunts are required.

42. AC Power and Energy Measurement

Power consumed by the load in the AC circuit
current, equal to the time-average product
instantaneous values ​​of voltage and load current.
If voltage and current vary sinusoidally (as
this usually happens), then the power P can be represented in
form P = EI cosj, where E and I are effective values
voltage and current, and j is the phase angle (shift angle)
sinusoidal voltage and current.
If voltage is expressed in volts and current in amperes,
then the power will be expressed in watts.
The cosj multiplier, called power factor,
characterizes
degree
synchronicity
fluctuations
voltage and current.

43.

WITH
economic
points
vision,
the most
important
electrical quantity - energy.
Energy W is determined by the product of power and
time of its consumption. In mathematical form this is
is written like this:
If time (t1 - t2) is measured in seconds, voltage e in volts, and current i in amperes, then the energy W will be
expressed in watt-seconds, i.e. joules (1 J = 1 W*s).
If time is measured in hours, then energy is measured in watt-hours. In practice, it is more convenient to express electricity in
kilowatt-hours (1 kW*h = 1000 Wh).

44. Induction electricity meters

An induction meter is nothing more than
as a low-power AC motor with
two windings - a current and a voltage winding.
A conductive disk placed between the windings
rotates
under
action
torque
moment,
proportional to power consumption.
This moment is balanced by the currents induced in
disk with a permanent magnet, so that the rotation speed
disk is proportional to power consumption.

45.

The number of revolutions of the disk for a given time
proportional to the total electricity received during
this is consumer time.
The number of revolutions of the disk is counted by a mechanical counter,
which shows electricity in kilowatt-hours.
Devices of this type are widely used as
household electricity meters.
Their error is usually 0.5%; They
characterized by a long service life under any
permissible current levels.

The content of the article

ELECTRICAL MEASUREMENTS, measurement of electrical quantities such as voltage, resistance, current, power. Measurements are made using various means - measuring instruments, circuits and special devices. The type of measuring device depends on the type and size (range of values) of the measured value, as well as on the required measurement accuracy. The basic SI units used in electrical measurements are volt (V), ohm (Ω), farad (F), henry (H), ampere (A), and second (s).

STANDARDS OF UNITS OF ELECTRICAL QUANTITIES

Electrical measurement is the determination (using experimental methods) of the value of a physical quantity expressed in appropriate units (for example, 3 A, 4 V). The values ​​of units of electrical quantities are determined by international agreement in accordance with the laws of physics and units of mechanical quantities. Since “maintaining” units of electrical quantities determined by international agreements is fraught with difficulties, they are presented as “practical” standards for units of electrical quantities. Such standards are supported by state metrological laboratories in different countries. For example, in the United States, the National Institute of Standards and Technology bears the legal responsibility for maintaining standards for units of electrical quantities. From time to time, experiments are carried out to clarify the correspondence between the values ​​of the standards of units of electrical quantities and the definitions of these units. In 1990, state metrology laboratories of industrialized countries signed an agreement to harmonize all practical standards of units of electrical quantities among themselves and with international definitions of units of these quantities.

Electrical measurements are carried out in accordance with state standards of units of voltage and direct current, direct current resistance, inductance and capacitance. Such standards are devices that have stable electrical characteristics, or installations in which, on the basis of a certain physical phenomenon, an electrical quantity is reproduced, calculated by known values fundamental physical constants. Watt and watt-hour standards are not supported, since it is more appropriate to calculate the values ​​of these units using defining equations that relate them to units of other quantities.

MEASURING INSTRUMENTS

Electrical measuring instruments most often measure instantaneous values ​​of either electrical quantities or non-electrical quantities converted into electrical ones. All devices are divided into analog and digital. The former usually show the value of the measured quantity by means of an arrow moving along a scale with divisions. The latter are equipped with a digital display that shows the measured value in the form of a number. Digital instruments are preferable for most measurements because they are more accurate, easier to take readings and generally more versatile. Digital multimeters ("multimeters") and digital voltmeters are used to measure DC resistance, as well as AC voltage and current, with medium to high accuracy. Analog devices are gradually being replaced by digital ones, although they are still used where low cost is important and high accuracy is not needed. For the most accurate measurements of resistance and impedance, there are measuring bridges and other specialized meters. To record the progress of changes in the measured value over time, recording instruments are used - strip recorders and electronic oscilloscopes, analog and digital.

DIGITAL INSTRUMENTS

All digital meters (except the simplest ones) use amplifiers and other electronic components to convert the input signal into a voltage signal, which is then converted to digital form by an analog-to-digital converter (ADC). A number expressing the measured value is displayed on a light emitting diode (LED), vacuum fluorescent or liquid crystal (LCD) indicator (display). The device usually operates under the control of a built-in microprocessor, and in simple devices the microprocessor is combined with an ADC on a single integrated circuit. Digital devices are well suited to work when connected to an external computer. In some types of measurements, such a computer switches the measuring functions of the device and gives data transmission commands for their processing.

Analog-to-digital converters.

There are three main types of ADCs: integrating, successive approximation, and parallel. An integrating ADC averages the input signal over time. Of the three types listed, this is the most accurate, although the slowest. The conversion time of the integrating ADC ranges from 0.001 to 50 s or more, the error is 0.1–0.0003%. The error of the successive approximation ADC is slightly larger (0.4–0.002%), but the conversion time is from ~10 µs to ~1 ms. Parallel ADCs are the fastest, but also the least accurate: their conversion time is about 0.25 ns, the error is from 0.4 to 2%.

Discretization methods.

The signal is sampled in time by quickly measuring it at individual points in time and holding (saving) the measured values ​​while they are converted to digital form. The sequence of obtained discrete values ​​can be displayed on the display in the form of a waveform; by squaring these values ​​and summing, you can calculate the root mean square value of the signal; they can also be used to calculate rise time, maximum value, time average, frequency spectrum, etc. Time sampling can be done either over a single signal period ("real time"), or (with sequential or random sampling) over a number of repeating periods.

Digital voltmeters and multimeters.

Digital voltmeters and multimeters measure a quasi-static value of a quantity and indicate it in digital form. Voltmeters directly measure only voltage, usually DC, while multimeters can measure DC and AC voltage, current, DC resistance and sometimes temperature. These most common general purpose test instruments, with measurement accuracy ranging from 0.2 to 0.001%, can have a 3.5 or 4.5 digit digital display. A “half-integer” character (digit) is a conditional indication that the display can show numbers beyond the nominal number of characters. For example, a 3.5-digit (3.5-digit) display in the 1-2V range can show voltages up to 1.999V.

Impedance meters.

These are specialized instruments that measure and display the capacitance of a capacitor, the resistance of a resistor, the inductance of an inductor, or the total resistance (impedance) of the connection of a capacitor or inductor to a resistor. Instruments of this type are available to measure capacitance from 0.00001 pF to 99.999 µF, resistance from 0.00001 ohm to 99.999 kohm, and inductance from 0.0001 mH to 99.999 H. Measurements can be made at frequencies from 5 Hz to 100 MHz, although one device does not cover the entire frequency range. At frequencies close to 1 kHz, the error can be as low as 0.02%, but the accuracy decreases near the boundaries of the frequency ranges and measured values. Most instruments can also display derived values, such as the quality factor of a coil or the loss factor of a capacitor, calculated from the main measured values.

ANALOG DEVICES

To measure voltage, current and resistance at direct current, analog magnetoelectric devices with a permanent magnet and a multi-turn moving part are used. Such pointer-type devices are characterized by an error of 0.5 to 5%. They are simple and inexpensive (for example, automotive instruments that indicate current and temperature), but are not used where any significant accuracy is required.

Magnetoelectric devices.

Such devices use the force of interaction between the magnetic field and the current in the turns of the winding of the moving part, which tends to turn the latter. The moment of this force is balanced by the moment created by the opposing spring, so that each current value corresponds to a certain position of the arrow on the scale. The moving part has the shape of a multi-turn wire frame with dimensions from 3-5 to 25-35 mm and is made as light as possible. The moving part, mounted on stone bearings or suspended on a metal strip, is placed between the poles of a strong permanent magnet. Two spiral springs that balance the torque also serve as conductors for the winding of the moving part.

A magnetoelectric device reacts to the current passing through the winding of its moving part, and therefore is an ammeter or, more precisely, a milliammeter (since the upper limit of the measurement range does not exceed approximately 50 mA). It can be adapted to measure higher currents by connecting a low-resistance shunt resistor in parallel with the moving part winding so that only a small fraction of the total current being measured is branched into the moving part winding. Such a device is suitable for currents measured in many thousands of amperes. If you connect an additional resistor in series with the winding, the device will turn into a voltmeter. The voltage drop across such a series connection is equal to the product of the resistance of the resistor and the current shown by the device, so its scale can be calibrated in volts. To make an ohmmeter from a magnetoelectric milliammeter, you need to connect resistors in series to it and apply a series connection to it. constant pressure, for example from a battery. The current in such a circuit will not be proportional to the resistance, and therefore a special scale is needed to correct the nonlinearity. Then it will be possible to directly read the resistance on the scale, although not with very high accuracy.

Galvanometers.

Magnetoelectric devices also include galvanometers - highly sensitive devices for measuring extremely small currents. Galvanometers do not have bearings; their moving part is suspended on a thin ribbon or thread, a stronger magnetic field is used, and the pointer is replaced by a mirror glued to the suspension thread (Fig. 1). The mirror rotates along with the moving part, and the angle of its rotation is estimated by the displacement of the light spot it casts on a scale installed at a distance of about 1 m. The most sensitive galvanometers are capable of giving a scale deviation of 1 mm with a change in current of only 0.00001 μA.

RECORDING DEVICES

Recording instruments record the “history” of changes in the value of the measured quantity. The most common types of such instruments include strip chart recorders, which record a curve of change in value with a pen on a chart paper tape, analog electronic oscilloscopes, which display the process curve on the screen of a cathode ray tube, and digital oscilloscopes, which store single or rarely repeated signals. The main difference between these devices is the recording speed. Strip recorders, with their moving mechanical parts, are most suitable for recording signals that change over seconds, minutes, or even more slowly. Electronic oscilloscopes are capable of recording signals that change over time from millionths of a second to several seconds.

MEASURING BRIDGES

A measuring bridge is usually a four-arm electrical circuit composed of resistors, capacitors and inductors, designed to determine the ratio of the parameters of these components. A power source is connected to one pair of opposite poles of the circuit, and a null detector is connected to the other. Measuring bridges are used only in cases where the highest measurement accuracy is required. (For medium-accuracy measurements, it is better to use digital instruments because they are easier to handle.) The best AC transformer measuring bridges have an error (ratio measurement) of the order of 0.0000001%. The simplest bridge for measuring resistance is named after its inventor, Charles Wheatstone.

Double DC measuring bridge.

It is difficult to connect copper wires to a resistor without introducing contact resistance of the order of 0.0001 ohms or more. In the case of a resistance of 1 Ohm, such a current lead introduces an error of the order of only 0.01%, but for a resistance of 0.001 Ohm the error will be 10%. Double measuring bridge (Thomson bridge), the diagram of which is shown in Fig. 2, is intended for measuring the resistance of small-value reference resistors. The resistance of such four-pole reference resistors is defined as the ratio of the voltage at their potential terminals ( R 1 , R 2 resistors R s And R 3 , p 4 resistors Rx in Fig. 2) to current through their current terminals ( With 1 , With 2 and With 3 , With 4). With this technique, the resistance of the connecting wires does not introduce errors into the result of measuring the desired resistance. Two additional arms m And n eliminate the influence of the connecting wire 1 between terminals With 2 and With 3. Resistance m And n these shoulders are selected so that the equality is satisfied M/m= N/n. Then, changing the resistance R s, reduce the imbalance to zero and find

Rx = R s(N/M).

AC measuring bridges.

The most common AC measurement bridges are designed to measure at either line frequency 50–60 Hz or audio frequencies (usually around 1000 Hz); specialized measuring bridges operate at frequencies up to 100 MHz. As a rule, in AC measuring bridges, instead of two arms that precisely set the voltage ratio, a transformer is used. Exceptions to this rule include the Maxwell-Wien measuring bridge.

Maxwell-Wien measuring bridge.

Such a measuring bridge makes it possible to compare inductance standards ( L) with capacitance standards at an unknown operating frequency. Capacitance standards are used in high-precision measurements because they are simpler in design than precision inductance standards, more compact, easier to shield, and create virtually no external electromagnetic fields. The equilibrium conditions for this measuring bridge are: L x = R 2 R 3 C 1 and Rx = (R 2 R 3) /R 1 (Fig. 3). The bridge is balanced even in the case of an “impure” power supply (i.e. a signal source containing harmonics of the fundamental frequency), if the value L x does not depend on frequency.

Transformer measuring bridge.

One of the advantages of AC measuring bridges is the ease of setting the exact voltage ratio using a transformer. Unlike voltage dividers built from resistors, capacitors or inductors, transformers maintain a constant voltage ratio over a long period of time and rarely require recalibration. In Fig. Figure 4 shows a diagram of a transformer measuring bridge for comparing two impedances of the same type. The disadvantages of a transformer measuring bridge include the fact that the ratio specified by the transformer depends to some extent on the frequency of the signal. This leads to the need to design transformer measuring bridges only for limited frequency ranges in which rated accuracy is guaranteed.

Grounding and shielding.

Typical null detectors.

In AC measuring bridges, two types of null detectors are most often used. The null detector of one of them is a resonant amplifier with an analog output device that shows the signal level. Another type of null detector is a phase-sensitive detector that separates the unbalance signal into active and reactive components and is useful in applications where only one of the unknown components (say, inductance) needs to be accurately balanced L, but not resistance R inductors).

MEASUREMENT OF AC SIGNALS

In the case of time-varying AC signals, it is usually necessary to measure some of their characteristics associated with the instantaneous values ​​of the signal. Most often, it is desirable to know the RMS (rms) AC electrical values, since the heating power at 1 VDC corresponds to the heating power at 1 Vrms AC. Along with this, other quantities may be of interest, for example the maximum or average absolute value. The root mean square (effective) value of the voltage (or strength) of an alternating current is determined as the square root of the time-averaged square of the voltage (or current):

Where T– signal period Y(t). Maximum value Y max is the largest instantaneous value of the signal, and the average absolute value YAA– absolute value averaged over time. With a sinusoidal oscillation Y eff = 0.707 Y max and YAA = 0,637Y Max.

AC voltage and current measurement.

Almost all instruments for measuring AC voltage and current show a value that is proposed to be considered as the effective value of the input signal. However, cheap instruments often actually measure the average absolute or maximum value of the signal and calibrate the scale so that the reading corresponds to the equivalent effective value, assuming the input signal is a sinusoidal waveform. It should not be overlooked that the accuracy of such devices is extremely low if the signal is non-sinusoidal. Instruments capable of measuring the true rms value of AC signals can be based on one of three principles: electronic multiplication, signal sampling, or thermal conversion. Devices based on the first two principles, as a rule, respond to voltage, and thermal electrical measuring instruments – to current. When using additional and shunt resistors, all devices can measure both current and voltage.

Electronic multiplication.

Squaring and averaging over time the input signal to some approximation is carried out electronic circuits with amplifiers and nonlinear elements to perform such mathematical operations, like finding the logarithm and antilogarithm of analog signals. Devices of this type can have an error of the order of only 0.009%.

Signal sampling.

The AC signal is converted into digital form using a high-speed ADC. The sampled signal values ​​are squared, summed, and divided by the number of sampled values ​​in one signal period. The error of such devices is 0.01–0.1%.

Thermal electrical measuring instruments.

The highest accuracy of measuring the effective values ​​of voltage and current is provided by thermal electrical measuring instruments. They use a thermal current converter in the form of a small evacuated glass container with a heating wire (0.5–1 cm long), to the middle part of which a thermocouple hot junction is attached with a tiny bead. The bead provides thermal contact and at the same time electrical insulation. With an increase in temperature, directly related to the effective value of the current in the heating wire, a thermo-EMF (direct current voltage) appears at the output of the thermocouple. Such converters are suitable for measuring AC current with a frequency from 20 Hz to 10 MHz.

In Fig. Figure 5 shows a schematic diagram of a thermal electrical measuring device with two thermal current converters selected according to parameters. When AC voltage is applied to the circuit input V ac at the thermocouple output of the converter TS 1 DC voltage occurs, amplifier A creates a direct current in the heating wire of the converter TS 2, in which the latter's thermocouple produces the same DC voltage, and a conventional DC meter measures the output current.

Using an additional resistor, the described current meter can be converted into a voltmeter. Since thermal electrical meters directly measure currents only from 2 to 500 mA, resistor shunts are needed to measure higher currents.

AC power and energy measurement.

The power consumed by the load in an AC circuit is equal to the time-average product of the instantaneous values ​​of voltage and load current. If the voltage and current vary sinusoidally (as is usually the case), then the power R can be represented in the form P = EI cos j, Where E And I are the effective values ​​of voltage and current, and j– phase angle (shift angle) of voltage and current sinusoids. If voltage is expressed in volts and current in amperes, then power will be expressed in watts. cos multiplier j, called power factor, characterizes the degree of synchronism of voltage and current fluctuations.

WITH economic point From a perspective, the most important electrical quantity is energy. Energy W is determined by the product of power and the time of its consumption. In mathematical form this is written like this:

If time ( t 1 - t 2) measured in seconds, voltage e- in volts, and current i– in amperes, then energy W will be expressed in watt-seconds, i.e. joules (1 J = 1 Wh s). If time is measured in hours, then energy is measured in watt-hours. In practice, it is more convenient to express electricity in kilowatt-hours (1 kWh h = 1000 Wh).

Time-sharing electricity meters.

Time-sharing electricity meters use a very unique but accurate method of measuring electrical power. This device has two channels. One channel is an electronic key that allows or does not pass the input signal Y(or reversed input signal - Y) to a low pass filter. The state of the key is controlled by the output signal of the second channel with the “closed”/“open” time interval ratio proportional to its input signal. The average signal at the filter output is equal to the time average of the product of the two input signals. If one input signal is proportional to the load voltage and the other is proportional to the load current, then the output voltage is proportional to the power consumed by the load. The error of such industrial counters is 0.02% at frequencies up to 3 kHz (laboratory ones are about only 0.0001% at 60 Hz). As high-precision instruments, they are used as standard counters for checking working measuring instruments.

Sampling wattmeters and electricity meters.

Such devices are based on the principle of a digital voltmeter, but have two input channels that sample current and voltage signals in parallel. Each discrete value e(k), representing the instantaneous values ​​of the voltage signal at the time of sampling, is multiplied by the corresponding discrete value i(k) current signal received at the same time. The time average of such products is the power in watts:

An adder that accumulates the products of discrete values ​​over time gives the total electricity in watt-hours. The error of electricity meters can be as little as 0.01%.

Induction electricity meters.

An induction meter is nothing more than a low-power AC electric motor with two windings - a current winding and a voltage winding. A conductive disk placed between the windings rotates under the influence of a torque proportional to the power consumed. This torque is balanced by currents induced in the disk by a permanent magnet, so that the rotation speed of the disk is proportional to the power consumption. The number of revolutions of the disk for a given time is proportional to the total electricity received by the consumer during this time. The number of revolutions of the disk is counted by a mechanical counter, which shows electricity in kilowatt-hours. Devices of this type are widely used as household electricity meters. Their error is usually 0.5%; They have a long service life at any permissible current levels.

Literature:

Atamalyan E.G. and etc. Instruments and methods for measuring electrical quantities. M., 1982
Malinovsky V.N. and etc. Electrical measurements. M., 1985
Avdeev B.Ya. and etc. Fundamentals of metrology and electrical measurements. L., 1987



Energy saving and energy efficiency of industry cannot be imagined without electrical measurements, since it is impossible to save what you do not know how to account for.

Electrical measurements are carried out in one of the following types: direct, indirect, cumulative and joint. Name direct view speaks for itself, the value of the required value is determined directly by the device. An example of such measurements is determining power with a wattmeter, current with an ammeter, etc.

Indirect view consists in finding a value based on the known dependence of this value and the value found by the direct method. An example is determining power without a wattmeter. Using the direct method, I, U, phase are found and the power is calculated using the formula.

Cumulative and joint species measurements consist in the simultaneous measurement of several quantities of the same name (cumulative) or not of the same name (joint) quantities. Finding the required quantities is carried out by solving systems of equations with coefficients obtained as a result of direct measurements. The number of equations in such a system must be equal to the number of required quantities.

Direct measurements As the most common type of measurements, they can be made by two main methods:

• direct assessment method
• comparison method with measure.

The first method is the simplest, since the value of the desired value is determined on the scale of the device.

This method determines the current strength with an ammeter, the voltage of voltmeters, etc. Advantage this method can be called simplicity, but the disadvantage is low accuracy.

Measurements by comparison with a measure are performed using one of the following methods: substitution, opposition, coincidence, differential and zero. A measure is a kind of reference value of a certain quantity.

Differential and zero methods– are the basis for the operation of measuring bridges. With the differential method, unbalanced indicating bridges are made, and with the zero method, balanced or zero ones are made.

In balanced bridges, comparison occurs using two or more auxiliary resistances, selected in such a way that with the resistances being compared they form a closed circuit (four-terminal network), powered from a single source and having equipotential points detected by a balance indicator.

The ratio between auxiliary resistances is a measure of the relationship between the quantities being compared. The balance indicator in DC circuits is a galvanometer, and in AC circuits a millivoltmeter.

The differential method is otherwise called the difference method, since the measuring instrument is affected precisely by the difference between the known and desired current values. The null method is an extreme case differential method. For example, in the indicated bridge circuit, the galvanometer shows zero if the equality is met:

R1*R3 = R2*R4;

From this expression it follows:

Rx=R1=R2*R4/R3.

Thus, it is possible to calculate the resistance of any unknown element, provided that the other 3 are exemplary. The direct current source should also be exemplary.

Contrasting method- otherwise this method is called compensation and is used for direct comparison of voltage or EMF, current and indirectly for measuring other quantities converted into electrical quantities.

Two counter-directed EMFs, not connected to each other, are switched on to a device that balances the branches of the circuit. In the picture: you need to find Ux. Using an exemplary adjustable resistance Rk, a voltage drop Uk is achieved such that it is numerically equal to Ux.

Their equality can be judged by the readings of a galvanometer. If U and Ux are equal, no current will flow in the galvanometer circuit, since they are oppositely directed. Knowing the resistance and current value, we determine Ux using the formula.

Substitution method– a method in which the desired value is replaced or combined with a known standard value, equal in value to the substituted one. This method is used to determine the inductance or capacitance of an unknown value. An expression that determines the dependence of frequency on circuit parameters:

fo=1/(√LC)

On the left, the frequency f0 is set by the RF generator, on the right side are the values ​​of the inductance and capacitance of the measured circuit. By selecting the resonance frequency you can determine unknown values on the right side of the expression.

An indicator of resonance is an electronic voltmeter with a high input resistance, the readings of which will be greatest at the moment of resonance. If the inductor being measured is connected in parallel with a reference capacitor and measured resonant frequency, then the value of Lx can be found using the above expression. The unknown capacity is located similarly.

First, the resonant circuit, consisting of inductance L and a model capacitor Co, is tuned to resonance at frequency fo; at the same time, the values ​​of fo and the capacitance of the capacitor Co1 are recorded.

Then, in parallel with the model capacitor Co, a capacitor Cx is connected by changing the capacitance of the model capacitor to achieve resonance at the same frequency fo; Accordingly, the required quantity is Co2.

Match method– a method in which the difference between the desired and known quantity determined by the coincidence of scale marks or periodic signals. A striking example The application of this method in life is to measure the angular velocity of rotation of various parts.

To do this, a mark is applied to the object being measured, for example, a small mark. When a part with a mark is rotated, a strobe light is directed at it, the blinking frequency of which is initially known. By adjusting the frequency of the strobe, you can ensure that the mark stays in place. In this case, the rotation frequency of the part is taken equal to the blinking frequency of the strobe light.

ELECTRICAL MEASUREMENTS AND INSTRUMENTS

3.1. The role of measurements in electrical engineering

In any field of knowledge, measurements are extremely important, but they are especially important in electrical engineering.

A person senses mechanical, thermal, and light phenomena with the help of his senses. We, although approximately, can estimate the size of objects, the speed of their movement, and the brightness of luminous bodies. For a long time this is how people studied the starry sky.

But you and I react in exactly the same way to a conductor whose current is 10 mA or 1 A(i.e. 100 times more).

We see the shape of the conductor, its color, but our senses do not allow us to assess the magnitude of the current. In the same way we are completely indifferent to magnetic field, created by the coil, the electric field between the plates of the capacitor. Medicine has established a certain influence of electric and magnetic fields on the human body, but we do not feel this influence, and the magnitude electromagnetic field we cannot evaluate.

The only exceptions are very strong fields. But here, too, the unpleasant tingling sensation, which can be noticed while walking around the eye of a high-voltage transmission line, will not allow us to even approximately estimate the value electrical voltage in line.

All this forced physicists and engineers from the first steps of research and application of electricity to use electrical measuring instruments.

Instruments are the eyes and ears of an electrical engineer. Without them he is deaf and blind and completely helpless. Millions of electrical measuring instruments are installed in factories and research laboratories. Each apartment also has a measuring device - an electric meter.

The readings (signals) of electrical measuring instruments are used to assess the operation of various electrical devices and the condition of electrical equipment, in particular the state of insulation. Electrical measuring instruments are distinguished by high sensitivity, measurement accuracy, reliability and ease of implementation.

The successes of electrical instrument making led to the fact that other industries began to use its services. Electrical methods began to be used to determine sizes, speeds, mass, and temperature. Even an independent discipline has emerged “ Electrical measurements of non-electrical quantities”.

The readings of electrical measuring instruments can be transmitted over long distances (telemetering), they can be used for direct impact on production processes(automatic regulation); with their help, the progress of controlled processes is recorded, for example by recording on tape, etc.

The use of semiconductor technology has significantly expanded the use of electrical measuring instruments.

To measure any physical quantity means to find its value experimentally using special technical means.

Bench testing of the latest equipment is unthinkable without electrical measurements. Thus, when testing a turbogenerator with a power of 1200 MW At the Elektrosila plant, measurements were taken at 1,500 points.

The development of electrical measuring instruments has led to the use of microelectronics in them, which makes it possible to measure physical quantities with an error of no more than 0.005-0.0005%.

3.2. Basic concepts, terms and definitions

The results of theoretical activities without verification by experiment are unreliable. Measuring equipment during an experiment gives results that indicate the quality and quantity of products, the correctness of technological processes, distribution, consumption and manufacturing. At the same time, electrical measurements, due to low energy consumption, the possibility of transmitting measured values ​​over a distance, high speed of measurements and transmission, as well as high accuracy and sensitivity, turned out to be preferable.

Electrical measurements and instruments, methods and means of ensuring their unity, methods of achieving the required accuracy - all this relates to metrology, and the principles and methods for establishing optimal norms and rules of interaction - to standardization.

IN Russian Federation standardization and metrology are combined into a single public service- State Committee of Standards. In 1963, GOST 9867-61 introduced the International System of Units (SI) based on the meter ( m), kilogram ( kg), seconds ( With), ampere ( A), kelvin ( TO) and candelas ( cd).

Issues of electrical measurements and instruments are easier to understand if the content of the terms and definitions is known.

Metrology- the science of measurements, methods and means of ensuring their unity, and methods of achieving the required accuracy.

Measurement- finding the value of a physical quantity experimentally using special technical means.

Measurement result- the value of a physical quantity found by measurement.

Measure- a measuring instrument designed to reproduce a physical quantity of a given size (for example, the unit of measurement of light - cd).

Transducer- a measuring instrument for generating a signal of measuring information in a form convenient for transmission, further conversion, processing (or storage), but not amenable to direct perception by an observer. The primary measuring transducer is a sensor.

Measuring device- a measuring instrument designed to generate a signal of measuring information in a form accessible to direct perception by an observer.

3.3. Measurement methods. Measurement error

For various measured electrical quantities there are their own measuring instruments, so-called measures. For example, normal elements serve as measures of EMF, measuring resistors serve as measures of electrical resistance, measuring inductors serve as measures of inductance, capacitors of constant capacitance serve as measures of electrical capacitance, etc.

In practice, various methods are used to measure various physical quantities. The latter, depending on the method of obtaining the result, are divided into straight And indirect. At direct measurement the value of the quantity is obtained directly from experimental data. At indirect measurement the desired value of a quantity is found by counting using a known relationship between this quantity and values ​​obtained from direct measurements. Thus, the resistance of a section of a circuit can be determined by measuring the current flowing through it and the applied voltage, followed by calculating this resistance from Ohm’s law. The most widely used methods in electrical measuring technology are direct measurement methods, since they are usually simpler and require less time.

In electrical measuring technology they also use comparison method, which is based on a comparison of the measured value with a reproducible measure. The comparison method can be compensatory or bridge. Application example compensation method serves to measure voltage by comparing its value with the value of the EMF of a normal element. Example bridge method is to measure resistance using a four-arm bridge circuit. Measurements using the compensation and bridge methods are very accurate, but they require more sophisticated measuring equipment.