Topic: Electrical measuring instruments and measurements of electrical quantities. Topic: Electrical measuring instruments and measurements of electrical quantities Measurement of electrical quantities during operation
Measuring instrument - a technical device intended for measurements, having standardized metrological characteristics, reproducing and (or) storing a unit of physical quantity, the size of which is assumed to be unchanged (within the limits of the established error) for a known time interval. This definition reveals the essence of the measuring instrument, which consists in the ability to store (or reproduce) a unit of physical quantity, as well as the invariability of the size of the stored unit. These factors determine the possibility of performing a measurement.
By purpose measuring instruments are divided into measures, measuring transducers, measuring instruments, measuring installations and measuring systems.
Measure - a measuring instrument designed to reproduce and (or) store a physical quantity of one or more specified dimensions, the values of which are expressed in established units and are known with the required accuracy.
The following types of measures are distinguished:
● unambiguous measure - the measure reproduces a physical quantity of the same size;
● multi-valued measure - the measure reproduces a physical quantity of different sizes;
● set of measures - set of measures different sizes the same physical quantity;
● store measures ~ a set of measures structurally combined into a single device, which contains devices for connecting them into various combinations. For example, a store electrical resistance provides a range of discrete resistance values.
Some measures simultaneously reproduce the values of two physical quantities. A measure is necessary in the comparison method to compare the measured value with it and obtain its value.
Transducer - a technical device with standardized metrological characteristics, used to convert a measured value into another value or measuring signal, convenient for processing, storage, further transformations, indication or transmission. Its operating principle is based on various physical phenomena. The measuring transducer converts any physical quantities (electrical, non-electrical, magnetic) into an electrical signal.
By the nature of the transformation a distinction is made between analog and analog-to-digital converters (ADCs), which convert a continuous value into a numerical equivalent, and digital-to-analog converters (DACs), which perform the inverse conversion.
Locally in the measuring room the converter circuits are divided into a primary one, which is directly affected by the measured physical quantity; intermediate, included in the measuring circuit after the primary; converters designed for large-scale conversion, i.e. to change the value of a quantity a certain number of times; transmitting, reverse for inclusion in the circuit feedback and etc.
Measuring transducers include transducers AC voltage to DC, measuring voltage and current transformers, current dividers, voltage dividers, amplifiers, comparators, thermocouple, etc. Measuring transducers are part of any measuring device, measuring installation, measuring system or are used together with any measuring instrument.
Measuring device(IP) is a measuring instrument designed to obtain the values of the measured physical quantity in the established range. There are indicating and recording instruments, digital and analogue.
Measuring setup— a set of functionally combined measures, measuring transducers, measuring instruments and other devices. Designed for measuring one or more physical quantities and located in one place, for example, an installation for measuring the characteristics of a transistor, an installation for measuring power in three-phase circuits, etc.
Measuring system - a set of functionally combined measures, measuring instruments, measuring transducers, computers and other technical means located at different points of a controlled object for the purpose of measuring one or more physical quantities characteristic of this object and generating signals for various purposes.
Depending on the purpose, measuring systems are divided into measuring information, monitoring, technical diagnostics, etc. Microprocessor measuring systems - control computing systems with microprocessor (MP) as an information processing node. In general, the MP includes: an arithmetic-logical unit, a block of internal registers for temporary storage of data and commands, a control device, internal bus lines, input-output data buses for connecting external devices.
LECTURE No. 1
Subject:ELECTRICAL INSTRUMENTS AND MEASUREMENTS OF ELECTRICAL QUANTITIES
1. General information about electrical measuring instruments
Electrical measuring instruments are designed to measure various quantities and parameters electrical circuit: voltage, current, power, frequency, resistance, inductance, capacitance and others.
In the diagrams, electrical measuring instruments are depicted with conventional graphic symbols in accordance with GOST 2.729-68. Figure 1.1 shows the general designations of indicating and recording devices.
Rice. 1.1 Symbols of electrical measuring instruments.
To indicate the purpose of an electrical measuring device, a specific symbol established in the standards or a letter designation of the units of measurement of the device according to GOST in accordance with Table 1.1 are entered into its general designation.
Table 1.1
Name units | Name units | Symbol |
|
Milliamp | |||
microamp | |||
Millivolt | |||
Kilowatt | |||
Power factor |
2. Electromechanical measuring instruments
According to the principle of operation, electromechanical devices are divided into devices of magnetoelectric, electromagnetic, ferrodynamic, induction, and electrostatic systems. Symbols of systems are given in table. 1.2. The most widespread devices are the first three types: magnetoelectric, electromagnetic, electrodynamic.
Table 1.2
Device type | Symbol | Type of current being measured | Advantages | Flaws |
|
electric | Constant | High accuracy, scale uniformity | Unresistant to overloads |
|
|
magnetic | Variable constant | Simplicity of the device, resistant to overloads | Low accuracy, sensitive to interference |
|
|
dynamic | Variable constant | High accuracy | Low sensitivity sensitive to interference |
|
Induction | Variable | High reliability, resistant to overloads | Low accuracy |
3. Application areas of electromechanical devices
Magnetoelectric devices: panel and laboratory ammeters and voltmeters; zero indicators when measuring in bridge and compensation circuits.
In industrial installations alternating current low frequency, most ammeters and voltmeters are devices of the electromagnetic system. Laboratory instruments of class 0.5 and more accurately can be manufactured to measure direct and alternating currents and voltage.
Electrodynamic mechanisms are used in laboratory and model instruments for measuring direct and alternating currents, voltages and powers.
Induction devices based on induction mechanisms are used mainly as single- and three-phase AC energy meters. According to accuracy, meters are divided into classes 1.0; 2.0; 2.5. The CO meter (single-phase meter) is used to account for active energy (watt-hours) in single-phase circuits. To measure active energy in three-phase circuits, two-element inductive meters are used, the counting mechanism of which takes into account kilowatt-hours. To account for reactive energy, special inductive meters are used, which have some changes in the design of the windings or in the switching circuit.
Active and reactive meters are installed at all enterprises to pay energy supply organizations for the electricity used.
Principle of selection of measuring instruments
1. By calculating the circuit, determine the maximum values of current, voltage and power in the circuit. Often the values of the measured quantities are known in advance, for example, mains voltage or battery voltage.
2. Depending on the type of quantity being measured, direct or alternating current, the device system is selected. For technical measurements of direct and alternating current, magnetoelectric and electromagnetic systems are chosen, respectively. In laboratory and precise measurements, a magnetoelectric system is used to determine direct currents and voltages, and an electrodynamic system is used for alternating current and voltage.
3. Select the measurement limit of the device so that
the measured value was in the last, third part of the scale
device.
4. Depending on the required measurement accuracy, select a class
accuracy of the device.
4. Methods for connecting devices to a circuit
Ammeters are connected in series with the load, voltmeters are connected in parallel, wattmeters and meters, as having two windings (current and voltage), are connected in series - in parallel (Fig. 1.2.).
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Rice. 1.3. Methods for expanding the measurement limits of instruments.
The division price of multi-limit ammeters, voltmeters, and wattmeters is determined by the formula:
P" in the most significant digit) and change the polarity of the input signal when the "-" sign in the most significant digit flashes.
Measurement error of the VR-11 A multimeter.
Constant voltage: ±(0.5% Ux +4 digits).
AC voltage: ±(0.5% Ux + 10 digits),
where Ux is the instrument reading;
zn. - unit of the lowest rank.
Advantages electronic devices: high input impedance, which allows measurements without affecting the circuit; wide measurement range, high sensitivity, wide frequency range, high measurement accuracy.
6. Errors of measurements and measuring instruments
The quality of measurement tools and results is usually characterized by indicating their errors. There are about 30 types of errors. Definitions are given in the literature on measurements. It should be borne in mind that the errors of measuring instruments and the errors of measurement results are not identical concepts. Historically, some of the names of the types of errors were assigned to the errors of measuring instruments, others to the errors of measurement results, and some are applied to both.
The methods for presenting the error are as follows.
Depending on the problems being solved, several methods of representing the error are used; absolute, relative, and reduced are most often used.
Absolute error – measured in the same units as the quantity being measured. Characterizes the magnitude of the possible deviation of the true value of the measured value from the measured value.
Relative error– the ratio of the absolute error to the value of the quantity. If we want to determine the error over the entire measurement interval, we must find the maximum value of the ratio over the interval. Measured in dimensionless units.
Accuracy class– relative error, expressed as a percentage. Typically, the accuracy class values are selected from the following range: 0.1; 0.5:1.0; 1.5; 2.0; 2.5, etc.
The concepts of absolute and relative errors apply to both measurements and measuring instruments, and the given error evaluates only the accuracy of measuring instruments.
Absolute measurement error is the difference between the measured value of x and its true value chi:
Usually the true value of the measured quantity is unknown, and instead of it in (1.1) one substitutes the value of the quantity measured by a more accurate device, that is, one that has a smaller error than the device that gives the x value. The absolute error is expressed in units of the measured value. Formula (1.1) is used when checking measuring instruments.
Relative error https://pandia.ru/text/78/613/images/image020_7.gif" width="99" height="45"> (1.2)
Based on the relative measurement error, the measurement accuracy is assessed.
The reduced error of a measuring device is defined as the ratio of the absolute error to the standard value xn and is expressed as a percentage:
(1.3)
The normalizing value is usually taken equal to the upper limit of the working part of the scale, in which the zero mark is at the edge of the scale.
The given error determines the accuracy of the measuring device, does not depend on the measured value and has a single value for a given device. From (1..gif" width="15" height="19 src="> the greater, the smaller the measured value x in relation to the measurement limit of the device xN.
Many measuring instruments differ in accuracy classes. Instrument accuracy class G is a generalized characteristic that characterizes the accuracy of the instrument, but is not a direct characteristic of the accuracy of the measurement performed using this instrument.
The accuracy class of the device is numerically equal to the greatest permissible reduced basic error, calculated as a percentage. The following accuracy classes are established for ammeters and voltmeters: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0; 5.0. These numbers are plotted on the instrument scale. For example, class 1 characterizes the guaranteed error limits as a percentage (± 1%, for example, of the final value of 100 V, i.e. ± 1V) under normal operating conditions.
According to the international classification, devices with an accuracy class of 0.5 and more accurately are considered accurate or exemplary, and devices with an accuracy class of 1.0 and coarser are considered working. All devices are subject to periodic verification for compliance of metrological characteristics, including accuracy class, with their passport values. In this case, the reference device must be more accurate than the one being verified through the class, namely: verification of a device with an accuracy class of 4.0 is carried out by a device with an accuracy class of 1.5, and verification of a device with an accuracy class of 1.0 is carried out by a device with an accuracy class of 0.2.
Since both the instrument’s accuracy class G and the measurement limit XN are indicated on the instrument scale, the absolute error of the instrument is determined from formula (1.3):
https://pandia.ru/text/78/613/images/image019_7.gif" width="15 height=19" height="19"> With The accuracy class of the device G is expressed by the formula:
from which it follows that the relative measurement error is equal to the accuracy class of the device only when measuring the limiting value on the scale, i.e. when x = XN. As the measured value decreases, the relative error increases. How many times is XN > x, how many times > G. Therefore, it is recommended to select the measurement limits of the indicating device so as to take readings within the last third of the scale, closer to its end.
7. Presentation of measurement results for single measurements
The measurement result consists of an assessment of the measured value and the measurement error, which characterizes the accuracy of the measurement. According to GOST 8.011-72, the measurement result is presented in the form:
where A is the measurement result;
Absolute error of the device;
P - probability, during statistical data processing.
In this case, A and https://pandia.ru/text/78/613/images/image023_5.gif" width="15" height="17"> should not have more than two significant figures.
Methods and means of measurement, testing and control
Acquiring an inheritance
To acquire an inheritance, the heir must accept it. Acceptance of an inheritance can be accomplished in several ways. Firstly, by submitting a written application for acceptance of the inheritance to a notary at the place of opening of the inheritance or an application for the issuance of a certificate of inheritance. Secondly, the heir is recognized as having accepted the inheritance if he has performed actions indicating this, in particular: taken possession or management of inherited property; took measures to preserve inherited property; made at his own expense expenses for the maintenance of this property; paid the debts of the testator at his own expense or received funds due to him from his debtors.
The inheritance can be accepted within six months from the date of opening of the inheritance. The inheritance can be accepted after the expiration of the six-month period, if all other heirs agree to this and they expressed their consent in writing, having certified the document by a notary. Another case of extending the period is hereditary transmission. If the heir died before accepting the inheritance, then the right to accept the inheritance passes to the heir of this heir. The heir may renounce all or part of the inheritance; he may or may not indicate the persons in whose favor he renounces the inheritance. The refusal can be addressed only to heirs by law, but of any order. The Civil Code of the Russian Federation establishes some preferential rights of inheritance for a number of heirs: an heir who had an immovable thing in common ownership with the testator has a priority right over other heirs to receive this thing on account of his property share; an heir who constantly used an immovable thing has a priority the right to receive it; the heir who lived together with the testator on the day the inheritance was opened has a priority right to receive ordinary household furnishings on account of his share. The heir to a share in any consumer cooperative has the right to become a member of this cooperative or receive a share in cash.
Lecture 1
main
1. Markov, N.N. Design, calculation and operation of control and measuring instruments and instruments: textbook. for technical schools / N.N. Markov, G.M. Ganevsky. - M.: Mechanical Engineering, 1993. – 416 p.
2. Belkin, I.M. Means of linear-angular measurements / I.M. Belkin. – M.: Mechanical Engineering, 1987. – 368 p.
additional
3. Sorochkin, B.M. Means for linear measurements / B.M. Sorochkin, Yu.Z. Tenenbaum, A.P. Kurochkin, Yu.D. Vinogradov. – L.: Mechanical engineering. Lenigr. department, 1978. – 264 p.
4. Kulikovsky, K.L. Methods and measuring instruments: textbook. manual for universities / K.L. Kulikovsky, V.Ya. Cooper. – M.: Energoatomizdat, 1986. – 448 p.
5. Tartakovsky, D.F. Metrology, standardization and technical means measurements: textbook. for universities / D.F. Tartakovsky, A.S. Yastrebov. – M.: Higher. school, 2001. – 205 p.
Measurement, testing and control are components ensuring product quality.
Measurement - the process of comparing a physical quantity with a certain value taken as a unit. Units of physical quantities are established by relevant documents (GOST R).
Together with the term “measurement”, and sometimes instead of it, the term “control” is used, for example, they say “measurement and control means”.
Control - a type of measurement in which, as a result of the comparison (measurement) process, the compliance of the measurement object (control) with the specified limit values of physical quantities is established.
The results of control are given not in the form of a value of a physical quantity, but in the form of information about the suitability or unsuitability of the controlled object or parameter.
Based on the results of control, actions are often taken to control the production process, and also the division of controlled objects into size groups within certain values or the division of controlled parts into suitability groups (suitable and defective) is carried out. The term "control" is most often used when using gauges and automatic means measurements.
There are very frequent cases when measurements are made for the purpose of control, the value of the measured size is found, then compared with the permissible largest and lowest values and determine the suitability or unsuitability of the part.
Measurement electrical quantities at industrial enterprises provides control of technological processes (TP), monitoring compliance established mode work, equipment operation monitoring, electrical equipment insulation monitoring and electrical networks, conditions that allow operating personnel to navigate during emergency conditions.
Instruments for measuring electrical quantities must meet the requirements for the accuracy class of measuring instruments (not lower than 2.5), and the measurement limits of instruments. Measuring instruments must be installed at points from which control is carried out.
Measurement of current, voltage and power is carried out in circuits of all voltages, where it is necessary for the systematic monitoring of TP or equipment. At substations, voltage measurement is allowed only on the low voltage side, unless the installation of voltage transformers on the HV side is required for other purposes. Voltage measurements should also be carried out in power converter circuits, batteries, chargers and rechargers, in circuits of arc suppression reactors. Power is measured in the circuits of active and reactive power generators, in the circuits of synchronous compensators - reactive power, and in step-down transformers, depending on the voltage - active and reactive power.
Accounting for active and reactive power and energy, as well as monitoring the quality of electricity for settlements between an energy-saving organization and a consumer, is carried out, as a rule, at the border of the balance sheet of the electrical network. Electricity metering is carried out based on measurements electrical energy using meters, as well as information and measuring systems. Application automated systems electricity metering and control increases the efficiency of metering. Various multifunctional meters are used in electrical installations. They can be used for daily and monthly recording of electricity consumption, recording electricity consumption on the first day of the month, after a power failure, 30-minute power value, attempts of unauthorized access to memory, changing seasonal time, etc.
Active electricity metering should provide the ability to draw up electricity balances for consumers, control over consumers’ compliance with specified consumption modes and electricity balances, consumer payments for electricity at current tariffs (including multi-rate and differentiated), and the ability to manage electricity consumption. Accounting for reactive electricity should provide the ability to determine the amount of reactive electricity received by the consumer from the power supply organization or transferred to it, if these data are used to make calculations or monitor compliance with the specified operating mode of compensating devices.
When determining the amount of electricity, only the transformation ratios of the measuring transformers are taken into account. The measured electricity is equal to the difference in the readings of the meter's counting mechanism multiplied by the transformation ratio; the introduction of other correction factors is not allowed.
According to the connection diagram to the electrical circuit, meters are divided into devices direct connection and transformer. In addition, meters are available in analogue and electronic types. Until now, analogue induction meters such as SAZU-670M, SR4U-I673 and others are widely used for measuring active and reactive energy. At the same time, electronic meters became widespread. Energy measurement by electronic meters is based on converting analog AC current and voltage input signals into a count pulse or code. Structural scheme an electronic counter based on amplitude and pulse-width modulation is shown in Fig. 9.17.
Electronic multi-tariff meters of the SEA32 type of various designs are designed to measure active energy in three-phase alternating current networks with a frequency of 50 Hz and are used as an energy increment sensor in the automated control system for monitoring and accounting of electrical energy (ASCAE) and telemetering of power.
Meters of the SE3000 type are used to measure active and reactive energy and power in three phases in three-phase three- and four-wire alternating current circuits and organize multi-tariff metering (number of tariffs - 4) of electricity at industrial enterprises and facilities.
Rice. 9.17. Block diagram of an electronic meter
Scheme direct connection three-phase meters in electrical installations with a voltage of 380/220 V in four-wire networks, designed for rated currents 5; 10; 20; 50 A, shown in Fig. 9.18, turning on the meter through the measuring transformers in Fig. 9.19. The connection circuit is ten-wire.
Rice. 9.18. Connection diagram for the SET4-1 direct-flow meter
Rice. 9.19. Scheme for connecting a three-element counter of type SA4U-I672M to a four-wire network with separate current and voltage circuits
Connecting each of the three measuring elements of the meter requires obligatory observance of the polarity of the connection of the current circuits and their correspondence to their voltage. Reverse switching polarity of the TA primary winding or its secondary winding causes a negative torque acting on the meter disk. The circuit provides a standardized measurement error. The connection of the neutral wire is mandatory.
The connection circuits for the reactive energy meter type SR4U-I673 and the active energy meter are no different (Fig. 9.20). Current circuits These meters are connected in series, the voltage circuits are connected in parallel. The internal connection diagrams for reactive and active energy meters are different. Due to the internal connection circuit of the coils, designed for a voltage of 380 V, an additional 90° phase shift is performed between the magnetic fluxes.
Three-phase transformer universal meters SETA and SET4 are designed for measuring active and reactive energy in three-phase three- and four-wire AC circuits 380/220 V, 50(60) Hz and are used for energy needs at a voltage of 100/57.7 V, and ST1 meters , SET3, “TRIO”, “SOLO” - for accounting for the consumption of active and reactive energy in everyday life and at work.
Rice. 9.20. Scheme for connecting counters to measure active
and reactive energy in a 380/220 V network
TsE6807 meters are designed for measuring active energy in single-phase two-wire AC networks of 220 V, 40 (60) Hz; they can be used as energy consumption increment sensors for remote information-measuring systems for accounting and distribution of automated energy supply systems; ESch TM201 meters are also used there. Single-phase single-tariff meters TsE6807P, CE101, CE200, as well as multi-tariff meters CE102, CE201 are designed for electricity metering in the household and small-motor sectors of electricity consumption, and have protection against under-metering and theft of electricity.
Three-phase single-tariff meters TsE6803V, TsE6804, CE300, CE302 are designed for metering electricity in three-phase alternating current circuits in the household, small-motor and industrial sectors of electricity consumption, and multi-tariff meters TsE6822, CE301, TsE6850M, CE303, CE304 - in industrial sectors of electricity consumption.
Multifunctional microprocessor electricity meters of the TsE6850, TsE6822, and other similar modifications are designed to measure active and reactive electricity and power, depending on the functional purpose. The functional set of parameters can be as follows:
· commercial accounting of intersystem flows, generation and consumption of electricity in energy systems, at network and industrial enterprises;
· power accounting in regional, territorial network and industrial enterprises, in small and medium-sized businesses, in housing and communal services;
· electricity metering in industrial and household sector(residential and public buildings, cottages, dachas, garages) when supplying consumers from a three-phase network, in industrial premises when supplying consumers from single-phase network;
· technical and commercial accounting of generation and consumption of active and reactive energy;
· registration of a daily schedule of half-hourly capacities (loads) with a storage depth of up to 45 days;
· measurement of instantaneous values of primary network parameters ();
· measurement of reactive power as part of ASKUE.
Measuring transducers are used to convert the measured electrical quantity (current, voltage, power, frequency) into a unified output signal of direct current or voltage or frequency. Measuring transducers are used in systems automatic regulation and management of electric power facilities in various industries, as well as for monitoring the current value of measured quantities.
In the field of electrical measuring technology upper class complexity, measuring and computing complexes (MCS), information measuring systems (IMS) are used, designed to receive, convert, store and present measurement information.
The measuring and computing complex measures constant voltages and does the conversion analog signals V digital code and digital-to-analog conversion of signals arriving through input channels.
Multifunctional ICS type K734 are designed for collecting, converting, measuring, presenting, recording and storing information about various parameters of electrical signals.
Modern multifunctional converters include converters of the PC 6806 type, designed to measure active and reactive energy in the forward and reverse directions (consumed and returned), frequency, current, voltage, active and reactive power for each phase of the network. They are used for commercial and technical metering of electricity as part of ASKUE. Depending on the purpose, they perform the functions of telecontrol, telesignaling, indication of measured and calculated parameters on the built-in digital indicator, recording maximum power in each tariff zone, archiving parameters and events with real time stamps, etc.
Questions for self-control
1. What types of errors do measuring current transformers have and what do they depend on?
2. Name the main design features of the current transformers used.
3. Explain the principle of operation of a DC instrument transformer.
4. What types of voltage transformers exist and what are their features when used in measuring circuits?
5. Name the accuracy classes of voltage and current transformers.
6. Name the types of meters used to account for active and reactive energy.
7. What types of meters are used in ASKUE systems?
8. Name the types of multifunctional converters.
9. Draw phasor diagrams of the voltage transformer.
10. Draw phasor diagrams of the current transformer.
11. What types of errors do voltage transformers have?
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Methods for measuring currents and voltages depend on the magnitude and type of these electrical quantities.
For determining small direct currents Both direct and indirect measurements can be used. In the first case, the current can be measured with mirror galvanometers and pointer magnetoelectric instruments. The smallest current that can be measured with a mirror galvanometer is approximately 10 "pA, and a pointer magnetoelectric device allows you to measure a value of 10 6 A.
Indirectly, the unknown current is determined by the voltage drop across a high-resistance resistor or by the charge accumulated by a capacitor. The instruments used are ballistic galvanometers with a minimum measurable current of 10‘ 12 A and electrometers with a minimum measurable current of 10 17 A.
Instruments are called electrometers high sensitivity by voltage with input resistance up to 10 15 Ohms. The electrometer mechanism is a type of electrostatic device mechanism that has one movable and several fixed electrodes at different potentials.
The quadrant electrometer is shown in Fig. 2.1.
Rice. 2.1.
The device has a moving part 1 with a mirror 2, which is mounted on a suspension 3 and located inside four fixed electrodes 4, called quadrants. Measured voltage Their is connected between the moving part and the common point, and constant voltages are supplied to the quadrants from auxiliary sources U, whose values are equal but opposite in sign. The deviation of the moving part in this case is equal to
where C is the capacitance between the movable electrode and two interconnected quadrants, M- specific counteracting moment, depending on the design of the suspension. The deflection of the moving part, and therefore the sensitivity of the electrometer, is proportional to the auxiliary voltage U, the value of which is usually chosen within the range of up to 200 V. The sensitivity of quadrant electrometers with an auxiliary voltage of 200 V reaches 10 4 mm/V.
TO average currents and voltages conditionally we can include currents in the range from 10 mA to 100 A and voltages from 10 mV to
600 V. Direct and indirect measurements can be used to measure average direct currents. To measure voltages, only direct measurements are used.
In direct measurements, current and voltage can be measured with instruments of magnetoelectric, electromagnetic, electrodynamic and ferrodynamic systems, as well as electronic and digital instruments. Voltage can be measured with instruments of electrostatic systems and direct current potentiometers.
Most precision instruments magnetoelectric systems designed to measure average currents and voltages have an accuracy class of 0.1.
In cases where it is necessary to measure voltage or current with high accuracy, DC potentiometers, digital voltmeters and ammeters are used. The accuracy class of the most accurate potentiometers is 0.001, digital voltmeters - 0.002, and digital ammeters - 0.02. Current measurement using a potentiometer is carried out indirectly, while the desired current is determined by the voltage drop across a reference resistor. The advantage of potentiometers and digital instruments is their low power consumption.
Measurement high currents and voltages carried out using attenuators. Shunting magnetoelectric devices makes it possible to measure direct currents up to several thousand amperes. Typically, multiple shunts connected in parallel are often used to measure large currents. Several identical shunts are connected to the bus break, and the conductors from the potential terminals of all shunts are connected to the same device.
Electrostatic voltmeters allow you to measure voltages up to 300 kV. To determine higher voltage values, instrument transformers are used.
For rate alternating currents and voltages use the concepts of effective or root mean square value, amplitude or maximum value and average rectified value.
The effective, amplitude and average rectified values are related to each other through the curve shape coefficient and the amplitude coefficient.
The waveform factor is
Where Ua- effective value of the signal, U cp - average rectified signal value.
The signal amplitude factor is defined as
Where Uа - amplitude value signal.
The values of these coefficients depend on the shape of the voltage or current curve. For sine wave = 1.11 and k a = l/2 = 1.41. From here, by measuring one of the three above-mentioned values of the measured quantity, the rest can be determined.
With a non-sinusoidal signal, the closer it is to a rectangular shape, the closer to unity the coefficients will be kf And to i. For a narrow and sharp curve shape of the measured value, these coefficients will be more significant.
Devices of electrodynamic, ferrodynamic, electromagnetic, electrostatic and thermoelectric systems respond to the effective value of the measured quantity. Rectifier system devices respond to the average rectified value of the measured value. Devices electronic system, both analog and digital, depending on the type of AC-to-DC voltage measuring converter, can respond to the effective, average-rectified or amplitude value of the measured value.
Voltmeters and ammeters of all systems are usually calibrated in effective values with a sinusoidal current waveform. With a non-sinusoidal waveform, devices that respond to an average rectified or amplitude value of current or voltage will experience an additional error, since the coefficients kf And to a with a non-sinusoidal curve shape they differ from the corresponding values for a sinusoid.