Methods for measuring direct currents and voltages. DC voltage measurement methods

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Federal Agency for Education

State educational institution

higher professional education

"Omsk State Technical University"

Department of Information and Measuring Technology

Abstract on the topic:

"Voltage measurement methods direct current»

Performed:

student of IE-417 group

Vasilyeva E.Yu.

Checked:

teacher

Saifutdinov K.R.

DC voltage measurement

Direct assessment devices. When using the direct assessment method, the voltmeter is connected in parallel with the section of the circuit where the voltage is to be measured. When measuring the voltage across the load R in a circuit with an energy source, the EMF of which is E and the internal resistance Rst, the voltmeter is turned on in parallel with the load (Fig. 1). If the internal resistance of the voltmeter is equal to Rv, then the relative voltage measurement error

where and is the actual value of the voltage across the load R before the voltmeter is turned on; ux is the measured value of the voltage across the load R.

Since the ratio R / Rv is inversely proportional to the ratio of the power consumption of the voltmeter Pv to the power of the circuit P, then

Thus, the smaller Pv and Rist, the smaller the error.

Voltage measurement in DC circuits can be performed by any DC voltage meters (magnetoelectric, electrodynamic, electromagnetic, electrostatic, analog and digital voltmeters). The choice of a voltmeter is determined by the power of the measurement object and the required accuracy. The range of measured voltages ranges from fractions of a microvolt to tens of kilovolts.

Fig 1. An equivalent circuit of a voltmeter of a magnetoelectric system (a) and a circuit for connecting it to a voltage measurement circuit (b)

If the required measurement accuracy, the permissible power consumption can be provided by devices of the electromechanical group, then this simple method of direct reading should be preferred. For voltage measurements with higher accuracy, instruments based on comparison methods should be used. Analog and digital readings can be used with any measurement method.

Measuring DC Voltage by Comparison Method

In devices for measuring DC voltage, the following comparison methods are widely used: compensation and differential.

The compensation method is based on equilibration (compensation) of the measured voltage with the known voltage drop across the reference (measuring) resistor. The indicator device registers the equality of the measured and compensating values.

The compensation method is characterized by high accuracy, determined by the accuracy of the measure and the sensitivity of the indicator. Potentiometers, potentiometric and integropotentiometric digital voltmeters are based on this method.

With the differential method, complete balancing does not occur. The device measures the difference between the measured value and the measure and is calibrated in the units of the measured value. The measured value is determined by the value of the measure and the readings of the device. This method allows high accuracy results to be obtained even with relatively coarse difference measuring instruments. However, the implementation of this method is possible only under the condition of reproducing with high accuracy the measure, the value of which is chosen close to the value of the measured quantity.

Let the value of the measured voltage ux be written as

where urev is the value of the reference voltage (measure); - voltage of uncompensation, measured by the measuring device; a - measurement error of the difference uх - urev.

Since urev is much larger, the relative measurement error ux is much less than the relative measurement error. If urev \u003d 9.9 V, \u003d 0.1 V, then (0.01%). Thus, a relatively coarse instrument can be used to achieve this high accuracy. However, in this measurement, it is necessary to use a very accurate measure urev, the value of which is determined with an even smaller (than 0.01%) error.

DC Potentiometers

voltage direct current voltmeter

Measurement of current and voltage by analog devices of direct assessment is performed at best with an error of 0.1%. More accurate measurements can be made using the compensation method. Devices based on the compensation method are called potentiometers or compensators. Mainly, voltage compensation or EMF circuits are used (Fig.2.a), electric current (fig. 2. 6) and balanced bridge. When measuring voltage, the most widely used voltage compensation circuit (Fig. 2.a).

Figure: 2. Structural compensation schemes constant voltage (a) and current (b)

In this circuit, the measured voltage uх is balanced by the known compensation voltage uk, which is opposite in sign. The voltage drop uk is created by the current Iр across a variable reference resistor Rk. The change in the resistance of the resistor Rk occurs until uk is equal to ux. The moment of compensation (equilibration) is determined by the absence of current in the indicator circuit I. Changing the compensation voltage u \u003d IpRk can be carried out by changing the resistance Rk with a constant value of the operating current Iр.

The advantage of the compensation method is the absence at the moment of full compensation of the current from the source of the measured EMF in the compensation circuit. In this case, it is the EMF value that is measured, and not the voltage at the source terminals. In addition, the absence of current in the zero indicator circuit eliminates the influence of the resistance of the connecting wires on the measurement result. In this case, the output resistance of the compensator is equal to infinity, i.e., with full compensation, the power from the measurement object is not consumed.

Simplified circuit diagram, which underlies almost all DC potentiometers, is shown in Fig. 3. It contains three circuits: an exemplary EMF circuit, which includes a source of exemplary EMF Eobr. exemplary resistor Robr and indicator AND; a working or auxiliary circuit containing an auxiliary power supply Ev, an adjusting resistor Rp, a store of compensation resistance Rk and an exemplary resistor Rrev; a measuring circuit consisting of a source of measured EMF Еx, indicator И and a box of compensating resistance Rk.

Figure: 3. Simplified circuit diagram of a DC potentiometer

Work begins with setting the operating current in the working circuit of the compensator using an auxiliary source. Ebr. The value of the operating current Iр is controlled by the EMF of an exemplary normal element. For this, at position 1 of the switch P, using a rheostat Rp, such a value of Ip is set so that the voltage drop created by it across the resistor Rrev is equal to the EMF of the normal element Erev. With compensation, AND will show the absence of current in the circuit of the normal element:

where is the value of the exemplary resistor Rrev with EMF compensation Erev.

To measure Ex, switch P is put in position 2 and by adjusting the compensating resistor Rk the current Vepi I is brought to zero again, while

where is the value of the compensating resistor Rk when compensating the EMF Ex.

Since at the moment of equilibrium there is no current in the indicator circuit, we can assume that the input resistance Rin of the potentiometer (from the side of the measured EMF) is equal to infinity, that is, with voltage compensation (EMF)

This shows one of the main advantages of the compensation measurement method - the absence of power consumption from the measurement object. From the equation Ex \u003d it can be seen that the unknown voltage is compared with an exemplary measure - the EMF of a normal element. The average value of the EMF of nascent normal elements at a temperature of 20 "C is known with an accuracy of the fifth decimal place and is equal to Erev \u003d 1.0186 V. Since the unknown EMF Ex is related to the EMF of a normal element Erev by the ratio, therefore, the accuracy of the measurement result is determined by the accuracy fitting exemplary Rrev and compensating Rk resistors.

The accuracy of setting the balancing torque is determined by the sensitivity of the zero indicator.

Consequently, the accuracy of the compensation circuit is determined by the accuracy of setting and maintaining the operating current Iр, the accuracy of manufacturing and fitting the exemplary Rо6p and compensating Rk resistors, and the sensitivity of the indicator.

One of the main characteristics of a potentiometer is its sensitivity. The sensitivity S of the potentiometer is understood as S \u003d SiSk, where Si is the indicator sensitivity; Sk is the sensitivity of the compensation circuit.

The sensitivity of the indicator is determined by the meter used, therefore, to determine S, it is necessary to find the sensitivity of the compensation circuit Sk. The sensitivity of the compensation circuit is determined by the ratio of the current increment in the indicator arising. When an EMF increment appears in the balanced circuit, to this increment, i.e. Sk \u003d

Current increment

where Ri is the resistance of the indicator; Rх is the resistance of the source of the measured EMF Ех. Therefore, the sensitivity of the potentiometer

The sensitivity of the circuit should be selected in strict accordance with the permissible measurement error under the condition

This expression allows you to determine the required sensitivity of the zero pointer. As zero pointers, highly sensitive devices of direct detection, autocompensation and photocompensation amplifiers, etc. are used. Model resistance boxes are used as a compensating resistor Rk. The exemplary resistor Rrev is constructively a resistance box, consisting of two parts: constant resistance and the so-called temperature decade. This decade allows you to adjust in accordance with the actual value of the EMF Erev at a given temperature, which provides an accurate setting of the operating current Iobr.

According to the resistance value of the measuring circuit, potentiometers are divided into low-resistance and high-resistance. Low-resistance potentiometers (with resistance less than 1000 Ohm) are used to measure low voltages (up to 100 mV), high-resistance

(with resistance more than 1000 Ohm) - for measuring voltages up to 1 - 2.5 V.

The compensation measurement method is one of the most accurate. DC potentiometers are available in accuracy classes 0.0005; 0.001; 0.002; 0.005; 0.01; 0.02; 0.05; 0.1; 0.2.

By the method of introducing the compensating value, potentiometers are divided into non-automatic, semi-automatic and automatic. In non-automatic expansion joints most of the measured voltage is compensated manually, and the rest - automatically.

Differential voltmeters

Differential voltmeter is an advanced DC potentiometer that combines a manual or automatic balancing potentiometer and a direct judgment microvoltmeter to measure the uncompensated portion of the measured voltage. It is distinguished by high accuracy, resolution and low consumption from the voltage source under study.The functional diagram of the differential voltmeter is shown in Fig. 4.

A decade potentiometer, consisting of an exemplary source of EMF Erev and a multistage voltage divider Rk, is the basis of a differential voltmeter and serves to balance the input voltage. The difference between the input and compensating voltages is measured with a direct evaluation microvoltmeter. Thus, the differential voltmeter is an incompletely balanced compensation circuit in which the voltage is determined from the reading of the decade potentiometer and from the reading of the measuring device. The current flowing into a circuit is determined by the uncompensated difference between the measured and reference voltages and the impedance of the circuit.

Figure: 4. Simplified diagram of a differential voltmeter

Fig. 5. Functional diagram of a manual differential digital voltmeter

The differential measurement method is implemented in a number of serial digital voltmeters. A functional diagram of one of these voltmeters is shown in Fig. 5.

The device uses a combination of the bit-coding method at the first stage and the time-pulse method at the second stage of converting the measured voltage.

The measuring part of the device includes an input voltage divider D, a scaled amplifier MU, a voltage source for TSC compensation, and a voltage-time converter PNV. The voltage-time converter converts the input voltage of the amplifier into a proportional time interval tinf. Information about the beginning and end of the information pulse and the polarity of the converted voltage is transmitted to the digital part of the device through pulse transformers Tr1, Tr2, which provide a good voltage isolation of the analog and digital parts of the device due to the high insulation resistance between the windings. The digital part of the device converts information to a form convenient for display and recording by a recorder.

Voltage measurement is performed in two stages. At the first stage (position 1 of the switch CL1), the transmission coefficient of the scale amplifier is equal to unity, and the compensating voltage is equal to zero. The pulses of the stable frequency generator of the RNG fо through the controlled key Kl2 and the logical key circuit Kl3 during the time tinf1 are fed to the input of the counter of the most significant bits of SC1 and are indicated, respectively, by the lamps of the most significant bits. At the second stage of measurement, the key Kl1 is transferred to position 2. At the same time, on a command from the control and synchronization unit, the transmission coefficient of the scale amplifier increases, and the digital code of the number received in the high-order bits is rewritten from SC1 to the memory circuit of the arithmetic device AU1, which controls the TSC. As a result, a Compensating voltage appears at the output of the TSC, which corresponds to the code of the number of high-order bits.

The voltage difference amplified by the amplifier is converted into the time interval tinf2. during which the pulses of a stable frequency f2 are fed to the input of the counter of the least significant bits of SC2.

Information about the sign of the non-compensation signal from the NVD enters the command generator of the control and synchronization unit of the WCD, which determines the type of operation: addition or subtraction of the results of the first and second measurement stages performed by the arithmetic device AU1. The numerical value of the result of the algebraic summation of the codes of the numbers of the counters Сч1 and Сч2 and its sign are indicated by a digital indicator.

Conversion of voltage into a time interval is carried out by the method of tracking balancing of the measured voltage with a linearly varying compensation voltage.

The voltmeter provides DC voltage measurement in the range from 5-10-6 to 1000 V on four subranges: 5-10-6-1; 5-10-5-10; 5-10-4-100; 5-10-3 - 1000 V. The measurement error, depending on the sub-range, is 0.3-0.05% of the measurement limit. The input resistance is 10 MΩ at the limits of 1 and 1000 V, 1 MΩ at the limit of 100 V and 0.1 MΩ at the limit of 10 V. The voltmeter automatically provides information about the polarity of the measured voltage and has an output for writing information to the DAC in binary decimal code.

Differential voltmeters with manual balancing of the measured voltage provide higher accuracy. The device uses a differential measurement method that combines a multi-decade, manually balanced compensation voltage source and a digital microvoltmeter that measures the uncompensated part of the input voltage.

The voltmeter consists of an input voltage divider, a six-decade TSC with manual balancing, and a comparison device, which is an autocompensating digital microvoltmeter containing a direct current amplifier, a voltage-time converter and a digital reading device (DPC).

The most important unit that determines the accuracy of a differential voltmeter is the TSC. The simplest option for constructing an adjustable TSC is a reference voltage source loaded with a scale converter. In this case, large-scale conversion can be carried out using resistive, inductive or pulse voltage dividers.

In differential voltmeter circuits, a pulse divider is preferred. The main advantages of pulse dividers are:

Figure: 6. Schematic diagram of a pulse divider of an exemplary voltage (a), voltage diagrams (b) and an equivalent circuit of the divider (c)

· Lack of precision resistors in their circuit;

· High accuracy and stability of the output voltage;

· Insignificant influence of climatic influences on the division accuracy.

In the simplest case, a pulse divider is an averaging device, the input of which is periodically supplied with an exemplary voltage urev. In fig. 6, a shows a schematic diagram of a pulse voltage divider with a COP filter as an averaging device. During the time, the input of the KS-filter is connected to uobr, and during the time t2 - to the common bus. The average value of the filter output voltage (uout in Fig. 6.6) is a function of the voltage urev and the duty cycle of the pulses controlled by the state of the key K:

This expression is equivalent to the equality connecting the output voltage of a conventional resistive divider (Fig. 6, c), while the accuracy of the transfer coefficient of the pulse divider depends on the accuracy of the ratio and the stability of the time intervals t1 and t2, which can be achieved with high accuracy due to the formation of time intervals by division the frequency of the master oscillator, the absolute accuracy and long-term stability of the frequency of which do not matter.

Modern differential voltmeters are devices with a complex circuit architecture that include elements of analog and computer technology that solve specific problems. automatic regulation, information conversion, computer technology, etc. The highest accuracy and sensitivity of differential voltmeters is provided by the iterative-compensation method of measurement, in which the measured voltage is compensated by the voltage of the built-in source (digital-to-analog converter with pulse-width modulation of the reference voltage).

The combination of these methods makes it possible to automate the measurement processes, implement auto-calibration (automatic self-verification) and diagnostics.

Based on this method, a new generation voltmeter was made, which significantly differs from traditional devices of a similar purpose.

The construction of the device is based on the principle of functional and constructive division of the device into functional (analog) and control (digital) parts (Fig. 7).

The digital part of the voltmeter includes a built-in micro-computer with a rigid program, which controls, together with the front panel controls and interface communication devices, the operation of the voltmeter. The micro-computer provides control of the functional (analog) part of the BF, the front panel and the interface for communication with the public channel KOP, as well as the mathematical processing of measurements and the process of auto-calibration of the device.

The composition and relationship of the main nodes of the functional block are shown in Fig. 8. The circuit for automatic selection of the ATP measurement limits provides normalization of the input signal, which varies in a wide voltage range, in terms of level and polarity. The ATP circuit divider is calibrated automatically by connecting an auto-calibration source to its voltage input. A digital-to-analog converter DAC with a voltage regulation range from 0 to 11.999999 V generates a compensating voltage in the modes of voltage measurement and its increments. The DC amplifier UPT with a differential comparison circuit operates with two transmission coefficients set by the coupling divider kupt \u003d 1 (in the voltage measurement mode up to 10-7 V) and kupt \u003d 100 (when measuring the voltage up to 10-7 V). The integrating analog-to-digital converter ADC has three and a half digits and is connected to the output of the DCA, depending on the set sensitivity, directly or through the katzp divider (1: 100). The difference between the compensating and measured voltages is fed to the input of the ADC with a transfer ratio of 0.01 (kupt \u003d 1. Katzp \u003d 0.01); 1 (kupt \u003d l, katp \u003d 1) and 100 (kupt \u003d 100, katp \u003d 1). The interface of the control and analog parts of the device, the formation of information exchange channels between them is carried out by the executive BSI interface unit.

The operation of the functional block in the mode of measuring voltages and voltage increments follows the algorithm in Fig. nine.

The measured voltage Ux is fed to the inverting input of the DCS through the ATP circuit (see Fig. 8) of measurement and polarity, which ensures signal transmission in a strictly defined polarity and at one of the transmission ratios kp \u003d 1: 1; 1:10; 1: 100.

At stage 1, after choosing the measurement limit, with the minimum sensitivity of the amplifying path, kp \u003d 0.01 and zero voltage at the DAC output, the measured voltage is converted into code. The received code is entered into the three most significant bits (1-3) of the digital-to-analog converter, which creates a compensating voltage at the non-inverting input of the DCT.

At stage 2, the obtained difference is measured in order to determine the subsequent digits (3-5) of the numerical expression of the input signal.

At stage 3, the result of the first two measurements is written into the DAC and the measurement of digits 5-7 of the input signal is carried out at the maximum sensitivity of the amplifying circuit. In the steady state ADC mode, the current voltage value is measured, which is summed up with the voltage of the digital-to-analog converter and is displayed in a single count on the digital display of the device. The formation of a single reading according to the results of measurements of the three described stages is conventionally shown on the mnemonic diagram in the upper right corner in Fig. 9. When the ADC counter overflows (capacity 2000 characters), the transition to the previous stage of the voltmeter is performed, which can be seen from the operation algorithm diagram.

Depending on the required resolution, the operation of the device can be limited to two measurement stages (with the possibility of indicating four or five most significant digits) or three (with the possibility of indicating six or seven digits of the measured voltage).

Figure: 7. Functional diagram of the microprocessor-based voltmeter-calibrator: BSI - executive interface unit; AVP - automatic selection of measurement limits; BPC- digital part power supply unit; E-screen; AK - automatic calibration

One of the main components ensuring the accuracy of the device is the DAC, which converts the control code into a constant voltage by means of a continuous sequence of pulse width modulated pulses of a fixed amplitude and repetition rate, followed by the selection of the average voltage value of the specified sequence of pulses by an averaging filter !.

Analysis of the operation of a DAC with pulse-width modulation makes it possible to distinguish in its structure the following components (Fig. 10) reference voltage source of the ION; code-time converter PKV, providing high-precision conversion of the code into the duration of width-modulated pulses of a fixed frequency; pulse voltage divider IDN, providing with the help of a key (keys) the formation of pulses with an amplitude determined by and and a duty cycle set by the PKV; filter.

Figure: 8. Functional diagram of the analog block of the device: TSC - source of calibrated voltage; IDN - pulse voltage divider; PKV - code-time converter; PNK - voltage-to-code converter

A pulse voltage divider regulates the voltage separately within the three senior decades (1-3), providing the basic metrological characteristics of the device, and within the lower decades (4-6). The summation of the voltages of the older and lower decades is carried out using a divider formed by the resistances of the resistors R of the summing grid (12 resistors of 2.21 MΩ each), and the resistor R1 \u003d 90.9 MΩ, to which the pulse voltage of the IDN of the lower decades is divided by half. The summing point is also supplied with voltage from the zero correction DAC and the correction voltage. The Zero Correction DAC is also intended to compensate for the offset of the DCC zero during autocalibration. The correction is necessary to compensate for the dynamic error of the keys. Keys Kl, switching the reference voltage, are made on complementary MOS transistors and are controlled from a digital circuit (not shown in Fig. 10). Formation of control width-modulated signals is performed by PKV.

The PKV code-time converter is built according to the scheme with a three-decadal clocking counter and code comparators. The clocking counter has a division ratio of N \u003d 1200. In the state of the counter 000, a pulse of the initial setting of the RS flip-flops is generated (Tg1, Tg2 to state 1. The pulses generated by the comparators and returning the RS-triggers of the high and low decades to state 0 (initial) are generated in the moment of coincidence of the counter code and control codes of the most significant and least significant digits, respectively.To form a twelve-phase signal from a single-phase one, a 24-bit shift register is used, which is clocked by a sequence of pulses representing the sum of the signal to zero the counter of the least significant bits (second and third decades) and the signal of coincidence of these decades ...

A significant volume of flows of measuring and control information between the two parts of the device required the organization of special communication channels and the creation of appropriate interface devices for servicing these channels and an executive interface unit (see Fig. 8).

The main tasks of the executive interface unit are to receive the control information of the control unit, transfer the ADC information to the control unit and generate trunk communication signals within the analog part (BF). The connection between the BF and the digital part is realized through three communication channels: one channel is used to transfer control information to the functional block (information input channel), the other channel is to transfer ADC information to the control unit (information output channel); I / O synchronization is carried out via the third channel - the synchronization channel - by signals transmitted from the control unit.

Figure: 9, Algorithm of the device in the voltage measurement and voltage increment mode

Figure: 10. Functional diagram of the DAC

Fig. 11 Function block control command distributor

Figure: 12. Block diagram of the control unit

The transmission of information in the channels is carried out through pulse transformers to ensure galvanic isolation.

In fig. 11 shows a simplified diagram of the distribution of commands to control a functional block. All receiving registers that carry out direct control are connected by information inputs in parallel to the data bus. The information is written into the register whose address is set on the address bus (in binary code) at the moment when the enable pulse appears on the bus (write enable).

The analog-to-digital converter used in the device implements the principle of double integration. The ADC is started by an external command generated in the control unit.

The control unit CU (Fig. 12) is designed to implement the relationship between the functional unit and the operator (directly or through the CPC). The structure and principle of operation of the control unit are determined by the tasks of implementing the above algorithms for the operation of the device, tasks of automatic calibration, information processing and interface. The functions performed by the control unit can be divided into two types: functions for exchanging information with the external environment (operator or KOP) and functions for controlling the analog unit during measurements. The BU operation is based on an embedded micro-computer based on a microprocessor. In general, the CU consists of a micro-computer containing a central processing unit (CPU) board, a read-only memory device (ROM), and a random access memory (RAM). The ROM stores the complete operating program that was programmed when the device was released and unchanged during the entire service life, RAM is used to store the indicated data, the results of intermediate calculations and other variables that are stored only during the operation of the device. The second part of the control unit is communication devices or interfaces connecting the micro-computer with various units of the device. Information about the time intervals required for auto-calibration of the device, about the temperature inside the analog block of the device is presented by the control unit synchronization unit.

The CPC interfaces perform the function of connecting the device with the CPC. On the one hand, it is connected to the system bus of the control unit, on the other hand, to a switch that sets the operating mode of the device when remote control... The CPC interface implements mechanical, electrical and partially logical compatibility with the public channel. The indicator interface unit controls the front panel of the device: the indicator board and the button board. It uses progressive methods of interaction of a micro-computer with the front panel - multiplex indication and scanning of a button matrix in order to detect a pressed button.

The interface unit controlling the BSU carries out a special (serial) connection between the micro-computer and the functional unit.

All CU boards are connected by a single bus system. Any exchange of information inside the control unit and with the functional section is carried out via the system bus of the control unit by the master module - the central processor of the CPU, i.e. one of the devices participating in the exchange is always the CPU, and the other is determined work program... So, for example, if information from an analog block needs to be written to RAM, then it will be received by the CPU and then transferred from the CPU to RAM. The scheme of the software for the operation of the device (Fig. 13) together with the block diagram of the control unit (Fig. 12) allow us to trace the operation of the device as a whole.

Figure: 13. Algorithm of the control unit

When the device is plugged into the network, "power-up" cleaning is performed: keeping the CPU in its original state until the voltage of the power supplies reaches the nominal values, after which the self-test program starts executing - a self-test and a subroutine that performs the initial settings. The autotesting program checks all the control units and the operability of the communication channel with the analog unit. In case of failure of any unit, the indicator board displays the mnemonic designation "NOT WORK - XX", where XX is a decimal number from 00 to 99, corresponding to the type of malfunction. In the event of a malfunction, the display lights up led indicator "Renouncement".

There are two ways to exchange information between the CPU and external devices: programmable and interrupt.

In the first case, the exchange of information with the external device is carried out according to the current program, and the CPU must periodically contact the external device, determining whether it has new information. In the second method of exchange, the processor's work under the current program is interrupted if a signal is received from an external device about its readiness to exchange information, and it goes to the service routine of this device. After completing the service, the processor continues executing the interrupted program.

The microcomputer implements an eight-level priority interrupt system, which allows servicing eight external devices, and requests with a higher priority level can be interrupted by subroutines serving requests of a lower priority level, but not vice versa.

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General information.

Currents and voltages are the most common electrical quantities to be measured. This explains the wide range of current and voltage measuring instruments produced by the industry. The choice of a measuring instrument can be determined by a combination of factors: the estimated size of the measured value, the type of current (direct or alternating), the frequency, the required measurement accuracy, the conditions of the experiment (laboratory, shop, field, etc.),

Figure: 15-1. Circuit for measuring current with an ammeter

Figure: 15-2. Voltage measurement circuit with a voltmeter

the influence of external conditions (temperature, magnetic field, vibrations, etc.), etc.

Determination of stress values \u200b\u200bis carried out, as a rule, by direct measurements; currents - in addition to direct measurements, indirect measurements are widely used, in which the voltage drop across a resistor with a known resistance included in the circuit of the measured current is measured. The current value is found according to Ohm's law:. In this case, the error in the measurement result is determined by the voltage measurement error and the error caused by the difference nominal value resistance from the true resistance value The error can be found according to the rules for processing observation results in indirect measurements (see § 14-2).

Measurements of currents and voltages are always accompanied by an error due to the resistance of the measuring instrument used. The inclusion of a measuring instrument in the investigated circuit distorts the mode of this circuit. So, for example, the inclusion of an ammeter with resistance in the circuit shown in Fig. 15-1, will lead to the fact that instead of the current that flowed in this circuit before the ammeter was turned on, after the ammeter was turned on, the current will flow.The error is the greater, the more resistance ammeter. A similar error occurs when measuring voltages. For example, in the circuit shown in Fig. 15-2, when a voltmeter with a resistance to measure the voltage between the points is turned on, the circuit mode is also violated, since instead of the voltage that was in the circuit before the voltmeter was turned on, after it was turned on, the voltage

The lower the resistance of the voltmeter, the greater the error.

An indirect indicator of the resistance of measuring instruments is the power consumed by the instrument from the circuit in which

measurement is made. When a current flows through an ammeter with a resistance, the power consumed by the ammeter, The power consumed by the voltmeter, is determined by the expression where is the voltage measured by the voltmeter; - internal resistance of the voltmeter. Consequently, the less the power consumed by the measuring instrument from the circuit where the measurement is made, the less the error from distortion of the circuit mode when measuring currents and voltages. Of the measuring instruments used to measure currents and voltages, compensators (potentiometers), electronic and digital devices have the lowest power consumption from the measurement circuit. Among electromechanical devices, the least power is consumed by magnetoelectric and electrostatic devices. The very low power consumed from the measurement circuit by the compensators allows them to measure not only voltages, but also EMF.

The range of measured currents and voltages is very wide. For example, in biological research, space research, measurements in a vacuum, it is necessary to measure direct currents that make up fractions of femtoamperes, and in powerful power plants, at non-ferrous metallurgy, chemical industry - currents reaching hundreds of kiloamperes. To measure currents and voltages in such a wide range of values, the domestic industry produces various measuring instruments that provide the ability to measure in certain subranges. Means for measuring currents and voltages are made, as a rule, multi-range. To expand the limits of current measurements, shunts and DC measuring transformers are used in DC circuits and AC measuring transformers in AC circuits. To expand the limits of voltage measurements, voltage dividers, additional resistors and voltage measuring transformers are used.

The entire range of measured currents and voltages can be conditionally divided into three sub-ranges: small, medium and large values. The most secured means of measurement is the sub-range of average values \u200b\u200b(roughly: for currents - from milliamperes to tens of amperes; for voltages - from millivolts to hundreds of volts). It is for this sub-range that measuring instruments have been created with the smallest error in measuring currents and voltages. This is not accidental, since additional difficulties arise when measuring small and large currents and voltages.

Figure: 15-3. Scheme of the influence of own resistive and capacitive couplings

Figure: 15-4. Diagram of the influence of insulation resistance on the division ratio of the voltage divider

An external alternating magnetic field can also introduce significant distortions due to the EMF induced in the wires and other elements of the circuit connecting the source of a small measured value with the measuring instrument.

It is not possible to completely eliminate the influence of the noted factors. Therefore, measurements of low currents and voltages are carried out with a greater error.

Measurements of high currents and voltages have their own characteristics and difficulties. For example, when measuring large DC currents using shunts, a lot of power is dissipated on the shunts, leading to significant heating of the shunts and the appearance of additional errors. To reduce the dissipated power and eliminate overheating, it is necessary to increase the size of the shunts or apply special additional measures for artificial cooling. As a result, shunts are cumbersome and expensive. When measuring high currents, it is very important to monitor the quality of the contact connections through which the current flows. Poor quality of the contact connection can not only distort the circuit mode and, consequently, the measurement result, but also lead to burnout of the contact due to the high power dissipated on the contact resistance. When measuring high currents, additional errors may arise from the influence on the measuring instruments of a strong magnetic field created around the tires by the flowing current.

When measuring high voltages, the requirements for the quality of insulating materials used in measuring instruments increase, both to reduce errors arising from leakage currents through the insulation, and to ensure the safety of service personnel. For example, if a voltage divider is used to expand the measurement limits, then with an increase in the measured voltage, the resistance of the divider must be increased. When measuring high voltages, the resistance of the divider can be comparable to the insulation resistance, which will lead to an error in voltage division and, therefore, to an error in measurements. Fig. 15-4, illustrating the effect of insulation on the division factor, it follows that instead of the nominal division factor, the real division factor will be determined by the expression where the sign means parallel connection... The difficulty in accounting for the actual division ratio is that the insulation resistance can vary depending on the state environment (dustiness, humidity, etc.).

Hence, when measuring high currents and voltages, in addition to the usual errors, errors arise due to the specifics of these measurements.

The characteristic change in the measurement error depending on the size of the measured value is illustrated (Fig. 15-5) qualitatively (for clarity, the variable

Figure: 15-5. Change in the error of DC measurements depending on the size of the measured value

Figure: 15-6. Change in the measurement error of alternating current (tens of milliamperes) depending on the frequency

scale along the axes) on the example of working instruments for measuring direct currents produced by the industry.

When measuring alternating currents and voltages, the frequency of the measured value is of great importance. The frequency range of measured currents and voltages is very wide: from fractions of a hertz (infra-low frequencies) to hundreds of megahertz and more.

currents and voltages, which is explained by the above reasons. When measuring at frequencies below 20 Hz, difficulties arise due to insufficient inertia of the moving part of electromechanical devices. When measuring time variables, the torque acting on the moving part of the device also changes over time. With a decrease in the frequency of the torque, the inertia of the moving part is insufficient to obtain a steady deflection of the pointer. This feature is strongly manifested at infra-low frequencies. Overcoming this difficulty by increasing the inertia of the moving part of the measuring mechanism is impractical, since this will decrease the sensitivity of the measuring instrument. Therefore, to measure currents and voltages of infra-low frequencies, special devices for averaging (integrating) the measured values \u200b\u200bare required. Among the mass-produced measuring instruments, it is worth mentioning thermoelectric devices, for example, an ammeter of the type that measures alternating currents with a frequency of 1 Hz. The integration function of these devices is performed by a thermoelectric converter.

In fig. 15-6 qualitatively (for clarity, a variable scale along the axes is used) illustrates the characteristic change in the measurement error depending on the frequency using the example of working instruments for measuring alternating currents (tens of milliamperes) produced by the industry.

Measurements of direct currents and voltages.

The highest accuracy of measurements of direct currents and voltages is determined by the accuracy of state primary standards of the unit of direct current electric current (GOST 8.022-75) and the unit electromotive force (GOST 8.027-81). State primary standards ensure the reproduction of the corresponding unit with the standard deviation of the measurement result not exceeding 4-10-6 for the direct current strength and for the EMF, with a non-excluded systematic error not exceeding, respectively, Of the working measuring instruments for direct currents and voltages, compensators give the smallest measurement error direct current. For example, a type compensator (potentiometer) has an accuracy class of 0.0005 and allows you to measure constant EMF and voltages in the range from 2.1211111 V. Direct currents are measured using compensators indirectly using coils electrical resistance... When using coils of electrical resistance type of accuracy class 0.002 and a compensator of the type, currents can be measured with an error of not more than Compensators are used for accurate measurements of constants

Table 15-1 (see scan)

currents, EMF and voltages and for verification of less accurate measuring instruments.

The most common means for measuring direct currents and voltages are ammeters (micro-, milli-, kiloammeters) and voltmeters (micro-, milli-, kilovoltmeters), as well as universal and combined devices (for example, microvoltnanoammeters, nanovoltammeters, etc.). Widely used measuring instruments for direct currents and voltages are presented in table. 15-1 and 15-2.

Electrometers and photovoltaic instruments are used to measure very small direct currents and voltages. As an example, we can specify digital universal microvoltmeters-electrometers of the type with a range of DC measurements from to and of the type with a range of current measurements from and to. An example

Table 15-2 (see scan)

photovoltaic devices is a nanovoltammeter of the type having the smallest measurement range of direct currents nA and constant voltages. When measuring small and medium values \u200b\u200bof direct currents and voltages, digital and magnetoelectric devices are most widely used. Measurements of large direct currents are carried out, as a rule, with magnetoelectric kiloammeters using external shunts, and very large currents - using direct current transformers. Magnetoelectric and electrostatic kilovoltmeters are used to measure high DC voltages. Measurements of DC currents and voltages can be

perform with other devices (see tables 15-1 and 15-2). It should be borne in mind that electrodynamic ammeters and voltmeters are rarely used for technical measurements of currents and voltages in DC circuits. They are more often used (along with digital and magnetoelectric instruments of high accuracy classes) as exemplary instruments for the verification of measuring instruments of a lower accuracy class. Table 15-1 and 15-2 thermoelectric devices are not indicated, since it is impractical to use them in direct current circuits due to the relatively large power they consume from the measurement circuit.

Measurements of alternating currents and voltages.

Measurements of alternating currents and voltages are based on a state special standard that reproduces the current strength in the frequency range Hz (GOST 8.183-76), and a state special standard that reproduces the voltage 0.1-10 V in the frequency range Hz (GOST 8.184-76). The accuracy of these standards depends on the size and frequency of the reproduced quantities. The standard deviation of the measurement result for an alternating current standard with non-excluded systematic error For an alternating voltage standard, these errors are, respectively,

Working instruments for measuring alternating currents and voltages are ammeters (micro-, milli-, kiloammeters), voltmeters (micro-, milli-, kilovoltmeters), alternating current compensators, universal and combined devices, as well as recording devices and electronic oscilloscopes.

A feature of measuring alternating currents and voltages is that they change over time. In general, a time-varying quantity can be fully represented by instantaneous values \u200b\u200bat any time. Time-variable quantities can also be characterized by their individual parameters (for example, amplitude) or integral parameters, which are used as the effective value

rectified mean

Table 15-3 (see scan)

and average

where is a time-varying quantity. Thus, when measuring alternating currents and voltages, their effective, amplitude, average rectified, average and instantaneous values \u200b\u200bcan be measured. In practice electrical measurements most often it is necessary to measure sinusoidal alternating currents and voltages, which are usually characterized by the rms value. Therefore, the overwhelming majority of measuring instruments for alternating currents and voltages are calibrated in rms values \u200b\u200bfor a sinusoidal current or voltage waveform.

Measurements of the effective values \u200b\u200bof alternating currents and voltages are carried out by various measuring instruments,

Table 15-4 (see scan)

measuring instruments are provided by rectifying devices. They have a relatively wide range and when measuring alternating voltages... These devices are made, as a rule, multidimensional. It should also be noted that when the rectifier is turned off, these devices are used as magnetoelectric devices for measuring direct currents and voltages. Due to this versatility and small size, rectifier devices are widely used in laboratory and industrial practice.

Alternating currents over a kiloampere and alternating voltages over a kilovolt are measured using external measuring current or voltage transformers by electromagnetic, rectifier and electrodynamic devices. Measurements of high alternating voltages (up to the direct connection of measuring instruments allow electrostatic kilovoltmeters, for example, a kilovoltmeter of the type

In the widest frequency range, when measuring alternating currents, thermoelectric and electronic devices work, and when measuring alternating voltages, electronic and electrostatic devices. Thermoelectric voltameters are of limited use due to the high power they consume from the measurement circuit, therefore, in table. 15-4 they are not shown. Electrodynamic and electromagnetic devices operate in the narrowest frequency range. The upper limit of their frequency range usually does not exceed 4 kilohertz units. It should be borne in mind that the figures given in table. 15-3 and 15-4, characterize the limiting capabilities of various devices. At the same time, it is impossible to unambiguously link the numbers characterizing the upper limits of the measurement range with the numbers characterizing the frequency range. The relationship between the range of measured values \u200b\u200band the frequency range is different for different measuring instruments. However, you can indicate a general rule: with an increase in the value of the measured value, the upper limit of the frequency range, as a rule, decreases. At the same time, another regularity is observed, noted earlier: with increasing frequency, the measurement error increases. For example, a thermoelectric milliammeter of accuracy class 1.0 at the measurement limit of 100 mA has an upper cutoff frequency of 50 MHz, and at the limit of 300 mA - 25 MHz. The same device allows measuring current up to 100 mA at a frequency of up to 100 MHz and current up to 300 mA at a frequency of up to 50 MHz with an error of no more than

When measuring the effective values \u200b\u200bof alternating currents and voltages, the shape of the curve of which differs from

sinusoidal, an additional error occurs. This error is minimal for measuring instruments operating in a wide frequency band, provided that the output signal of these instruments is determined by the effective value of the input quantity. The least sensitive to changes in the shape of the waveform of alternating currents and voltages are thermoelectric, electrostatic and electronic devices.

The most accurate measurements of the rms values \u200b\u200bof sinusoidal currents and voltages can be carried out by electrodynamic devices, digital devices and AC compensators. However, the measurement error of alternating currents and voltages is greater than that of direct ones. For example, an alternating current compensator of the type in the frequency range from 40 to 60 Hz measures EMF and voltages with a minimum permissible basic error.The same accuracy in a wider frequency range is provided by electrodynamic ammeters and milliammeters of the type and voltmeters of the type

Let's note some features of measuring currents and voltages in three-phase circuits. In the general case, in asymmetrical three-phase circuits, the number of necessary instruments for measuring currents and voltages corresponds to the number of measured quantities, if each measured quantity is measured by its own device. When measuring in symmetrical three-phase circuits, it is enough to measure the current or voltage in only one line (phase), since in this case all linear (phase) currents and voltages are equal to each other. The relationship between line and phase currents and voltages depends on the load switching circuit. It is known that for symmetrical three-phase circuits, this connection is determined by the ratios: when connecting the load with a star and when connecting the load with a triangle. In single-ended three-phase circuits, when measuring currents and voltages with instrument transformers, you can save on the number of instrument transformers used. For example, in Fig. 15-7, and a diagram of measurements of three line currents is shown using two measuring current transformers, and in Fig. - a similar circuit for measuring line voltages. These circuits are based on the known relationships for three-phase circuits: intended for measuring the effective values \u200b\u200bof the current in the phases To measure the average rectified currents and voltages, the shape of the curve of which differs from the sinusoidal one, you need to use measuring instruments with an output signal determined by the average rectified value of the input quantity. These tools include rectifiers and some electronic and digital devices. When calibrating these means in the effective values \u200b\u200bof the sinusoid, the measured rectified average value is found by dividing the readings of the devices by a factor of 1.11.The error from changing the shape of the curve of currents and voltages for these devices is the smaller, the wider their frequency range. For measuring the amplitude values \u200b\u200bof currents and voltages, the shape of the curve of which differs from the sinusoidal one, it is necessary to use measuring instruments, the output signal of which is determined by the amplitude value of the input quantity. Some electronic devices belong to such means. When calibrating these devices in the effective values \u200b\u200bof the sinusoid, the measured amplitude value is found by multiplying the readings of the devices by a factor of 2. Pulse electronic devices are used to measure the amplitudes of pulse currents and voltages.

The average value of an AC current or voltage characterizes the DC component contained in the measured current or voltage. Magnetoelectric devices are usually used to measure the average values \u200b\u200bof alternating currents and voltages.

Instantaneous values \u200b\u200bof alternating currents and voltages are measured by recording devices and electronic oscilloscopes, the main characteristics of which are given in § 6-6 and 9-1. It should be borne in mind that the instantaneous values \u200b\u200bcan be used to determine other values \u200b\u200bof currents and voltages (average, average rectified, effective, amplitude).

Direct current and voltage measurements are performed using instruments of magnetoelectric, electromagnetic, electrodynamic systems, voltage is also measured by electrostatic and electronic voltmeters. In addition, DC compensators are used for more accurate measurements.

Magnetoelectric measuring mechanisms are directly micro- and milliammeters or millivoltmeters, and in combination with shunts and additional resistances, respectively, ammeters and voltmeters.

To measure and detect low currents (10 -11 - 10 -5 A) and voltages (less than 10 -4 V) galvanometers are used - highly sensitive measuring mechanisms, usually of a magnetoelectric system. Unlike devices whose scales are calibrated in measured values, galvanometers have an unnamed scale, the division value of which is indicated in the device's passport data or is determined experimentally.

Measurement of direct currents and voltages can be made using ammeters and voltmeters of electromagnetic and electrodynamic systems. They are mainly used for measurements in AC circuits.

Electrostatic measuring devices are electrostatic voltmeters because they can directly measure voltage. The range of voltages they measure ranges from tens of volts to hundreds of kilovolts. To measure voltages up to 3 kV, measuring mechanisms with varying activity of the electrode surface are used. Voltmeters are made single-range and multi-range, portable (up to 30 kV) and stationary (for measuring high voltages, over 30 kV).

The accuracy class of modern electrostatic voltmeters reaches 0.1 and even 0.05 (C-71), but most often devices of classes 1.5 are made; 2 and 2.5. To reduce the influence of external electrostatic fields, electrostatic shielding is used. The measurement limits are expanded using resistor voltage dividers.

The main advantages of electrostatic voltmeters are: very low intrinsic power consumption (high input resistance, 10-10 ohms), the ability to measure AC and DC voltages, the ability to directly measure high voltages. The disadvantages include low sensitivity and unevenness of the scale.

Measurement of direct voltages from fractions of a volt to several kilovolts can be carried out using electronic voltmeters, which contain a measuring mechanism and a tube or transistor DC amplifier. There are several types of electronic DC voltmeters, but all of them are characterized by the structural diagram shown in Figure 6.1 in Fig. 6.1.

Figure: 6.1.

The input device (voltage divider), which is supplied with voltage U X, allows you to change the measurement limits and provides a high input resistance of the device.

A magnetoelectric microammeter with a measurement range of 50 to 500 μA is usually used as a measuring mechanism.

DC amplifiers are designed to increase the sensitivity of the device, increase the power of the measured signal to a level at which the required deflection of the pointer of the measuring mechanism is provided. The amplifiers have high input impedance and low output impedance. This ensures the matching of the input resistance of the voltmeter (10 - 20 MΩ) with the low internal resistance of the microammeter. Most often, amplifiers are made in the form of feedback bridge circuits.

Electronic voltmeters with pointer reading have the following features: high input resistance and, therefore, low power consumption from the object of measurement; high sensitivity with a large measurement range; ability to withstand overload; relatively low measurement speed (due to the inertia of the magnetoelectric measuring mechanism); the need for power supply (mains or battery); large errors (basic reduced error 2 - 3%).

Nowadays, of course, digital voltmeters have become more widespread - devices with a digital reading device and an analog-to-digital converter, in which voltage (or other physical quantities; frequency, phase shift, etc.) are automatically converted into a digital code. Such devices have a number of advantages over analogue devices: they have a wide range of measured voltages (from 1 mV to 1000 V), high-speed performance, allow measurements with small errors (0.01 - 0.005), since the principle of operation of most devices is based on the comparison method, and digital readout eliminates reading error. Digital voltmeters also allow you to enter measurement data directly into computers, which makes it possible to further process the obtained data more quickly.

The disadvantages include the complexity of the device, less reliability and high cost.

There are various principles for building digital DC voltmeters:

By the type of elements used in the circuits, they are divided into:
- electromechanical;
- electronic;
- combined.
By the method of analog-to-digital conversions, they are divided into devices with:
- spatial coding;
- intermediate transformation (in time interval, frequency, phase, etc.);
- balanced reference voltage (most accurate).

MINISTRY OF EDUCATION AND SCIENCE OF THE RUSSIAN FEDERATION

Federal State Budgetary Educational Institution

higher professional education

"NATIONAL RESEARCH

TOMSK POLYTECHNICAL UNIVERSITY "

Automation of Heat and Power Processes

Laboratory report No. 3

RESEARCH METHODS FOR MEASURING DC CURRENT AND VOLTAGE

on the course "Metrology, standardization and certification"

Student gr. _____________ Pirnazarov M.I

Completed

Student gr ________________ Usmonov E.B

Supplier ________________ Medvedev V.V.

Tomsk-2015

Introduction

The purpose of the work is to study various types of measurements, as well as in the practical development of direct and indirect methods for measuring electrical quantities (direct current and voltage).

1.study of measurement classification;

2.measurement of direct current and indirect current;

.build graphs;

.measurement of DC voltage by direct, m and indirect methods.

Direct and indirect measurements of U and I

Direct measurements are those in which the desired value of the quantity is found directly from the experimental data (according to the MT readings).

Indirect measurements are called measurements in which the desired value of a quantity is found on the basis of a known relationship between this quantity and quantities measured by the direct method.

where Y is the sought, indirectly measured value ;, x2, x3,… xn are the values \u200b\u200bmeasured by the direct method;

Direct method of measuring DC voltage

For direct measurement of DC voltage, it is necessary to assemble the circuit shown in Fig. 2.

Figure 2 - Diagram of the experimental circuit

Table 1 - DC voltage measurement results

Rotation angle of the P2 regulator "Setting + U", ° Results of direct voltage measurement, V Results of indirect voltage measurement, V 00,010,56603,813,811209,569,2118014,2714,33

Indirect method for measuring DC voltage

Voltage and current in a DC circuit are related by Ohm's law:

where U is the sought-for voltage value, V, is the measured DC value, A, - known resistance value, Ohm.

therefore, the magnitude of the DC voltage in the circuit can be estimated by measuring the magnitude of the current in the circuit.

To indirectly measure the DC voltage of the circuit, you must collect

Figure 3 - Diagram of the experimental circuit

Table 2 - DC measurement results

Rotation angle of regulator P2 "Setting + U", ° Results of current measurement, mA 00.02604.131208.46 18013.30

The DC voltage values \u200b\u200bin the circuit are calculated according to the formula (2) and are entered in table 1.

According to the data in Table 1, in one coordinate system, we build graphs of the dependence of the results of direct and indirect measurements on the value of the angle of rotation of the regulator P2. Show graphically the absolute measurement error. Make a conclusion about the nature of the error.

Figure 4 - Graph of dependence of the results of direct and indirect measurements of current on the value of the angle of rotation of the regulator.

Direct and indirect methods for measuring direct current

Putting together the circuit shown in Fig. 3 and enter the data into table 3.

voltage power current measurement

Table 3 - DC measurement results

Result of direct measurement of direct current, mA Value of direct current voltage in the circuit, V Result of indirect measurement of direct current in the circuit, mA Absolute error of indirect measurement, mA I10.59-0.58-0.56I21.931.502.63I37.116.571.89I411.3110.682, 62

We also enter the data in table 3.

CONCLUSION

We studied various types of measurements, and practically mastered the direct and indirect methods of measuring electrical quantities (direct current and low pressure).

Control questions

Give examples of direct, indirect, aggregate, and joint measurements /

What measurements (indirect / direct) do you consider more accurate and ovens

A feature of measuring alternating currents and voltages is that they change over time. In general, a time-varying quantity can be fully represented by instantaneous values \u200b\u200bat any time.

Time-varying quantities can also be characterized by their individual parameters (eg, amplitude) or integral parameters.

Integral parameters include:

effective value -,

rectified mean - ,

average value - ,

where x (t) is a time-varying quantity.

Thus, when measuring alternating currents and voltages, their effective, amplitude, average rectified, average and instantaneous values \u200b\u200bcan be measured. In the practice of electrical measurements, it is most often necessary to measure sinusoidal alternating currents and voltages, which are usually characterized by the rms value. Therefore, the overwhelming majority of measuring instruments for alternating currents and voltages are calibrated in rms values \u200b\u200bfor a sinusoidal current or voltage waveform.

Small alternating currents are measured with digital, electronic and rectifier devices, small alternating voltages - with electronic voltmeters. The widest range of measurements of alternating currents with direct connection of measuring instruments is provided by rectifying devices. They also have a relatively wide range when measuring alternating voltages. These devices are usually made multi-range.

It should also be noted that when the rectifier is turned off, these devices are used as magnetoelectric devices for measuring direct currents and voltages. Due to this versatility and small size, rectifier devices are widely used in laboratory and industrial practice.

Alternating currents above a kiloampere and alternating voltages above a kilovolt are measured using external measuring current or voltage transformers by electromagnetic, rectifier and electrodynamic devices.

Measurements of high alternating voltages (up to 75 kV) with direct connection of measuring instruments allow carrying out electrostatic kilovoltmeters.

Active power is measured with a wattmeter, and reactive power measured with a varmeter.

Measurement of high powers.When measuring high powers, a current transformer and a voltage transformer are used.

The connection diagram is shown in Figure 8.4.

Figure 8.4 Wattmeter connection diagram for measurement

large capacities

8.3 Measurement of currents and voltages in three-phase circuits

In the general case, in asymmetrical three-phase circuits, the number of necessary instruments for measuring currents and voltages corresponds to the number of measured quantities, if each measured quantity is measured by its own device. When measuring in symmetrical three-phase circuits, it is enough to measure the current or voltage in only one line (phase), since in this case all linear (phase) currents and voltages are equal to each other. The relationship between line and phase currents and voltages depends on the load switching circuit.

In single-ended three-phase circuits, when measuring currents and voltages with instrument transformers, you can save on the number of instrument transformers used.

For example, figure 8.5 shows a measurement circuit for three line currents using two measuring current transformers, and figure 8.6 shows a similar measurement circuit for line voltages (V1 - U AB, V2 - U B C, V3 - U C A).

Figure 8.5

Figure 8.6

These diagrams are based on known relationships for three-phase circuits.

It should be borne in mind that for the correct summation of currents, it is necessary to monitor the correct connection of the generator terminals of the measuring transformers. Incorrect connection of the generator clamps of one of the transformers (in the primary or secondary circuit) will lead to a change in the phase of one of the summed currents, and the result will be incorrect. The circuit for measuring line voltages works in a similar way. Similar circuits can be used to measure phase currents and voltages.

To measure currents and voltages in three-phase circuits, you can use measuring instruments for these quantities, designed for single-phase circuits. In addition to these tools, the industry produces special instruments for measuring in three-phase circuits, which make it possible to more quickly and conveniently perform the necessary measurements.

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