Measurement is the process of finding experimentally the value of a physical quantity using special technical means. Electrical measuring instruments are widely used in monitoring the operation of electrical installations, monitoring their condition and operating modes, and taking into account consumption and quality electrical energy, during repair and adjustment of electrical equipment.
Electrical measuring instruments are electrical measuring instruments designed to generate signals that are functionally related to the measured physical quantities in a form understandable to an observer or an automatic device.
Electrical measuring instruments are divided into:
- by the type of information received on instruments for measuring electrical (current, voltage, power, etc.) and non-electrical (temperature, pressure, etc.) quantities;
- according to the measurement method - for direct assessment devices (ammeter, voltmeter, etc.) and comparison devices (measuring bridges and compensators);
- according to the method of presenting measured information - analog and discrete (digital).
The most widely used analog devices for direct assessment are classified according to the following criteria: type of current (direct or alternating), type of measured quantity (current, voltage, power, phase shift), principle of operation (magnetoelectric, electromagnetic, electro- and ferrodynamic), accuracy class and operating conditions.
To expand the measurement limits of electrical devices running on direct current, shunts (for current) and additional resistances Rd (for voltage) are used; on alternating current, current transformers (tt) and voltage transformers (tn).
Instruments used to measure electrical quantities.
Voltage measurement is carried out with a voltmeter (V), connected directly to the terminals of the area under study electrical circuit.
Current measurement is carried out with an ammeter (A), connected in series with the elements of the circuit under study.
Measurement of power (W) and phase shift () in circuits alternating current carried out using a wattmeter and a phase meter. These devices have two windings: a fixed current winding, which is connected in series, and a moving voltage winding, connected in parallel.
Frequency meters are used to measure alternating current frequency (f).
To measure and account for electrical energy - electrical energy meters connected to the measuring circuit similarly to wattmeters.
The main characteristics of electrical measuring instruments are: accuracy, reading variations, sensitivity, power consumption, reading settling time and reliability.
The main parts of electromechanical devices are the electrical measuring circuit and the measuring mechanism.
The measuring circuit of the device is a converter and consists of various connections active and reactive resistance and other elements depending on the nature of the transformation. The measuring mechanism converts electromagnetic energy into mechanical energy necessary for the angular movement of its moving part relative to the stationary one. The angular movements of the pointer a are functionally related to the torque and counteracting moment of the device by a transformation equation of the form:
k is the design constant of the device;
Electrical quantity under the influence of which the arrow of the device deviates by an angle
Based given equation it can be argued that if:
- input quantity X to the first power (n=1), then a will change sign when the polarity changes, and the device cannot operate at frequencies other than 0;
- n=2, then the device can operate on both direct and alternating current;
- the equation includes more than one quantity, then you can choose any one as the input, leaving the rest constant;
- two quantities are input, then the device can be used as a multiplying converter (wattmeter, counter) or dividing converter (phase meter, frequency meter);
- with two or more input values on a non-sinusoidal current, the device has the property of selectivity in the sense that the deviation of the moving part is determined by the value of only one frequency.
Common elements are: a reading device, a moving part of the measuring mechanism, devices for creating torque, counteracting and calming moments.
The reading device has a scale and a pointer. The interval between adjacent scale marks is called a division.
The instrument division value is the value of the measured quantity that causes the instrument needle to deflect by one division and is determined by the dependencies:
Scales can be uniform or uneven. The area between the initial and final values of the scale is called the range of instrument readings.
The readings of electrical measuring instruments differ somewhat from the actual values of the measured quantities. This is caused by friction in the measuring part of the mechanism, the influence of external magnetic and electric fields, and temperature changes environment etc. The difference between the measured Ai and actual Ad values of the controlled quantity is called the absolute measurement error:
Because absolute error does not give an idea of the degree of accuracy of measurements, then use the relative error:
Since the actual value of the measured quantity during measurement is unknown, the accuracy class of the device can be used to determine it.
Ammeters, voltmeters and wattmeters are divided into 8 accuracy classes: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0. The number indicating the accuracy class determines the largest positive or negative basic reduced error that a given device has. For example, for an accuracy class of 0.5, the given error will be ±0.5%.
| Parameter name | Ammeters E47 | Voltmeters E47 |
| System | electromagnetic | electromagnetic |
| Information output method | analog | analog |
| Measuring range | 0...3000 A | 0...600 V |
| Installation method | on the shield panel | on the shield panel |
| Switching method | <50 А- непосредственный, >100 A - via current transformer with 5 A secondary current | direct |
| Accuracy class | 1,5 | 1,5 |
| Limit of permissible basic error of instruments, % | ±1.5 | ±1.5 |
| Rated operating voltage, no more | 400 V | 600 V |
| Permissible long-term overload (no more than 2 hours) | 120% of the final value of the measuring range | |
| Average time to failure, not less, h | 65000 | 65000 |
| Average term service, at least, years | 8 | 8 |
| Ambient air temperature, °C | 20±5 | 20±5 |
| Frequency of the measured value, Hz | 45...65 | 45...65 |
| Mounting plane position | vertical | vertical |
| Dimensions, mm | 72x72x73.5 96x96x73.5 | 72x72x73.5 96x96x73.5 |
Electrical measuring instruments (ammeters and voltmeters) series E47
They are used in low-voltage complete devices in electrical distribution networks of residential, commercial and industrial facilities.
E47 ammeters - analog electromagnetic electrical measuring instruments - are designed to measure current in AC electrical circuits.
E47 voltmeters - analog electromagnetic electrical measuring instruments - are designed to measure voltage in alternating current electrical circuits.
Wide measurement range: ammeters up to 3000 A, voltmeters up to 600 V. Accuracy class 1.5.
Ammeters designed to measure currents above 50 A are connected to the circuit being measured via a current transformer with a rated secondary operating current of 5 A.
Operating principle of ammeters and voltmeters of the E47 series
E47 ammeters and voltmeters are devices with an electromagnetic system. They consist of a round coil with movable and stationary cores placed inside. When current flows through the turns of the coil, a magnetic field is created that magnetizes both cores. As a result.
the like poles of the cores repel each other, and the movable core turns the axis with the arrow. To protect against negative influence external magnetic fields, the coil and cores are protected by a metal shield.
The principle of operation of magnetoelectric system devices is based on the interaction of the field permanent magnet and conductors with current, and the electromagnetic one - on drawing the steel core into a stationary coil when there is a current in it. The electrodynamic system has two coils. One of the coils, movable, is mounted on an axis and is located inside the stationary coil.
The principle of operation of the device, the possibility of its operation in certain conditions, the possible maximum errors of the device can be established according to symbols, printed on the dial of the device.
For example: (A) - ammeter; (~) - alternating current ranging from 0 to 50A; () - vertical position, accuracy class 1.0, etc.
Current and voltage measuring transformers have ferromagnetic magnetic cores on which the primary and secondary windings are located. The number of turns of the secondary winding is always greater than the primary.
The terminals of the primary winding of the current transformer are designated by the letters L1 and L2 (line), and the secondary windings by I1 and I2 (measurement). According to safety regulations, one of the terminals of the secondary winding of the current transformer, as well as the voltage transformer, is grounded, which is done in case of insulation damage. The primary winding of the current transformer is connected in series with the object being measured. The resistance of the primary winding of the current transformer is small compared to the consumer resistance. The secondary winding is connected to the ammeter and current circuits of devices (wattmeter, meter, etc.). The current windings of wattmeters, meters and relays are rated at 5A, voltmeters, voltage circuits of wattmeters, meters and relay windings are rated at 100 V.
The resistances of the ammeter and the current circuits of the wattmeter are small, so the current transformer actually operates in the short circuit. The rated current of the secondary winding is 5A. Current transformer ratio equal to the ratio primary current to the rated current of the secondary winding, and for a voltage transformer - the ratio of the primary voltage to the secondary rated.
The resistance of the voltmeter and voltage circuits of measuring instruments is always high and amounts to at least a thousand ohms. In this regard, the voltage transformer operates in idle mode.
The readings of devices connected through current and voltage transformers must be multiplied by the transformation ratio.
TTI current transformers
TTI current transformers are intended: for use in electricity metering schemes for settlements with consumers; for use in circuits commercial accounting electricity; for transmitting a measurement information signal to measuring instruments or protection and control devices. The transformer housing is non-separable and sealed with a sticker, which makes access to the secondary winding impossible. The secondary winding terminals are covered with a transparent cover, which ensures safety during operation. In addition, the lid can be sealed. This is especially important in electricity metering circuits, as it helps prevent unauthorized access to the secondary winding terminals.
The built-in tinned copper busbar of the TTI-A modification makes it possible to connect both copper and aluminum conductors.
Rated voltage - 660 V; nominal network frequency - 50 Hz; transformer accuracy class 0.5 and 0.5S; rated secondary operating current - 5A.
| Transformer modifications | Rated primary current of the transformer, A |
| TTI-A | 5; 10; 15; 20; 25; 30; 40; 50; 60; 75; 80; 100; 120; 125; 150; 200; 250; 300; 400; 500; 600; 800; 1000 |
| TTI-30 | 150; 200; 250; 300 |
| TTI-40 | 300; 400; 500; 600 |
| TTI-60 | 600; 750; 800; 1000 |
| TTI-85 | 750; 800; 1000; 1200; 1500 |
| TTI-100 | 1500; 1600; 2000; 2500; 3000 |
| TTI-125 | 1500; 2000; 2500; 3000; 4000; 5000 |
Electronic analog devices are a combination of various electronic converters and a magnetoelectric device and are used to measure electrical quantities. They have high input impedance (low energy consumption from the measurement object) and high sensitivity. Used for measurements in high and high frequency circuits.
The operating principle of digital measuring instruments is based on converting the measured continuous signal into an electrical code displayed in digital form. The advantages are small measurement errors (0.1-0.01%) in a wide range of measured signals and high performance from 2 to 500 measurements per second. To suppress industrial interference, they are equipped with special filters. Polarity is selected automatically and indicated on the reading device. Contains output to a digital printing device. They are used to measure voltage and current, as well as passive parameters - resistance, inductance, capacitance. Allows you to measure frequency and its deviation, time interval and number of pulses.
Resistance, capacitance and inductance are the main parameters of electrical circuits, the measurement of which is often encountered in practice. There are many methods for measuring them, and the instrument-making industry produces a wide range of measuring instruments for this purpose. The choice of a particular measurement method and measuring equipment depends on the type of parameter being measured, its value, the required measurement accuracy, characteristics of the measurement object, etc. For example, measuring the resistance of solid conductors is usually carried out using direct current, since the device for measuring in In this case, it is simpler in design and cheaper than a similar device for measuring alternating current. However, measurement in environments with high humidity, or grounding resistance is carried out only on alternating current, since the measurement result on direct current will contain large errors due to the influence of electrochemical processes.
Basic methods and means of measuring the resistance of an electrical circuit to direct current
The range of resistances measured in practice is wide (from 10 8 to 10 ohms), and it is conventionally divided according to resistance values into small (less than 10 ohms), medium (from 10 to 10 6 ohms) and large (over 10 6 ohms), in each of which has its own characteristics for measuring resistance.
Resistance is a parameter that appears only when passing through a circuit electric current, so measurements are carried out with the device running or a measuring device with its own current source is used. Care must be taken to ensure that the resulting electrical value correctly reflects only the resistance being measured and does not contain unnecessary information that is perceived as a measurement error. Let us consider from this point of view the features of measuring small and large resistances.
When measuring small resistances, such as transformer windings or short wires, current is passed through the resistance, and the voltage drop that occurs across this resistance is measured. In Fig. 10.1 shows the connection diagram for measuring resistance K x short conductor. The latter is connected to the current source I through two connecting conductors with their own resistance I p. At the junction of these conductors with the measured resistance, transition contact resistances /? j. Meaning Me and depends on the material of the connecting conductor, its length and cross-section, the value of /? k - on the area of the contacting parts, their cleanliness and compression force. So the numeric values Me and and depend on many reasons and it is difficult to determine them in advance, but they can be given an approximate estimate. If the connecting conductors are made short copper wire with a cross section of several square millimeters
Rice. 10.1.
conductor
meters, and the contact resistances have a clean and well-compressed surface, then for approximate estimates we can take 2(Me and + I k)* 0.01 Ohm.
As the measured voltage in the circuit of Fig. 10.1 can be used 11 p, I 22 or?/ 33 . If selected II p, then the measurement result reflects the total resistance of the circuit between terminals 1-G:
Yats = ?/,//= Poison+ 2(L I + L K).
Here the second term represents the error, the relative value of which 5 in percent is equal to:
5 = I ~ Yah 100 = 2 KP + Yak 100.
k x*x
When measuring small resistances, this error can be large. For example, if we take 2(Me and + I k)* 0.01 Ohm, a I x = 0.1 Ohm, then 5 * 10%. Error 5 will decrease if you select And 22:
I'm 22 = and 22 /1 = I x + 2Ya K.
Here, the resistance of the supply wires is excluded from the measurement result, but the influence of Lc remains.
The measurement result will be completely free from influence I p And I'm to if you choose?/ 33 as the measured voltage.
Connection diagram I in this case it is called four-clamp: the first pair of 2-2" clamps is intended for supplying current and is called current clamps, the second pair of 3-3" clamps is for removing voltage from the measured resistance and is called potential clamps.
The use of current and potential clamps when measuring small resistances is the main technique for eliminating the influence of connecting wires and transition resistances on the measurement result.
When measuring large resistances, for example, the resistance of insulators, they do this: voltage is applied to the object, and the resulting current is measured and the value of the measured resistance is judged from it.
When testing dielectrics, it should be borne in mind that their electrical resistance depends on many conditions - ambient temperature, humidity, leakage on a dirty surface, the value of the test voltage, the duration of its action, etc.
Electrical circuit resistance measurement DC in practice, it is most often carried out by the ammeter and voltmeter method, ratiometric or bridge method.
Ammeter and voltmeter method. This method is based on separate current measurements I in the circuit of the measured resistance K x and voltage And on its terminals and subsequent calculation of the value based on the readings of measuring instruments:
I x = u/i.
Usually current / is measured with an ammeter, and voltage And - voltmeter, this explains the name of the method. When measuring high-resistance resistances, such as insulation resistance, the current is / small and is measured with a milliammeter, microammeter or galvanometer. When measuring low resistance resistances, for example a piece of wire, the value turns out to be small And and millivoltmeters, microvoltmeters or galvanometers are used to measure it. However, in all these cases, the measurement method retains its name - ammeter and voltmeter. Possible schemes switching on of devices is shown in Fig. 10.2, a, b.
Rice. 10.2. Circuits for measuring small (A) and large (b) resistance
ammeter and voltmeter method
The advantage of the method lies in the simplicity of its implementation, the disadvantage is that it is relatively high precision measurement result, which is limited by the accuracy class of the measuring instruments used and the methodological error. The latter is due to the influence of the power consumed by the measuring instruments during the measurement process, in other words, the final value of the ammeter’s own resistance I'm A and voltmeter I'm u.
Let us express the methodological error through the parameters of the circuit.
In the diagram of Fig. 10.2, A the voltmeter shows the voltage value at the terminals I, and the ammeter is the sum of currents 1 U +/. Therefore, the measurement result I, calculated from instrument readings will differ from I:
l_ and and I*
I + 1 Y and/I x + and I 1 + I x/I y "
Relative measurement error in percent
- 1 + I x/I y
Here the approximate equality is valid, since when proper organization the experiment assumes the fulfillment of the condition I y » I x.
In the diagram of Fig. 10.2, 6 The ammeter shows the current value in the circuit with I, and the voltmeter is the sum of the voltage drops by I x and and ammeter and A. Taking this into account, we can calculate the measurement result from the instrument readings:
+ I am A.
C + C l
The relative measurement error in percentage in this case is equal to:
From the obtained expressions for relative errors it is clear that in the diagram in Fig. 10.2, A the methodological error of the measurement result is influenced only by the resistance I have; to reduce this error it is necessary to ensure the condition I x « I y. In the diagram of Fig. 10.2, b the methodological error of the measurement result is influenced only by I am A; reduction of this error is achieved by fulfilling the condition I x » I A. Thus, when practical use This method can be recommended as a rule: small resistances should be measured according to the diagram in Fig. 10.2, A When measuring high resistances, preference should be given to the circuit in Fig. 10.2, b.
The methodological error of the measurement result can be eliminated by introducing appropriate corrections, but for this you need to know the values I'm A And I'm u. If they are known, then from the measurement result according to the diagram in Fig. 10.2, b value should be subtracted I am A; in the diagram of Fig. 10.2, A the measurement result reflects parallel connection resistance I And I'm therefore the meaning I calculated by the formula
If at this method If you use a power source with a previously known voltage, then there is no need to measure the voltage with a voltmeter, and the ammeter scale can be immediately calibrated in the values of the measured resistance. The operation of many models of direct assessment ohmmeters produced by industry is based on this principle. A simplified circuit diagram of such an ohmmeter is shown in Fig. 10.3. The circuit contains an EMF source, an additional resistor I and an ammeter (usually a microammeter) A. When connecting the measured resistance to the terminals of the circuit I current occurs in the circuit I, under the influence of which the movable part of the ammeter rotates through an angle a, and its pointer deviates by A scale division:
WITH/ I'm a + I'm A + I
Where WITH, - division price (constant) of the ammeter; I A - ammeter resistance.
Rice. 10.3. Schematic diagram ohmmeter with series connection
measured resistance
As can be seen from this formula, the ohmmeter scale is nonlinear, and the stability of the calibration characteristic requires ensuring the stability of all quantities included in the equation. Meanwhile, the power source in this kind of devices is usually implemented in the form of a dry galvanic cell, the emf of which drops as it is discharged. To correct for the change?, as can be seen from the equation, it is possible by appropriate adjustment WITH" or I am. In some ohmmeters WITH, regulated by changing the induction in the gap of the ammeter's magnetic system using a magnetic shunt.
In this case, the constancy of the relationship is maintained ё/С, and the calibration characteristic of the device retains its value regardless of the value e. Adjustment WITH, is done as follows: the terminals of the device to which it is connected K x, short-circuited (I x = 0) and by adjusting the position of the magnetic shunt, ensure that the ammeter pointer is set to the zero scale mark; the latter is located at the extreme right point of the scale. This completes the adjustment, and the device is ready to measure resistance.
In combined devices ampere-voltmeters adjustment WITH, is unacceptable, since this will lead to a violation of the calibration of the device in current and voltage measurement modes. Therefore, in such devices the correction for changes in EMF e is introduced by adjusting the resistance of a variable additional resistor. The adjustment procedure is the same as in devices with magnetic induction in the working gap controlled by a magnetic shunt. In this case, the calibration characteristic of the device changes, which leads to additional methodological errors. However, the circuit parameters are chosen so that the indicated error is small.
Another way to connect the measured resistance is possible - not in series with the ammeter, but in parallel with it (Fig. 10.4). Dependency between I and the angle of deflection of the moving part in this case is also nonlinear, however, the zero mark on the scale is located on the left and not on the right, as is the case in the previous version. This method of connecting the measured resistance is used when measuring small resistances, as it allows you to limit the current consumption.
Electronic ohmmeter can be implemented on the basis of a direct current amplifier with a high gain,
Rice. 10.4.
measured resistance
example, on an operational amplifier (op-amp). The diagram of such a device is shown in Fig. 10.5. Its main advantage is the linearity of the scale for reading measurement results. The op-amp is covered by negative feedback through the measured resistor I, stabilized supply voltage?/0 is applied to the amplifier input through an auxiliary resistor/?, and a voltmeter is connected to the output RU With large own coefficient op-amp gain, low output and high input resistances, the op-amp output voltage is:
and for given values and 0 and /?, the scale of the measuring device can be calibrated in resistance units to read the value K x, Moreover, it will be linear within the range of voltage changes from 0 to?/out max - the maximum voltage at the output of the op-amp.
Rice. 10.5. Electronic ohmmeter
From formula (10.1) it is clear that the maximum value of the measured resistance is:
", t „ =- ",%="? 00.2)
To change the measurement limits, switch the values of the resistor resistance /?, or voltage?/ 0.
When measuring low-resistance resistances, you can swap the measured and auxiliary resistors in the circuit. Then the output voltage will be inversely proportional to the value I:
and wx = - and 0 ^. (10.3)
It should be noted that this method inclusion does not allow measuring low-resistance resistances less than tens of Ohms, since the internal resistance of the reference voltage source, which amounts to fractions or units of Ohms, turns out to be connected in series with the measured resistance and introduces a significant error in the measurements. In addition, in this case, the main advantage of the device is lost - the linearity of the measured resistance reading, and the zero shift and the amplifier input current can introduce significant errors
Let's consider a special circuit for measuring low resistances, free from these disadvantages (Fig. 10.6). Measurement resistor I along with resistor I 3 forms a voltage divider at the op-amp input. The voltage at the output of the circuit in this case is equal to:
Rice. 10.6.
If you select " I, then the expression will be simplified and the instrument scale will be linear with respect to I:
An electronic ohmmeter does not allow you to measure reactance, since the inclusion of the measured inductance or
capacitance into the circuit will change the phase relationships in the circuit feedback The op-amp and formulas (10.1)-(10.4) will become incorrect. In addition, the op-amp may lose stability, and generation will occur in the circuit.
Ratiometric method. This method is based on measuring the ratio of two currents /, and /2, one of which flows through a circuit with a measured resistance, and the other through a circuit whose resistance is known. Both currents are created by one voltage source, so the instability of the latter has virtually no effect on the accuracy of the measurement result. The schematic diagram of an ohmmeter based on a ratiometer is shown in Fig. 10.7. The circuit contains a measuring mechanism based on a ratiometer, a magnetoelectric system with two frames, one of which creates a deflecting torque when current flows, and the other creates a restoring torque. The measured resistance can be connected in series (Fig. 10.7, A) or in parallel (Fig. 10.7, b) relative to the frame of the measuring mechanism.
Rice. 10.7. Ohmmeter circuits based on a ratiometer for measuring large (A)
and small (b) resistance
Serial connection is used when measuring medium and high resistances, parallel connection is used when measuring small resistances. Let's consider the operation of an ohmmeter using the example of the circuit in Fig. 10.7, A. If we neglect the resistance of the windings of the ratiometer frames, then the angle of rotation of the moving part a depends only on the resistance ratio: where /, and /2 are the currents through the ratiometer frames; I 0 - resistance of the ratiometer frames; /?, - known resistance; I - measured resistance.
The resistor resistance /? sets the range of resistances measured by an ohmmeter. The ratiometer's supply voltage affects the sensitivity of its measuring mechanism to changes in the measured resistance and should not be lower than a certain level. Typically, the supply voltage of ratiometers is set with some margin so that its possible fluctuations do not affect the accuracy of the measurement result.
The choice of supply voltage and the method of obtaining it depend on the purpose of the ohmmeter and the range of measured resistances: when measuring small and medium resistances, dry batteries, accumulators or power supplies from an industrial network are used, when measuring high resistances - special generators with voltages of 100, 500, 1000 V and more.
The ratiometric method is used in ES0202/1G and ES0202/2G megaohmmeters with an internal electromechanical voltage generator. They are used to measure large (10..10 9 Ohms) electrical resistances, to measure insulation resistance electrical wires, cables, connectors, transformers, windings electric machines and other devices, as well as for measuring surface and volume resistance of insulating materials.
When measuring electrical insulation resistance using a megohmmeter, one should take into account the temperature and humidity of the surrounding air, the value of which determines possible uncontrolled current leaks.
Digital ohmmeters are used in research, testing and repair laboratories, industrial enterprises manufacturing resistors, i.e. where increased measurement accuracy is required. These ohmmeters provide manual, automatic and remote control measurement ranges. Displays information about the measurement range, numerical value measured value is produced in parallel binary decimal code.
The block diagram of the Shch306-2 ohmmeter is shown in Fig. 10.8. The ohmmeter includes a conversion block/indication block 10, Control block 9, power supply, microcomputer 4 and the results output block 11.
Rice. 10.8. Block diagram of ohmmeter type Shch306-2
The conversion block contains an input scaler 2, an integrator 8 and control unit 3. The measured resistor 7 is connected to the feedback circuit of the operational amplifier. Depending on the measurement cycle, a current corresponding to the measurement range is passed through the resistor being measured, including additional current caused by the zero offset of the operational amplifiers. From the output of the scale converter, the voltage is supplied to the input of the integrator, made according to the principle of multi-cycle integration with measurement of the discharge current.
The control algorithm ensures the operation of a large-scale converter and integrator, as well as communication with a microcomputer.
In the control unit, time intervals are filled with clock pulses, which then arrive at the inputs of four counters of high and low digits. The information received at the outputs of the counters is read into the random access memory (RAM) of the microcomputer.
Retrieving information from the control unit about the measurement result and operating mode of the ohmmeter, processing and bringing the data to the form required for display, mathematical processing of the result, outputting data to the auxiliary RAM of the control unit, controlling the operation of the ohmmeter and other functions are assigned to the microprocessor 5, located in the microcomputer unit. Stabilizers are located in the same block 6 for powering ohmmeter devices.
The ohmmeter is built on microcircuits with a high degree of integration.
Specifications
Measuring range 10L..10 9 Ohm. Accuracy class for measurement limits: 0.01/0.002 for 100 Ohm; 0.005/0.001 for 1.10, 100 kOhm; 0.005/0.002 for 1 MOhm; 0.01/0.005 for 10 MΩ; 0.2/0.04 for 100 MOhm; 0.5/0.1 for 1 GOM (the numerator shows the values in the mode without data accumulation, the denominator shows the values with accumulation).
Number of decimal places: 4.5 in ranges with an upper limit of 100 MΩ, 1 GΩ; 5.5 in other ranges in mode without summation, 6.5 in mode with summation.
Portable digital multimeters, for example the M83 series produced Mazes/i can be used as ohmmeters of accuracy class 1.0 or 2.5.
Electrical measurements include measurements of physical quantities such as voltage, resistance, current, and power. Measurements are made using various means - measuring instruments, circuits and special devices. The type of measuring device depends on the type and size (range of values) of the measured value, as well as on the required measurement accuracy. The basic SI units used in electrical measurements are volt (V), ohm (Ω), farad (F), henry (H), ampere (A), and second (s).
Electrical measurement is the determination (using experimental methods) of the value of a physical quantity expressed in appropriate units.
The values of units of electrical quantities are determined by international agreement in accordance with the laws of physics. Since “maintaining” units of electrical quantities determined by international agreements is fraught with difficulties, they are presented as “practical” standards for units of electrical quantities.
Standards are supported by state metrological laboratories different countries. From time to time, experiments are carried out to clarify the correspondence between the values of the standards of units of electrical quantities and the definitions of these units. In 1990, state metrological laboratories of industrialized countries signed an agreement on the harmonization of all practical standards of units of electrical quantities among themselves and with international definitions units of these quantities.
Electrical measurements are carried out in accordance with state standards of units of voltage and direct current, direct current resistance, inductance and capacitance. Such standards are devices that have stable electrical characteristics, or installations in which, based on a certain physical phenomenon an electrical quantity is reproduced, calculated from the known values of the fundamental physical constants. Watt and watt-hour standards are not supported, since it is more appropriate to calculate the values of these units using defining equations that relate them to units of other quantities.
Electrical measuring instruments most often measure instantaneous values of either electrical quantities or non-electrical quantities converted into electrical ones. All devices are divided into analog and digital. The former usually show the value of the measured quantity by means of an arrow moving along a scale with divisions. The latter are equipped with a digital display that shows the measured value in the form of a number.
Digital instruments are preferable for most measurements, as they are more convenient to take readings and, in general, are more versatile. Digital multimeters ("multimeters") and digital voltmeters are used to measure DC resistance, as well as AC voltage and current, with medium to high accuracy.
Analog devices are gradually being replaced by digital ones, although they are still used where low cost is important and high accuracy is not needed. For the most accurate measurements of resistance and impedance, there are measuring bridges and other specialized meters. To record the progress of changes in the measured value over time, recording instruments are used - strip recorders and electronic oscilloscopes, analog and digital.
Measurements of electrical quantities are one of the most common types of measurements. Thanks to the creation of electrical devices that convert various non-electrical quantities into electrical ones, methods and means of electrical instruments are used in measuring almost all physical quantities.
Scope of application of electrical measuring instruments:
· Scientific research in physics, chemistry, biology, etc.;
· technological processes in energy, metallurgy, chemical industry and etc.;
· transport;
· exploration and production of mineral resources;
· meteorological and oceanological work;
· medical diagnostics;
· manufacture and operation of radio and television devices, aircraft and spacecraft and so on.
A wide variety of electrical quantities, wide ranges of their values, requirements for high measurement accuracy, a variety of conditions and areas of application of electrical measuring instruments have led to a variety of methods and means of electrical measurements.
The measurement of “active” electrical quantities (current strength, electrical voltage, etc.), characterizing the energy state of the measured object, is based on the direct impact of these quantities on the sensitive element and, as a rule, is accompanied by the consumption of a certain amount of electrical energy from the measured object.
Measurement of "passive" electrical quantities ( electrical resistance, its complex components, inductance, dielectric loss tangent, etc.), characterizing the electrical properties of the measurement object, requires recharging the measurement object from an external source of electrical energy and measuring the parameters of the response signal.
Methods and means of electrical measurements in DC and AC circuits differ significantly. In alternating current circuits, they depend on the frequency and nature of the change in quantities, as well as on what characteristics of variable electrical quantities (instantaneous, effective, maximum, average) are measured.
For electrical measurements in DC circuits, magnetoelectric measuring instruments and digital measuring devices are most widely used. For electrical measurements in alternating current circuits - electromagnetic instruments, electrodynamic instruments, induction instruments, electrostatic instruments, rectifier electrical measuring instruments, oscilloscopes, digital measuring instruments. Some of the listed instruments are used for electrical measurements in both AC and DC circuits.
The values of the measured electrical quantities are approximately within the following limits: current strength - from to A, voltage - from to V, resistance - from to Ohm, power - from W to tens of GW, alternating current frequency - from to Hz. The ranges of measured values of electrical quantities have a continuous tendency to expand. Measurements at high and ultra-high frequencies, measurement of low currents and high resistances, high voltages and characteristics of electrical quantities in powerful power plants have been allocated to sections that develop specific methods and electrical measuring instruments.
The expansion of the measurement ranges of electrical quantities is associated with the development of technology for electrical measuring transducers, in particular with the development of technology for amplifying and weakening electric currents and voltages. Specific problems of electrical measurements of ultra-small and ultra-large values of electrical quantities include the fight against distortions accompanying the processes of amplification and weakening of electrical signals, and the development of methods for isolating a useful signal from a background of noise.
The limits of permissible errors in electrical measurements range from approximately units to %. For relatively rough measurements, direct measuring instruments are used. For more accurate measurements, methods are used that are implemented using bridge and compensation electrical circuits.
The use of electrical measurement methods for measuring non-electrical quantities is based either on the known relationship between non-electrical and electrical quantities, or on the use of measuring transducers (sensors).
To provide collaboration sensors with secondary measuring instruments, transmitting electrical output signals of sensors over a distance, increasing the noise immunity of transmitted signals, a variety of electrical intermediate measuring transducers are used, which, as a rule, simultaneously perform the functions of amplification (less often, attenuation) of electrical signals, as well as nonlinear transformations in order to compensate for the nonlinearity of sensors .
Any electrical signals (values) can be supplied to the input of intermediate measuring transducers; unified electrical signals of direct, sinusoidal or pulsed current (voltage) are most often used as output signals. AC output signals use amplitude, frequency, or phase modulation. Digital converters are becoming increasingly widespread as intermediate measuring converters.
Comprehensive automation scientific experiments And technological processes led to the creation complex means measuring installations, measuring and information systems, as well as to the development of telemetry technology and radio telemechanics.
Modern development electrical measurements are characterized by the use of new physical effects. For example, at present, quantum effects of Josephson, Hall, etc. are used to create highly sensitive and high-precision electrical measuring instruments. Electronics achievements are widely introduced into measurement technology, microminiaturization of measuring instruments is used, their coupling with computer technology, automation of electrical measurement processes, as well as unification of metrological and other requirements for them.
Plan
Introduction
Current meters
Voltage measurement
Combined devices of the magnetoelectric system
Universal electronic measuring instruments
Measuring shunts
Instruments for measuring resistance
Determination of ground resistance
Induction
Bibliography
Introduction
Measurement is the process of finding the value of a physical quantity experimentally, using special technical means - measuring instruments.
Thus, measurement is an informational process of obtaining, experimentally, a numerical relationship between a given physical quantity and some of its values, taken as a unit of comparison.
The result of a measurement is a named number found by measuring a physical quantity. One of the main tasks of measurement is to assess the degree of approximation or difference between the true and actual values of the measured physical quantity - measurement error.
The main parameters of electrical circuits are: current, voltage, resistance, current power. Electrical measuring instruments are used to measure these parameters.
Measuring the parameters of electrical circuits is carried out in two ways: the first is a direct measurement method, the second is an indirect measurement method.
The direct measurement method involves obtaining the result directly from experience. An indirect measurement is a measurement in which the desired quantity is found on the basis of a known relationship between this quantity and the quantity obtained as a result of direct measurement.
Electrical measuring instruments are a class of devices used to measure various electrical quantities. The group of electrical measuring instruments also includes, in addition to the measuring instruments themselves, other measuring instruments - gauges, converters, complex installations.
Electrical measuring instruments are classified as follows: according to measured and reproducible physical quantity(ammeter, voltmeter, ohmmeter, frequency meter, etc.); by purpose (measuring instruments, measures, measuring transducers, measuring installations and systems, auxiliary devices); by the method of providing measurement results (displaying and recording); by measurement method (direct assessment devices and comparison devices); by method of application and design (panel, portable and stationary); according to the principle of operation (electromechanical - magnetoelectric, electromagnetic, electrodynamic, electrostatic, ferrodynamic, induction, magnetodynamic; electronic; thermoelectric; electrochemical).
In this essay I will try to talk about the device, the principle of operation, give a description and brief description electrical measuring instruments of electromechanical class.
Current measurement
Ammeter is a device for measuring current in amperes (Fig. 1). The scale of ammeters is calibrated in microamperes, milliamperes, amperes or kiloamperes in accordance with the measurement limits of the device. In an electrical circuit, the ammeter is connected in series with the section of the electrical circuit (Fig. 2) in which the current is measured; to increase the measurement limit - with a shunt or through a transformer.
The most common ammeters are those in which the moving part of the device with the pointer rotates through an angle proportional to the magnitude of the current being measured.
Ammeters are magnetoelectric, electromagnetic, electrodynamic, thermal, induction, detector, thermoelectric and photoelectric.
Magnetoelectric ammeters measure direct current; induction and detector - alternating current; ammeters of other systems measure the strength of any current. The most accurate and sensitive are magnetoelectric and electrodynamic ammeters.
The principle of operation of a magnetoelectric device is based on the creation of torque due to the interaction between the field of a permanent magnet and the current that passes through the winding of the frame. An arrow is connected to the frame, which moves along the scale. The angle of rotation of the arrow is proportional to the current strength.
Electrodynamic ammeters consist of fixed and moving coils connected in parallel or in series. The interaction between the currents that pass through the coils causes deflections of the moving coil and the arrow connected to it. In an electrical circuit, the ammeter is connected in series with the load, and when high voltage or high currents - through a transformer.
Technical data of some types of domestic ammeters, milliammeters, microammeters, magnetoelectric, electromagnetic, electrodynamic, and thermal systems are given in Table 1.
Table 1. Ammeters, milliammeters, microammeters
| Instrument system | Device type | Accuracy class | Measurement limits |
| Magnetoelectric | M109 | 0,5 | 1; 2; 5; 10 A |
| M109/1 | 0,5 | 1.5-3 A | |
| М45М | 1,0 | 75mV | |
| 75-0-75mV | |||
| M1-9 | 0,5 | 10-1000 µA | |
| M109 | 0,5 | 2; 10; 50 mA | |
| 200 mA | |||
| М45М | 1,0 | 1.5-150 mA | |
| Electromagnetic | E514/3 | 0,5 | 5-10 A |
| E514/2 | 0,5 | 2.5-5 A | |
| E514/1 | 0,5 | 1-2 A | |
| E316 | 1,0 | 1-2 A | |
| 3316 | 1,0 | 2.5-5 A | |
| E513/4 | 1,0 | 0.25-0.5-1 A | |
| E513/3 | 0,5 | 50-100-200 mA | |
| E513/2 | 0,5 | 25-50-100 mA | |
| E513/1 | 0,5 | 10-20-40 mA | |
| E316 | 1,0 | 10-20 mA | |
| Electrodynamic | D510/1 | 0,5 | 0.1-0.2-0.5-1-2-5 A |
| Thermal | E15 | 1,0 | 30;50;100;300 mA |
Voltage measurement
Voltmeter - direct reading measuring device for determining voltage or EMF in electrical circuits (Fig. 3). Connected in parallel to the load or source of electrical energy (Fig. 4).
According to the operating principle, voltmeters are divided into: electromechanical - magnetoelectric, electromagnetic, electrodynamic, electrostatic, rectifier, thermoelectric; electronic - analog and digital. By purpose: direct current; alternating current; pulse; phase sensitive; selective; universal. By design and method of application: panel; portable; stationary. Technical data of some domestic voltmeters, millivoltmeters of magnetoelectric, electrodynamic, electromagnetic, and thermal systems are presented in Table 2.
Table 2. Voltmeters and millivoltmeters
| Instrument system | Device type | Accuracy class | Measurement limits |
| Electrodynamic | D121 | 0,5 | 150-250 V |
| D567 | 0,5 | 15-600 V | |
| Magnetoelectric | M109 | 0,5 | 3-600 V |
| M250 | 0,5 | 3; 50; 200; 400 V | |
| М45М | 1,0 | 75 mV; | |
| 75-0-75 mV | |||
| 75-15-750-1500 mV | |||
| M109 | 0,5 | 10-3000 mV | |
| Electrostatic | C50/1 | 1,0 | 30 V |
| C50/5 | 1,0 | 600 V | |
| C50/8 | 1,0 | 3 kV | |
| S96 | 1,5 | 7.5-15-30 kV | |
| Electromagnetic | E515/3 | 0,5 | 75-600 V |
| E515/2 | 0,5 | 7.5-60 V | |
| E512/1 | 0,5 | 1.5-15 V | |
| With electronic converter | F534 | 0,5 | 0.3-300 V |
| Thermal | E16 | 1,5 | 0.75-50 V |
For measurements in direct current circuits, combined instruments of the magnetoelectric system, ampere-voltmeters, are used. Technical data on some types of devices are given in Table 3.
Table 3. Combined devices of the magnetoelectric system.
| Name | Type | Accuracy class | Measurement limits |
| Millivolt-milliammeter | M82 | 0,5 | 15-3000 mV; 0.15-60 mA |
| Voltammeter | M128 | 0,5 | 75mV-600V; 5; 10; 20 A |
| Ampere-voltmeter | M231 | 1,5 | 75-0-75 mV; 100-0-100 V;0.005-0-0.005 A; 10-0-10 A |
| Voltammeter | M253 | 0,5 | 15mV-600V; 0.75 mA-3 A |
| Millivolt-milliammeter | M254 | 0,5 | 0.15-60 mA; 15-3000 mV |
| Microamperevoltmeter | M1201 | 0,5 | 3-750 V; 0.3-750 µA |
| Voltammeter | M1107 | 0,2 | 45mV-600V; 0.075 mA-30 A |
| Milliamp-voltmeter | М45М | 1 | 7.5-150 V; 1.5 mA |
| Volt-ohmmeter | M491 | 2,5 | 3-30-300-600 V; 30-300-3000 kOhm |
| Ampere-voltmeter | M493 | 2,5 | 3-300 mA; 3-600 V; 3-300 kOhm |
| Ampere-voltmeter | M351 | 1 | 75mV-1500V;15uA-3000mA;200Ohm-200Mohm |
Technical data on combined instruments - ampere-voltmeters and ampere-voltmeters for measuring voltage and current, as well as power in alternating current circuits.
Combined portable instruments for measuring direct and alternating current circuits provide measurement of direct and alternating currents and resistances, and some also provide element capacitance in a very wide range, are compact, and have autonomous power supply, which ensures their wide application. The accuracy class of this type of DC device is 2.5; on variable – 4.0.
Universal electronic measuring instruments
The main parameters of electrical circuits are: for a direct current circuit, resistance R,
for AC circuit active resistance ,
inductance 
, capacity 
, complex resistance 
.
The following methods are most often used to measure these parameters: ohmmeter, ammeter - voltmeter, bridge. The use of compensators for measuring resistance 
already discussed in paragraph 4.1.8. Let's consider other methods.
Ohmmeters. Directly and quickly the resistance of DC circuit elements can be measured using an ohmmeter. In the diagrams presented in Fig. 16 THEM- magnetoelectric measuring mechanism.
At a constant supply voltage 
the readings of the measuring mechanism depend only on the value of the measured resistance 
.
Therefore, the scale can be graduated in units of resistance.
For a series circuit of connecting an element with resistance 
(Fig. 4.16, 
)
pointer deflection angle
,
For a parallel circuit (Fig. 4.16, 
)
,Where 
- sensitivity of the magnetoelectric measuring mechanism; 
- resistance of the measuring mechanism; 
- resistance of the additional resistor. Since the values of all quantities on the right side of the above equations, except 
,
then the angle of deviation is determined by the value 
.
The ohmmeter scales for both circuits are uneven. In a series circuit, unlike a parallel circuit, the zero of the scale is aligned with the maximum angle of rotation of the moving part. Ohmmeters with a series circuit are more suitable for measuring high resistances, and those with a parallel circuit are more suitable for measuring small ones. Typically, ohmmeters are made in the form of portable devices of accuracy classes 1.5 and 2.5. As a power source 
battery is used. The need to set zero using a corrector is a major drawback of the ohmmeters considered. This disadvantage is absent in ohmmeters with a magnetoelectric logometer.
The connection diagram for the ratiometer in the ohmmeter is shown in Fig. 4.17. In this scheme 1 and 2
- ratiometer coils (their resistance 
And 
);
And 
- additional resistors permanently included in the circuit.
,
then the deviation of the logometer needle 
,
i.e. the angle of deviation is determined by the value 
and does not depend on voltage
.
Ohmmeters with a logometer have different designs depending on the required measurement limit, purpose (panel or portable device), etc.
Ammeter-voltmeter method. This method is an indirect method for measuring the resistance of elements of direct and alternating current circuits. An ammeter and a voltmeter measure the current and voltage across the resistance, respectively. 
the value of which is then calculated using Ohm's law: 
. The accuracy of determining resistance by this method depends both on the accuracy of the instruments and on the switching circuit used (Fig. 4.18, 
And 
).
When measuring relatively small resistances (less than 1 ohm), the circuit in Fig. 4.18, 
preferable, since the voltmeter is connected directly to the resistance being measured 
, and the current 
, measured by an ammeter, is equal to the sum of the current in the measured resistance 
and current in a voltmeter 
, i.e. 
. Because 
>>
, That 
.
When measuring relatively high resistances (more than 1 Ohm), the circuit in Fig. 4.18, 
, since the ammeter directly measures the current in the resistance 
,
and the voltage 
,
measured by a voltmeter is equal to the sum of the voltages on the ammeter
and measured resistance 
, i.e. 
. Because 
>>
, That 
.
Schematic diagrams of switching on devices for measuring the impedance of elements 
AC circuits using the ammeter-voltmeter method are the same as for measuring resistance 
.
In this case, based on the measured voltage values 
and current 
determine the total resistance 
.
Obviously, this method cannot measure the argument of the resistance being tested. Therefore, the ammeter-voltmeter method can measure the inductance of coils and the capacitance of capacitors, the losses in which are quite small. In this case
;
.