Electrical instruments are designed to measure electrical quantities. The instruments studied in this work belong to the group of indicating devices, i.e., direct action (the desired value of a physical quantity is determined directly from the instrument reading). According to the method of converting electrical energy supplied to the device into mechanical energy of movement of the moving part and according to the design features of the measuring mechanism, devices are divided into magnetoelectric, electromagnetic, electrodynamic, electrostatic, ferrodynamic, induction, etc.
The symbol for the operating principle of an electrical measuring instrument is indicated on its scale (Table 1).
Table 1
Symbols on the instrument scale
|
Conditional designation |
Conditional designation |
||
|
Magnetoelectric |
Ferrodynamic | ||
|
Magnetoelectric with thermoelectric converter |
Electrostatic | ||
|
Magnetoelectric with rectifier |
Ferrodynamic | ||
|
Electromagnetic |
Induction |
Magnetoelectric devices in which a current-carrying coil interacts with the field of a permanent magnet are widely used for measuring direct current (ammeters) and voltage (voltmeters). Such devices have a uniform reading scale and high sensitivity. To expand the measurement limit, ammeters are included in the circuit using shunts, and voltmeters with additional resistance.
Connection diagrams for magnetoelectric devices are shown in Fig. 1, where r A , r V– resistance of measuring instruments; R d – additional resistor; R w – shunt resistance, which can be built into the device or switched on separately; I, I A– direction of current in the circuit and coil; U V , U d, U– respectively, the voltage on the measuring device, resistor R d, measurable.
B electromagnetic devices, the magnetic field of a stationary coil acts on a movable ferromagnetic plate, moving it relative to the coil. In electromagnetic ammeters, the coil is connected to the network in series. The measurement limit is set by changing the number of turns of the measuring coil. To measure significant alternating currents and voltages, current and voltage measuring transformers are used. Voltmeter coils are connected to the network through a large additional resistance. Electromagnetic devices are simple, reliable, can withstand significant overloads, can be used in direct and alternating current circuits, but they have low sensitivity, low accuracy, uneven scale, and consume high power.
IN electrodynamic devices use the interaction of the fields of two coils with current; such devices operate on both direct and alternating current as ammeters, voltmeters, wattmeters, and phase meters. The main disadvantages of electrodynamic devices are the influence of external magnetic fields and weak torque.
IN ferrodynamic Electrodynamic system devices use a steel magnetic core in the measuring mechanism. The use of steel reduces the accuracy of the device due to the influence of hysteresis and eddy currents, and greatly complicates the design of the device. For these reasons, ferrodynamic instruments are of little use for precise measurements and are used mainly as recording instruments and panel wattmeters (the latter do not have the disadvantages of electrodynamic wattmeters and are much more accurate than induction ones).
IN electrostatic devices use the energy of the electric field of a system of electrodes to move the moving part of the measuring mechanism. Such devices have an almost uniform scale, are used to measure only direct and alternating current voltages from 10 to tens of kilovolts, have the highest accuracy class (0.05) and do not consume active power.
IN induction In devices, torque is created by the interaction of currents induced in the moving part of the device, the metal disk, with the magnetic fluxes of stationary electromagnets. In an induction wattmeter, one coil is connected in series in the circuit, and the second in parallel, due to which the flux of the first coil is proportional to the current I, and the second – voltage U. The measuring mechanism of the induction system is also used in AC electrical energy meters.
Digital meters(TsIP) have the following advantages over analogue ones: high measurement accuracy, wide range, indication of results in digital form, speed, ability to output information to a computer, automatic measurement process, choice of measurement limits.
Most CIPs have several measurement ranges for which limit values are indicated. The range is selected manually or automatically. Switching the range is accompanied by a change in the position of the decimal point on the digital readout device (DRO). The measurement accuracy is determined by the quantization error, which depends on the number of digits of the DOC.
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, in monitoring their condition and operating modes, in taking into account the consumption and quality of electrical energy, in the 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 section of the electrical circuit under study.
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 alternating current circuits is 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 of active and reactive resistance and other elements, depending on the nature of the conversion. 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 on this 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, changes in ambient temperature, etc. The difference between the measured Ai and actual Ad values of the controlled quantity is called the absolute measurement error:
Since the absolute error does not give an idea of the degree of measurement accuracy, the relative error is used:
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 service life, not less, 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 the negative influence of 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 of a permanent magnet and conductors with current, and the electromagnetic system is based on the retraction of a steel core into a stationary coil when there is 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 the 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 resistance of the ammeter and the current circuits of the wattmeter are small, so the current transformer actually operates in short circuit mode. The rated current of the secondary winding is 5A. The transformation ratio of a current transformer is equal to the ratio of the 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 current.
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 commercial electricity metering schemes; 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.
Chapter VI
ELECTRICAL INSTRUMENTS AND MEASUREMENTS
§ 67. General information
Electrical measuring instruments are used to measure various electrical quantities: current, voltage, resistance, power, energy, as well as many non-electric quantities, including temperature, pressure, humidity, speed, liquid level, material thickness, etc.
Due to the fact that there are no absolutely accurate instruments, the readings of electrical measuring instruments differ somewhat from the actual value of the measured values.
The difference between the measured and actual value of a quantity is called absolute error of the device. If, for example, the current in the circuit is I = 10 A, and the ammeter connected to this circuit shows I unit = 9.85 A, then the absolute error of the instrument reading is
Δ A = I meas - I = 9,85 - 10 = -0,15 a. (94)
Reduced instrument errorγ pr is called the ratio of the absolute error Δ A to the largest value A max that can be measured with a given instrument scale:
The reduced error of a device under normal operating conditions (temperature 20° C, absence of ferromagnetic masses near the device, normal operating position of the scale, etc.) is called main instrument error.
Example. Let when measuring the current strength I = 4 A under normal conditions we used an ammeter with a scale of 0 - 10 A and it showed that the current in the circuit is 4.1 A. Calculate the basic (reduced) error of the device, which characterizes its accuracy.
Solution .
Depending on the permissible basic error, electrical measuring instruments are divided into eight accuracy classes: 0.05; 0.1; 0.2; 0.5; 1; 1.5; 2.5; 4.
The accuracy class number shows the value of the permissible basic (reduced) error Δ A max of the device as a percentage, regardless of the sign of the error.
Accuracy class
A device whose accuracy class is expressed by a smaller number allows you to perform measurements with greater accuracy.
Knowing the accuracy class of the device and the largest value of the quantity that can be measured with a given scale of the device, you can determine the largest possible absolute error of the measurement performed:
Example. Let us assume that the maximum current that can be measured with this ammeter is 15 A, and the accuracy class of the device TO = 4.
Determine the largest possible absolute error when taking a measurement at any point on the scale.
Solution .
The closer the measured value is to the largest value that the device can measure, the smaller the relative error is, all other things being equal. This circumstance should be taken into account when choosing the measuring limit of the device to perform the measurement.
Electrical measuring instruments are classified according to the type of quantity being measured, the principle of operation, the degree of accuracy and the type of current being measured, in addition, they are divided into operational groups.
According to the type of quantity being measured, instruments are divided into ammeters, voltmeters, ohmmeters, wattmeters, counters, electric thermometers, electric tachometers (measuring the number of revolutions per minute), etc.
According to the principle of operation of the measuring mechanism, devices can be of the following systems: electromagnetic, magnetoelectric, electrodynamic, ferrodynamic, induction, rectifier, thermoelectric, electronic, vibration and electrostatic.
Depending on the type of current that the instruments are designed to measure, they are divided into instruments that measure alternating current, direct current, and instruments that measure alternating and direct currents.
They produce devices of three main operational groups: A, B And IN. The symbols of electrical measuring instruments of different operational groups are given in table. 7.
On the scale of each electrical measuring instrument, symbols indicate the necessary information about the design and operation of the device. For example, on the voltmeter scale (Fig. 79) it is indicated: voltmeter (V) of the electromagnetic system; designed to measure alternating voltage (~) in the range from 0 to 250 V; when measuring voltage, the device should be installed vertically (⊥); insulation tested voltage 2 kv; accuracy class 1.5; serial number 5140; year of manufacture 1966; operational group.
Electronic measuring instruments have increased speed, high sensitivity and a fairly wide frequency range. They are used to measure certain electrical quantities - voltage, current, resistance and other parameters.
These devices are divided into analog and digital models. These models differ from each other in that they have different forms of information reproduction - using a digital monitor or an arrow. Today, electronic digital measuring instruments are the most popular, since mechanical options are inferior in the accuracy of the information displayed. However, the affordable cost persuades many to buy mechanical devices.
Voltage indicators and indicators
They are used to determine the presence or absence of current in the network for electrical appliances whose power does not exceed 1000 V. The operating principle is the conversion of electrical signals into light signals. The device has a scale and a light indicator, with which you can simply understand whether there is voltage in the network. If there is no glow, then this indicates its breakage or absence. Indicators can also measure the phases of alternating current and the polarity of direct current.
Voltmeter, ammeter, ohmmeter
An electronic device is used to measure current, voltage, power, resistance, capacitance, inductance, etc. They can combine converters from the measured quantity to direct voltage, that is, current, they can also combine a magnetoelectric device and are distinguished by high sensitivity, wide frequency range and low power consumption.
A detectable voltage is supplied to the output of the amplifier through a divider, and the output voltage after the amplifier is calculated by a magnetoelectric device. The main error of this voltmeter is 0.5…1.0 percent.
An AC voltmeter is an electronic instrument designed to measure and convert alternating voltage to direct voltage. Voltmeters are divided depending on the measured alternating voltage: root mean square values, average rectifier values and amplitude values.
The ohmmeter is not available as a separate device; its functions are performed by an electronic voltmeter. The ohmmeter is equipped with a converter, which is an amplifier surrounded by negative feedback by measured and reference resistors. Therefore, the voltage measured by an electronic voltmeter is proportional to the resistance of the resistor being detected. This circuit is very popular for measuring resistance from 10 to 1000 MΩ.
Frequency meter and oscilloscope
The frequency meter uses the principle of charging and discharging a capacitor and is combined with an analog output mechanism designed to determine the average amount of force flowing through the capacitor as it periodically recharges relative to the detected frequency.
In order to study the behavior of signals over time, an electronic oscilloscope is used, which makes it possible to directly observe or record the shape of non-periodic and periodic signals. Due to the fact that the moving part of the oscilloscope is made of electrons, it has virtually no inertia and can be used to measure quantities with a frequency of up to several hundred megahertz and non-periodic operations, the duration of which reaches a fraction of microseconds.
These instruments for measuring current and voltage also have a high input resistance and high sensitivity. However, they also have disadvantages, namely low measurement accuracy (10 percent error), structural and electrical complexity, and high cost. Moreover, if we compare the oscilloscope with other electronic measuring instruments, it is the most difficult to operate and requires certain personnel qualifications.
The oscilloscope has become widely used for measuring the phase and frequency of electrical oscillations. In addition, it is possible to study vibrations of various forms.
As a rule, this device is used for short-term current measurement without breaking the circuit. Due to the fact that current is supplied to the coil from the line being detected, it is possible not to break the circuit during operation - this is the primary principle of operation of this electronic device. Clamp meters can be analog or digital. The main functions they perform are: measuring AC voltage, DC voltage, resistance, AC current, temperature.
This is a device that combines almost all instruments designed to measure current and voltage,” as well as other parameters. It may contain an ammeter, a voltmeter, an ohmmeter and similar electronic devices. Due to their simple design and positive properties, these multimeters have been very well known for many years. Multimeters come in varying degrees of accuracy, which directly determines their cost, so before choosing this electrical measuring device, you need to decide on the tasks that it will perform.
Repair of electronic devices
Due to the fact that the designs of measuring instruments are varied, it is very difficult to describe all the processes of disassembly and assembly. However, most of the processes are common to any instrument design.
Homogeneous repair processes can be performed by specialists of different qualifications. Devices of class 1 - 1.5 - 2.5 - 4 must be repaired by specialists whose qualifications are 4-6 categories. Complex and special devices should be repaired by electromechanics of the 7th-8th category.
In general, the processes of disassembling and assembling electrical measuring instruments are critical processes, so they must be performed carefully and carefully. In case of careless disassembly, individual parts may deteriorate, which will lead to the addition of new faults. Before you begin disassembly, you should consider the general order of operations.
Complete disassembly of an electronic device is carried out during a major overhaul, which involves rewinding coils, frames, resistances, manufacturing or replacing damaged and burnt parts. It provides for the separation of all parts of the device from each other.
When a medium repair is performed, all parts of the device are not completely disassembled, but are limited to only removing the moving part, changing the bearings, refilling the cores, restoring the moving part, adjusting and adjusting the indications of the mechanism. Re-calibration during a mid-life repair should only be carried out when the scale has become dull and dirty. In other cases, the scale should be kept with the same marks. An indicator of a high-quality average repair is the production of a device with the same scale.
To disassemble and assemble devices, you will need watch tweezers, screwdrivers, small electric soldering irons, watch cutters, oval pliers, pliers, specially made keys, etc.
After a complete repair of the device, it is checked whether the moving part moves freely, the internal part is inspected, and the readings of the repaired and standard apparatus are recorded during measurements of the determined value from zero to maximum and back.
Measurement is the determination of the value of a physical quantity experimentally using special technical means. Measurements are made in generally accepted units.
The main elements of the measurement process: measurement object, measured quantity, measuring instrument, measurement principle, measurement method, measurement conditions, measurement result, measurement error, human operator performing the measurements (measurement subject).
Object of measurement– this is a complex, multifaceted phenomenon or process (for example, electrical oscillations at the output of a self-generator), characterized by many individual physical parameters. One of these parameters that interests us and is subject to measurement is called a measured physical quantity (for example, the oscillation frequency of a self-oscillator).
Measuring instrument is a technical tool used in measurements and having standardized metrological properties.
Measuring principle is a set of physical phenomena on which measurements are based (for example, the resonant principle of frequency measurement).
Measurement method is a set of techniques for using principles and measuring instruments (for example, a method of comparing the measured frequency with a known frequency).
Electrical methods for measuring electrical and non-electrical quantities have a number of advantages compared to other measurement methods: low energy consumption; possibility of remote transmission of measurement information; high measurement speed; high accuracy and sensitivity.
Measurement technique in contrast to a method, it includes a detailed procedure for the measurement process using specific methods and measuring instruments.
No matter how carefully the measurement is carried out, its result will contain some inaccuracy, which is characterized by an error. Measurement error is the deviation of the measurement result from the true value of the measured value.
The widely used term measurement accuracy characterizes the quality of measurements, reflecting the closeness of their results to the true value. Greater accuracy corresponds to less measurement error.
The value of a physical quantity found by measuring it is called the result of the measurement. The measurement result can be obtained as a result of one observation or by processing the results of several
observations. In this case, observation is understood as an experimental operation in which one numerical value of a quantity is obtained.
The Republic of Belarus has introduced the International System of Units, abbreviated SI. The basic units of this system are: meter ( m), kilogram ( kg), second ( s), ampere ( A), kel-vin ( TO), mole ( mol) and candela ( CD), additional - angular units: radian (rad) and steradian (sr). In addition to the basic and additional ones, derived units are established.
Technical means used in electrical measurements and having standardized errors are divided according to their purpose into measures, measuring transducers, electrical measuring instruments, electrical measuring installations and measuring systems.
A measure is a measuring instrument designed to reproduce the value of a physical quantity of a given size with a certain accuracy. There are single-valued measures, for example, a resistance measuring coil, a capacitor, and multi-valued (variable values), as well as sets and stores of measures, i.e., sets of measures for reproducing a number of the same values of quantities of various sizes (stores of resistances, capacitances).
Measuring transducers are designed to generate measurement information signals in a form convenient for transmission, further conversion and processing, but not amenable to direct perception by an observer. Some of them - shunts, voltage dividers, instrument transformers, amplifiers - can convert electrical quantities into electrical ones, but necessary for the consumer, others - thermoelectric thermometers, strain gauges, inductive converters - non-electric quantities into electrical ones.
Electrical measuring instruments are electrical measuring instruments designed to generate measurement information signals in a form convenient for direct perception by an observer (for example, a voltmeter, ammeter, wattmeter, phase meter).
Electrical measuring instruments are classified according to their purpose, design, type of quantity being measured, principle, operating conditions, accuracy class and other criteria.
Depending on the type of quantity being measured (for example, voltage, current, power), electrical measuring instruments are divided into ammeters, voltmeters, wattmeters, etc. and combined ones, measuring two or more quantities (for example, ampere-voltmeters).
Electrical measuring instruments whose readings are continuous functions of the measured quantities are called analog instruments. Electrical measuring instruments that automatically produce discrete signals of measuring information, the readings of which are presented in digital form, are called digital instruments.
An electrical measuring installation consists of a number of measuring instruments (gauges, measuring transducers, instruments) and auxiliary devices located in one place. Electrical measuring installations are used for checking and calibrating electrical measuring instruments and testing magnetic and electrical insulating materials.
Depending on the method of obtaining the result, two measurement methods are distinguished: straight And indirect.
Direct is a measurement whose result is obtained directly from experimental data. This includes measurements of various physical quantities using instruments calibrated in established units, for example, measuring current with an ammeter, conductor resistance with an ohmmeter, temperature with a thermometer, etc. Direct measurements are widely used because of their simplicity and speed of obtaining results.
Indirect is a measurement in which the desired value of a quantity is determined on the basis of a known mathematical relationship between it and quantities obtained from direct measurements. For example, power P in DC circuits is calculated using the formula: R= U I; voltage U in this case, it is measured with a voltmeter, and the current I– ammeter; resistor value R = U/I– based on measured voltage values U and current I. Indirect measurements are used, as a rule, only in cases where direct measurements cannot be used.
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