Circuit Diagram 31-Analysis of Measurement Circuit Principle

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Circuit Diagram 31-Analysis of Measurement Circuit Principle [Copy link]

Commonly used measuring instruments include AC (DC) ammeter, AC (DC) voltmeter, active power meter, reactive power meter, frequency meter, power factor meter, active energy meter, etc.

1. Current measurement circuit

The instrument used to measure current in the measurement circuit is the ammeter. The ammeter must be connected in series in the circuit, as shown in the figure below. When measuring DC current, a DC ammeter must be used, and attention must be paid to the fact that the positive and negative polarities of the ammeter must be consistent with the polarity of the power supply. They must not be connected incorrectly, otherwise the pointer of the ammeter will deflect in the opposite direction, causing damage to the ammeter. The wiring method is shown in the figure below a); when measuring AC current, an AC ammeter must be used, which has no distinction between positive and negative polarities. The wiring method is shown in the figure below b).

When measuring large AC current, current transformer is generally used to expand the range of the ammeter. Current transformer is a current conversion device that converts large AC current into small current for use as current element in electrical measuring instruments and relay protection devices.

The graphic symbol, text symbol and wiring of current transformer are shown in the figure below. The ammeter range that matches the current transformer is 5A, but when the scale is made on the dial, it is generally marked according to the primary rated current. For example, when an ammeter is used with a current transformer to measure a large current, assuming that the current ratio of the current transformer is 100/5A, when the primary current is 100A, the actual current flowing through the ammeter (the secondary current of the current transformer) is 5A. At this time, the pointer of the ammeter is at the full scale, and the full-scale current value is not marked with 5A, but marked with 100A, so that the reading indicated on the ammeter is the value of the primary current. This can avoid current conversion in measurement. 344652Current transformer measurement circuit in three-phase circuit: 1. The connection between an ammeter and a current transformer is shown in the figure a). This connection method is used to measure single-phase load current or one phase current in a three-phase symmetrical load.

2. The star connection consisting of three current transformers is shown in the following figure b). This circuit uses three ammeters and three current transformers to measure the current of each phase in a three-phase power system.

3. The incomplete star connection consisting of two current transformers and three ammeters is shown in the following figure c). This wiring method is used to measure the current of each phase in a three-phase symmetrical load. As can be seen from the figure, the current in the ammeter at the common node is equal to the vector sum of the U and W phase currents.

2. Voltage measurement circuit

The voltmeter for measuring the voltage in the circuit must be connected in parallel with the circuit being measured, as shown in the figure below. The following figure a) shows the wiring method for measuring the DC voltage of the load with a DC voltmeter. When wiring, pay attention to the polarity of the voltmeter corresponding to the polarity of the voltage being measured to avoid reverse deflection of the pointer and damage to the voltmeter. The following figure b) shows the wiring method for measuring the load voltage with an AC voltmeter. There is no polarity requirement when wiring.

When measuring AC high voltage (more than 500V), a voltage transformer is generally used to expand the range of the voltmeter. The voltage transformer is a step-down transformer with a primary winding with far more turns than the secondary winding. The rated voltage of the primary winding of the voltage transformer adopts different voltage levels, while the rated voltage of the secondary winding is 100V, which brings great convenience to the measurement. The symbol of the voltage transformer in the circuit is shown in the figure below a), and the figure below b) is the wiring method of the voltage transformer in the circuit. The primary winding AX of the voltage transformer is connected in parallel with the load to be measured, and the secondary winding az is connected to the voltmeter. If the number of turns of the primary winding is N1 and the number of turns of the secondary winding is N2, then the voltage ratio K=N1/N2=U1/U2. If the voltmeter reading is U2, the measured voltage U1=KU2, so that the range of the voltmeter can be expanded to K times the original.

3. Power measurement circuit

The circuit power is measured by a power meter. The power meter (also known as a wattmeter) is an electric system instrument, in which the current coil is connected in series with the load (it has two current coils, which can be connected in series or in parallel to obtain two current ranges), and the voltage coil is connected in parallel with the power supply. The same-name ends of the current coil and the voltage coil (marked with *) must be connected together.

1. Measurement of single-phase power

The figure below is a single-phase power measurement circuit. The power meter W consists of a voltage coil and a current coil. The current coil is connected in series with the ammeter and then connected to the load Z. The voltage coil is connected in parallel with the load. The same-name ends of the two coils are connected and then connected to the positive end of the power supply.

After the power is turned on, the power meter displays the load power. There are voltmeters and ammeters in the circuit, which can measure voltage and current at the same time.

2. Measurement of three-phase power

1) Three-phase four-wire power supply

The measurement circuit is shown in the figure below. The readings of the three single-phase power meters are W1, W2, and W3, so the three-phase power P=W1+W2+W3. This measurement method is called the three-wattmeter method.

2) Three-phase three-wire power supply

In a three-phase three-wire power supply system, regardless of whether the three-phase load is symmetrical or not, and regardless of whether the load is connected in star or triangle, the two-watt method can be used to measure the active power of the three-phase load. The measurement circuit is shown in the figure below. 3) Measure the reactive power of three-phase symmetrical load In a three-phase symmetrical system, the three-phase voltages are completely symmetrical and the load impedances of each phase are exactly the same, then the currents of each phase are also completely symmetrical. At this time, only one power meter is needed to measure the active power of one phase load, and then multiply it by 3 to get the total power of the three phases.

The measurement circuit is shown in the figure below.

4) Power factor measurement of load

The measurement circuit is shown in the figure below

4. Electric energy measurement circuit

The electric energy meter (watt-hour meter) is an instrument for measuring electric energy. The electric energy meter can measure both AC and DC electric energy. The electric energy meter that measures AC electric energy can be divided into active electric energy meter and reactive electric energy meter. The electric energy meter that measures active electric energy can be divided into single-phase electric energy meter and three-phase electric energy meter.

1. Measurement circuit of single-phase active energy meter

The energy meter is connected to a single-phase 220V AC circuit to measure the energy consumed by the single-phase load. This wiring method is called direct-entry wiring. The direct-entry connection method of a single-phase active energy meter is generally divided into jump-in wiring and straight-in wiring, as shown in the figure below.

The jump-in wiring method is characterized by the connection of terminals 1 and 3 of the energy meter to the power supply, and terminals 2 and 4 to the load. The characteristic of the wiring method during input is that the terminals are connected in sequence according to the number of the energy meter terminals, that is, terminals 1 and 2 are connected to the power supply, and terminals 3 and 4 are connected to the load. When the current in the measured circuit is very large, and the rated current of the energy meter cannot meet the requirements, the current transformer can be used for measurement. The wiring of a single-phase active energy meter with a current transformer is shown in the figure below. At this time, the current coil of the energy meter is no longer connected in series in the load circuit, but is connected to the secondary winding of the current transformer. The primary winding of the current transformer is the load line (power line) to be measured. It should be noted that the connection method of the same-name ends of the two windings of the current transformer and the same-name ends of the two coils of the energy meter cannot be wrong, otherwise the energy meter may be reversed.

Requirements for measuring and wiring of single-phase active energy meters:

1) Before wiring, check whether the model and specifications of the energy meter are compatible with the load. The rated voltage of the energy meter should not be lower than the power supply voltage, and the rated current should not be less than the load current. When equipped with a current transformer, it should not be less than the secondary maximum load current of the transformer. 2) The connecting wires of the energy meter must be single-strand insulated copper wires. The cross-sectional area of the current loop wire shall not be less than 2.5 square millimeters, and the cross-sectional area of the voltage loop wire shall not be less than 1.5 square millimeters. 3) The wires connecting the energy meter and other components shall not have joints in the middle. 4) The neutral wire should be connected from one terminal of the energy meter and then connected from the other terminal. 5) The switch and fuse should be connected to the load end of the energy meter. 6) Pay attention to the polarity of the energy meter when wiring, that is, the voltage coil and current coil marked with "*" must be connected to the phase line of the power supply. 7) When wiring the current transformer, connect the K2 terminal of the current transformer to the ground. 8) The crimping screws connecting all the terminals of the energy meter to the wires must be tightened, and the wire ends must be clearly numbered. 2. Direct-entry wiring and requirements for three-phase active energy meters 1) The direct-entry wiring of three-phase three-wire active energy meters is shown in the figure below. The figure is the direct-entry wiring method of DS type 5~10A three-phase three-wire active energy meters. For three-phase three-wire active energy meters above 10A, the internal wiring is different, but the external wiring is basically the same. 2) The direct wiring of three-phase four-wire active energy meter is shown in the figure below. 344664 3) 3) The wiring of three-phase active energy meter with current transformer is shown in the figure below.34)]The wiring of the three-phase active energy meter with current transformer is shown in the figure below.