The principle of measuring the active power of a three phase circuit using the two watt meter method

Principle: If there are two power meters reading W1 and W2, then P=W1+W2=UICOS(30°- UICOS(30°+ where is the impedance angle of the load (i.e., the power factor angle) The two power meter readings have the following relationship with

When the load is pure resistance, =0,w1=w2, when the load power factor cos =, = 60°, there will be a power meter reading of zero; 3 When the load power factor cos, 60°, then there is a power meter reading is negative.

Symmetrical three-phase power can be achieved with a single-phase power meter (Watts.

For the asymmetric three-phase power of the three-wire system, it is measured by two single-phase power meters (or a two-dimensional three-phase power meter composed of two single-phase power meters), which are called the one-watt meter method and the two-watt meter method respectively.

Three-phase circuit. total power. It is equal to the sum of the power of each phase. Three-phase circuits are divided into symmetrical three-phase circuits and asymmetrical three-phase circuits; Power has average power (i.e., active power) and reactive power.

and apparent power.

Assuming that AB and BC are connected to two watt meters, then phase B can be regarded as the zero line (assuming that the potential of phase B is 0), then the two meters measure the single-phase voltage of phase A and C relative to the neutral line (phase B) respectively (in fact, AC and BC, which are line voltages), and the power of the two single-phase meters adds up to the total active power.

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According to Kirchhoff's current law, the three-phase power test for three-phase three-wire systems is independent of whether the system is symmetrical or not.

The one-watt method can only be used for power testing of three-phase symmetrical circuits, where the sum of the three-phase power is equal to three times the single-phase power reading.

The two-watt method uses two power meters to measure the total power of a three-phase circuit, and the sum of the three-phase power is equal to the sum of the readings of the two single-phase power meters. It is theoretically based on Kirchhoff's current law, which is applicable to three-phase power testing in three-phase three-wire systems, regardless of whether the system is symmetrical or not.

Three-phase load. Instantaneous power p=pa+pb+pc=3upipcos=p. It can be seen that the instantaneous power of the three-phase load in a symmetrical three-phase circuit is a time-independent fixed value, which is equal to the average power p. If the load is a three-phase motor, because its instantaneous power does not change with time, its instantaneous torque is also constant, so the motor runs smoothly.

This is one of the advantages of symmetrical three-phase circuits. A three-phase system in which the instantaneous power of the three phases is constant is called a balanced three-phase system.

The sum of the currents of the three phases is 0, and the sum of any two phases can be measured by the two-watt meter method to obtain the third phase.

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In layman's terms, a three-phase energy meter is equivalent to 'packaging' three single-phase energy meters together.

Assuming that the active power of the three single-phase energy meters is Pa, Pb, and PC respectively, Pa=uaiacos, Pb, and PC are the same, and UA=ub=uc, ia=ib=ic.

Therefore, when calculating the active power, the calculation formula is, p=3uicos.

Reactive power], in the three-phase circuit, the voltage 380V is generally referred to as the line voltage, to convert it into the phase voltage and divide by 3, then each phase power becomes the root 3/s of the UICOS. If there are three phases, then 3/3 of uicos multiplied by 3 is the power of three phases, i.e. 3 uicos, line voltage = 3 phase voltage. And there are two wiring methods for three-phase circuits:

1. The load is Y connection, line voltage = 3 phase voltage, line current = phase current, bring the phase voltage and phase current into the previous formula, you get p = 3uicos, where you and i are line voltage line current.

Second, the load is connected, line voltage = phase voltage, and line current = 3 phase current, the result is the same.

The above is for the three-phase balanced voltage and balanced load.

Finally, it is added that the single-phase electric energy meter should be matched with TA wiring, and its wiring method is: No. 1 hole is S1 hole (usually also written K1, that is, TA), No. 2 hole is S2 hole (usually also written K2), No. 3 hole is a voltage hole, and No. 4 hole is a neutral line hole.

The two-table method is suitable for the three-phase three-wire system because it is based on the premise that ia+ib+ic=0. Assuming that the two meters are connected between the AC phase and the BC phase respectively, then the sum of the three-phase power p=ua*ia+ub*ib+uc*ic=ua*ia+ub*ib-(ia+ib)*uc=ia*(ua-uc)+ib*(ub-uc)=ua*ia+ubc*ib, and the sum is exactly the sum of the powers of the two meters.

In three-phase three-wire symmetry comes.

The circuit uses the two-table method to measure the instantaneous value of the power from the three phases:

The sum of the power of the two tables is: UAC*ia+UBC*IB=(UA-UC)*IA+(UB-UC)*IB

ua*ia+ub*ib-uc*（ia+ib）=ua*ia+ub*ib+uc*ic

Because the three are symmetrical, ia+ib+ic=0, i.e., -uc*(ia+ib)=uc*ic

The sum of the readings of the two meters is the three-phase power.

1. Three-phase four-wire system.

For the load of three-phase star connection of power supply (i.e., Y0 connection method), the active power of each phase can be measured by a power meter Pa, Pb, PC, and the sum of the three-phase power (P=Pa+PB+PC) is the total active power of the three-phase load (the so-called one watt.

The table method is to use a single-phase power meter to measure the active power of each phase separately).

3. In the three-phase three-wire power system, regardless of whether the three-phase load is symmetrical or not, and whether the load is a star connection or an angular connection (i.e., the connection method), the total active power of the three-phase load can be measured by the two-watt meter method. The measurement line is shown in the figure.

4. If the load is inductive or capacitive, and the phase is different.

At 60°, one of the power meter pointers in the line will be back-biased (negative readings will occur for digital power meters), at which point the two terminals of the power meter current coil should be placed.

Swap (voltage coil terminals cannot be swapped) and the reading should be recorded as a negative value.

5. For the three-phase three-wire power supply of the three-phase symmetrical load, the total reactive power of the three-phase load can be measured by the one-watt meter method.

Total reactive power. In addition to the one connection method (IU, UVW) shown in the figure above, there are two other connection methods, which can be connected as (IV, UUW) or (IW, UUV).

Three-phase circuit power test.

The one-watt method can only be used for power testing of three-phase symmetrical circuits, where the sum of the three-phase power is equal to three times the single-phase power reading.

The two-watt method uses two power meters to measure the total power of the three phases of a three-phase circuit, and the sum of the three-phase powers is equal to the sum of the readings of the two single-phase power meters. The theoretical basis for this is Kirchhoff's law of currents.

Three-phase power testing for three-phase three-wire systems, regardless of whether the system is symmetrical or not.

Principle: If there are two power meters reading W1 and W2, then P=W1+W2=UICOS(30°- UICOS(30°+ where is the impedance angle of the load (i.e., the power factor angle) The two power meter readings have the following relationship with

When the load is pure resistance, =0,w1=w2, when the load power factor cos =, =60°, Hu Bo has a power meter reading of zero; 3 When the load power factor cos, 60°, then there is a power meter reading is negative.

Symmetrical three-phase power can be measured with a single-phase power meter (watt meter), and for three-wire asymmetrical three-phase power, it is measured with two single-phase power meters (or a two-dimensional three-phase power meter composed of two single-phase power meters), which are called one-watt meter and two-watt meter respectively.

The total power of the three-phase circuit. It is equal to the sum of the power of each phase. Three-phase circuits are divided into symmetrical three-phase circuits and asymmetrical three-phase circuits; Power is divided into average power (i.e., active power), reactive power and apparent power.