Power and measurement methods of three-phase circuits

The average power absorbed by the three-phase load is equal to the sum of the average power absorbed by each phase load, that is:

(4-5-1)

In the formula, , , are the effective values ​​of the three-phase voltages respectively; , , are the effective values ​​of the currents of each phase; , , represent the phase differences between the voltages and currents of each phase.

In a symmetrical three-phase circuit, the effective values ​​of each phase voltage and each phase current are equal, that is ,

, the load impedance of each phase is the same, so the phase difference between the voltage and current of each phase is also equal, , then the average power of the symmetrical three-phase load is

(4-5-2)

For a three-phase symmetrical load connected in Y shape, the line voltage and line current are substituted into the above formula:

(4-5-3)

If the load is connected in a triangle shape, then , , after substituting into formula (4-5-2), we get the same three-phase power expression as above. It can be seen that whether it is a Y-shaped or a triangle-shaped connection, formula (4-5-3) can be used to calculate the average power of a symmetrical three-phase circuit. It should be pointed out here that in the formula, is the power factor of one-phase load, and is the phase difference between phase voltage and phase current.

The reactive power of the three-phase circuit can also be written as

For a symmetrical three-phase circuit, we have:

(4-5-4)

The three-phase apparent power is defined as:

For a symmetrical three-phase circuit:

(4-5-5)

The power factor of a three-phase load is defined as:

(4-5-6)

If it is a symmetrical three-phase circuit, the power factor of the three-phase circuit load is equal to the power factor of the single-phase load.