A dynamometer type three-phase power factor meter gives correct readings only when the load is balanced.
The basic principle of dynamometer type three-phase power factor meter of operation of this instrument is similar to that of dynamometer type wattmeter i.e. when the field produced by moving system tries to come in line with the field produced by the fixed coil, deflecting torque torque is exerted on the moving system which deflects the pointer attached to it.
It consists of two fixed coils FF connected in series in one of the phase and carries the load current. The moving coils are so placed that the angle between their planes is 1200. They are connected across two different phases of the supply circuit. Each coil has a series resistance. There is no necessity for phase splitting by artificial means, since the required phase displacement between currents in the two moving coils can be obtained from the itself.
When the instrument is connected in the circuit, under balanced load conditions, the phase angle through which the pointer is deflected from the unity power factor position is equal to the phase angle of the circuit, because the two moving coils are fixed 1200 apart. The deflections in these instruments are independent of frequency and waveform, since the currents in the two moving coils are affected in the same way by change of frequency.