Synchronous motor power factor

Assume that a synchronous motor is driving a constant torque load. The active power converted by the machine is constant, no matter what the value of the field current is, since the motor speed is a constant. Thus, Using the phasor diagram below, we analyse the variation of the power factor angle of a synchronous motor when the rotor field excitation is varied. Figure 7a shows a synchronous motor initially operating at a lagging power factor. Now, increase its field current and see what happens to the motor. Note that an increase in field current increases the magnitude of EA but does not affect the real power supplied by the motor. The power supplied by the motor changes only when the shaft load torque changes. Since a change in IF does not affect the shaft speed nm, and since the load attached?to the shaft is unchanged, the real power supplied is unchanged. Of course, VT is also constant, since it is kept constant by the power source supplying the motor. The distances proportional to power on the phasor diagram (EA sin? and IA cos ?) (must therefore be constant. When the field current is increased, EA must increase, but it can only do so by sliding out along the line of constant power.For conversion of a certain amount of active electrical power into mechanical power, a certain amount of magnetic flux is required. In the case of a lagging power factor, the rotor field current is so small that some reactive power is required from the stator power supply, and hence the stator current lags the terminal voltage. This state is known as under excitation. When the rotor field current is just enough to produce the required magnetic flux, a unit power factor is obtained. If the rotor field current is more than required the spurious reactive power is to be exported to the power lines of the power supply. This state is known as over excitation.

In practice, because of this feature, synchronous motors are often run at no active load as synchronous condensers(capacitor) for the purpose of power factor correction.