Prior to understanding this synchronous motor excitation, it should be remembered that any electromagnetic device must draw a magnetizing current from the ac source to produce the required working flux. This current lags by almost 90° to the supply voltage. In other words, the function of this magnetizing current or lagging VA drawn by the electromagnetic device is to set up the flux in the magnetic circuit of the device.
The synchronous motor is doubly fed electrical motor i.e it converts electrical energy to mechanical energy via magnetic circuit. Hence it comes under electromagnetic device. It receives 3 phase ac electrical supply to its armature winding and DC supply is provided to rotor winding. Synchronous motor excitation refers to the DC supply given to rotor which is used to produce the required magnetic flux.
One of the major and unique characteristics of this motor is that it can be operated at any power factor leading, lagging or unity and this feature is based on the excitation of the synchronous motor.
When the synchronous motor is working at constant applied voltage V, the resultant air gap flux as demanded by V remains substantially constant. This resultant air gap flux is established by the co operation of both AC supply of armature winding and DC supply of rotor winding
CASE 1: When the field current is sufficient enough to produce the air gap flux, as demanded by the constant supply voltage V, then the magnetizing current or lagging reactive VA required from ac source is zero and the motor operate at unity power factor. The field current, which causes this unity power factor is called normal excitation or normal field current.
CASE 2: If the field current is not sufficient enough to produce the required air gap flux as demanded by V, additional magnetizing current or lagging reactive VA is drawn from the AC source. This magnetizing current produces the deficient flux (constant flux- flux set up by dc supply rotor winding). Hence in this case the motor is said to operate under lagging power factor and the is said to be under excited.
CASE 3: If the field current is more than the normal field current, motor is said to be over excited. This excess field current produces excess flux ( flux set up by DC supply rotor winding – resultant air gap flux) must be neutralized by the armature winding. Hence the armature winding draws leading reactive VA or demagnetizing current leading voltage by almost 90o from the AC source. Hence in this case the motor operate under leading power factor.
This whole concept of excitation and power factor of synchronous motor can be summed up in the following graph. This is called V curve of synchronous motor
Conclusion: An overexcited synchronous motor operate at leading power factor, under-excited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power factor.