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Synchronous Motor Excitation
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) which 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.