V and Inverted V curves of Synchronous Motor
Electromagnetic devices draw a magnetizing current from the a.c source, in order to establish the working flux. This magnetizing current lags the applied voltage by almost 900.
A synchronous motor is a double-excited machine, its armature winding is energised from an a.c source and its field winding from d.c source. When synchronous motor is working at constant applied voltage, the resultant air gap flux demanded by applied voltage remains constant. This resultant air gap flux is established by both a.c in armature winding and d.c in the field winding. If the field current is sufficient enough to set up the air-gap flux, as demanded by constant applied voltage then magnetizing current or lagging reactive VA requied from the a.c source is zero and therefore motor operates at unity power factor. This field current, which causes unity power factor operation of the synchronous motor, is called normal excitation or normal field current. If the current less than the normal excitation, i.e the motor is under excited, then the deficiency in flux must be made up by the armature winding m.m.f. In order to do the needful, the armature winding draws a magnetizing current or lagging reactive VA from the a.c source and as a result of it, the motor operates at a lagging power factor. In case the field current is made more than its normal excitation, i.e the motor is over-excited, operates at leading power factor. Fig(1) shows the variation of armature current and power factor with field current at no load, half load and full load conditions.