EE213 - 39b - Effect of changes in field current in synchronous motors

A synchronous motor behaves either as an inductive load or as a capacitive load depending upon the field current supplied to the field windings. This lecture explains this phenomenon with the help of phasor diagrams. If a synchronous motor is over-excited - supplied with high field current - the motor behaves as capacitive load and it supplies reactive power. And if the motor is under-excited - supplied with low field current - it behaves as an inductive load and consumes reactive power. The in-between situation is when the motor behaves as a resistive load. Furthermore, the magnitude of the armature current drawn by the motor also depends upon the field current for a given load. The relationship between the armature current and field current is described by a v-curve, due to its similarity with the English alphabet v.