SOLUTION

• In general, the counter emf or back emf of any motor must be very nearly equal and opposite to the impressed terminal voltage so that if the latter is constant, the counter emf will be substantially constant.
• Now the counter emf is proportional to the speed and to the field flux, and since the speed is constant in the case of the synchronous motor, the field flux must likewise be substantially constant within the normal limits of operation.
• It follows, then, that if the field excitation is increased, thereby tending to increase the field flux that can vary only slightly, there must be an automatic change of the armature MMF in order to offset the effect of the increased field excitation
• The armature current must therefore contain a leading component, that a leading current in a synchronous motor exerts a demagnetizing effect.
• A weakening of the field excitation tends to draw a lagging current from the source of supply.

In short:- The magnitude of a field flux in a 3-phase synchronous machine remains constant at all loads because this motor runs at a constant speed for that The magnitude of field flux must be constant. The field is supplied from a d.c. source and the stator coils with a three-phase current.