A synchronous generator are runs at a constant speed and draws its excitation from a power source external or independent of the load or transmission network it is supplying. It has an exciter that enables the synchronous generator to produce its own reactive power and to also regulate its voltage. Synchronous generators can operate in parallel with the utility or in stand-alone. Synchronous generators require a speed reduction gear.
The synchronous motors are specifically designed to maintain constant speed, with the rotor synchronous to the rotating field. It requires a separate source of DC for the field. It also requires special starting components. These include a salient-pole field with starting grid winding. The rotor of the conventional type motor is essentially the same as that of the salient-pole AC generator. The stator windings of induction and synchronous motors are essentially the same. It differs from the induction motor in several ways. If supplied with proper voltage, a dc generator operates satisfactorily as a dc motor, and there is practically no difference in construction and rating between the two. The rotating fields are generally of the salient pole type.