The Start of Synchronous Motors

If we observe the operation of the synchronous motors, it is clear that the rotor must be brought to a sufficient speed close to the synchronous, in order to synchronize with the rotating field. Some means to make this happen is to use a motor coupled to the axis of the synchronous motor, which if it is induction must have at least a pair of poles less than those of the synchronous motor and the use of the compensating windings.
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When starting with compensating windings, we must short-circuit the DC winding while applying alternating current to the stator, bringing the motor to its empty speed as an induction electric motor. The short-circuit is then removed from the field and DC voltage is applied to this winding by adjusting to the current (the DC voltage level or the field resistance value can be varied) to be small . When starting under load the best way is to use a winding rotor in place of the winding in the cage on the polar faces, that is to say, the so-called winding winding type rotor, where we use coupled to the field winding, a three-phase resistance. The motor starts with all external resistance per phase and with the field winding short-circuited.

The Start of Synchronous Motors