Electrical Engineering ⇒ Topic : Synchronous motor - starting
As evlier stated, the synchronous motor must be brought to a speed sufficiently close to synchronous speed in order to lock into synchronism with the rotating field. The means by which it is brought up to speed are :
1. A.D.C. motor coupled to the synchronous motor shaft. This method is sometimes used in laboratories with synchronous motors not equipped with damper windings. Generally, the synchronous motor is intended as the constant speed primemover for the D.C. generator.But in order to bring the motor upto synchronism the D.C. generator is operated as motor, and the A.C. synchronous dynamo is synchronized to the A.C. supply as an alternator. Once in parallel with the supply, the synchronous dynamo is operated as a motor. The D.C. 'motor' will now act as a generator if its field current is increased so that its generated e.m.f. exceeds the D.C. bus .
2. Using the field excited generator as a D.C. motor. This method is the same as the first, except that the exciter (D.C. shunt generator) is operated as a motor, and the A.C.synchronous dynamo is synchronize.d to the A. C. supply.
.3. A small induction motor of at least one pair of poles less than the synchronous motor. This method involves the same synchronizing procedure for A.C. synchronous motor as an alternator. At least one pair of poles fewer is required on the induction motor to compensate for the loss in induction motor speed due to slip.
In the above three methods the following conditions should be met with :
4. Using the damper windings as a squirrel-cage induction motor.
Note. It is practically impossible to start a synchronous motor with its D.C. field energized.Even when left-de-energized, the rapidly rotating magnetic field of the stator will induce extremely high voltages in the many turns of the field winding. It is customary, therefore, to short-circuit the D.C. field winding during the starting period; whatever voltage and current are induced in it may then aid in producing induction motor action. In very large synchronous motors, field sectionalising or field-splitting switches are used which short-circuit individual field windings to prevent umulative addition of induced voltages from pole to pole.
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