Methods of Starting of Synchronous Motors

Since synchronous motors are not self-starting inherently, we should have some means of starting synchronous motors.

The methods are given below:

1. Induction Motor Start

Damper windings, consisting of copper bronze bars embedded in slots in the pole faces and short-circuited at both ends are usually provided in synchronous motors which serve not only to damp out oscillations during hunting but are also suitably designed as a squirrel-cage winding for starting purposes. If the rotor reaches near synchronous speed, the field circuit is suddenly energised and the rotor and stator fields interlock each other.

2. Auxiliary Motor Start

Using an auxiliary motor, the synchronous motor is first run as an alternator and is synchronised with the three-phase bus-bars. After synchronisation, supply to the auxiliary motor is cut, and the alternator is made to run as the synchronous motor drawing power from 3-phases AC mains.

3. Using Resistor in the FieId Circuit

Instead of leaving the field circuit open during starting of the synchronous motor, it can be short-circuited and can be used as a damper winding. When an additional resistance of value 100 times that of the field resistance, is inserted in series with the field, the starting torque will be large. The field resistance is reduced as the motor accelerates until it becomes three to four times that of the field itself. This method can be used in machines where large starting torque is required.

4. Super Synchronous Start

The armature is not rigidly bolted to the bed-plates as in an ordinary motor. It is capable of rotating about the rotor shaft. But under normal conditions, it is held stationary by a brake band surrounding the frame of the armature core. The rotor is mechanically coupled to load. The brake band on the stator is released and when the line voltage is impressed on armature terminals, the stator will try to rotate. When the armature reaches near-synchronous speed, the rotor field is excited and the machine pulls into synchronism.

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