Feb 17, 2015

Methods of Starting a Synchronous Motor



We know that synchronous motor cannot start of its own. Hence some method must be used to start it. The starting of a synchronous motor from its standstill position can be obtained by the following methods.
Synchronous motor starting methods

Starting Using a Separate Induction Motor

The synchronous motor is made to reach its synchronous speed by using a separate small induction motor mechanically coupled to it. The number of poles in the synchronous motor must be higher than the number poles of the induction motor. This is to help the induction motor to run at the synchronous speed of the synchronous motor. The synchronous motor will have to synchronize with the bus bar, in this method. Afterwards the induction motor can be disconnected. Then the synchronous motor uniform of its own.
 

Starting by using a DC Motor coupled to Synchronous Motor

A synchronous motor can be started by using a DC motor. Firstly, the synchronous motor is driven by a DC motor and is brought to synchronous speed. The machine is then synchronized with the bus bars. When the synchronous machine is connected in parallel with the bus bars, the machine works as a motor. Then the DC machine coupled to the synchronous machine can be mechanically disconnected from the system or it can be drawn as a generator.
 

Starting using a Damper winding

A damper winding is made on the pole face slots. Copper aluminium bars are inserted in slots, on pole shoes, on each side of the poles, end rings short circuit, these bars. A squirrel cage winding is formed by these short circuited bars. A three phase supply is given to the stator.
 
Damper winding is used to start the synchronous motor
The synchronous motor, provided with a damper winding, starts as an induction motor. This will run at a speed near the synchronous speed. At this stage DC excitation is given to the field windings. The rotor will be pulled into synchronous speed. This is because the pole of the rotor, rotating magnetic slip speed only with respect to the stator rotating magnetic field. For higher output motors, the starting current drawn may be many times the full load current. So the starting current has to be limited to a safe value. For this purpose reduced voltage may be applied by an auto transformer. The voltage applied should be about 50 to 80 percent of the full line voltage. The connections for the auto transformer are shown in the figure. To reduce the supply voltage, the switch S1 is closed and S2 is kept open. When the motor picks up speed, S2 is closed and S1 is kept opened, to cut out the transformer.

In this method starting is effected as an induction motor. The starting torque produced is low. Hence high output motor may not be able to start with full load.

Starting Using a Separate Induction Motor

The synchronous motor is made to reach its synchronous speed by using a separate small induction motor mechanically coupled to it. The number of poles in the synchronous motor must be higher than the number poles of the induction motor. This is to help the induction motor to run at the synchronous speed of the synchronous motor. The synchronous motor will have to synchronize with the bus bar, in this method. Afterwards the induction motor can be disconnected. Then the synchronous motor uniform of its own.

Starting by using a DC Motor coupled to Synchronous Motor

A synchronous motor can be started by using a DC motor. Firstly, the synchronous motor is driven by a DC motor and is brought to synchronous speed. The machine is then synchronized with the bus bars. When the synchronous machine is connected in parallel with the bus bars, the machine works as a motor. Then the DC machine coupled to the synchronous machine can be mechanically disconnected from the system or it can be drawn as a generator.


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