Method to Starting the Synchronous Motor


Method to starting the synchronous motor

These motor is also known as constant speed motor. its operates in synchronism with line frequency. Synchronous motor is not self starting motor,hence external DC source is necessary. Basically there are three Method to Starting the Synchronous Motor.

  • Auxiliary Motor Starting: 
    The auxiliary motor is mechanically coupled to the synchronous motor. No load is put on the synchronous motor during starting. Therefore, the auxiliary motor has to overcome only the inertia of the synchronous motor and may have its rating much smaller than that of the synchronous motor being started.
    The auxiliary motor may be a dc shunt motor or an induction motor having the same number of poles as the synchronous motor or two poles less as compared to synchronous motor. The job of the auxiliary motor is to bring the synchronous motor to synchronous speed or near synchronous speed. When the speed of the synchronous motor almost equals the synchronous speed, it’s field winding is energized.
    The synchronous motor pulls into step and starts running at synchronous speed. However, in modern days this method of starting is hardly used.
  • By using damper windings or amortisseurs windings:  

This is the most used method of starting synchronous motor.In addition to the normal field winding an additional winding consisting of copper bars placed in the slots in the pole faces.Ends of the copper bars are short circuited with the help of end rings.This winding is called damper winding.This damper winding acts as a squirrel cage rotor of an induction motor.
First synchronous motor runs as induction motor due to the damper winding at sub synchronous speed.Then d.c. supply is given to the field winding. At some instant motor gets pulled into synchronism and starts rotating at a synchronous speed.
As rotor rotates at synchronous speed, the relative motion between damper winding and the rotating magnetic field is zero. Hence when motor is running as synchronous motor, there can not be any induced e.m.f. in the damper winding. So damper winding is active only at start, to run the motor as an induction motor at start.
  • By reducing the speed of the rotating magnetic field of the stator: 
  • To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications.

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