Why three Phase synchronous motor is not self starting

Three phase synchronous motor is not self-starting because synchronous motor rotation is based on locking mechanism. Just to get the feel of it imagine field of stator rotating at 1500 r.p.m. and rotor is stationary, it can't simply get locked to stator field due to its inertia. So, torque produced in the synchronous motor depends on the load angle for small values of load angle and to be precise depends on sin δ. The load angle (δ) is measured in degrees electrical. As load angle increases, the magnetic flux lines producing the force of attraction between the two get more and more stretched. This weakness the force maintaining the magnetic locking, though torque produced by the motor increases. As load angle increases upto 90 degree electrical i.e., half a pole pitch, the stretched flux lines get broken and hence magnetic locking between the stator and rotor no longer exists. The motor comes out of synchronism. So torque produced at load angle at 90 degree electrical is the maximum torque, a synchronous motor can produce, maintaining magnetic locking i.e., synchronism. Such a torque is called pull out torque. For understanding three phase synchronous motor, consider an instant when two poles are at such a position where stator magnetic axis is vertical, along A-B.



              At this instant, rotor is stationary and unlike poles will try to attract each other. Due to this rotor will be subjected to an instantaneous torque in anticlockwise direction. Now stator poles are rotating very fast i.e., at a speed Ns r.p.m. due to inertia, before rotor hardly rotates in the direction of anticlockwise torque, to which its is subjected, the stator poles change their positions. Consider an instant half a period latter where stator poles are exactly reversed but due to inertia rotor is unable to rotate from its initial position.
              

               At this instant, due to the unlike poles trying to attract each other, the rotor will be subjected to a torque in a clockwise direction. This will tend to rotate rotor in the direction of rotating magnetic field. But before this happens, stator poles again change their positions reversing the direction of the torque exerted on the rotor. As a result, the average torque exerted on the rotor is zero. and hence the synchronous motor is not self starting. Damper winding provides smooth starting and compensating the transient effect or unbalanced condition. Damper winding in synchronous machine is placed in the coil slot and short-circuited.


  • Why single phase induction motor is not self-starting:-
                 Single phase induction motor is not self starting because the magnetic field of stator and rotor opposes each other and cancel out each other. We can start single phase induction motor by shaded pole motor.

                  It is a small squirrel cage motor in which auxiliary winding is composed of cu ring surrounding a portion of each pole. This auxiliary single turn winding is called shading coil. Currents induced in this coil by magnetic field create a second electrical phase by delaying the phase of magnetic flux change for that shaded pole to provide rotating magnetic field.

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