Why Three Phase Synchronous Motor Is Not Self Starting?

Why Three Phase Synchronous Motor Is Not Self Starting?

A three-phase synchronous motor is not self-starting because synchronous motor rotation is based on the locking mechanism. Just to get the feel of it to imagine a field of the stator rotating at 1500 r.p.m. and the rotor is stationary, it can't simply get locked to the stator field due to its inertia. So, the 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 is the force maintaining the magnetic locking, though torque produced by the motor increases. As load angle increases up to 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 degrees 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 a three-phase synchronous motor, consider an instant when two poles are at such a position where the stator magnetic axis is vertical, along A-B.

              At this instant, the rotor is stationary and unlike poles will try to attract each other. Due to this rotor will be subjected to an instantaneous torque in an anticlockwise direction. Now stator poles are rotating very fast i.e., at a speed Ns r.p.m. due to inertia, before the rotor hardly rotates in the direction of anticlockwise torque, to which it is subjected, the stator poles change their positions. Consider an instant half a period later 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 torque in a clockwise direction. This will tend to rotate the rotor in the direction of the 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 compensates the transient effect or unbalanced condition. Damper winding in the synchronous machine is placed in the coil slot and short-circuited.

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

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

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