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Synchronous motor is
Not self starting
Self staring
Both of above
None of above
Not self starting
A synchronous motor is not self-starting because its rotor must be synchronized with the rotating magnetic field produced by the stator. At standstill, the rotor inertia prevents it from accelerating to the synchronous speed instantaneously, causing the average torque to be zero.
A synchronous motor is not self-starting because its rotor must be synchronized with the rotating magnetic field produced by the stator. At standstill, the rotor inertia prevents it from accelerating to the synchronous speed instantaneously, causing the average torque to be zero.
NsтАЛ=P120fтАЛ тАФ Calculation of synchronous speed in RPM
╧ЙsтАЛ=P/22╧АfтАЛ тАФ Angular synchronous speed in rad/s
When AC supply is applied to the stator, it creates a revolving magnetic field (RMF) that rotates at the synchronous speed NsтАЛ=P120fтАЛ. Due to rotor inertia, the rotor cannot follow the RMF, and the torque oscillates rapidly between positive and negative half-cycles, resulting in zero net torque. To start the motor, an auxiliary means such as a damper winding (squirrel cage) or an external prime mover is required to bring the rotor close to NsтАЛ.
Synchronous motors run at a constant speed determined by supply frequency and poles.
The rotor poles lock into the rotating stator magnetic field.
Damper windings are used as a standard starting mechanism to overcome the lack of self-starting capability.
If load torque exceeds pull-out torque, the motor loses synchronism and stops (pole slipping).
Constant speed operation regardless of load
Ability to operate at leading power factor for power factor correction
High efficiency at low speed
Requires DC excitation for the rotor
Not self-starting without auxiliary starting devices
Complexity in control due to potential loss of synchronism
Power factor correction (Synchronous Condensers)
High-power constant speed industrial drives
Centrifugal pumps and compressors
A synchronous motor acts like an inductor when under-excited and like a capacitor when over-excited.
Option B is incorrect because the motor lacks a starting torque at zero speed due to inertia.
A is correct тАФ The synchronous motor is not self-starting due to the inertia of the rotor which cannot instantaneously track the revolving magnetic field of the stator.
Always remember: If a question asks about 'Self-Starting' in synchronous motors, look for terms like 'Damper Winding' or 'Induction Motor Start' as the mechanism to overcome this inherent limitation.