Synchronous Motors
A synchronous motor will always stop when

Supply voltage frequency changes
Load in motor varies
Excitation winding gets disconnected
Supply voltage fluctuates

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Synchronous Motors
The power factor of a synchronous motor is better than that of induction motor because

synchronous motor has large air gap
synchronous motor runs at synchronous speed
mechanical load on the motor can be adjusted
stator supply is relieved of responsibility of producing magnetic field

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