Synchronous Motors
A synchronous motor will always stop when

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

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Synchronous Motors
While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by

induction motor torque in field winding
induction motor torque in damper winding
eddy current and hysteresis torque in pole faces
All of the listed here

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