Synchronous Motors
The operating speed of a synchronous motor can be changed to new fixed value by

changing the load
changing the supply voltage
using brakes
changing frequency

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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

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

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