Synchronous motors are used where a very constant speed is required, where power factor correction is required, or for slow-speed machines. A synchronous motor is an AC motor that rotates at exactly the same speed as the rotating magnetic field of the stator. This means that they are not subject to the slip found in other poly-phase motors. They are most often used either for power factor correction, of to power large equipment such as compressors and pumps.

 

It is however, not an induction motor. It does not depend on induced current in the rotor to produce a torque. The beauty of this motor is that it will run at a constant speed regardless if the motor is run at no-load or full-load. In order to run at synchronous speed, the same speed as the stators rotating magnetic field, the rotor must have some form of DC excitation current to operate. An added advantage or application of this motor is that it has the ability to correct its own power factor or the power factor of other motors connected to the same service.

 

The motor will keep its fixed speed regardless of the torque required up until it reaches its stall torque rating. If the load becomes greater than the motors stall torque, the AC Synchronous Motor will stall and stop turning. The AC Synchronous motor is an effective way to obtain a fixed speed at a very low motor system cost. No expensive driver or amplifier is necessary. Most synchronous motors are used where precise timing and constant speed are required.