Lesson 24: Synchronous Motors: Starting Methods
100 year old 350 HP 2400 V motor for flour mill
This motor is designed to run at constant RPM (Hence it is a synchronous motor) but when it is started it takes a lot of energy to get it spinning because the load is great and the rotor and the rotating field aren’t in sync yet.
To get it going, the motor frame is allowed to spin freely at first. When the electricity is turned on, the frame starts to spin at full speed around the rotor. Once it reaches synchronous speed, the guy with the big wheel starts putting a brake on the frame, which slows the stator, and the rotor inside starts to spin to compensate. By slowly putting the brake on, the motor can be made to start up whatever the load is. Once the the stator stops spinning altogether the rotor is driving the load at the synchronous speed.