Every dryer motor iv ever disassembled has one...I just inherited an old radiator hot water pump motor and it has one...These are the only true AC motors i ever encounter.
there are two windings (stator) in these types of motors. one is the starting windings and the other is the run windings. the switch in question is for the starting windings. start windings provide higher torque for the motor to start and as the motor comes up to speed. the switch opens due to the centrifugal force and allows the run windings to take over.
