Abridgment of synchronous motor effects in induction machines

Abstract

Many induction motors do not have the smooth speed — torque curves which are to be expected from elementary theory. Many motors at no-load show a tendency to run at some speed far below that normally to be expected. This is sometimes called “sub-synchronous” speed; or the motor is said to be “crawling.” In the present paper, one cause of this phenomenon is explained. It is found that certain combination of phases, rotor slots, and poles cause the motor to run as a synchronous motor at some intermediate speed between zero and normal no-load induction-motor speed. This effect has been christened the “synchronous-motor effect” in induction machines and is shown to be due to the locking of harmonic fields generated by the stator winding with similar harmonic fields generated by the rotor windings. A method is given for avoiding this trouble by the proper selection of the number of rotor slots.