Low frequency applied to control problems

THE BEHAVIOR of an induction motor under normal conditions of applied voltage and frequency is well understood. The purpose of this discussion is to investigate induction motor behavior when low-frequency polyphase voltages are impressed on its primary, and to consider a means of efficiently generating the required low-frequency voltages. Decreasing the frequency of the voltages impressed upon the primary or stator winding of the induction motor slows the speed of rotation of the air gap flux. The speed of the air gap flux is proportional to the frequency. Decreasing the frequency reduces the motor speed, but has no effect upon the shape of the induction motor's speed-torque curve providing the magnitude of the air gap flux is maintained constant.