Variable speed drives (VSD) are adhered to induction motors in most of the present industrial applications. And since the world has become more conscientious to energy saving, inverters established them selves in domestic buildings and industry. Square wave inverters generate dominant harmonics of the 5th, 7th, 11th and 13th order. Inverters based on PWM strategy produce much higher harmonic order depending on the frequency modulation ratio. The presence of any sub-synchronous harmonics (sub-harmonics) is usually unheeded probably to their ineffective magnitude. The work in this paper is to investigate the effects of low frequencies on the performance of induction motors running near rated peed. Low frequencies are superimposed on the motor by a second inverter cascaded with the main inverter. The results shown in this paper manifest the significant effects of sub-harmonics on the speed stability of induction motors. The results obtained also show that sub-harmonics can adversely affect the motor performance even if they are present with small amplitudes. This work is expected to lead to the notion of using low frequency injection in speed control of induction motors.
