Speed Control of Induction Motor using Modified Pulse Width Modulation Technique

Abstract

Alternating Current (AC) motors have speed control capabilities only when operating from alternating current sources, and speed control is achieved by varying the applied voltage and/or the frequency of its inversion. All the known methods of controlling the speed of an AC motor using electronic devices are nothing but variations of either Phase Angle firing control (PFC) or Pulse Width Modulation firing control (PWM). Phase angle firing control is the most common method of speed control. It is cheap and reliable, but torque is low at reduced voltages and the motor may stall on load or not start at all. The Pulse width modulation firing control, on the other hand, produces an average voltage value which is directly proportional to the percentage on and off duty cycle of the applied voltage. It is efficient and can be used on all types of load, but the control mechanism is cumbersome and expensive. The aim of this research is to design a speed controller that brings an induction motor up to speed for a certain number of cycles, and then when the motor is up and running, control the applied 
voltage by phase angle firing for the remaining number of cycles. The overall effect is still PWM because the controller now averages the speed into a value that is proportional to the percentage duty cycle of the dwell time difference between the applied voltage and the speed induced stator emf. At first the general principles of both the PFC and PWM firing control schemes are discussed and a scheme is proposed, the validity of which is tested via simulation.