FPGA Based Implementation of Variable-Voltage Variable-Frequency Controller for a Three Phase Induction Motor

Abstract

This paper presents the design and implementation of a Variable-Voltage Variable-Frequency (VVVF) Controller based on Sinusoidal Pulse Width Modulation (SPWM) Technique for a 3 Phase Induction Motor using a Field Programmable Gate Array (FPGA). The work involves implementation of an Open loop control scheme for an induction motor. The technique is used extensively in the industry as it provides the accuracy required at minimal cost. Voltage/ frequency (v/f) controlled motors fall under the category of Variable Voltage Variable Frequency (VVVF) drives. To maintain maximum torque for a given working condition, the flux in the machine must be maintained constant. In other words, the ratio of Voltage to frequency must be held constant. For Variable Voltage Variable Frequency (VVVF) drives, there is a need to control the fundamental voltage of the inverter if its frequency (and therefore the frequency of the induction motor), need to be varied. To vary the fundamental component of the inverter, the Modulation Index of the carrier signal has to be changed. The speed at rated supply frequency is normally used as the base speed. At frequencies below the base speed, the supply magnitude needs to be reduced so as to maintain a constant Volt/Hertz. The FPGA controller is used to generate SPWM pulses based on the frequency input, that are used to control the inverter. The VVVF output of the inverter can be used as supply to a three phase induction motor and thereby speed of the motor can be controlled.