CONVERTOR-BASED SPEED CONTROL OF DOUBLY FED INDUCTION MACHINE

Abstract

The electric motor applications are increasing speedily with advancement in technology. With the technological progress the adjustable speed drives are preferred. So speed control of electric motors is always a challenge for researchers and technologists. In this thesis the speed control with the help of power electronic converter has been studied. The present thesis based on 1) mathematical modeling of doubly fed induction machine (DFIM) ii) rotor resistance controlled DFIM drive and variation of rotor resistance with the help of rectifier and chopper iii) variation of voltage and frequency on stator and rotor side iv) implementation of constant volts/hertz control v) implementation of vector control vi) simulation of drives with matlab simulink. To use the electrical energy efficiently doubly fed induction machine (DFIM) with the help of power electronics converter are widely used with voltage-controlled voltage source inverters for industrial, traction, large capacity pumps, and wind energy systems, flywheel energy storage applications, because of their high power density, flexibility, smaller speed limitation, part load operation, good transient response and high efficiency. As a first step towards the objective of this thesis, the speed control by inserting the external tv resistance in the rotor circuit has been studied. With this method having disadvantages like additional ohmic loss in rotor circuit hence reduced efficiency of the machine and also the loss of energy. If a step has taken to increase or decrease the frequency, flux try to increase or decrease, haveving constant flux machines converter based constant volts/hertz control for keeping flux constant has been implemented at rotor side of doubly fed induction machine (DFIM) for operation below synchronous speed. For high speed operation i.e the operation above synchronous speed the vector control scheme has been implemented due to some advantageous features like quicker system response, good transient response. Constant volts/hertz cannot be applied for operation above rated speed because we cannot increase the voltage above the rated value also scalar control system is a sluggish system due to inherent system coupling. Sinusoidal pulse width modulation (SPWM) has been implemented to generate gate signals for the inverter. The simulation study has been carried out for the drives in below synchronous speed operation, above synchronous speed operation with load variation.