Design and Implimentation of Soft Starter for Three Phase Induction Motor

Abstract

Three phase Induction motors draw such large currant during direct on line starting process that it can pull down the voltage of the power supply net, which will severely influence the other electrical devices in the same power net. On the other hand, it also makes the temperature of motor becomes higher, which may lead to motor damage. In order to minimize this effects conventional electro mechanic starters are implemented, but this conventional methods can not adjusts the stator voltage continuously during the start up process, so the start up current is still large. With the development of computer technology and power electronic technology, electronic starters are implemented in order to achieve the softly starting of Induction motor. Among these, thyristorized Soft Starters which apply reduced voltage to the motor are cheep, simple, reliable, and occupy less volume, and hence, their use is a viable solution to the starting problem of medium voltage large ac motors in applications where the starting torque requirement of load is not high. This report demonstrates the different starting methods of Induction motor and proposed Soft Starting controller for Induction motor. In Soft Start controller, the output voltage of the thyristor is not only related to the trigger angle, but also associated with the power factor angle which varies with the speed of the motor during Soft Start process. It’s known that the maximal torque of Induction motor is approximate in direct proportion to the square of the voltage of the stator. Accordingly, a closed loop control system is designed in which the voltage and power factor angles are used as a feed backs, which is solution to the true Soft Starter. The control strategy of Soft Starter is implemented by using TMS320F2811 DSP Processor.