Design & Implementation of Medium Voltage Soft Starter for Three Phase Induction Motor

Abstract

Induction motor is increasingly becoming more and more popular in industrial as well as domestic applications. It is very important to limit high inrush current drawn by motor during starting. Proper selection of medium voltage (MV) motor and its associated controller has become gradually more important. The problem is more severe in areas where the loads represent high portion of power demand. If medium voltage starters are substituted in such application, reduction of inrush current can be achieved and on other hand, energy saving can also be possible. In medium voltage soft starter fed induction motor drive, smooth acceleration with reduced stress on the mechanical drives system is achieved with enhanced life, reliability of belt, gear boxes, chain drives, motor bearings and shafts. Smooth acceleration also reduces stress on the electrical supply side and reducing voltage dips and brown out condition. Soft starter is a solid state device which regulates terminal voltage of induction motor by varying the firing angle of thyristor. Firing angle is non linear function of motor speed and torque, so it is indispensable to calculate the exact value for any operating condition. A novel electronic potential transformer (EPT) is devised to provide isolation between high voltage system and low voltage system. EPT eliminates bulky high voltage potential transformer.