DESIGN IMPROVEMENT OF INDUCTION MOTORS FOR ADJUSTABLE SPEED DRIVES

Abstract

The Induction motors are widely used in the industry, because of their simplicity, robustness & maintenance free operation. But to overcome the use of the DC motors they had to be supplied by the flexible three phase sinusoidal power supplies. This had been made possible with the advent of the solid state devices in the field of the ac drives. Nowadays, the advanced adjustable speed drives powered through power converters, are burning topic of discussion. They provide the desired flexibility in the speed torque characteristics of the Induction motors, as per the requirements. This is obtained at the cost of the high harmonic contents in the input power supply of the motor, injection of harmonics in the power distribution line and greater chance of failure of the Induction motor due to high losses and heating. A lot of modifications of the basic adjustable speed drives are available today, to overcome these problems in the adjustable speed drive, but less has been focused on the main part of the drive i.e. the motor itself. The design of the machine is the main factor which can compensate for the ill effects caused due to the harmonics present it the input of the motor. Also the design and the optimization procedure had reached the new heights due the introduction of the new machine design softwares and the computer aided design technologies. Here a new optimized design of the induction machine is presented with the help of the machine design software SPEED which is introduced by the University of Glasgow. The factors like temperature rise and maximum flux density are taken in consideration automatically in the SPEED software design, hence making the design optimization easier. The new machine is designed to give a suitable operation under adjustable speed drives with the harmonics present in the input power of the machine by reducing the losses in the machine and the wide range of the speed is available due to high breakdown torque. In the present work, the main considerations of the high efficiency and high breakdown torque are taken into consideration for the adjustable speed drive, to make the machine suitable for operation under the harmonics in supply. Different rotor bars are studied and the best among them is searched out.