EFFICIENCY OPTIMIZATION OF VECTOR CONTROLLED INDUCTION MOTOR DRIVE

Abstract

Induction motors are widely used in domestic, commercial and industrial application. About 80% of the auxiliaries used in thermal power plant are induction motors. And, in households almost all rotating machines are induction motors. Most of the applications of induction motors (IMs) are based on variation in speed according to the load demand. Induction machines are non linear machines and there exist high dynamic interactions. Hence, in order to emulate it as separately excited DC motor, vector control technique is used as this method removes the coupling effects (using a frame of reference like synchronous, stationary) which makes the response sluggish. Traditionally, PI controllers are generally used in order to control the speed but tuning is somewhat time consuming as values have to be changed every time when parameters are changed. In order to avoid this fuzzy controllers are used. Also, induction motors consume a major part of the total electrical energy production. Therefore, improving the efficiency of the induction motor is always been a major concern. Various essential strategies are adopted to ensure optimum efficiency operation of induction motor drive. This is usually realized either by selecting proper control strategies or by improvement in the design, materials and construction techniques or both. Design aspects involve shaping of rotor bars, use of copper material instead of aluminium, etc. The control strategies adopted to optimize the efficiency are search controller and loss model controller. Search controller method is a feedback method that finds maximum efficiency by adopting a search technique. The value of efficiency at a particular sampling instant is compared with the efficiency value of previous sampling instant and necessary control action is executed accordingly. Loss model controller depends on motor parameters because it is based on the modelling of the motor and losses to derive an objective function, which is optimized to yield maximum efficiency. Machines are designed to operate at rated flux because if gives fast transient response and high torque to ampere ratio. If rated flux is maintained at light loads, then core losses of the machine will be excessively high which will result in reduced efficiency. Here, loss model control technique will be used to reduce the losses and increase the torque per ampere ratio by making rated flux as increasing function of load.