STEADY STATE AND TRANSIENT ANALYSIS OF A CHOPPER CONTROLLED INDUCTION MOTOR

Abstract

There is growing demand for precise and variable speed drives with long term reliable performance. With rapid develop-merits in the technology of thyristors and solid state devices, electric drives with solid state control are becoming increasingly popular and are replacing all types of conventional drives. The induction motor seems to be an attractive proposition as a drive, because of its unmatched ruggedness, simple construction and low capital cost. The speed control of induction 7njtor may be accomplished by several methods, such as Pole changing, Pole amplitude modulation, Cascading, Stator voltage control, frequency control and rotor resistance control. Although rotor resistance control method is inefficient because of power wastage in addi-tional rotor resistances, yet in small size motors, additional rotor resistance is very small and consequently power wastage will be very small and Rotor resistance control becomes a convenient method of speed control. In chopper controlled slip ring induction motor, rotor voltage is rectified by an uncontrolled bridge and applied to an external circuit which is a variable resistance, variation being obtained through controlling the chopper duty cycle. In the present study a mathematical model for analysing the steady state and transient behaviour of chopper controlled slip ring induction motor has been developed. The syst,-.-.) i ; analyzed in a synchronously rotating reference frames The generalized two axis non-linear differential equations of the system are established from this model of the symmetrical induction machine. The complete steady state performance of the system is computed. A good correlation is observed between the computed and experimental results for the case of a plain induction motor. The investigation of transient behaviour of the system is of a great practical importance because of severe instantaneous torque generation during the transition period imposing undue strain on the mechanical parts of the drive. In order to investi-gate the transient behaviour, the non-linear 4iff erential equa-tions describing the dynamics of the system have been simulated on a digital computer and solved by the application of Runge* -Kutta Method. As the most important transients from electro-mechanical considerations are those of electromagnetic torques and speed, these are investigated in detail when the system is started from rest under no load and loaded conditions. The effects of system inertia and of applied voltage are also studied on transient torque and speed transients. Transients of stator current are also obtained and computed results are correlated with experimental results. This correlation is obtained for plain induction motor. This study has been confined to computation of only starting switching transients.