RESWITCHING TRANSIENTS IN INDUCTION MOTOR DRIVES

Abstract

Predicting transient performance of induction motor, for given bus-transfer conditions, is one of the important problems faced by systen designers. Looking to the literature published so far, it is noted that, most of the authors have considered speed of the motor during supply change-over inter-val, as constant. The work presented in this thesis gives in detail the calculation of reswitching( reclosing) transients in induction motor drives, by considering speed as one of the variables. The drive considered here is a slip ring induction motor worse slip power is wasted in externally connected re-sistance for the purpose of speed control. The variation of resistance is achieved by a chopper circuit connected in the rotor circuit. Induction motors, by and large, operate on uninterrupted power supply. Large induction motors, using high voltage supply with direct-on-line switching, however, face momentary interrup-tions of power supply due to two reasons - i) Fault in the supply system which takes about 10 cycles to clear. ii) Emergency his transfer. During this small period of interruption the motor terminals are open and a voltage appears across then which keeps on changing its phase and magnitude with time. This voltage is known as open-circuit or residual or motor-self-generated voltage. If the power supply is restored to motor at an instant when bus voltage and the residual voltage are out of phase by 180° and residual voltage is substantial in magnitude, the effective voltage would be very high and will produce excessive forces: This may damage the motor_ in follow-ing manner - i.) Stator coils may become loose in the slots ii) . The shaft may twist iii) The motor may even be ripped from its base plate in this work, the transient results have been tho rough-ly examined to evolve the safe time for reclosure which may be used to design the protective scheme of the motor to save it from this hazard. The mathematical model for analyzing the steady state and transient behaviour of chopper controlled slip ring induc-tion motor has been developed using synchronously rotating reference frame. The parameters involved in these equations are such that they can be easily measured experimentally at the terminals of the induction machine. Here the effect of saturation has not been taken into account. In order to investigate the transient behaviour, the non-linear differential equations 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 electromechanical considerations are those of electromagnetic torques and speed, these are investigated in detail. The effects of system inertia and of applied voltage are also studied on torque . r -sients. Experimental results are also included. This work will, thus, be found useful in two ways - i) It presents a general , approach of formulating mathematical model for induction motor, wnictl is useful in predicting the safe reclosure time for large machines. ii) The cho'per controlled slip ring induction motor drive has been studied and the results of reclosing transients provided.