(A) new indirect field oriented induction motor drive using active power to reactive power ratio

Abstract

An induction machine is much more difficult control plant than a dcmotor. This is not only due to fact three ac voltages and currents of varying amplitude and frequency have to be generated, but also because there is an intricate coupling between the control inputs and inner quantities, flux and torque. In addition, the rotor currents are inaccessible in case of the squirrel-cage induction motors. This limitation can be overcome by applying a field-oriented control method. In the high performance field oriented control system, the current components controlling the rotor flux and torque are decoupled so that independent control of rotor flux and torque are possible as in a separately excited dc motor. The key requirement to this control scheme is the information on the rotor flux vector. As long as the rotor flux vector detection is concerned, the direct sensing method of the air gap vector using Hall probe is most desirable in theory. However, from the viewpoint of practical applications, this method suffers from high cost and the unreliable flux measurement. Therefore, the indirect sensing methods of the rotor flux vector have been developed. The major drawback of this method is the sensitivity of the estimation to the change of motor parameters arising from a magnetic saturation and a temperature change. Any mismatch between the real parameters and the parameters used in the field-oriented controller will result in the deterioration of performance such as steady-state error and transient oscillations of rotor flux and torque. Thus, the motor parameters, especially rotor time constant, should be identified during normal operation. Also, the current should be controlled accurately since the stator current is considered as the control input. Consequently, the indirect field oriented control requires the current control and the rotor time constant identification. In addition, it is desired to control the rotor flux level for the field weakening. This dissertati...