Mathematical modeling of an induction motor with chopper-controlled rotor resistance

The effect of chopper frequency and relative pulsetime on the rotor's rectified current ripple and the motor's developed torque were investigated. The electrical drive (three phase uncontrolled bridge rectifier and a chopper controlled external resistance) provides contactless adjustment of rotor resistance by high switching ability of power semiconductor devices. Using a high switch power semiconductor such as an IGBT transistor for chopper switching allows a high chopper frequency, eliminating the smoothing inductor. This reduces volume and, therefore, drive system cost. The initial values of the current for “on” and “off” mode, which give the ripple, were determined by imposing the continuity and periodicity conditions. In a state-space approach, for constant or variable load torque, using the duty cycle a as parameter, the dynamic variations of the d.c. link current and rotor speed can be obtained. The measurements carried out on an experimental bench at load motor operation allows the analysis of the torque-speed and efficiency characteristics at different duty-cycle and/or chopper frequency. Finally, analytical efficiency characteristics, for the induction motor and for electrical drive with chopper frequency and relative pulsetime as parameters, were computed using a Matlab application.