Three-Phase Dynamic AC Braking of Induction Motors by Discontinuous Phase-Controlled Switching

The speed of three-phase induction motors can be controlled by three-phase AC regulators. The voltage control method is effective for compressor or fan-type loads only. Alternatively, it had also been controlled by single-phasing and integral-cycle control methods. However, the induced emf in windings and the decelerating rotor during the off-period, cause a large reswitching current and large transient-negative torque. Here, this phenomenon is used for braking applications. It is achieved by repeated re-switching of all the three-phase using discontinuous phase-control in the first-half cycle during each on-period. A combination of switching-instant control and integral-cycle control is employed for the voltage variation of all three phases of the stator input voltage. It controls the peak re-switching current as well as transient-negative torque. Thus, the generation of repeated transient negative torque opposes the rotor speed, and hence fast braking is achieved. Both PSCAD and MATLAB/Simulink software are used for analysis. It is found that simply the voltage across the switch gives the required sufficient condition of the reswitching-instant for braking. In conventional braking systems, a change in power circuit configurations and arrangements is necessary for applying the brake, which delays the operation. However, in this case, with a three-phase AC regulator circuit, no modifications are required in the power circuit. Only control signals are to be changed while the power circuit (three-phase AC voltage regulator) remains the same. Moreover, after braking, speed control can be resumed immediately.