Thyristor controlled AC induction motors using a discrete frequency control method

The AC thyristor controllers have been traditionally used for starting induction motors to reduce the current. In this work it is proposed that the utilization of the AC thyristor controllers be used like inverters by increasing the frequency in discrete steps until the line frequency is reached. These discrete steps are sub-multiples of the line frequency. They are generated by the omission or inclusion of the line frequency half cycles. The sequences voltages are predefined for each sub-multiple of the frequency. In order to obtain positive torque for all the frequencies, the system must be unbalanced. This strategy was simulated using EMTP. The traditional starting method was compared with the new discrete frequencies method in the simulations. The results were the reduction of RMS starting current and the possibilities to work at low velocities.