Comparison among three topologies of induction motor fed by inverter and controlled by DFC

Improving the performance of induction motor at start up has been the subject of many researches. So, several starting techniques based on AC thyristors have been developed in many industrial applications. One of these techniques is introduced in this paper which is starting of induction motor using the technique of discrete frequency control (DFC). The proposed technique consists of the generation of sub-harmonic voltages until reaching the line frequency (50Hz). This strategy leads to significant reduction of starting current and improvement of the starting torque. The technique of DFC is based in the use of ac thyristors, therefore it present several topologies concerning the connection mode between induction motor and inverter. So, we propose in this paper to compare between three proposed topology of them, and to establish the difference using a several controls rules generated by the strategy of DFC.