A Series Connected Three-Level Inverter Topology For Medium Voltage Squirrel Cage Motor Drive Applications

The application of PWM voltages using two level high voltage inverters to a squirrel cage induction motor (SQIM) can cause high bearing currents, heating of rotor shaft, voltage spike across the motor terminals, etc. The increase in the number of steps of the motor voltage and hence decreasing the dv/dt applied to the machine terminals can be a solution to this problem. The existing topologies that generate this multi-step voltage include cascading of a number of single-phase inverters or use of higher order multi-level inverters. In this paper a topology with series connection of three phase three-level inverters is proposed that addresses the problems of medium voltage drives. The design of the inverter topology and its various PWM techniques are presented in the paper. This inverter topology and its control are verified on a 7.5 hp squirrel cage induction motor (SQIM) drive. Experimental results validate the steady-state and dynamic performances of the drive.