A CASCADED H-BRIDGE AND NOVEL MULTILEVEL INVERTER TOPOLOGY FOR INDUCTION MOTOR DRIVE Ratnakumar*

The poor quality of voltage and current of a conventional inverter fed induction machine is due to the presence of harmonics and hence there is significant level of energy losses. The nine level inverter is used to reduce the harmonics. The inverters with a large number of steps can generate high quality voltage waveforms. The concept of multilevel inverters, introduced about 20 years ago entails performing power conversion in multiple voltage steps to obtain improved power quality, lower switching losses, better electromagnetic compatibility, and higher voltage capability. The benefits are especially clear for medium-voltage drives in industrial applications and are being considered for future naval ship propulsion systems. The application of pulse width-modulated (PWM) voltages using two-level high-voltage inverters to a squirrel-cage induction motor (SQIM) can cause 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 multistep voltage include cascading of a number of single-phase inverters or use of higher order multilevel inverters. In this paper, a topology with series connection of three-phase three-level inverters is proposed, which addresses the problems of medium-voltage drives.