Dual inverter control strategy for high speed operation of EV induction motors

ISA (integrated starter/alternator) will be used for cars soon with 42 volt system. ISA is a multi-functional integrated device that functions as starting motor, generator, flywheel, or torque assist to a combustion engine. But, other than thermal and cost problem, there is an inherent difficulty in the design of motor-inverter system. Specifically, it should produce 150 Nm starting torque and also generate electricity while engine is running 6000 rpm. To produce the required torque, ISA has normally as many as 12 poles. A great problem is that the generated voltage must be limited to under 42 volt even when 12 pole ISA is running 6000 rpm. Hence, the ISA design shows that it has as much as 10:1 speed range, i.e., the field weakening operation should be extended to 10 times higher than the rated speed. In this work, we are considering the use of induction machine instead of permanent synchronous machine. As an idea for solving the voltage limit problem, we are utilizing two inverters. Sharing the voltage requirements by dual inverters is the main idea. But, the secondary inverter only takes care of the reactive voltage component which grows very large in high speed operation. Therefore, the secondary inverter does not require the use of extra voltage source. A capacitor bank suffices the purpose of the secondary inverter. Finally, both simulation and experiment for confirmation are presented.