A Square-wave Controller for a high speed induction motor drive using a three phase floating bridge inverter

A square-wave voltage control scheme is presented for a high speed induction motor drive that uses a dual 3 phase inverter system and open ended motor windings. The drive main inverter bridge is connected to the dc battery source and a second inverter bridge is floating with no dc power source. The floating bridge dc voltage is allowed to naturally fluctuate and used to provide voltage boosting at high speeds, well above the maximum possible when using just the main bridge. This voltage boosting can also be used to compensate for fluctuations in the dc battery voltage. A second controller feature forces the main bridge to operate at a unity displacement power factor, lowering the main inverter losses and the rms current drawn from the battery - while maximizing the reactive support available to the machine. Significantly, these features are obtained inherently without having to switch between different inverter switching patterns or monitor the load current magnitude or phase. Experimental results are used to illustrate the principal of the natural reactive compensation of the floating bridge and to verify drive operation on a 2HP, 1800 rpm induction machine.