Overmodulation algorithm for space vector modulated inverters and its application to low switching frequency PWM techniques

An overmodulation algorithm is applied to four space vector-based synchronised PWM strategies, to extend their range of operation and enhance the DC bus utilisation. The waveform symmetries and the proportionality between the reference magnitude and the fundamental voltage generated are also maintained in the overmodulation zone. It is shown that the relationship between the control variable and the fundamental voltage generated differs widely with the strategy and the pulse number used in low switching frequency PWM techniques unlike that at high switching frequencies. As a result the boundaries between any two consecutive zones of modulation and the maximum modulation index also vary with the strategy and the pulse number used. Cases where operation is possible up to the six-step mode are distinguished from those cases where it is not. The performance of the different strategies is evaluated. It is shown theoretically as well as experimentally that the bus-clamping PWM strategies perform better than the conventional space vector strategy in the overmodulation zone