High power pulse-width modulated (PWM) inverters are generally operated at low switching frequency in order to avoid excessive switching losses. This produces increased current waveform distortions, additional machine losses and torque harmonics. It is therefore expedient to adopt optimal PWM strategies for their control.
The technique of optimizing the pulse sequences by off-line calculations is advantageous as far as the steady-state performance is concerned; however, the modulator is nonoptimal and unpredictible during transient operation. Any change of the steady-state leands to transient overcurrents. This drawback is eliminated by the implementation of a control algorithm for the harmonic currents. A trajectory controller ensures that the current vector is guided along precalculated optimal trajectories, maintaining optimal current waveforms also under transient operating conditions.
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