Control of PWM voltage inverters in the pulse dropping region

The overmodulation of pulse width modulated (PWM) inverters causes a nonlinearity in the feedforward channel. The type of modulator, sine wave, space vector or third harmonic, establishes the characteristics of the transition region's nonlinearity. The characteristics for a number of modulation strategies are introduced. Test results from commercially available volts per hertz (V/F) drives reveal their inability to provide rated voltage even at rated input conditions. The adverse effects of the overmodulation region on current regulated AC inverters are demonstrated by experimental results. A compensated modulation technique (CMT), adaptable to continuous and discontinuous modulators, provides the exact inverse of the nonlinearity; thus it produces a smooth transition to six-step operation without inducing a voltage transient. Experimental results presented in the paper demonstrate the CMT's smooth transition to six-step and the improved performance of the CMT-PWM. Finally, a comparison of the CMT with the other known overmodulation strategy shows the CMT provides a simple technique with essentially identical harmonic characteristics.<
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