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In order to minimize motor losses and avoid torque pulsations in a variable-speed electric drives, it is necessary that, at low speeds, the output voltage of the inverter is characterized as far as possible by a sinusoidal shape with variable amplitude and frequency. To this aim, inverters controlled by pulsewidth modulation techniques are employed. In particular, when the subharmonic method is used in an asynchronous way utilizing the natural sampling, a dc component and subharmonics may be present, with dangerous consequences to the electric motor. To overcome these drawbacks, one can use a modulation method called ``uniform sampling,'' with symmetrical or asymmetrical modulation. Single-and three-phase analytical solutions to cover also overmodulation conditions are presented for uniform sampling, and comparisons are made between the natural and the symmetrical and asymmetrical uniform-sampling methods. The harmonic content obtainable under overmodulation conditions is investigated, useful information about the first-harmonic behavior is provided as a function of the modulating depth, and the influence of the displacement angle between the modulating and carrier waves on the first harmonic of the output voltage is evidenced particularly at low frequency values.
Date of Publication: July 1985