Analysis of PWM Techniques using Uniform Sampling in Variable-Speed Electrical Drives with Large Speed Range

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 pulse-width modulation techniques are employed. In particular, when the sub-harmonic method is used in asynchronous way utilizing the natural sampling, a dc component and sub-harmonics may be present, with dangerous consequencies to the electric motor. To overcome these drawbacks, one can use a modulation method called "uniform sampling", with "symmetrical" or "asymmetrical" modulation. In this paper single and three-phase analytical solutions to cover also over-modulation 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 over-modulation conditions is investigated, useful information about the first-harmonic behaviour 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.