Q5Electric Drives

In a PWM inverter-controlled induction motor drive, a DC-link voltage (typically obtained by rectifying the fixed AC mains supply) is converted into a variable-frequency, variable-magnitude three-phase AC output by an inverter bridge, whose semiconductor switches (IGBTs, in modern drives) are rapidly turned on and off according to a PWM switching pattern — most commonly generated by comparing a sinusoidal reference waveform (at the desired output fundamental frequency and amplitude) against a much higher-frequency triangular carrier waveform (Sinusoidal PWM), or by the space vector modulation technique discussed in detail elsewhere in this general subject area.

Achieving variable motor speed: since the resulting inverter output, though composed of discrete switched voltage pulses, has an average value over each portion of the fundamental cycle that closely approximates the desired sinusoidal waveform at the commanded frequency, the induction motor connected to this output experiences an effectively sinusoidal supply at whatever frequency the inverter is commanded to produce. Because an induction motor's synchronous speed (and hence its approximate actual operating speed, accounting for the small slip) is directly proportional to this applied stator supply frequency (Ns=120f/P), commanding the PWM inverter to produce a different output frequency directly and continuously changes the motor's operating speed — the PWM technique itself provides the practical means of actually generating this required variable-frequency AC waveform from a fixed-frequency DC-link source, using only semiconductor switches operated in a simple on/off (switched) manner rather than requiring any analog/linear power amplification stage.

Role of voltage magnitude control: alongside frequency control, the PWM inverter also controls the magnitude of the output voltage (by varying the modulation index — the ratio of the reference signal's amplitude to the carrier signal's amplitude in SPWM, or the equivalent parameter in space vector modulation), which is essential for maintaining approximately constant motor flux across the controlled speed range (as required by V/f/scalar control, discussed in detail elsewhere in this paper) — without this coordinated voltage control alongside frequency control, the motor's air-gap flux would vary excessively as frequency is changed, causing either magnetic saturation (voltage too high for a given low frequency) or reduced torque capability (voltage too low for a given high frequency).

Practical benefits: PWM inverter control provides smooth, continuously variable, high-resolution control of both motor speed (via output frequency) and torque-producing capability (via coordinated voltage magnitude and, in more advanced vector/DTC control schemes, via direct current-vector control), while pushing the unavoidable switching-frequency harmonic content of the switched output waveform to frequencies well above the fundamental motor-driving frequency, where it is effectively filtered out by the motor's own inherent winding inductance, minimizing the actual harmonic distortion and associated additional losses experienced by the motor in normal operation — this combination of precise, continuous speed/frequency control together with acceptably low motor harmonic losses is precisely why PWM inverter-based variable-frequency drives have become the standard, near-universal technique for modern induction motor speed control across virtually all industrial, commercial, and residential variable-speed motor drive applications.