Q9Electric Drives
Question
Q.9. Explain the principle of PWM control in AC drives.
Answer
PWM control in AC drives generates a variable-frequency, variable-voltage three-phase output by comparing a sinusoidal (or space-vector-based) reference against a high-frequency carrier/switching pattern, producing a train of switched pulses whose average value over each fundamental cycle traces out the desired sinusoidal output waveform.
In AC drives, Pulse Width Modulation (PWM) control generates the required variable-frequency, variable-voltage three-phase output waveform (needed to control induction or synchronous motor speed) by rapidly switching the inverter's semiconductor devices on and off according to a specific switching pattern — most commonly by comparing three sinusoidal reference waveforms (one per phase, each at the desired fundamental output frequency and amplitude) against a much higher-frequency triangular carrier waveform (Sinusoidal PWM, SPWM), or by directly synthesizing the desired instantaneous output voltage vector using the space vector modulation technique discussed elsewhere in this paper. The resulting switched output, though composed of discrete voltage pulses of the fixed DC-link voltage magnitude, has an average value (over each portion of the fundamental cycle) that closely approximates the desired sinusoidal reference waveform, with the unwanted switching-frequency harmonic content pushed to frequencies well above the fundamental (and hence easily filtered by the motor's own inherent inductive filtering effect), providing effective, continuously variable control of both the output voltage magnitude and frequency needed for AC motor speed control.