- With reference to speed control of ac induction motors, explain the function of Pulse Width Modulation (PWM).
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Pulse Width Modulation (PWM) in AC Induction Motor Speed Control
Definition
Pulse Width Modulation (PWM) is a technique used in Variable Frequency Drives (VFDs) to control the effective voltage and frequency of the supply to an AC induction motor by rapidly switching semiconductor devices (e.g., IGBTs) on and off.
Function and Operation
- High-frequency switching
- The inverter switches (IGBTs/MOSFETs) are turned on and off at very high frequency.
- Instead of producing a sinusoidal waveform directly, the inverter creates a train of DC voltage pulses.
- Pulse width control
- The duration (width) of each pulse is varied in proportion to the desired instantaneous value of a sine wave.
- Narrow pulses = lower effective voltage.
- Wide pulses = higher effective voltage.
- Effective sinusoidal output
- The motor windings respond to the average (RMS) value of the pulses, not to the high-frequency switching.
- After filtering by the motor inductance, the result is a near-sinusoidal current.
- Speed control
- By varying the pulse pattern, the frequency of the simulated AC waveform is changed, adjusting the synchronous speed of the motor.
- At the same time, the voltage is adjusted proportionally (V/f control) to maintain constant flux and torque.
- Efficiency
- PWM minimises harmonic distortion and reduces motor heating compared to earlier square-wave inverters.
- Provides smooth torque and quiet operation.
Advantages of PWM control in induction motors
- Precise speed control over a wide range.
- High efficiency with reduced losses.
- Improved torque characteristics, especially at low speeds.
- Lower harmonic distortion compared to six-step inverters.
- Smooth acceleration and deceleration, reducing mechanical stress.
- Flexibility – can be used for soft-starting, braking, and energy saving at partial load.