Six-step switching vs. space-vector PWM
Two ways to approximate a sine wave, and why most drives use the smarter one.
- Explain six-step switching vs. space-vector pwm at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
The simplest possible switching pattern — six-step — gets you a crude AC waveform with very little computation. Modern drives almost always use space-vector PWM instead, trading switching losses for waveform quality and control bandwidth.
In depth
| Six-step | SVPWM | |
|---|---|---|
| Switching frequency | Equal to output frequency | Fixed carrier, e.g. 8–16 kHz |
| Harmonic content | High, strong 5th/7th harmonics | Low, pushed to carrier sidebands |
| DC-bus utilisation | Maximum (100%) | Up to ~90.7% without overmodulation |
| Torque ripple | Noticeable | Low |
| Typical use | Field-weakening / max speed | Normal operating range |
Space-vector PWM treats the three-phase voltage command as a single rotating vector and finds the combination of switch states, held for the right fractions of each carrier period, that reproduces it on average. This spreads switching losses evenly across all six devices and pushes unwanted harmonics up near the carrier frequency, where the motor's inductance filters them out easily.
PWM waveform simulator
Change carrier frequency and modulation index and watch the synthesised phase voltage and its harmonic content.
Duty cycle sets the average voltage. Carrier frequency sets how finely the DC bus is chopped: higher means less ripple and less audible whine, but more switching loss.
Near maximum speed, when there is no more voltage headroom for a sinusoidal pattern, drives fall back to six-step to extract every last volt from the DC bus — at the cost of extra torque ripple and acoustic noise.
Key takeaways
- Six-step maximises DC-bus utilisation but has poor harmonic content.
- SVPWM gives smooth torque and low harmonics across most of the operating range.
- Drives often blend both strategies depending on operating point.
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