Three-Phase Inverters and SPWM
Combine three half-bridges to create controlled three-phase voltage and motor current from a DC bus.
Three coordinated half-bridges are like three musicians playing the same pattern 120 electrical degrees apart; together they create a rotating electrical field.
A three-phase voltage-source inverter uses three half-bridge legs connected to a common DC bus. Each switch node is a pulse waveform, and the load responds mainly to line-to-line voltage. Motor inductance filters much of the switching ripple so phase current can be much smoother than the node voltage.
Sinusoidal PWM compares three phase references separated by 120 electrical degrees with a high-frequency carrier. Space-vector modulation is another way to schedule the available switching states. Both methods must obey dead time and never command both devices in one leg on together.
Electrical frequency sets the rotating-field speed. For a machine with p pole pairs, electrical frequency is fe = p × mechanical_rpm/60. Torque control also depends on current magnitude and phase relative to rotor flux; applying three sine-like voltages alone is not full field-oriented control.
The DC-link capacitor carries pulsed current, switching edges create common-mode voltage and braking can return energy to the bus. Gate isolation or level shifting, current sensing, fault shutdown, thermal design and EMI layout are essential parts of the inverter rather than accessories.
Combine three half-bridges to create controlled three-phase voltage and motor current from a DC bus.
Core ideas
- Three half-bridges share one DC link.
- Phase references are separated by 120 electrical degrees.
- Electrical frequency depends on pole pairs and mechanical speed.
fe = pole pairs × rpm / 60Electrical angle = pole pairs × mechanical angleWork through a practical example
1A motor has four pole pairs and runs at 1,500 rpm.
2Mechanical revolutions per second are 1500/60 = 25.
3Electrical frequency is 4 × 25 = 100 Hz.
4The PWM carrier must be much higher than 100 Hz, while current control, dead time and device switching limits determine the practical value.
Calling the pulsed switch-node waveform a clean sine wave, or confusing mechanical angle with electrical angle in a multi-pole motor.