How inverters drive a motor
Turning battery DC into synthesised three-phase AC, one switching decision at a time.
- Explain how inverters drive a motor at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
The inverter is the bridge between the battery's DC and the motor's need for precisely shaped three-phase AC — and it makes that shape purely by switching, not by generating a true sine wave.
In depth
Lab: Configure an inverter
Pick a semiconductor, switching frequency and current limit that stay inside the thermal budget.
Inverter loss has two parts. Conduction loss depends on the device's on-state resistance or saturation voltage and the current through it. Switching loss depends on the energy lost in each transition multiplied by how often you switch. Silicon carbide has far lower switching energy than an IGBT, which is what allows higher switching frequencies and smaller filters.
Six switches, three phases
A typical traction inverter has six power switches — two per phase, arranged as a top and bottom pair. By switching each pair rapidly on and off in a carefully calculated pattern, the inverter builds up an average voltage waveform on each phase that approximates a sine wave, even though every individual switching event is a hard on/off transition.
Three-Phase Power Calculator
Real power from line voltage, line current and power factor.
P = √3 × V_line × I_line × cos φIn a balanced three-phase system the √3 factor relates line and phase quantities. The power factor accounts for the share of current that does no useful work.
Example: √3 × 400 V × 250 A × 0.95 ≈ 164.5 kW.
- Field-oriented control keeps the effective power factor high across the map.
- Apparent power (kVA) sizes the inverter; real power (kW) moves the car.
- Reactive current still heats the windings even though it does no work.
Key takeaways
- The inverter converts DC into synthesised three-phase AC using six switches.
- Rapid switching builds an average sine-like waveform from discrete on/off pulses.
- SiC devices reduce switching losses versus traditional IGBTs.
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