What an inverter does
The box between the battery and the motor that turns DC into three-phase AC.
- Explain what an inverter does at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
An electric motor needs alternating current with a controllable frequency and amplitude, but the battery only stores DC. The traction inverter is the translator that sits between them, and getting it right is most of what makes an EV drive well.
In depth
Power electronics system explorer
Trace the path from battery to motor through the inverter and its supporting electronics.
From DC to rotating field
The inverter contains three half-bridges, one per motor phase. Each half-bridge can connect its output to the positive or negative DC rail. By switching rapidly and in a specific pattern, the inverter synthesises a voltage waveform that looks, on average, like a sine wave at whatever frequency the motor needs to spin at that instant.
The inverter's output frequency must track the motor's rotor speed multiplied by its number of pole pairs to stay synchronised.
The inverter is really a real-time computer with power switches attached: it runs current and torque control loops at tens of kilohertz.
Key takeaways
- The inverter converts fixed DC into variable-frequency, variable-amplitude three-phase AC.
- Three half-bridges, one per phase, are the fundamental building block.
- Electrical frequency scales with mechanical speed and pole-pair count.
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