The motor as a generator
The same machine that accelerates the car can decelerate it, simply by reversing the sign of its torque.
- Explain the motor as a generator at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real braking system
Overview
An electric traction motor is not a one-way device. Spin its rotor and it induces a voltage in its windings; if that voltage is allowed to drive current into a load, the machine is generating rather than motoring, and the reaction torque on the rotor opposes rotation — exactly the deceleration a brake needs.
In depth
Regenerative braking simulator
Drag the brake pedal or lift off the accelerator and watch torque, current and battery charge respond.
Motoring and generating are the same physics
A permanent-magnet synchronous motor produces torque proportional to the current in its windings and the flux linking the rotor magnets. In motoring mode the inverter forces current that is in phase with back-EMF to push the rotor forward. In generating mode the inverter instead extracts current that opposes rotation, converting mechanical energy into electrical energy which the inverter rectifies into DC for the battery.
| Mode | Torque sign | Power flow | Inverter role |
|---|---|---|---|
| Motoring | Positive (same direction as rotation) | Battery → wheels | Sources current in phase with back-EMF |
| Generating (regen) | Negative (opposes rotation) | Wheels → battery | Rectifies motor output, controls charge current |
| Coasting | Near zero | Negligible | Freewheels the motor, minimal current |
Induction motors need to be actively excited to generate, costing a little efficiency at low torque. Permanent-magnet motors generate as soon as they spin, which is one reason most modern EVs use them.
Key takeaways
- Motoring and generating differ only in the sign of torque relative to rotation.
- The inverter rectifies the motor's generated AC into DC to charge the battery.
- Permanent-magnet motors generate inherently; induction motors need active excitation.
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