Electric Motor, Controller and Drivetrain
What this domain covers
This domain checks your understanding of the traction motor, the controller/inverter, position sensors and the reducer, plus torque, speed and regenerative braking.
Core rules and concepts
- Motor types: PMSM and BLDC use permanent magnets and are common in EVs; induction motors have no magnets; switched reluctance motors are robust.
- Controller: reads throttle, brake, gear and sensor signals, then switches the inverter to control motor current, torque and speed.
- Inverter: converts DC from the pack to three-phase AC for the motor, and rectifies AC back to DC during regeneration.
- Sensors: Hall sensors, resolvers or encoders report rotor position; temperature sensors protect motor and controller.
- Drivetrain: most EVs use a single-speed reducer and differential; some use two motors for AWD.
- Regenerative braking: the motor acts as a generator, returning energy to the pack; it is limited by SoC, temperature and speed.
Worked example
A 48 V EV has an 8 kW controller. At full power, approximate pack current = 8000 ÷ 48 = 166.7 A. A 100 Ah pack at 1C can supply 100 A, so full power may be limited unless the pack has a higher C-rate or higher capacity. This explains why small packs cannot always deliver peak motor power.
Traps that cost marks
Do not say the controller charges the HV battery from the mains - that is the onboard charger. Do not say the motor makes DC; the inverter handles conversion. Do not assume regen works at 100% SoC or in very cold cells. Do not ignore a motor-position sensor fault; it can cause rough running or no start. Do not test motor phases with the HV system live unless trained and authorised.