Resolvers and Hall sensors — how EV motors know where they are

To switch the right motor phase at the right moment, the inverter needs to know the rotor's exact angle. Most EV traction motors use a resolver; some smaller motors use Hall-effect sensors.

Resolvers

A resolver is a rugged rotary transformer. An excitation signal goes in; two output windings return sine and cosine signals whose ratio encodes the angle. They tolerate heat and vibration well — but connectors, wiring and the decoding electronics can fail.

Hall sensors

Hall-effect sensors switch as magnets pass, giving a simpler digital position signal. The same principle sits inside many crank, cam and speed sensors on petrol cars.

The overlap

Diagnosing a noisy resolver signal and diagnosing a jittery tacho from a dirty CAS signal use the same mindset: scope the waveform, check grounds and shielding, and separate sensor from wiring from controller.