Hybrid and Electric Drive Systems
Architectures you must separate
48 V mild hybrids are not high-voltage systems. Full hybrids (HEV), plug-in hybrids (PHEV), and battery electrics (BEV) use HV traction. Hybrid drives are series (engine drives a generator), parallel (engine and motor both drive the wheels), or power-split, where a planetary gearset acts as an electronically controlled CVT. BEVs typically use a single-speed reduction gear, roughly 8:1 to 10:1, with a park pawl instead of a conventional gearbox.
The motor and its feedback
Most traction motors are permanent-magnet synchronous machines; some are induction or wound-rotor designs. The inverter needs rotor position from a resolver or encoder to commutate correctly. A permanent-magnet machine generates voltage whenever the rotor turns, and back-EMF rises with speed - never tow or roll an EV with the HV cables disconnected and the drive wheels on the ground.
Symptoms and where they point
Regenerative braking blends with friction brakes and is limited when the pack is full, cold, or hot. Hesitation, odd one-pedal feel, or a torque derate usually traces to pack temperature or SOC limits rather than a failed motor. A resolver fault, an inverter current-sensor fault, and a phase cable fault can all set similar "reduced power" messages, so compare commanded versus actual torque and motor speed in scan data before condemning parts.
Traps
Expecting regen to always work; diagnosing "slippage" in an eCVT that has no conventional clutches; ignoring a gear whine when reduction-gear oil is low; and assuming a hybrid needs no HV PPE because the pack is small.
Worked example: a motor spinning 3000 rpm through a 9.0:1 reduction turns the wheels at about 333 rpm.