Electric Machines, Inverters, and Power Electronics
Machines
Candidates must distinguish permanent-magnet synchronous machines (PMSM), induction machines, and brushless DC designs by construction and failure mode. In a PMSM the rotor carries permanent magnets; the inverter must know rotor position from a resolver or Hall sensors to commutate correctly. Back-EMF rises roughly linearly with speed, and field weakening extends the speed range by reducing flux-producing current. Understand that torque is produced by the interaction of stator current and rotor flux, and that the machine can generate - back-feeding the DC bus - whenever it is driven above its torque command.
Inverters and power electronics
The inverter is a three-phase bridge of IGBTs or SiC MOSFETs that converts DC to AC and, in regeneration, rectifies AC back to DC. Key components: DC bus capacitor, gate drivers, current sensors, temperature sensors, and a cooling circuit shared with the motor in many designs. Common faults are gate-driver and desaturation faults, capacitor degradation, coolant leaks, and resolver or harness issues. Phase-cable resistance should be essentially equal phase-to-phase; unequal readings point to a cable or connector, not the motor.
Traps
A motor that spins does not prove the resolver is calibrated - resolver offset learn is often required after replacement. A weak or failing 12 V supply can set inverter communication faults. Do not probe inverter output with a non-CAT, low-bandwidth meter; use a scope or insulation tester instead. Respect the strong magnetic field of PM rotors (pacemakers, ferrous tools).
Rule of thumb
If a hand-spun motor generates a small, roughly proportional voltage on all three phases, the winding and magnets are likely intact; an open or shorted phase produces a dead leg.