Electric Motors & Motor Controllers Interview Questions & Answers

eMobility Academy — Electric Motors & Motor Controllers interview questions

1. Q1.What types of motors are used in EVs?

   Fresher

   The main types are: PMSM (permanent magnet synchronous motor), used in most modern cars and many two-wheelers for high efficiency and power density; BLDC (brushless DC), which is a permanent magnet motor with trapezoidal back-EMF, very common in e-rickshaws, hub-motor scooters and e-bikes because of low-cost six-step control; induction motors, which have no magnets, are robust and cheap in materials, and were used in early Tesla Model S drive units, but are less efficient at light load; and switched reluctance and synchronous reluctance motors, which avoid rare earths and are under active development. Many recent cars use interior PMSM (IPMSM), which combines magnet torque with reluctance torque.

2. Q2.What is the difference between BLDC and PMSM?

   Fresher

   Both have permanent magnets on the rotor and windings on the stator. The difference is the back-EMF shape and how they are driven. A BLDC motor has concentrated windings that produce trapezoidal back-EMF and is normally driven with six-step (trapezoidal) commutation, where two phases conduct at a time based on Hall sensor signals. It is simple but has torque ripple at each commutation step. A PMSM has distributed or designed windings that produce sinusoidal back-EMF and is driven with sinusoidal currents, usually through field-oriented control, giving smooth torque, low noise and better efficiency. In practice many 'BLDC' hub motors are run with FOC too, so the boundary is about control method as much as construction.

3. Q3.Explain six-step commutation with Hall sensors.

   Fresher

   Three Hall sensors placed 120 electrical degrees apart give a 3-bit code that changes every 60 electrical degrees, producing six valid states (000 and 111 are invalid and indicate a fault). For each state the controller energises two phases, one high-side switch and one low-side switch, and leaves the third phase floating, so the stator field leads the rotor field by roughly 90 degrees for maximum torque. Speed and torque are controlled by PWM duty on the active switches. Reversing direction is done by changing the commutation table. It is cheap and robust, but each step causes torque ripple and audible noise, and the sensors need careful alignment. Sensorless six-step uses back-EMF zero-crossing detection on the floating phase instead.

4. Q4.What is field-oriented control (FOC)?

   Fresher

   FOC controls an AC motor like a separately excited DC motor by working in a frame that rotates with the rotor. Measured phase currents are transformed with the Clarke transform (three-phase abc to two-axis stationary αβ) and the Park transform (αβ to rotating dq using rotor angle). In the dq frame, Iq produces torque and Id controls flux. Two PI controllers regulate Id and Iq to their references, giving Vd and Vq. An inverse Park transform converts them back to αβ, and space vector PWM generates switching signals for the inverter. For a surface PMSM below base speed you set Id to zero and command Iq proportional to torque. FOC needs accurate rotor position from an encoder, resolver or sensorless observer.

5. Q5.How does a three-phase inverter work?

   Fresher

   A two-level three-phase inverter has six switches (MOSFETs, IGBTs or SiC MOSFETs) arranged as three half-bridges across the DC bus, each half-bridge driving one motor phase. By switching the high and low devices of each leg at high frequency with PWM, the inverter creates an average phase voltage that can be shaped into a sine wave of any frequency and amplitude, so it controls motor speed and torque. The two switches in a leg must never be on together, which would short the bus, so the controller inserts dead time between turning one off and the other on. A DC-link capacitor supplies ripple current. Freewheeling diodes, or the MOSFET body diode or synchronous rectification, carry current during dead time.

6. Q6.What is back-EMF and why does it limit motor speed?

   Fresher

   When the rotor spins, its magnets induce a voltage in the stator windings called back-EMF, proportional to speed (Ke × ω). The inverter must apply more voltage than the back-EMF to push current into the windings. As speed rises, back-EMF approaches the maximum voltage the inverter can produce from the DC bus, roughly Vdc/√3 per phase peak with SVPWM, and current, therefore torque, falls off. The speed where this happens at full torque is the base speed. Above base speed, the motor enters the constant power region only if the controller applies field weakening. So battery voltage directly affects top speed and high-speed power: a sagging pack at low SoC gives less top speed.

7. Q7.What are the rotor position sensors used in EV motors?

   Fresher

   Hall sensors are cheap digital sensors giving position every 60 electrical degrees; they suit six-step BLDC control and are common in two-wheelers, but FOC with Halls needs angle interpolation. Incremental or absolute encoders (optical or magnetic) give high resolution; magnetic encoders such as on-axis TMR or AMR ICs are growing in two-wheeler drives. Resolvers are rotary transformers that are extremely rugged against heat, vibration and contamination, and are the standard in passenger-car traction motors; they need a resolver-to-digital converter or software demodulation. Sensorless control estimates position from back-EMF or flux observers at medium and high speed, and from high-frequency injection at low speed. Each choice trades cost, robustness and low-speed performance.

8. Q8.What is regenerative braking from the motor controller's point of view?

   Fresher

   In regen, the controller commands negative torque, meaning negative Iq in FOC. The motor acts as a generator, current flows from the motor through the inverter back into the DC bus, and the bus voltage rises slightly to push charge into the battery. The controller must respect the battery's charge current limit from the BMS, the maximum bus voltage, and the motor's thermal limits. If the battery cannot accept current, for example when it is full, regen must be reduced or a brake resistor used. On three-wheelers and scooters, regen strength is typically tuned per drive mode and blended with friction brakes so the vehicle stops smoothly and the rear wheel does not lock.

9. Q9.What is SVPWM and why is it preferred over sine PWM?

   Experienced

   Space vector PWM treats the inverter's eight switching states as six active voltage vectors and two zero vectors. For any reference voltage vector, it applies the two adjacent active vectors and zero vectors for computed durations within each PWM period. Compared with carrier-based sine PWM, SVPWM gives about 15.5% higher DC bus utilisation, because the maximum linear phase voltage rises from Vdc/2 to Vdc/√3. That means more torque at high speed or a higher base speed for the same battery voltage. It also produces lower current harmonic distortion. In practice SVPWM is equivalent to sine PWM with a specific zero-sequence (third-harmonic-like) injection, and many controllers implement it that way. Overmodulation strategies extend it further at the cost of distortion.

10. Q10.Explain field weakening and MTPA.

    Experienced

    MTPA (maximum torque per ampere) chooses the Id and Iq combination that gives the most torque for a given current magnitude. For a surface PMSM, MTPA is simply Id = 0. For an interior PMSM, where Ld is less than Lq, a negative Id adds reluctance torque, from the torque equation T = 1.5·p·(ψ·Iq + (Ld − Lq)·Id·Iq), so MTPA uses negative Id even at low speed. Field weakening applies above base speed, when the voltage limit is reached: increasing negative Id counteracts magnet flux, reducing back-EMF so current can still be driven at higher speed, extending the constant-power region. Implementations use lookup tables from dynamometer calibration or a voltage-feedback loop that adjusts Id when the modulation index saturates.

11. Q11.How do you tune the current loop PI controllers in FOC?

    Experienced

    Start from the motor's phase resistance R and inductances Ld and Lq. A common method is pole-zero cancellation: set Kp = L × ωc and Ki = R × ωc, where ωc is the desired current-loop bandwidth, giving a first-order closed-loop response. Bandwidth is typically chosen around a tenth or less of the PWM switching frequency, accounting for sampling and computation delays. Add decoupling feed-forward terms (ω·L·I cross-coupling and back-EMF) so the d and q axes do not disturb each other at high speed. Add anti-windup on the integrators, because voltage saturates in field weakening. Validate with current steps on a locked rotor or dyno, then at speed, and watch for oscillation where inductance drops with saturation at high current.

12. Q12.IGBT, Si MOSFET or SiC MOSFET: how do you choose for a traction inverter?

    Experienced

    For 48 to 96 V two- and three-wheeler controllers, silicon MOSFETs (often 100 to 150 V rated, several in parallel) are the default: very low on-resistance, fast switching and low cost. For 400 V car inverters, IGBTs (650 to 750 V) have been the cost-effective standard, but they have a fixed collector-emitter voltage drop and tail-current switching losses. SiC MOSFETs have resistive conduction (very efficient at partial load, which dominates drive cycles), low switching loss and higher temperature capability, improving inverter efficiency by a few percent and therefore range; at 800 V, 1200 V SiC is now the standard choice. The trade-off is device cost, the need for fast gate drivers, and higher dv/dt, which stresses motor insulation and EMI design.

13. Q13.What is dead time and what problems does it cause?

    Experienced

    Dead time is the short interval, typically a few hundred nanoseconds to a couple of microseconds depending on the devices, when both switches in a leg are off to prevent shoot-through. During dead time, current flows through freewheeling diodes, so the actual output voltage depends on current direction rather than the PWM command. This causes voltage error, current distortion near zero crossing, torque ripple at low speed and audible noise, and it distorts sensorless estimators. Mitigations: minimise dead time with fast, well-matched gate drivers (SiC and GaN help), and apply dead-time compensation in firmware that adds a voltage correction based on the sign of phase current, with a smooth transition near zero current.

14. Q14.How do you measure phase currents in a motor controller?

    Experienced

    Options: low-side shunts in each leg's emitter or source, read through op-amps; cheap and common in two-wheelers, but current can only be sampled when the low-side switch is on, so sampling must be synchronised to the PWM centre and needs a minimum low-side on-time. Single-shunt in the DC link reconstructs phase currents from sample timing within the PWM period, saving cost but complicating low-modulation operation. Inline phase shunts with isolated or high common-mode amplifiers give continuous measurement. Hall-effect or fluxgate sensors on phase busbars are isolated and standard in high-voltage car inverters. Since Ia + Ib + Ic = 0, two sensors suffice, but a third improves fault detection. Offset calibration at start-up is essential.

15. Q15.How do you protect a motor controller from faults?

    Experienced

    Layers of protection: hardware over-current using comparators or gate-driver desaturation detection that shut down within microseconds; DC bus over-voltage and under-voltage protection, important during regen with a full battery; over-temperature on power devices (NTC on the heatsink or module) and motor windings with derating curves before shutdown; position sensor faults (invalid Hall states, resolver signal loss); phase loss and short detection; watchdogs and plausibility checks on torque commands. The safe state matters: for a PMSM at high speed, opening all switches can cause uncontrolled regeneration through the diodes if back-EMF exceeds bus voltage, so controllers may use an active short circuit (all low-side switches on) instead.

16. Q16.What is cogging torque and torque ripple, and how are they reduced?

    Experienced

    Cogging torque is the torque a permanent magnet motor produces with no current, caused by the magnets' attraction to stator teeth; you can feel it as notchiness when you turn a hub motor by hand. Torque ripple under load comes from cogging, non-sinusoidal back-EMF, current harmonics and commutation. Both cause vibration and noise, and matter at low speed. Design fixes: an appropriate slot-pole combination, skewing the stator or rotor, magnet shaping and pole-arc optimisation. Control fixes: use FOC rather than six-step, improve current sensing and dead-time compensation, and use harmonic injection or lookup-based compensation of known ripple. In interviews, mention that skewing reduces cogging but also slightly reduces torque.

17. Q17.How would you test and characterise a new traction motor?

    Experienced

    On a dynamometer, test the motor and inverter together. First measure parameters: phase resistance, Ld and Lq versus current (saturation), back-EMF constant from open-circuit voltage at known speed, and cogging torque. Then map torque versus speed at multiple bus voltages to confirm peak and continuous curves, and record the efficiency map using a power analyser on DC input, AC phases and shaft torque. Run thermal tests at continuous rating and peak duty to verify winding and magnet temperatures with the cooling system. Check NVH, insulation resistance and partial discharge for high-voltage machines, and demagnetisation margin at high temperature and maximum current. Use the results to build the calibration tables for MTPA and field weakening.

18. Q18.Why is there a push for rare-earth-free motors?

    Experienced

    High-performance PMSMs use NdFeB magnets with dysprosium or terbium for high-temperature coercivity. Rare-earth supply is concentrated, mainly in China, and prices are volatile; export restrictions on rare-earth magnets in 2025 directly disrupted Indian two-wheeler production. So OEMs and suppliers are investing in alternatives: ferrite-assisted synchronous reluctance, switched reluctance, induction, wound-rotor (electrically excited) synchronous motors such as Renault uses, and heavy-rare-earth-free magnet grades. The trade-offs are lower torque density, more complex control, and higher noise for reluctance machines. India's push for domestic magnet manufacturing is part of the same strategy. This is a good topic to show industry awareness.

19. Q19.How do you debug a motor that vibrates and draws high current at low speed under FOC?

    Experienced

    Suspect rotor angle error first. If the encoder or Hall offset is wrong, the controller injects current in the wrong axis, so torque per amp drops, current rises, and the motor vibrates or runs rough. Re-run the offset calibration (align the rotor with a known d-axis current and record the angle), and confirm the electrical-to-mechanical angle uses the correct pole-pair count. Check phase sequence between the motor wiring, the sensor direction and the firmware. Then check current sensing: offsets, gain mismatches between phases, and sampling alignment with PWM. Look at dead-time compensation and PI gains. Logging Id, Iq, angle and phase currents at high rate and plotting them almost always reveals the cause.

More free EV career resources: emobility.academy/career-resources