EV traction motors: BLDC, PMSM and induction

How BLDC, PMSM (surface and interior) and induction traction motors work and compare, the constant-torque and constant-power regions of the torque–speed curve, with BLDC, induction-motor power-flow, three-phase power and field-weakening numericals.

Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.

Why it matters

The traction motor decides how an EV accelerates, how far it goes on a charge and how much it costs. Indian e-scooters mostly use BLDC hub or mid-drive motors, most electric cars use permanent-magnet synchronous motors, and induction motors appear where magnet cost or high-speed efficiency matter. Choosing between them, and reading a motor's torque–speed curve, are core skills for EV drivetrain work.

Key ideas

What a traction motor must do. Give high torque from standstill (launch, gradients), keep roughly constant power over a wide speed range (overtaking, top speed), stay efficient over the drive cycle (mostly part load), be light and compact, quiet, robust and cheap.

Torque–speed envelope. From zero to base speed the motor can give its maximum torque, limited by current (and so by heating and inverter rating). Above base speed the back-emf reaches the voltage the inverter can supply, so the controller weakens the field and torque falls roughly as 1/ω — the constant-power region. Peak ratings (seconds to tens of seconds) are well above continuous ratings, which are set by cooling.

BLDC (brushless DC) motor.

  • Permanent magnets on the rotor, three-phase windings on the stator; the brushes and commutator of a DC motor are replaced by an electronic inverter (electronic commutation).
  • Designed for a trapezoidal back-emf and driven with six-step (120°) commutation: at any time two phases conduct, switched according to three Hall-effect sensors.
  • Simple, cheap controller; behaves like a DC motor (torque ∝ current, speed ∝ voltage). Drawbacks: torque ripple at each commutation step and audible noise.
  • Widely used in hub and mid-drive motors of Indian two- and three-wheelers.

PMSM (permanent-magnet synchronous motor).

  • Also PM rotor and three-phase stator, but designed for a sinusoidal back-emf and fed with sinusoidal currents by field-oriented control (FOC), using a resolver or encoder for rotor angle. The rotor turns exactly at synchronous speed — the controller sets the stator frequency from the measured rotor speed, so it cannot "slip".
  • Surface-mounted (SPM): magnets on the rotor surface, torque from magnet flux only. Interior (IPM): magnets buried in the rotor; extra reluctance torque and much better field weakening, so most EV cars use IPM motors.
  • Highest efficiency (often above 95 % peak) and power density. Drawbacks: rare-earth (NdFeB) magnet cost and supply risk, demagnetisation risk at high temperature, and uncontrolled generator voltage if the inverter fails at high speed.

Induction motor (asynchronous).

  • Squirrel-cage rotor of aluminium or copper bars; the rotating stator field induces rotor currents, which requires the rotor to run slightly slower than the field — slip.
  • No magnets: robust, cheaper materials, tolerant of high temperature and speed, and naturally easy to field-weaken; with no magnet drag it has low losses when freewheeling, which is why some dual-motor EVs use it on the secondary axle.
  • Drawbacks: rotor copper losses (proportional to slip) heat the rotor, which is hard to cool; lower efficiency at part load; needs magnetising current, so a lower power factor.

Other types. Switched reluctance (simple rotor, high torque ripple and noise), synchronous reluctance, and wound-field synchronous motors (rotor field from a winding instead of magnets) — all magnet-free options.

Control link. All of these are fed by a three-phase inverter (next topic), which sets frequency (speed) and current (torque), and runs the motor as a generator for regenerative braking.

Formulas

n_s = 120 f / p

  • n_s: synchronous speed (rpm), f: supply (electrical) frequency (Hz), p: number of poles.

s = (n_s − n_r) / n_s

  • s: slip of an induction motor (dimensionless), n_r: rotor speed (rpm).

P_cu,r = s × P_ag, P_mech = (1 − s) × P_ag, T = P_ag / ω_s

  • Induction motor: P_ag: air-gap power (W), P_cu,r: rotor copper loss (W), P_mech: gross mechanical power (W), ω_s: synchronous speed (rad/s).

E = K_e × ω, T = K_t × I, V = E + I × R

  • BLDC (DC-equivalent model): E: back-emf (V), K_e (V·s/rad) and K_t (N·m/A) are numerically equal in SI units, I: DC-link current (A), R: winding resistance seen by the drive (Ω).

T = 1.5 × p_p × [ψ_m i_q + (L_d − L_q) i_d i_q]

  • PMSM torque in dq frame (amplitude-invariant form): p_p: pole pairs, ψ_m: magnet flux linkage (Wb), i_d, i_q: d- and q-axis currents (A), L_d, L_q: inductances (H). The second term is reluctance torque (zero for SPM, where L_d = L_q).

P_in = √3 × V_L × I_L × cos φ, P_out = η × P_in, T = P_out / ω

  • Three-phase input power (W) from line voltage V_L (V), line current I_L (A) and power factor cos φ.

T = P_max / ω (above base speed, constant-power region)

Worked examples

Example 1 (standard). A 48 V BLDC hub motor has K_e = K_t = 0.9 (SI units) and R = 0.15 Ω. At 400 rpm it draws 60 A. Find torque, back-emf, required voltage, mechanical power, electrical input and efficiency (copper loss only).

  1. ω = 400 × 2π / 60 = 41.89 rad/s.
  2. T = K_t I = 0.9 × 60 = 54 N·m.
  3. E = K_e ω = 0.9 × 41.89 = 37.70 V.
  4. V = E + I R = 37.70 + 60 × 0.15 = 46.70 V (≤ 48 V, so achievable).
  5. P_mech = E I = 37.70 × 60 = 2262 W (check: T ω = 54 × 41.89 = 2262 W).
  6. P_in = V I = 46.70 × 60 = 2802 W; η = 2262 / 2802 = 0.807.

Answer: 54 N·m, 37.7 V, 46.7 V, 2.26 kW, 2.80 kW, about 81 %.

Example 2 (GATE level). (a) A 4-pole induction traction motor is fed at 150 Hz and runs at 4410 rpm with 50 kW crossing the air gap. Find slip, rotor copper loss, mechanical power and torque. (b) A traction motor has 100 kW peak power and a base speed of 4000 rpm. Find the peak torque and the torque available at 10,000 rpm.

  1. n_s = 120 × 150 / 4 = 4500 rpm; s = (4500 − 4410) / 4500 = 0.02.
  2. P_cu,r = 0.02 × 50 = 1.0 kW; P_mech = 0.98 × 50 = 49.0 kW.
  3. ω_s = 4500 × 2π / 60 = 471.2 rad/s; T = 50,000 / 471.2 = 106.1 N·m (check: 49,000 / 461.8 = 106.1 N·m).
  4. T_peak = 100,000 / (4000 × 2π / 60) = 100,000 / 418.9 = 238.7 N·m.
  5. T_10000 = 100,000 / (10,000 × 2π / 60) = 100,000 / 1047.2 = 95.5 N·m.

Answer: s = 0.02, 1.0 kW, 49.0 kW, 106.1 N·m; 238.7 N·m and 95.5 N·m.

Common mistakes

  • Using p as pole pairs in 120 f / p (it is the number of poles) or as poles in the dq torque formula (there it is pole pairs).
  • Saying a PMSM "slips" or "loses synchronism" in normal EV use. With rotor-position feedback the inverter always keeps it synchronous; loss of synchronism is a problem of open-loop, line-fed machines.
  • Treating BLDC and PMSM as identical. Both are brushless PM machines, but BLDC uses trapezoidal back-emf and six-step drive; PMSM uses sinusoidal back-emf and FOC.
  • Computing induction-motor torque from mechanical power with synchronous speed (use P_ag with ω_s, or P_mech with ω_r).
  • Forgetting √3 and the power factor in three-phase power.
  • Assuming the peak torque is available at all speeds; above base speed torque falls as P/ω.

For GATE ME

Expect questions using P = Tω, the synchronous-speed and slip relations, induction-motor power flow, three-phase power with power factor and efficiency, and motor torque–speed envelopes combined with vehicle road load and gear ratio. Practise rpm ↔ rad/s conversions and power-flow bookkeeping.

Quick check

  1. What is the synchronous speed of an 8-pole motor at 400 Hz?
  2. Why do most EV cars use interior PM motors?
  3. An induction motor's slip is 3 % with 40 kW air-gap power. What is the rotor copper loss?
  4. What sensors does a six-step BLDC drive commonly use?
  5. Why might an EV use an induction motor on its secondary axle?

Answers: 1. 120 × 400 / 8 = 6000 rpm. 2. High efficiency and power density plus reluctance torque and good field weakening for a wide constant-power range. 3. 1.2 kW. 4. Three Hall-effect sensors. 5. With no magnets it has very low drag losses when unpowered, and it is magnet-free and robust.

Interview questions

All Automotive Electronics and Electric Vehicles interview questions

Try answering each one aloud before you open it.

  1. 1.What is a BLDC motor and how does it differ from a brushed DC motor?Concept

    A BLDC (Brushless DC) motor is a type of electric motor that uses electronic commutation instead of mechanical brushes and a commutator. Unlike brushed DC motors, BLDC motors have a longer lifespan, higher efficiency, and require less maintenance due to the absence of brushes. The rotor in a BLDC motor is typically a permanent magnet, while the stator contains the windings.

  2. 2.Explain the working principle of a Permanent Magnet Synchronous Motor (PMSM).Concept

    A Permanent Magnet Synchronous Motor (PMSM) operates on the principle of synchronous speed, where the rotor rotates at the same speed as the magnetic field produced by the stator. The rotor contains permanent magnets, and the stator has windings that are energized to create a rotating magnetic field. This interaction between the magnetic fields of the rotor and stator produces torque, causing the rotor to turn.

  3. 3.What are the main advantages of using induction motors in electric vehicles?Concept

    Induction motors are favored in electric vehicles for their robustness, simplicity, and cost-effectiveness. They do not require permanent magnets, which can be expensive and subject to supply constraints. Induction motors also have a wide speed range and can handle high torque, making them suitable for various driving conditions. Additionally, they are known for their durability and low maintenance requirements.

  4. 4.Why are BLDC motors common in electric two- and three-wheelers?Application

    A BLDC motor has a permanent-magnet rotor and electronic commutation, so it has no brushes to wear and gives high efficiency and torque density, which suits hub or mid-drive motors at 48–72 V. Its six-step drive with three Hall sensors needs only a simple, cheap controller, and torque is roughly proportional to current, as in a DC motor. The trade-off is torque ripple and noise at each commutation step, which is why cars generally use sinusoidally driven PMSMs with field-oriented control instead.

  5. 5.What happens if the rotor speed of a PMSM does not match the synchronous speed?Application

    A PMSM produces steady torque only when the stator field rotates exactly at rotor speed. In an EV this is guaranteed by the controller: it reads rotor angle from a resolver or encoder and sets the stator current frequency and angle from it, so the motor cannot slip as an induction motor does. If a line-fed or open-loop PMSM is overloaded beyond its pull-out torque, or the position signal is wrong, the rotor falls out of step and the average torque collapses into pulsating torque with heavy current, vibration and heating. That is why position-sensor faults are treated as serious faults that shut the drive down.

  6. 6.How does the efficiency of an induction motor compare with that of a permanent-magnet (BLDC/PMSM) motor in EVs?Application

    Permanent-magnet motors are usually more efficient, especially at low speed and part load, because the magnets provide the field without magnetising current and there are no rotor copper losses; peak efficiencies above 95 % are common. An induction motor loses power in its rotor in proportion to slip and needs magnetising current, so its part-load efficiency is lower. At high speed and light load, however, a PM motor must spend current on field weakening and suffers magnet-induced iron losses even when coasting, while an induction motor can simply reduce its field. This is why some dual-motor EVs pair a PM motor with an induction motor.

  7. 7.Calculate the synchronous speed of a 4-pole PMSM connected to a 50 Hz supply.Numerical

    The synchronous speed (Ns) of a PMSM can be calculated using the formula: Ns = (120 × f) / P, where f is the frequency in Hz and P is the number of poles. For a 4-pole motor connected to a 50 Hz supply, Ns = (120 × 50) / 4 = 1500 RPM.

  8. 8.A BLDC motor has a back EMF constant of 0.1 V/rad/s. What is the back EMF generated at 3000 RPM?Numerical

    First, convert RPM to rad/s: 3000 RPM × (2π / 60) = 314.16 rad/s. The back EMF (E) is calculated using the formula: E = K × ω, where K is the back EMF constant and ω is the angular velocity in rad/s. Therefore, E = 0.1 V/rad/s × 314.16 rad/s = 31.416 V.

  9. 9.Explain why PMSMs are preferred for high-performance applications in electric vehicles.Application

    PMSMs, especially interior-magnet types, give the highest torque and power density and peak efficiencies above 95 %, so the motor is small and range is maximised. With field-oriented control they produce smooth torque with low ripple. Buried magnets add reluctance torque and allow strong field weakening, giving full torque up to base speed and a wide constant-power range above it. The drawbacks are rare-earth magnet cost, supply risk and the risk of demagnetisation at high temperature.

  10. 10.What are the challenges associated with using induction motors in electric vehicles?Application

    Induction motors have rotor copper losses proportional to slip, and heat generated in the rotor is hard to remove, which limits continuous rating. They need magnetising current, so power factor and part-load efficiency are lower than for PM motors, and for the same output they are usually somewhat larger and heavier. Field-oriented control needs an accurate rotor-flux estimate that depends on rotor resistance, which changes with temperature. Against this, they need no magnets, tolerate high temperature and speed, and are robust and cheaper in materials.

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