Switched-reluctance motor (SRM)

Reference notes

The switched-reluctance motor (SRM) is the simplest mainstream rotating machine: salient stator + salient rotor, both made of laminated iron, with no rotor windings or magnets. Torque comes from pure reluctance — the stator field pulls rotor teeth into alignment with energized stator poles. Use Next → to walk through the construction, the T = ½·i²·dL/dθ torque equation, the asymmetric half-bridge converter, and why SRM is making a comeback as a rare-earth-free EV traction candidate.

SRM construction

• Stator: salient poles (typically 6, 8, 12) wound with concentrated coils. Each phase = a pair of stator poles 180° apart, wound in opposing polarity.
• Rotor: salient laminated steel teeth (typically 4, 6, 8, 10). NO windings. NO permanent magnets. NO squirrel-cage. Just iron.
• Common topology: 8/6 — 8 stator poles in 4 phases (A, B, C, D), 6 rotor teeth.

For 8/6: 360 / (6 · 4) = 15° per excitation step. Higher pole counts (12/10, 12/8) give finer steps and lower torque ripple at the cost of more switches in the drive.

Torque equation

From magnetic co-energy. Two essential features:

1. i² term — torque is independent of current direction. Flip the current, torque is unchanged. So SRM drives use unipolar current and don't need full H-bridges.
2. dL/dθ sign controls motoring vs braking. Inductance L(θ) is low when rotor teeth are unaligned, peaks at alignment, drops past alignment.
   • Approaching alignment: dL/dθ > 0 → motoring torque.
   • At alignment: dL/dθ = 0 → no torque.
   • Past alignment: dL/dθ < 0 → braking torque.

The drive must turn each phase OFF exactly at the aligned position and energize the NEXT phase to keep producing motoring torque. Precise rotor position feedback is mandatory — SRM cannot be open-loop.

Asymmetric half-bridge converter

Because torque depends on i², the drive doesn't need bidirectional current. But each phase needs INDEPENDENT control. Standard SRM drive: one asymmetric half-bridge per phase, with 2 switches + 2 freewheel diodes:

• Magnetize — both switches ON, phase current builds from V_DC.
• Freewheel — top switch OFF, bottom switch ON; current circulates via bottom switch + top diode (zero applied voltage).
• Demagnetize — both switches OFF; current decays via both diodes back into V_DC, applying −V_DC across the winding to force fast turn-off.

4-phase SRM = 8 switches total (vs 6 in a standard 3-phase inverter), but each phase is electrically isolated. Fault tolerance: lose one phase, the motor still runs with 3/4 of its capability — useful for safety-critical applications.

Drawbacks

• Torque ripple — torque produced one phase at a time. Without sophisticated current shaping, 20–40 % peak-to-peak ripple is typical (vs < 2 % for a properly-driven PMSM with FOC). Mitigation: torque-sharing functions in DSP that ramp one phase off while ramping the next on; phase-current overlap during commutation.
• Acoustic noise — radial magnetic force on energized stator pole tips deforms stator laminations at the switching frequency, producing audible noise. The stator acts like a speaker driver. Mitigation: skewed poles, damping mounts, soft switching. Modern designs are quieter but still perceptibly louder than induction or PMSM equivalents.
• Average efficiency — 5–7 % below PMSM at full load. Better at part load than induction motors due to no rotor I²R.

Why SRM is making a comeback

• No rare-earth magnets — uses only standard silicon-steel laminations and copper. Avoids the NdFeB cost volatility and Chinese supply-chain concentration that affects PMSM economics.
• High-temperature operation — no rotor winding insulation, no magnets to demagnetize. Continuous rotor operation at 200 °C is straightforward. Attractive for electric aircraft and hot industrial environments.
• Fault tolerance — independent per-phase electronics; loss of one phase reduces capability but the motor keeps running.
• Maturing control — modern DSP-based torque-sharing functions and current profiling have tamed the traditional torque-ripple problem. Several EV OEMs are now evaluating SRM for next-generation drives.

Comparison with neighboring machines

Applications

• Aerospace — engine starter-generators in jet aircraft, where a single machine handles starting torque AND continuous generation, in a hot environment.
• Domestic appliances — washing machine drum drives (Maytag Neptune), vacuum cleaner motors.
• Servers / HVAC fans — high-speed cooling fans where simplicity and reliability matter.
• EV traction (emerging) — Land Rover, several Chinese OEMs, multiple research consortia are evaluating SRM as PMSM alternatives.

Keyboard shortcuts

• The cross-section panel cycles through phase A → B → C → D as the rotor turns; the L(θ) panel shows the motoring region (dL/dθ > 0) shaded orange and braking region shaded green.