Synchronous reluctance motor (SynRM)
Salient steel rotor + sinusoidal 3-φ supply. Pure reluctance torque T = (3/2)(P/2)(L_d − L_q)·i_d·i_q. NO magnets, NO windings on rotor. FOC same as PMSM. Modern rare-earth-free PMSM alternative (ABB, Siemens, Bosch).
Step 1 — Synchronous reluctance motor: salient rotor + sinusoidal 3-φ
Reference notes
The synchronous reluctance motor (SynRM) has a salient steel rotor driven by sinusoidal 3-φ supply — no windings, no magnets, no cage. Torque comes purely from reluctance. Use Next → to walk through construction, the dq-frame torque equation, comparison with PMSM / induction / SRM, FOC control, and the rare-earth-free comeback narrative in 2020s industrial drives.
What it is
- 3-φ stator with sinusoidal supply (same as standard 3-φ motors).
- Salient laminated steel rotor — no windings, no magnets, no cage.
- Torque from reluctance: rotor's high-permeance (d) axis aligns with rotating stator field.
- Synchronous operation: rotor runs at exactly synchronous speed n_s = 120·f/P.
Max torque at δ = 45 electrical degrees; rotor pulls out beyond this.
Rotor construction
- Transverse-laminated with flux barriers — standard radial laminations punched with arc-shaped slots along the q-axis. Slots = high reluctance in q. Saliency ratio L_d/L_q = 3–7. Easy to manufacture, dominant in industrial SynRM products.
- Axial-laminated — thin steel laminations stacked with non-magnetic insulating layers, magnetic axis perpendicular to the laminations on one axis and parallel on the other. Saliency 5–10, highest performance, complex manufacturing.
- PM-assisted SynRM — adds modest ferrite magnets to the flux barriers. Boosts PF and torque density without rare-earth content. Examples: ABB SYRA.
Comparison with neighbors
| Motor | Rotor | Magnets | η | PF |
|---|---|---|---|---|
| Induction (squirrel cage) | Cage | No | 85–92 % | 0.85–0.90 |
| PMSM (IPM) | Buried NdFeB | YES (rare-earth) | 92–96 % | 0.90–0.95 |
| SynRM | Salient steel with flux barriers | No | 90–95 % | 0.60–0.80 |
| SRM | Salient steel | No | 85–92 % | n/a (switched DC) |
Control: same FOC architecture as PMSM
- 3-φ inverter with SVPWM, current sensors, rotor position encoder or resolver.
- Software: Clarke + Park transforms to dq-frame, PI on i_d and i_q, outer-loop speed / torque PI.
- Critical difference from PMSM: i_d is NEVER zero for SynRM. Torque equation has i_d·i_q product — both components must be non-zero.
- MTPA strategy: i_d = i_q at 45 electrical degrees (max torque per amp).
- Above base speed, field weakening adjusts the operating angle to extend speed range.
- Sensorless variants use saliency tracking (high-frequency injection) — the salient rotor naturally provides position signals.
Power factor — the SynRM trade-off
SynRM PF is fundamentally limited by saliency:
For typical saliency L_d/L_q = 5: PF_max ≈ 0.67. For 8: PF_max ≈ 0.78. Below PMSM's 0.95. Practical operation: 0.6–0.8 PF. Implication: inverter sized ~20–30 % larger than PMSM for same shaft power. PM-assisted SynRM (ferrite magnets in flux barriers) raises PF without rare-earth cost.
Modern industrial adoption
- ABB SynRM — IE5 efficiency-class industrial motors, 1–500 kW, with matching ABB drives.
- Siemens SIMOTICS SynRM — targeted at HVAC fans, pumps, conveyors.
- Bosch Rexroth ServoX — SynRM servo motors.
- ABB SYRA — synchronous reluctance + ferrite-assisted, hybrid for higher PF.
Market share of SynRM in new industrial drives grew from < 1 % (2015) to 5–8 % (2024), driven primarily by rare-earth supply-chain concerns and IE5 efficiency targets. Forecasts project continued growth into 2030.
Decision matrix
- PMSM — when highest efficiency / power density matters and rare-earth supply is acceptable.
- SynRM — when rare-earth-free is required and the larger inverter for lower PF is acceptable. Similar efficiency to PMSM.
- Induction — when cheap, robust, mature solution suffices.
- SRM — for high-temperature, fault-tolerant, or specialty applications. Accepts torque ripple and acoustic noise.