What is a synchronous reluctance motor?
Working principle
A synchronous reluctance motor (SynRM) has a rotor made only of laminated steel: no magnets, no cage and no windings. Curved air slots called flux barriers make the rotor magnetically lopsided, so the inductance is high along the direct axis (Ld) and low along the quadrature axis (Lq). The stator field pulls the easy flux path, the d-axis, towards itself, and that pull is the torque: T = 1.5·p·(Ld − Lq)·id·iq. With Ld = Lq there would be no torque at all.
Saliency and power factor
The saliency ratio ξ = Ld/Lq measures how well the barriers work. A larger ξ gives more torque per ampere and a better power factor, which can never exceed (ξ − 1)/(ξ + 1). With ξ = 6 the ceiling is about 0.71, and real machines run at roughly 0.6 to 0.8. Without a rotor field the stator must also supply all the magnetizing current, which is why the power factor is lower than in a permanent-magnet motor.
Inverter control and MTPA
A SynRM cannot start or run from the mains; an inverter must create the rotating field and steer the current vector. The drive splits the current into id (flux) and iq (torque). With linear iron, torque per ampere is highest at a current angle of 45° from the d-axis, the maximum torque per ampere (MTPA) point; saturation moves it higher. Other strategies, such as maximum power factor, trade extra current for a better cos φ.
Speed, field weakening and applications
The rotor turns exactly at synchronous speed, 120·f/p, with no slip. Above base speed the inverter voltage is the limit, so the drive lowers id (field weakening) and raises iq to hold the torque, until the available torque falls. With no rotor losses and no magnets, SynRMs run cool, are cheap and are easy to recycle. They are used in pumps, fans and compressors as IE4 and IE5 alternatives to induction motors.
Frequently asked questions
- How can a motor make torque without magnets or windings on the rotor?
- Through reluctance. The flux barriers make the rotor easy to magnetize along the d-axis and hard along the q-axis. The stator field tries to line the easy axis up with itself, and the resulting pull is torque, proportional to (Ld − Lq)·id·iq. Set the saliency ratio to 1 in the simulator and the torque disappears.
- Why is the power factor of a SynRM low?
- The rotor brings no flux of its own, so the stator current must supply all of it, which makes the current mostly reactive. In a lossless machine the power factor cannot exceed (ξ − 1)/(ξ + 1), so only a high saliency ratio gives a good value. Change ξ and watch the limit in the chart.
- Can a synchronous reluctance motor run directly on the mains?
- No. It has no cage to start it and no field to hold synchronism, so it needs an inverter that ramps the frequency and sets the current angle. The motor then follows the commanded speed exactly. Raise the speed above 100 % and the simulator shows the drive weakening the field.