SynRM reluctance electric motor

The reluctance motor (SynRM and PMSynRM)

The EMCE reluctance motor of Energica (source Energica Motor Company)

Today, permanent magnet synchronous motors (PMSMs) are the dominant technologies for many applications, including traction. High efficiency, high torque density, and desirable wide speed range performance of these motors have made the technology popular among manufacturers. The most important elements of PMSMs are permanent magnet (PM) materials, which includes the rare-earth elements. The dramatic rise and fall of the price of the rare-earth magnets have directed the research towards rare-earth-free machines to replace high-performance PMSMs.
Synchronous reluctance (SynRM) motor is an answer of that research; it is characterized not only by its high energy efficiency but also by its reliability and low maintenance needs.

An example of a SynRM motor

What is a SynRM? SynRMs work on a very elegant principle that has been known for a long time, but only since the recent rise of sophisticated control systems has it been possible to exploit fully these super-efficient electrical machines.
In a SynRM, the rotor is designed to produce the smallest possible magnetic reluctance (the resistance to the flow of a magnetic field) in one direction and the highest reluctance in the direction perpendicular. The control system steers the stator magnetic field, so it “rotates” around the motor. This with the directionally unequal magnetic reluctance properties of the rotor generate a reluctance torque which rotate the rotor in the direction with minimum magnetic resistance at the same frequency of the stator magnetic field.
SynRM technology achieves the performance of the permanent magnet motor with better simplicity, as SynRMs do not feature rare-earth-based components like permanent magnets. The rotor has neither magnets nor windings and suffers virtually no power losses.

The principle of operation of a SynRM motor

SynRM technology also offers lower winding temperatures and lower bearing temperatures. These lower operating temperatures have multiple benefits, including longer insulation life and longer bearing lifetime.
Furthermore, the rotor of SynRM motors is potentially less expensive than PM motors because it doesn’t need high cost rare-earth-based components.
The average torque is due to the rotor anisotropy which is achieved by means of a high number of flux barriers (air gaps).
The so-called “ribs” in tangential and radial direction are usually adopted in the SynRel rotor geometry to lower the mechanical stress in the rotor iron and the maximum deformation at the air gap due to the centrifugal forces.

Permanent magnets can be inset in the flux barriers to assist the synchronous reluctance motor improving its capabilities but avoiding the use of rare earth permanent magnets. This type of motor is called PMSynRM. The main advantages of the permanent magnet assistance is an increase of the main torque density and of the power factor.
Besides, another special SynRM which has hybrid rotor is put forward for EV: rotor is manufactured combined by two parts in series which are SynRM topology and surface mounted permanent magnet.

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