Development of Rotor for Permanent Magnet Assisted Synchronous Reluctance Motor

Abstract

The main disadvantage of synchronous reluctance motor (SynRM) is its poor power factor. To improve the power factor a small amount of permanent magnet material can be added to the rotor of SynRM that is then called permanent magnet assisted synchro-nous reluctance motor (PMaSynRM). The objectives of this thesis were: how the addi-tion of magnets affects the performance and the temperature rise of SynRM, whether cheaper ferrite magnets can be used instead of NdFeB magnets, and whether the place-ment of the magnets in the rotor structure matters.

To study these research questions, three types of motors were designed: SynRM, PMaSynRM with NdFeB magnets, and PMaSynRM with ferrite magnets. Each design had the same rotor lamination structure and the same stator of an ordinary induction motor. Thus, the only difference in the rotor designs was the number and the material of magnets. The investigated operating points were 5 kW/350 V, 11 kW/425 V, and 15 kW/450V, and the rotational speed was 1500 rpm.

The simulations showed that the placement of the magnets within the rotor structure affects its performance, especially in the case of NdFeB magnets. The optimal number and placement of the magnets was decided based on these simulations. In total six pro-totypes were built and tested: one without magnets, two with ferrite magnets and three with NdFeB magnets.

Both the simulation and the measurement results showed that the addition of magnets can improve the power factor of SynRM. Also, minor improvement was seen in effi-ciency. In addition, temperature rise in the windings decreased considerably mainly due to the smaller current.

As expected, NdFeB magnets in the rotor improved the performance of the motor more than ferrite magnets. Thus the choice of magnets is a matter of cost. However, simula-tions showed that ferrite magnets at least in these designs are in dire risk of demagneti-zation in a fault situation.