Analysis and Design Remarks of Variable Flux Reluctance Motors

The Variable Flux Reluctance Motor (VFRM) is a double-salience synchronous machine which includes two windings in the stator and no winding or permanent magnet in the rotor. One of the two stator winding is a three-phase AC winding, formed by non-overlapped coils, referred to as armature winding. The other is supplied by a DC current with the task of creating the main flux, thus it is referred to as field winding (or excitaton winding). The flux produced by such a winding is modulated by the salient rotor so that an electro-motive force is induced in the armature winding. There is an increasing interest on such a VFRM, due to some its peculiar characteristics. The absence of permanent magnets makes this motor quite cheap. The rotor structure is robust, so that it is suitable for high speeds. In addition, it is possible to vary the induced voltage on the armature by means of the field winding, but this does not require any slip ring and brushes since the winding is located on the stator. However, the stator slots share two windings, so that the current densities are quite high. Then, there is flux variation in the rotor iron, with consequent rotor iron losses. Finally, the power factor is relatively low if the machine is not properly designed. This paper aims to highlight the advantages and drawbacks of the VFRM and gives some suggestion for a proper design.