Issues with the design of brushless doubly-fed reluctance machines: Unbalanced magnetic pull, skew and iron losses

In this paper some fundamental design issues concerning the brushless doubly-fed reluctance machine are addressed. The literature has several examples of electrical machines with different pole number. It has become apparent that a radially-laminated ducted rotor is a better option. In this paper it is illustrated that the pole number combination should not have pole pair number varying by one, otherwise unbalanced magnetic pull occurs. In addition, it is shown that to reduce voltage ripple due to the interaction of the slotting between the stator slots and rotor ducts, two axial rotor sections can be used with a small degree of skew between them. The example machine design is a 2 MW design as used previously, this has 48 stator slots. The machine results are couched in terms of a 4/8 pole and a 4/6 pole combination (for UMP calculation). Finite element analysis and analytical algorithms are used in the paper.