Modular 3-phase permanent magnet brushless machines for in-wheel applications

The paper describes the merits of a recently developed form of 3-phase permanent magnet brushless ac machine in which the concentrated coils of each stator phase are either wound on adjacent teeth or on alternate teeth. Such a machine is often referred to as modular, and offers a number of significant advantages over conventional permanent magnet brushless machines. For example, it results in a smaller number of slots for a given number of poles, which is a distinct manufacturing advantage, and yields a fractional number of slots per pole, which is conducive to a low cogging torque. It also enables a significant increase in the machine inductance to be achieved to facilitate constant power operation over a wide speed range by flux-weakening. However, the torque in modular machines is developed by the interaction of a high-order stator space harmonic mmf with the permanent magnets, since the fundamental stator mmf has fewer poles than the permanent magnet rotor. Hence, significant eddy currents may be induced in the rotor by the fundamental and low-order space harmonic mmfs. The eddy current loss can, however, be reduced by segmenting the magnets. Given that modular machines combine the high specific power and efficiency of conventional permanent magnet brushless machines with a high machine inductance, to enable wide speed range, constant power operation, their potential for low cost manufacture, and the fact that they have an inherently low cogging torque, they are eminently suitable for in-wheel traction applications.