In-wheel PM motor: compromise between high power density and extended speed capability

Purpose ‐ Today's brushless permanent magnet (PM) drive systems usually adopt motors including the advancements in magnet technology, e.g. better thermal characteristics and higher magnetic strength. By this means, they become capable in the roughest applications yet maintain a high accuracy at competitive prices. These drive systems are not only appreciated for their high performance, but they are also advantageous for the applications requiring tough, dependable, and continuous-duty operations, e.g. hybrid or complete electrical vehicles, extruders, wire drawers, winders, cranes, conveyors, and roll formers. The purpose of this paper is to provide an extended comparative study of the different motor configurations for the hybrid electric drive application, aiming at a compromise between high power density and extended speed capability. Design/methodology/approach ‐ To suit strict design requirements, such as the very limited volumetric envelope, high-output power, wide constant power speed range, and pre-selected in-direct cooling system, the constraint variants of possible motor types are researched. Findings ‐ Considerably, high torque density and an extended speed range limit the options of PM rotor configurations for this motor design. The considered rotor configurations are the surface PM (SPM) and interior PM (IPM) types. The advantage of the (non-salient) SPM configuration is its applicability with higher levels of magnetic flux densities without causing significant saturation in the rotor. On the other hand, an IPM rotor, which places the magnets in special rotor slots, open or closed (by saturation bridges), can operate on both the reluctance torque and the magnet alignment torque. This generally leads to a better performance in a wide speed range. However, this advantage can be eliminated by severe iron saturation resulting from the required high-specific power. Originality/value ‐ The most appropriate rotor configuration will finally be selected between the two considered types, depending on detailed electromagnetic and thermal analysis. This paper usefully studies the correlation between the motor parameters required for high power density and field-weakening performance.