Fractional-Slot Surface Mounted PM Motors with Concentrated Windings for HEV Traction Drives

High-power density and efficiency resulting from elimination of rotor windings and reduced magnetic-flux losses have made the rare earth permanent magnet (PM) motor a leading candidate for the Department of Energy's Office of FreedomCAR and Vehicle Technologies (FCVTs) traction drive motor. These traction drives are generally powered by radial-gap motors, having the magnets on or embedded in a rotating cylinder separated from the inside surface of a slotted cylindrical stator by an annular gap. The two main types of radial-gap PM rotors are those with magnets mounted on the surface of a supporting back iron, called PM surface mounted (PMSM) motors, and those with magnets mounted in slots in the rotor, called interior PM (IPM) motors. Most early PM motor research was on the PMSM motor, which was thought to have an inherently low stator inductance. A low stator inductance can lead to currents dangerously exceeding rated current as the back-emf across the inductance increases with speed; consequently, part of the attempted solution has been to increase the stator inductance to reduce the rate of current rise. Although analysis suggested that there should be no problem designing sufficiently high stator inductance into PMSMs, attempts to do so were often not successful and a motor design was sought that would have a higher intrinsic inductance. Commercial research at Toyota has focused on IPM motors because they can achieve a high-saliency ratio, which helps them operate over a high constant power speed ratio (CPSR), but they are more difficult to fabricate. The Oak Ridge National Laboratory's (ORNL) position has been to continue research on brushless direct current (dc) motors (BDCMs) because of ease of fabrication and increased power output. Recently there has been a revival of interest in a fractional-slot PMSMs [15] made with concentrated windings because they possess three important features. First, they can increase the motor's inductance sufficiently to reduce the characteristic current to value of the rated current, which will enable them to operate at high CPSR. This feature also limits short-circuit fault currents. Second, their segmented structure simplifies assembly problems and is expected to reduce assembly costs. Third, the back-emf waveform is nearly sinusoidal with low cogging. To examine in depth this design ORNL entered into a collaborative agreement with the University of Wisconsin to build and test a 6 kW laboratory demonstration unit. Design, fabrication, and testing of the unit to 4000 rpm were completed during FY 2005. The motor will be sent to ORNL to explore ways to control its inverter to achieve higher efficiency during FY 2006. This paper first reviews the concept of characteristic current and what is meant by optimal flux weakening. It then discusses application of the fractional-slot concentrated winding technique to increase the d-axis inductance of a PMSM showing how this approach differs from an integral-slot motor with sinusoidal-distributed windings. This discussion is followed by a presentation of collaborative analyses and comparison with the University of Wisconsin's measured data on a 6 kW, 36-slot, 30-pole motor with concentrated windings. Finally ORNL presents a PMSM design with integral-slot windings that appears to meet the FreedomCAR Specifications, but has some disadvantages. Further collaboration with the University of Wisconsin is planned for FY 2006 to design a motor that meets FreedomCAR specifications