IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 5, SEPTEMBER/OCTOBER 2006 1185 Application of Direct-Drive Wheel Motor for Fuel Cell Electric and Hybrid Electric Vehicle Propulsion System Khwaja M. Rahman, Member, IEEE, Nitin R. Patel, Member, IEEE, Terence G. Ward, James M. Nagashima, Member, IEEE, Federico Caricchi, Member, IEEE, and Fabio Crescimbini, Member, IEEE Abstract—This paper presents a gearless wheel motor drive system speciﬁcally designed for fuel cell electric and hybrid elec- tric vehicle propulsion application. The system includes a liquid- cooled axial ﬂux permanent-magnet machine designed to meet the direct-drive requirements. The machine design implements techniques to increase the machine inductance in order to improve machine constant power range and high-speed efﬁciency. The im- plemented technique reduces machine spin loss to further improve efﬁciency. The machine design also optimizes the placement of magnets in the rotor to reduce cogging and ripple torque. An original cooling system arrangement based on the use of high ther- mal conductivity epoxy joining machine stator and liquid-cooled aluminum casing allows the very effective removal of machine power loss. Design details and experimental results are presented. Index Terms—Axial ﬂux, component, direct drive, wheel motor. I. I NTRODUCTION T HE ELECTRIC traction system is an important com- ponent of fuel cell electric and hybrid electric vehicle propulsion systems. Generally, in most propulsion applications, an ac machine is connected to the wheels by reduction gears and mechanical differential. In some vehicle drive arrangements, high-speed low-torque wheel motors requiring gear reduction are used, and in these cases, either a gear motor assembly is mounted inside the wheel or a chassis-mounted motor is con- nected to the wheel through gear reduction. Further simpliﬁca- tion of the vehicle drive arrangement results in the elimination of the gear being interposed between motor and wheel, and this calls for the use of wheel motors being designed for high- Paper IPCSD-06-029, presented at the 2004 Industry Applications Society Annual Meeting, Seattle, WA, October 3–7, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Elec- tric Machines Committee of the IEEE Industry Applications Society. Man- uscript submitted for review November 1, 2004 and released for publication June 1, 2006. K. M. Rahman is with General Motors Hybrid Power Train Engineering, Troy, MI 48083 USA (e-mail: khwaja.rahman@gm.com). N. R. Patel, T. G. Ward, and J. M. Nagashima are with the Advanced Tech- nology Center, General Motors, Torrance, CA 90505 USA (e-mail: nitinkumar. patel@gm.com; james.nagashima@gm.com). F. Caricchi is with the Department of Electrical Engineering, Univer- sity of Rome “La Sapienza,” 00184 Rome, Italy (e-mail: Fred@elettrica. ing.uniroma1.it). F. Crescimbini is with the Department of Mechanical and Industrial Engi- neering, University of Rome “Tre,” 00146 Rome, Italy (e-mail: crescimbini@ ieee.org). Color versions of Figs. 1, 3, 4, 7, 8, and 10–13 are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TIA.2006.880886 torque low-speed application. In this regard, this paper presents a gearless wheel motor drive system. Gearless wheel motor drive systems for fuel cell electric or hybrid vehicles have advantages over the classical construction with one central machine. Mounting the motors directly to the wheels simpliﬁes the mechanical layout. The gearless wheel motor drive system will reduce the drive line components, thus improving the overall reliability and efﬁciency. This option will also reduce the drive line weight since mechanical differential and gear reduction are not used. However, due to the elimina- tion of gears, the machine needs to produce the total torque directly into the wheel shaft. Hence, the size and the weight of the machine tend to grow. Therefore, the major challenge in a gearless system is to keep the size and the weight of the machine low for direct in-wheel mounting. Unsprung mass should be carefully evaluated when designing wheel motors. There should be a tradeoff between the size, power, and weight of the machine. Compared to conventional designs, the axial ﬂux permanent- magnet (AFPM) machine has favorable characteristics con- cerning efﬁciency and speciﬁc torque. For this reason, it is especially suited for direct-drive applications [1]–[5]. This pa- per presents a wheel direct-drive system with an axial ﬂux ma- chine targeted for fuel cell electric and hybrid electric vehicle propulsion systems. The axial ﬂux machine in this system is directly coupled with the wheel without gear reduction. The machine has been designed, built, and tested in the dyno and also in a mule vehicle. This paper presents the design of a gearless wheel motor drive system and the dyno test data to validate the drive system performance. II. REVIEW OF AFPM MACHINES Flux in an axial ﬂux machine ﬂows axial to the direction of rotation. The AFPM machine is basically equivalent to the conventional radial ﬂux surface-mount permanent-magnet (SMPM) machine geometry. Therefore, they suffer from the well-known low-inductance problem. However, this problem can be substantially mitigated as discussed in the following. High torque density and good efﬁciency of the AFPM machines have been veriﬁed theoretically [6]. It has been shown that an improvement of two times the density of an induction machine is possible even with ferrite magnets. The gain in density can exceed three times if high-energy magnets are used. The AFPM machine has favorable geometry for in-wheel mounting and 0093-9994/$20.00 © 2006 IEEE