UC Davis Institute of Transportation Studies

Propulsion systems designed specifically for electric vehicles (EVs) are currently produced in small volumes and sold at high costs, although some components used in hybrid EVs are beginning to see production in higher volumes. At present, there are two primary choices of motor technology for use in EV drivetrains. Most vehicles in pilot-scale production today use alternating current (AC) induction systems, but some vehicles, such as the Toyota RA V4, use systems based on brushless permanent magnet (BPM) motors. Both AC induction and BPM systems offer similar advantages over conventional direct current (DC) brush motors. These include lighter motor weights, higher efficiencies, and lower service requirements (the brushes in DC brush motors wear out and require replacement). In general, AC induction motors provide high efficiencies over a wide range of operation, while BPM motors provide higher peak efficiencies. BPM motors also tend to be lighter, but they use rare earth magnets that are somewhat costly at present. Both of these motor types require complicated control systems relative to DC brush motors, in order to operate from a DC source. We analyze both AC induction and BPM systems because both are good choices for use in EVs, and it is not clear which system will prove to be the most popular. The control systems needed for these types of motors are costly and complex, but the necessary electronics, particularly insulated-gate bipolar transistor (IGBT) power switching devices, have been improving rapidly. Continued progress in IGBT technology is expected, particularly with regard to the saturation characteristics of the devices and their switching energies, and inverters in general are expected to progress in terms of not only the cost and performance of the IGBT silicon chips, but also in packaging, controls, processors, and transducers (Hodkinson, 1997). Recent statements by EV project managers at GM and Ford reflect the progress that has been made in reducing the cost and complexity of EV motor controllers over the past few years. Bob Purcell of GM reports that the second generation EV-1 motor controller has only three IGBTs, while the first generation had six. The new IGBTs have twice the power handling capability of the old ones, with equal precision levels. Overall, the new electric drive control system has half the mass, one-third fewer parts, and half the cost of the first generation system (Purcell, 1998). John Wallace of Ford reports similar progress in the development of its system (Wallace, 1998.