Advanced Automotive Technology: Visions of a Super-Efficient Family Car

special types such as the switched reluctance motor. The major advantages of the series-wound or

separately excited DC motors are that they are easy to control, which makes the control system

relatively inexpensive, and that they are technologically mature. For high power applications,

however, they are large, heavy, inefficient, and require maintenance. Consequently, they are

considered unsuitable for modem EV’s. Switched reluctance motors are still in the research stage,

and are discussed later in this section.

AC motors can be classified as asynchronous (or induction type) or as synchronous. The

asynchronous induction motor is the workhorse of industry in constant speed applications, and

has also emerged a prime contender for EVs, as it requires almost no maintenance and can be

manufactured relatively cheaply, although the variable speed electronic controls required for a

vehicle application are expensive. In an EV application, the controller transforms the DC from the

battery to AC (with a frequency from O to 400 Hz^125 ). Pulse width modulation schemes use

chopping frequencies typically in the range of 10 to 20 kHz. The system works well but requires

high current owing to the relatively low-power factors (which are proportional to the phase angle

between voltage and current waveforms). Asynchronous induction motors designed by

Westinghouse for EVs have shown high efficiency, and peak motor plus controller efficiencies of

91 percent to 92 percent have been achieved.^126

As induction motor size is reduced, “ripple” currents create higher losses, and one way to

circumvent this problem is by operating with higher chopping frequency. DOE is sponsoring

research into induction motors^127 that are half the size of the current best motors used in EV

applications and use electronic controllers that operate at chopping frequencies of 80 kHz.

However, available high-power electronic controllers of the IGBT (Insulated Gate Bipolar

Transistor) type cannot operate at high frequency. Instead, MOSFET (Metal Oxide-Silicon Field

Effect)-type controllers can be used, though at lower efficiency, or else more expensive control

systems are required.

Synchronous motors can be further classified into the permanent magnet type and the

electrically excited type. The latter type is considered to be too expensive for EV use, and most

research has focused on the permanent magnet synchronous (PMS) motor. The use of these

magnets allows the creation of a magnetic field without attendant electrical losses, so that these

motors are very efficient at their best operating point. Recent breakthroughs in magnetic materials

have allowed the development of very powerful lightweight permanent magnets, such as those

made from samarium-cobalt alloys.^128

Torque in an electric motor is proportional to the magnetic flux times current. Because the
PMS motor has constant magnetic flux, it produces constant torque with increased rpm, and,
hence, requires higher voltages to increase rpm. To reduce voltage requirements at higher motor
speeds, flux must be reduced or else the motor rpm range is restricted. Many PMS motors used in
EVs utilized a two-speed transmission to restrict the range of operating rpm. New methods have
been developed to decrease the magnetic flux above certain rpm, however, either by designing the

125 A hertz, or Hz in abbreviated form, is a unit of frequency equal to one cycle per second.

126 Westinghouse, brochures on EV motor controllers.

127 Siegel, see footnote 113.

128 Scheurer and Goubeau, see footnote 77.