stator winding to create a reverse magnetic force or by using electronic phase advance.^129 The
field-weakening requirement reduces efficiency of such motors at high rpm, although such a
solution is superior to using a two-speed transmission. One company, Unique Mobility, has
developed lightweight PMS motors that are up to 93 percent efficient (peak), including the
controller loss.^130 Unique Mobility also claims that its motors do not require a two-speed
transmission, unlike earlier PMS designs, and such claims are supported by the BMW El design.
The switched reluctance motor has been a subject of intense research, as it has the potential to
be very efficient and very cheap. Its design simplicity is an attraction, and it has the capability to
operate with reduced power even if one winding fails. New designs are said to reach efficiency
levels comparable to those of PMS motors.^131 The motors are still under development, however;
current designs are still fairly bulky, and there is some lingering controversy about whether torque
pulsation problems have been solved. Most industry experts contacted by OTA do not believe
switched reluctance motors can be commercialized before 2005, and some question whether
they will ever be commercialized.
Table 3-13 provides an auto manufacturer’s subjective rating of the near-term candidates for
EV propulsion motors, using 27 criteria.^132 If all criteria are equally weighted, then the AC
induction motor appears to be the choice with the best characteristics overall. PMS motors
may be the choice, however, if efficiency, size, and weight are regarded as more important
than low cost, simplicity, and durability. These conclusions do not appear to be controversial
with most of the EV supplier community.
There appears to be a widespread misconception that electric motor efficiency is always high,
over 90 percent. Indeed, both the AC induction motor and PMS motor have displayed peak
efficiency of over 90 percent--at times, as high as 96 percent.^133 However, efficiency is a function
of load and speed, and peak efficiency is attained only at midspeed, high-load conditions. At low
speed and low load, efficiency falls to 80 percent or less. Hence, a powerful motor used in an EV
to provide high peak performance will operate at city speeds in the low efficiency part of its
operating envelope. Even low-powered EVs--which should be comparatively efficient in low-
speed travel--have reported motor average efficiencies over the city cycle in the range of 65
to 75 percent.^134Controller efficiencies have also improved but suffer at high current conditions typical of low-
speed, high-load operation--a condition frequently imposed on urban EVs At high voltages (over
200 V), most controllers use the efficient IGBT-type power-switching transistors, although
MOSFET-type transistors can be adequate at lower voltages. Controllers generally have an
efficiency of 94 to 95 percent (nominal), but their efficiencies are lower at high-current
conditions. It is now typical to plot the efficiency of the motor and controller together, and an
(^129) J. Lutz and C. Cambier, "Phase Advanced Operation of a PMS Motor Drive System,” paper presented at the 12th International EV
Symposium, December 1994. (^130) S. Ericksson, "Drive Systems with PMS Motors," Automotive Engineering, February 1995.
(^131) IEEE Transactions on Power Electronic, January 1995.
(^132) Daimler-Benz, presentation to OTA May 1994.
(^133) Ericksson, see footnote 130.
(^134) Data provided by Volkswagen and BMW to OTA.