example is provided in figure 3-5.^135 These plots, however, are sometimes generated for a
constant input voltage, whereas the voltage from a battery declines with increasing current,
causing motor efficiency to decline from published values at high loads.Unlike IC engines that produce nearly constant torque over a wide operating rpm range,
electric motors are designed to operate at constant torque from zero rpm to the motor design
“base rpm” or “comer point,”followed by operation at nearly constant power with rpm (in other
words, torque declines as motor speed increases). Motors in EV applications are rated at peak
output, which can be sustained for two to three minutes before overheating, and continuous
output is usually restricted to 50 percent to 60 percent of peak output; these ratios are similar to
the maximum peak output to maximum continuous output ratio required for a light-duty vehicle.
The availability of high torque at low rpm allows a motor to match the characteristics of an IC
engine with higher maximum or rated output at city speeds. For example, Westinghouse claims
that its 100 HP electric motor provides better performance than a 125 HP V-6 engine up to 60
mph. At higher vehicle speeds, the motor’s lower HP translates to reduced performance. It should
be noted that an IC engine’s performance also depends on the transmission ratios which determine
the ratio of engine rpm to vehicle speed, so that the Westinghouse example is not necessarily
applicable to all vehicles.
Although there are millions of multiple-kilowatt electric motors in operation today, there
remains some disagreement about how much EV motor and controllers will cost. Current
industrial-grade variable speed motor systems in the 10 to 20 kW range cost about $200/kW--far
too expensive for EV use. However, motor manufacturers claim that these motors are a factor of
six heavier than advanced motors for EV use, although it is unclear whether motor costs are
driven primarily by material input costs. Discussions with motor manufacturers reveal that their
goal is to match the cost of a current IC engine of similar performance capability. Based on
confidential information provided by two motor manufacturers, the cost to the auto manufacturer
of an induction motor/controller manufactured at high volume (~100,000 units per year) will be:
Cost ($) = 300+30* Peak kWHence, the cost of a 60 kW system (80 HP peak) is about $2,1OO. This estimate is consistent
with the DOE research goal of a $2,000 powertrain for a 75 HP system. Manufacturers stated
that the motor itself costs about one-third of the total, or $700 in this example, and the controller
costs two-thirds, or $1,400. Motor manufacturers believe that this is a realistic cost goal,
although these costs are almost an order of magnitude lower than current variable-speed
drive motor costs. PMS motors are expected to cost 15 to 20 percent more than induction
motors of the same rating.
Others claim that even more substantial cost reductions are possible. For example, the DOE is
sponsoring research into high frequency induction motors; preliminary estimates of motor plus
controller costs are $600 to $700 for a 60 kW system.^136 Motor manufacturers do not believe
these claims, as they feel there are problems with high-frequency motor drives that are not easily
135 Motor efficiency data provided by Ford.(^136) W.L. Siegel, ‘Electric and Hybrid Propulsion Systems Development” paper presented at the Automotive Technology Development
Contractors Coordination Meeting, U.S. Department of Energy, October 1994.