Lightweight Electric/Hybrid Vehicle Design

TOUQUE
(a) (b)
Fig. 1.8 Torque–speed curves for 45 kW vehicle (a) and each motor (b).
Current EV design approaches 11
1.2.9 ELECTRICAL SYSTEM DESIGN CHALLENGE
What are the design problems for the electrical system? The first one is cost. Unless the final
product is attractive to the consumer, we do not have a market. Where are we now? For 1000 off
systems at 45 kW, a brushless DC motor would cost £1000, a controller £2000, and a battery
£2000 (lead–acid). These 1992 prices will reduce with mass production. The second design
challenge is one of methodology. Electric vehicles have been traditionally built by placing motor
and batteries then spreading the electrical system over the vehicle. This needs to change. Polaron
would like to suggest a modular approach to the problem whereby sealed batteries and controller
power electronics are in one unit and the motor is in fact the second. The third design challenge is
one of compatibility. Low performance vehicles can be built with 110 V electrical systems. However,
as the power increases this is not practical. But both fuel cells and batteries are low voltage heavy
current devices – how can this conflict be addressed?
The solution is to use power conversion. In Fig. 1.9 a 100 stage fuel cell is integrated with a 216
V battery to give a stabilized 300 V DC rail. The motor and controller are then built at 300 V
where the currents are significantly reduced on the 100 V system. As the power level rises, voltages
up to 500 V DC can be anticipated. However, when the power conversion is switched off the
highest voltage will be the battery voltage. This additional power conversion will be needed for
another reason. If vehicles are equipped with small booster batteries for acceleration, the DC link
voltage will change significantly according to load conditions. The power conversion provides a
means of stabilizing for this variation.
1.2.10 MOTOR TYPES AND LOCATIONS (FIG. 1.10)
Which is the best type of motor? Answer – the cheapest. Which is the cheapest motor? Answer –
the lightest. Which is the lightest motor? Answer – the most efficient. On this criteria, there is no
doubt that a permanent magnet brushless DC motor would sweep the board. However, our
Fig. 1.9 Fuel-cell power conversion.
TOUQUE