Prospects for EV package design

Electric traction was viable even before 1910 when Harrods introduced their still familiar delivery truck, with nickel–iron battery, which is still in daily use and speaks volumes for the longevity and reliability of the electric vehicle. But very important structural changes have taken place since. At the turn of the Twentieth century EVs did not have solid-state controls and sophisticated control was achieved using primitive contactor technology. Amazingly successful results were obtained with contactor-changing and field-weakening resistor solutions, as is seen in the Mercedes vehicle described at the end of the Introduction.

But the ‘writing on the wall’ for the first generation of electric cars appeared in the World War I period with the development of electric starters for thermal engines. This was followed by unprecedented improvement, by development, of the piston engine and the success of the Ford Model T generation vehicles in the 1920s which substantially outperformed early electric cars.

From then on until 1960 when high-power solid-state switching devices were developed and EVs were basically used for delivery and other secondary applications. Between 1960–1980, a new generation of EVs was developed, of which mechanical-handling trucks and golf-carts were the

most notable. These were based on Brushed DC motors with lead–acid batteries and some millions of golf-carts are in use today. However, with growing pollution, and fuel-availability problems in the 1970s, there was an impetus to try to build a successful passenger car, with the realization that it would take an order-of-magnitude improvement in technology to make this happen. There was, however, a significant advance with the coming of power transistors in place of thyristors, and big improvements in drive controllers resulted, epitomized by the successful Curtis controller. This was a field-effect transistor chopper that was to become almost universally used in low power DC vehicles. High performance AC drives also came into being, and four machines thus came to do battle for the EV market.

4.7.1 MOTOR CONSTRAINTS

There is now general acceptance that the brushless DC motor will be the one used now and in the future. At a conference in Toronto in June 2000, GM gave details of its latest version, with inverter, in the Precept car. Compared with their earlier induction motor drives, they have halved the size by going to the permanent-magnet motor as well as reducing current consumption, for equivalent performance, by a factor of 1.8. They thus also have an inverter of half the size of that required for an induction motor, and this has resulted, too, in substantial manufacturing cost reduction.

The disadvantage of brushed DC motors is the high unit weight for the performance obtained (and the commutator is moisture sensitive, perhaps beyond the capability of tolerating a high-pressure car wash). This is quite acceptable in industrial trucks where extra weight is often required to counterbalance handling of the payload at high moment arms, but not of course in cars. Because the frequency of the commutator in a brushed DC machine is 50 Hz, compared with 1 kHz for a brushless PM machine, the latter is smaller and lighter; also the electronics can switch at 20 times the equivalent rate of mechanical brushes, which is the basis for the weight advantage. A 45 kW brushed machine weighs 140 kg, and typically runs at 1200–5000 rpm, while an equivalent powered brushless machine operates at 12 000 rpm and weighs less than 20 kg. There can of course be a 5 kg weight penalty for a reduction gearbox but even so there is a 75% weight reduction overall.

The inverter is also the cheapest of those used with any ‘AC-type’ motor.

The other contenders were the switched reluctance motor (SRM) and AC induction motor, plus the permanent magnet Pancake motor (by Lynch) which serves the specialized light vehicle market but is non-scaleable technology. Key result of the investigation into comparative methods was that one had to look at the development of the whole drive package and not just the traction motor. While once people balked at the $200 required for the permanent magnets required in brushless motors, in 2000 they appreciate that some $1000 of power electronics is saved in the inverter. The induction motor was put ‘out of court’ because traction operation requires constant power over a 4:1 speed range. Since voltage, current, speed (V, I, N) characteristics show that to do this an induction motor goes from 0.5 V at double current at the bottom end of the speed range, to V and I at the top end, so an inverter that can supply double current is required. With a brushless DC machine it can be designed to require V and I at both minimum and maximum speeds, so only half the size of converter is required. It also has a further advantage that scratches SRM from the equation. Because the SRM is force commutated (current interruption is very dissipative , a ‘hard-switching’ turn-off), power losses in the inverter are very significant compared with that associated with a brushless machine which at high speed operates with a leading power factor and so has hardly any switching losses in the inverter (which may be of very compact construction). The SRM also exhibits significant acoustic noise due to the magnetostriction of its operating dynamics.

With the 4 or 6 coils in the machine which have to be moved relative to one another forces of

attraction between them are up to ten times greater than the developed torque of the motor; the framework of the motor can physically distort and considerable noise thus generated.

4.7.2 WHEEL MOTORS AND PACKAGE DESIGN

While wheel motors are ideal for low speed vehicles the problem of high suspended mass rules them out for cars. Road damage can be caused at wheel hop frequencies and the perceived threat of losing traction on one wheel, by a single motor failure, would prevent any safety authority from issuing a certificate of roadworthiness. Use of such devices as active suspension makes them possible on medium speed urban buses where road wheel tyres can be as much as one metre in diameter and large brake assemblies reduce the relative weight of wheel motors. Motors driving individual back wheels are a possibility in commercial vehicles, where traction and steer forces are not shared by individual tyres, and 4 × 4 drives with a single motor power source are ideal for more expensive cars, which could tolerate the cost of multiple control systems. Wheel motors could see wider application if steels with adequate magnetic properties could be developed for lighter-weight PM motors at reasonable cost. Expensive military vehicles use such a steel, called Rotalloy, but it costs some £15 per kg in 2000. Such vehicles sometimes have individually steered and driven wheels which enable them to move sideways so perhaps cheaper future alloys of this type will improve parking manoeuvres.

At the present time safety authorities are unlikely to certificate cars with electrical rather than mechanical differential gears but a number of drive-by-wire solutions may become more feasible on EVs. Introduction of 5 kW, 42 V electrical systems is a strong possibility, that could see the replacement of many hydraulic, pneumatic and mechanical controls by electrical ones monitored electronically. These drive-by-wire systems will be a prelude to convoy control of vehicles on motorways. Many development pains have yet to be cured, however, though EV technology will be helpful in the implementation. Other advances such as starter-alternators are likely to be found on thermal-engined hybrid vehicles; these use a new kind of power electronics with silicon-carbide switching devices cooled by hot water from the engine cooling system, allowing semiconductors to operate safely at 250^oC. First is due on the Mercedes 500 to be introduced in 2001 and fitment to all European and American cars is expected in two years’ time.

4.7.3 ALTERNATIVE AUXILIARY POWER

Consideration of photovoltaic power is often a pastime of EV promoters but 10–15% light to electricity conversion efficiency has precluded serious traction usage so far, though use as an auxiliary power source is important. Even at high noon in the tropics solar radiation can only generate 1 kW/m² which means that the solar cell will produce only 150 W for each square metre.

In the Honda Solar Challenger, 8 m² of solar cells generates 1–1.5 kW, which would be nowhere near enough to provide propulsion and hotel loads (‘parasitic’ loads such as lighting and air conditioning) for a conventional car. The most hopeful traction application is for electric scooters operating in the tropics where a reasonable size photocell array, carried in the panniers then unfolded and left out in the sun, could charge the battery of a Honda 50 electric scooter in 6 hours and provide traction for 50 miles without energy being drawn from the grid important in isolated areas.

Photocells are also useful on battery-electric vehicles to ensure that the battery never gets fully discharged (particularly important with lead–acid types). They are valuable sources of auxiliary power for cooling purposes, either lowering interior temperatures on cars parked in the sun or providing refrigeration power to keep gaseous fuels in liquid form. The transformation in usage that could follow an increase in conversion efficiency may be realizable before long if the reported

Fig. 4.8(a) Complete GM fuel-cell chassis with POX converter capable of up to 70 kW output at 300 V DC (including AC drive train), (b) Gasoline to hydrogen (POX) converter close up.

(a)

intensity of research bears fruit. BP and Sanyo are both world leaders in this and already enjoy market success with static arrays of low efficiency cells in tropical countries.