472wheel 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 x 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 250oC. 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.

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