AutomobileTechnology

Electric drive vehicle

Introduction

The pressure to produce a non-fossil-fuel vehicle is increasing. Indeed, recent legislation has set the requirement for the production of zero emission vehicles (ZEVs). The development of the electric vehicle is still in a state of flux, but some major manufacturers now have electric vehicles available for sale to the general public.

In 1990, General Motors announced that its EV, the ‘Impact’, could accelerate to 100 km/h in just 8 s, had a top speed of 160 km/h (100 mile/h) and had a range of 240 km between charges. Running costs were about double the fossil-fuel equivalent but this cost was falling. The car was a totally new design with drag-reducing tires and brakes which, when engaged, act as generators.

The car was powered by a 397 kg array of advanced gel electrolyte lead-acid batteries (32 at 10 V) and two small AC electric motors to drive the front wheels. The recharging time was about 2 hours but this could be reduced to 1 hour in an emergency. This was very impressive, but things have moved on still.

Electric drive vehicle layout

Note that the drive batteries are often a few hundred volts, so a lower 12/24 V system is still required for ‘normal’ lighting and other systems. Some of the components shown are optional

EV batteries

A number of options are available when designing the electric car but, at the risk of over-simplification, the most important choice is the type of batteries. Currently the main advantage of lead-acid batteries is the existing mature technology, which is accepted by the motor industry. The disadvantage is their relatively low specific power.

The sodiumsulphur battery is a good contender but has a far greater cost and new technologies are needed to cope with the operating conditions such as the high temperatures. Significant developments are occurring in relation to lithium-based batteries. However, most batteries in general use are lead-acid or nickel-based.

Asynchronous motor

The asynchronous motor is often used with a squirrel cage rotor made up of a number of pole pairs. The stator is usually three-phase and can be star or delta wound. The rotating magnetic field in the stator induces an EMF in the rotor which, because it is a complete circuit, causes current to flow.

This creates magnetism, which reacts to the original field caused by the stator, and hence the rotor rotates. The amount of slip (difference in rotor and field speed) is about 5% when the motor is at its most efficient.

 

EC motors (electronically controlled)

The EC motor is, in effect, half way between an AC and a DC motor. Its principle is very similar to the synchronous motor above except the rotor contains permanent magnets and hence no slip rings. It is sometimes known as a brushless motor. The rotor operates a sensor, which provides feedback to the control and power electronics. This control system produces a rotating field, the frequency of which determines motor speed.

DC motor – series wound

The DC motor is a well proven device and has been used for many years on electric vehicles such as milk floats and fork lift trucks. Its main disadvantage is that the high current has to flow through the brushes and commentator.

DC motor – separately excited shunt wound

The fields can be controlled either by adding a resistance or using chopper control in order to vary the speed. Start-up torque can be a problem but, with a suitable controller, can be overcome. This motor is also suitable for regenerative braking by increasing field strength at the appropriate time. Some EV drive systems only vary the field power for normal driving and this can be a problem at slow speeds due to high current.

Summary

The concept of the electric vehicle is not new, the essential battery technology was developed in the late 19th century and many such cars were being manufactured by the year 1900. Although some models achieved high speeds at that time, the electric car was generally slow and expensive to operate. Its range was also limited by its dependence on facilities to recharge the battery.

Many of these problems have been overcome, but not all of them. Cost is still an issue, but ‘cost’ is a relative value and when the consequences of pollution are considered the ‘cost’ may not be as high as it appears.

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