ELECTRIC VEHICLES

A typical electric vehicle consists of a battery, an electric motor, an engine controller and regenerative brakes. An electric motor powers the vehicle using electric energy stored in the battery. An electric motor can also act as an on board generator to charge the battery by producing electricity while the vehicle is decelerating.

Different types of direct and alternating current electric motor are used in electric vehicles; this includes the induction motor, permanent motor and synchronous motor. The induction motor has many advantages; the most important might be that it is very simple to construct, it also has the lowest price because it has no permanent magnet, no brushes, no position sensor, and no starting mechanism since it is self-starting. Speed control is easy; this is achieved by simply controlling the frequency of the alternating current with a variable frequency drive. But there are some disadvantages, the induced current in the coil cause efficiency losses. The induced losses also mean induced heat in the rotor and thus the need for cooling the rotor. It is neither the lightest nor the most compact rotor.

The advantages of the permanent magnet motor are that it is the lightest and smallest motor. It is also the most silent and efficient motor. The biggest disadvantage is that it needs the rare earth metal magnets that are scare and are expensive. They are also more complex to run, requiring a position sensor, starter mechanism and a more advanced controller.

The synchronous motor important advantage includes, it has torque and efficiency (especially at higher speeds) that is comparable to permanent magnet motors. It does not need permanent magnets, which makes it more rugged, cheaper and better for the environment. However it has some disadvantages; efficiency at lower speeds creates more lagging compared to permanent magnet motor. Therefore there is always a higher inherent noise and torque ripple.

The battery in an electric vehicle stores electricity energy that the electric motor uses to power the vehicle. Most electric vehicles use lithium ion batteries. They have certain advantages over other battery types (like sodium-sulfur(Na-S),nickel-cadmium(Ni-Cd), and others) including higher energy storage capacity and long life spans. Despite this they lose capacity over time as a result of ageing and repeated charging cycles.

The electric motor controller governs the performance of the electric motor, including regulating the amount of power that the battery supplies to the motor. The electric vehicle controller operates between the batteries and the motor to control the electric vehicle speed and acceleration. The controller transforms the battery direct current into alternating current (for ac motor only) and regulates the energy flow from the batteries. The controller will also reverse the motor rotation so that the vehicle can go in reverse.

Electric vehicles with direct current motors require a simple variable resistor type controller to control the acceleration and the speed of the vehicle with this type of controller, full current and power is drawn from the battery all of the time. At slow speed, when full power is not needed a high resistance is used to reduce the current of the motor. With this type of system, a large percentage of the energy is wasted as an energy loss in the resistor. The only time that all of the available power was used is at high speeds. Modern controllers adjust speed and acceleration by an electronic process called pulse width modulation switching devices, such as silicone controlled rectifiers which rapidly interrupt (turn on and then off) the electricity flow to the motor. High power (high speed and acceleration) is achieved when the intervals (when the current is turned off) are short. Low power (low speed and or acceleration) occurs when the intervals are longer. The controllers on most vehicles also have a system for regenerative braking. Regenerative braking is a process by which the motor is used as a generator to recharge the batteries when the vehicle is slowing down. During regenerative braking, some kinetic energy normally absorbed by the brakes and turned into heat is converted to electricity by the motor and controller and is used to recharge the batteries. Regenerative braking not only increases the range of the electric vehicles by 5 to 10 per cent, it also decreases brake wear and reduces maintenance cost. However it is pertinent to note that while ac motor is less expensive and lighter in weight, the dc motor has simpler controller making the direct current motor and controller combination less expensive. The main disadvantages of the alternating current motor is the cost of electronic package needed to convert or invert the battery direct current to alternating current for the use of the motor. Despite this, a large number of electric vehicles are using alternating current motors and controller system because of their improved motor efficiency and their light weight.

Furthermore with electric vehicles, where the dominant power is electric, it will be inconvenient to use the proven hydraulic system for its steering, since it will further add complexity to the vehicle in terms of the additional weight and energy required in powering the system. Thus an electrical power steering is the standard feature of electric vehicles. Since the electrical power system only needs to draw electric power to drive, usually a variable assist rack and pinion electric power steering instead of a hydraulic power steering system.

The advantages of the electric vehicle over other conventional vehicles are too numerous to mention; one of the advantages is the possible configurations of the motor. In the single configuration, it uses one motor to drive either the front axle or the rear axle of the electric vehicle. In the dual configuration, it uses two motors to drive the electric vehicle, one for the front axle and the other for the back or rear  axle or alternatively both motors can be placed on either the front or rear axle to provide increased torque, compared to the single motor configuration. Otherwise, the two motors can be used to drive the left and right wheel independently on either the front or rear axle, meaning there is more control over the vehicle when cornering or turning. Though most electric vehicles are single configuration, the dual configuration is employed for vehicles with higher torque needs or capability. Also using separate motors to drive each of the four wheels is possible. This can be achieved using an in wheel, where the motor is placed within the wheel of the vehicle.

Electric vehicle engines are superior in every respect to other conventional vehicle engine, since it only has one moving part, that is the rotor compared for instance with the internal combustion engine with more than a hundred of different parts. Therefore it is not surprising that they are lighter, smaller, compact, more durable, cheaper, and far less noisy, cause no exhaust and are virtually maintenance free.

The only disadvantages of electric vehicles compared to other conventional vehicles, is in the energy storage and the energy infrastructure. In both cases, there is progress; for instance the storage capacity and charging infrastructure of the battery has grown tremendously over the years. Today the lithium ion battery which is the battery of choice of most electric vehicle manufacturer can be said theoretically to hold more energy than gasoline. They are in fact four times more efficient and the weight is no longer a constraint resulting in a lighter vehicle needing fewer raw materials which of course will eventually be translated to a cheaper vehicle with less drag and more overall efficiency.

 

 

 

Sources

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• Chan C.C and Chau K.I.(2001) modern electric vehicle technology. Oxford university press,