Electric Cars : Structure

Structure of Electric Cars

Electric vehicles are one of the simplest forms of self propelled mechanical transport. In the basic design, the drive train of the car is made up of a battery array connected to an electric motor via a switch. The amount of electricity that is allowed to pass through to the electric motor and gear systems is controlled such that the electric motor drives the wheels in the most efficient manner. Thus, the key elements that differentiate an electric car from other cars and also form the heart and core of this type are:

1. Electric Motor: The car may run on AC or DC motors. If it uses a DC motor, the car may run on any voltage between 96V and 192V of the rating around 20kW. Motors work on the principle of electromagnetic induction where change in magnetic flux causes the central shaft to rotate. DC motors can be overdriven at a value much greater than the normal operating point thus offering short burst of increased horsepower. Overdriving for longer spells lead to overheating of the motor to a point where it may be destroyed. In case of AC motors, 3 phase motors are generally used which run at 220-240 Volts AC along with 300 Volt battery packs. AC motors have the ease of availability in various sizes, shapes and power ratings in contrast to DC motors and also have a ‘regenerative braking’ feature by virtue of which, the motor can act as a generator to charge the batteries while braking.

2. Motor Controller: This is the part of the system which would control the amount of current being supplied to the motor depending on amount of pressure on the accelerator pedal. The accelerator is connected to potentiometers which act as variable resistors to provide the signal on how much power to deliver. When the pressure on accelerator is zero, no power is delivered and full power is delivered when pedal has been fully pressed. In case of DC motors the controller would chop the values of DC supply voltage to obtain a current with average value that is proportional to the amount of pressure applied to the accelerator. This chopping or pulsing takes place at a rate of more than 15000 cycles per second to avoid the humming sound in the human hearing range. In case of three phase AC motors, the controllers need to create 3 pseudo sine waves by taking DV voltage from batteries and then pulsating them on and off while inverting the polarities 60 times a second. This requires 6 sets of transistors, three for the pulsing and the other 3 for inversion of the three signals. This is a little more complex than a simple DC motor controller.

3. Batteries: Batteries are the parts that hold the energy reserve for the entire car operation. These store energy in the form of chemical energy and convert them back to electrical energy when required. Despite of all the advances in technology, battery still remains the weakest link in the chain. Lead acid batteries find predominant use in today’s cars. These are heavy, bulky, take a lot of time to charge and yet have limited capacity and life. Better replacements in the form of LiMH batteries do exist, which not only double the range of cars but also have significantly longer lives, are present but at present are too expensive to invest in. Fuel Cells offer the most attractive solution to all these problems along with being environment friendly, but still need a lot of R&D before they enter the mainstream market.