This guide is part two on how to build an electric car, continuing on the previous post. The drive train of a pure electric vehicle is the easiest configuration, the main components are the electric motor and the battery. True enough there are some other (vital) components that are required, such as a convertor between the electric motor and the battery.
The rather simple configuration of an electric vehicle drive train.
The electric motor is there to move the vehicle forward, the battery is there to store the power. The convertor in the middle is used to control the amount of power that moves from the battery to the motor and ensure the voltages on each side remain within a specified range. For a more easier understanding of what the converter does, compare it with a valve in a water line; where the valve is turned more open when the a consumer wants to increase the flow of water. Similar, when a driver wants to increase speed, he/she pushes down the accelerator pedal which controls the converter. The converter then knows how much power to allow from the battery to the motor.
The electric motor is also used during the process that is called regenerative braking. This process allows to recoup the energy from braking or decelerating the car. Basically what happens is that the vehicle slows down on the friction of the motor, which acts as a generator. Power then flows from the motor through the converter and into the battery to be used at a later time. During this process the converter’s job is to ensure that the voltages on both end remain within the specifications and try to balance out between what amount of power is available and allowed at that time into the battery. The goal in this case for the converter is to recoup as much energy as possible, but protect the battery and ensure it stays in good condition. In case the power that is generated becomes too much for the converter or the battery to handle, mechanical brakes can be used to take out the excess energy.
With regards to what the specified conditions the converter has to keep the voltages; that really depends on the battery and the motor that are chosen in the configuration. Better is to first check what motor and battery are required/desired for the application and then check the specifications of the available models before looking at the converter.
To determine which type of electric motor to pick the two most important factors to consider are the total mass of the vehicle and the driving characteristic that is required or desired. The heavier a vehicle, the bigger the motor will have to be. Also include the added mass for the batteries and the optional large cargo loads that you want to take along with it. The method to determine the size of your motor/battery will be discussed in a next post.