Why Will Future Automobiles be Electric? – How to Choose a Car for Your Electric Motor
By John Talbot
(Under the Direction of the Lord Jesus, Prov 3:6; Col 3:17)
In previous articles we have seen how to choose a motor and a Variable Speed Drive (VSD) for an electric car. In this article we will describe how to choose a car for your electric motor. You may be wanting to:
Convert an existing car to electric operation, using a battery, or
Convert an existing car to electric operation, using the Talbot Electric grid connection, or
Build a car from scratch for electric operation, using a battery, or
Build a car from scratch for electric operation, connecting to the electricity supply grid.
Whichever it is, we will show you how to go about it. Assuming that your choice is 1, you may have an old car with a worn out gasoline engine that you would like to convert to electric, or you may be looking to purchase an old car from a breaker's yard. If the latter, bear in mind that older technology cars are easier to work on. Try to find a car that is reasonably sound except for the engine, which latter will be discarded. If it looks a bit shabby you can easily have it reupholstered and repainted after conversion. Then you will have a fine-looking electric car that could last you a lifetime. If you want it as a local runabout choose a small car. You can use a smaller battery and motor. Decide on the top speed that you want. That will determine the size of motor. The average light car requires only 5kW to maintain a speed of 60km/h (37.3 mph) on a level road. If you want to do 100km/h (62.1mph) you will need a motor of 15kW. On my website you will find a list of suitable 400Hz motors ranging from 5.6kW up to 150kW. My company does not sell these. If you send me an email from my website I will tell you where you can get them.
If your choice is 2 you will not need a large lithium-ion battery. Your power will come from the grid. In an emergency, your normal 12 volt battery will be sufficient. Whichever case you choose, the first thing is to remove the gasoline engine from the car. The gearbox (“transmission”) may be retained. On a 400Hz supply these motors rotate at speeds of 11 400r/m to 12 000r/m, depending on load. Many of the motors are available with a reduction gear. Various ratios are available. Choose one that gives a reduction of between 2:1 (for a high top speed) and 4:1 (for a lower top speed). Retaining the car's original transmission will enable excellent acceleration and the ability to climb steep hills. Some of the motors are water cooled. Should you use one of these, retain the car's radiator. Join hoses between the radiator and the motor, with an electric water pump in between. This will provide excellent cooling and allow the rated power of the motor to be exceeded. It is advisable to put thermistors in contact with the stator windings to warn of excessive heat build-up. A three-phase induction motor has an efficiency of 90% or better. Thus only 10%, at most, of the input energy is manifested as heat. Compare this with a gasoline engine, which at 60km/h has an efficiency of around 14%, meaning that 86% of the energy of combustion of the fuel has to be removed by the radiator and exhaust system. Such effective cooling will allow the motor to be operated continuously well above its rated power.
If you intend building a car from scratch, consider making the body of spheroidal shape, like that of the model “69zulu.” Make it in two halves in fiberglass reinforced epoxy resin and join them. This will give you a very light car with a low drag coefficient. A 5kW motor in each wheel fairing will give more than adequate performance. A 5.6kW motor is only 6 inches (150mm) in diameter and 9.5 inches (241mm) long and weighs only 6.8kg (15 lb). Using separate motors on each wheel eliminates the need for a differential.
In order to be able to control the speed of your car you will need a VSD. If you are an electronic whizkid you can make your own. Otherwise there are many models available commercially. Unfortunately, most of the latter are for 50 or 60 hertz. If you plan to use a battery, you will need one of about 280 volts in order to get 200 volts 3-phase after inversion.
Any problems, feel free to write to me. In any case, let me know how it goes. Good luck!