Why Will Future Automobiles Be Electric? How to Make a Variable Speed Drive For Your Electric Car
In the previous article in this series we saw that the most suitable motor for an electric car is a 400Hz three-phase induction motor. Operated on a fixed frequency supply this type is essentially a constant speed motor. To be able to vary the speed of our car we need to vary the frequency of the supply. The speed of the motor will then be proportional to the frequency. To avoid damaging the motor the voltage of the supply should be varied in the same proportion as the frequency. The device for achieving this is called a Variable Speed Drive (VSD). There are various types but the most suitable for our purpose is the type that uses Insulated Gate Bipolar Transistors (IGBT's) to form a three-phase inverter with Pulse Width Modulation (PWM). The easiest will be to purchase a VSD with the motor. These are usually designed to operate on a 200 volt or 208 volt three-phase AC supply. If you are planning to use a battery for your car, you will need to remove, or bypass, the rectifier on the input side. If you are electronically inclined, you may prefer to build your own. On my website you will find references to a number of useful papers on the subject.
If you are not electronically inclined, your best bet may be to buy a 200 volt 60Hz motor complete with matching VSD. It will be considerably heavier than a 400Hz outfit, but still acceptable for a local runabout. There are several manufacturers in the far east that can supply good units at a reasonable price. If you are able to connect to a Talbot Electric grid system, these will be suitable. If you have to use a battery, you will need a 200 volt lithium-ion battery. Typically, these lose storage capacity by about 20% per year and will need to be replaced at three to four year intervals. The cost will be comparable with the use of gasoline. For those east of the Atlantic where the standard 3-phase mains voltage is 400 or 415, there is a wide selection of 50Hz motors and VSD's ranging from 60 watts up to 350kW. These are ideal for heavy vehicles. Operating these from a battery is not recommended as the battery will need to be huge.
Several useful articles are to be found in the Wikipedia under the heading Variable Frequency Drives.