It all depends on the type of motors that are used in the electric car.
If the car has DC motors then the speed is controlled by chopping the DC to the motor. The speed is therefore controlled by the duty cycle. If the duty cycle is 50% ie half of the time on/ half of the time of the motor will go at half speed. As the time on/time off ratio increases so does the speed of the DC motor.
If the vehicle has AC motors then first the DC from the batteries has to be converted to AC by some form of inverter. On method to alter the speed is to increase the frequency of the AC to the motor. The higher the frequency the higher the speed. The other is vector control.
See this link for more information and maybe a better explanation than mine.
Which ever way its done it all comes down to electronics. You push the 'gas' pedal and this information is processed and the proportional amount of power is applied to the motors.
Dear Laffalot,
Thanks a lot for your valuable info. it is specially valuable for me living in a backward part of India.
I think if, an AC motor is used taking AC power directly from the Alternator then actually the IC engine can be made to burn fuel very very leanly.
What is your opinion?
Dhritiman
If I understand you question correctly an alternator driven by an engine is used to power an AC motor which in turn propels the car ?
I should imagine it is possible to drive an AC motor from an Alternator, and the Alternator from the engine but in my opinion it would be very inefficient.
As an engine rotates the burning petrol is converted into energy in the form of expanded gasses which push down the pistons. However not all the possible energy is obtained from the petrol and used to propel the car. Some of the energy is wasted in the form of heat, noise etc. so already we have loss with the engine.
Now as the engine powers the Alternator again there are losses as noise, heat, friction on the bearings, magnetic loss, copper loss etc. So the losses are compounding with the losses of the engine.
Now as the alternator powers the AC motor again there is heat, noise, bearing friction etc.
So not very efficient even if you can run the engine leanly. Much better have the engine power the wheels directly through a gearbox as is normally the case.
Another factor to consider is the power requirements of the AC motors. They will probably exceed anything an alternator can generate.
Now there are some cars which are hybrid, they contain both a petrol and electric power.
The petrol powers the car when speed, acceleration or power are needed and the batteries power the car when your around town.
Dear LAffalot,
Thanks Again. I found those sites extremely informative.
If the power generated by the alternator is used to drive four traction motors driving each wheel, then I think torque available would be more and weight with these changes would be lesser.
Also it would have better all-terrain capabilities.
But as you have said the alternator should have geater power output.
Dhritiman
