Why Will Future Automobiles Be Electric? How to Control the Speed of Electric Vehicles
November 17th, 2009 |
One of the big advantages of operating electric vehicles directly from the grid is that their maximum speed can be controlled externally by setting the voltage of the supply. This would make it impossible for vehicles to exceed the speed limit. Traffic police could be spared for other duties and the accident rate would be reduced, since many traffic accidents are caused by speeding. Those abominable speed humps would no longer be necessary. In places where pedestrians walk, such as parking lots, the voltage could be set as low as 15 volts. This would limit the speed of vehicles to 20km/h (12.4 miles per hour) and eliminate any possibility of electric shock or electrocution. The system would then be intrinsically safe.
Many towns in the United States have a speed limit of 25 miles per hour (40km/h). This could be enforced by limiting the supply voltage to about 30 volts. This is still a reasonably safe voltage. Nevertheless, the connection method is so designed that it would very difficult for a would-be suicide to make contact with the conductors. Accidental contact would be virtually impossible.
It is intended that three-phase induction motors will be used. Motors of this type operating on a fixed frequency supply are essentially constant speed motors. However the speed can be varied over a wide range by varying the frequency. For example a two-pole 60Hz motor would rotate at about 3400 revolutions per minute (r/m) on optimum load. Reducing the frequency to 30Hz would reduce the speed to 1700r/m. So as not to damage the motor, the voltage should be reduced in the same proportion as the frequency. In our example, the voltage would need to be halved. If the motor was intended to operate on 208 volts (a common 3-phase voltage in the United States), halving the frequency would mean that the voltage would need to be reduced to 104 volts. If the voltage were reduced to 15, the corresponding frequency would be 3.46Hz, and the motor would rotate at 196r/m. The gearing could be chosen so that at this speed the vehicle would be doing 16km/h (10 miles per hour). Increasing the frequency while keeping the voltage the same would increase the speed a little but the torque would soon become inadequate and the vehicle speed would be limited to about 13 miles per hour, which would be a suitable speed in parking lots. In a similar way speed limits could be enforced by the appropriate choice of voltage.
In this article we have seen how the maximum speed of vehicles in various speed zones could be controlled by setting an appropriate voltage. In the next article we will take this discussion further and show how space and weight can be saved by the correct choice of electric motor.
By John Talbot
(Under the Direction of the Lord Jesus, Prov 3:6; Col 3:17)
The Lord Jesus Christ appeared to the author, John Talbot, in a vision, on September 30th 1974 and called him to follow Him. He is the author of several books on the subject. He is now the CEO of Talbot Electric (Pty) Ltd, a research and development company incorporated in the Republic of South Africa. For more information please visit http://www.johntal.com/home.htm.
