Why Will Future Automobiles Be Electric? - Little Known Facts About Electric Motors

In 1956 an article appeared in an American magazine that predicted that there would be an energy crisis about 15 years from that time. They were not far out, the first energy crisis made its appearance in 1973. They forecast the eventual downfall of the internal combustion engine and considered alternatives for its replacement. The electric motor, they said, would not qualify, as it was too heavy. In those days, the conventional DC traction motor was all that was available. In that respect they were right, the DC motor was too heavy to give the kind of performance required to satisfy drivers used to gasoline powered cars. The three-phase induction motor had a superior power-to-mass ratio but was essentially a constant speed device, and so was not considered suitable.

The decade of the 60's brought power semiconductor devices which made the solid state Variable Speed Drive (VSD) possible. The speed of a three-phase induction motor could then be varied by varying the frequency and the voltage.

In the United States the usual supply frequency is 60Hz, while in the rest of the world the frequency is 50Hz. A 60Hz two-pole motor rotates at about 3420 r/m on optimum load. The corresponding speed for a four-pole motor is 1710 r/m.

For a 50Hz motor the corresponding speeds are 2850 r/m and 1425 r/m. The power-to-mass ratios of such motors are similar to those of a high speed diesel engine.

To obtain better power-to-mass ratios one can go to higher frequencies. The speed of rotation of a motor with a given number of poles is proportional to the frequency. High frequency motors rotate at high speeds and therefore have improved power-to-mass ratios. The electrical equipment in aircraft is required to have triple redundancy. This means that three motors must be provided for the same particular task, any one of which must be capable of taking over the duties of the other two. With such a stringent requirement, mass and volume become very critical. Thus the motors and alternators used on aircraft usually operate on a frequency of 400Hz but some use 1200Hz. Space and mass requirements are also important considerations on ships. Ships normally have electrical systems operating on 400Hz.

The following information will give an idea of the advantages of using high frequency motors:

A 60Hz motor with two pole pairs rotates at 2840r/m and has a power-to-mass ratio of 250W/kg. A 400Hz motor with two pole pairs rotates at 22 720r/m and has a power-to-mass ratio of 2000W/kg. For four pole motors, the corresponding figures are, 1704r/m and 125W/kg for 60Hz, and 11 360r/m and 1000W/kg for 400Hz.

High frequency motors have small size and mass. For a given power their torque is much smaller than that of a low frequency motor. They make up for it by rotating at proportionately high speeds. Since the torque is small, the motor can be made smaller and lighter. The parameters in the foregoing table refer to motors with an aluminum frame and aluminum squirrel cage.

We see that it is possible for electric motors to compare favorably with internal combustion engines in power-to-mass ratio. Further, they are more compact and easier to package and do not require all the auxiliary equipment of an internal combustion engine, such as radiator, exhaust system, starter motor, alternator, transmission, etc.

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) Limited, a research and development company incorporated in the Republic of South Africa. For more information visit [http://www.johntal.com/home.htm]

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