COMPUTERS FOR MOTORS

An alternating current (AC) motor (of the most common type) uses a three-phase rotating magnetic field which induces currents in the rotor, and these currents, without any commutation, make the rotor chase after the rotating field. Because of its inherent simplicity, such an AC motor is cheaper (about $40 per kW vs. $240 for DC), is more robust, and more easily maintained. Moreover, AC power is available virtually everywhere; DC must be either generated by the user or made from AC by rectifying it.

All of which seems to stack the odds pretty heavily against DC motors.

Why would they be used?

In part, for special reasons: They deliver a high torque at starting speeds, which is why they are used in traction, e.g. for the flywheel bus. But at constant speeds, both AC and DC motors are pretty efficient--90% or better from 50 HP up.

The overriding objection to AC motors is the cost of providing them with an efficient speed control. Good induction motor controllers cost about $700 per kW, or almost 18 times as much as the motor itself (there are cheaper controllers, but they waste energy); the cost for a DC controller is about $240/kW, or about the same as for the motor. Thus, for motors of 10 HP (7.43 kW) or more, the DC motor wins heavily on total cost.

The trouble with AC motor regulation used to be that the only efficient way of changing the motor speed was to change the frequency of the alternations of AC, which is fixed at 60 hertz (cycles/sec). But with today's power electronics, it is no longer difficult to change the frequency. The AC is rectified into DC, and the level of the DC is varied, not continuously, but in steps, and this works almost as well as a smooth AC wave. (There are also other methods.) This synthesized AC, however, has a frequency of whatever we please to make it, and we are no longer tied to the power company's 60 hertz. Moreover, if we control the controller with a microprocessor (computer on a chip), it will adjust the frequency to the load and desired speed as instructed by a program of almost limitless sophistication.

Whenever there are large and profitable energy savings of this type to be made, one can be sure of two effects: Free enterprise will compete to get in on it; and Sen. Metzenbaum of Ohio will do his best to kill such initiative by bureaucratic measures attempting to legislate technology. He did not disappoint us: He sponsored the Industrial Equipment Efficiency Act of 1979, which would mandate efficiencies in the same wonderful way as has been done for automobile mileage.

Best known among the companies moving into the field is Exxon, which has acquired Reliance Electric of Cleveland and promises to market AC speed control devices for between $10 and $40 per HP by the mid-eighties--1/10 of the present price. But other companies are already delivering, among them Electronic Relays, Inc. of Downers Grove, Ill., and Wandlass Inc. of Tustin, Calif., and large corporations such as Westinghouse and Gould are also moving into the field.

[More: "Inverter-fed induction motor drives," Electr. Rev., 18 Jan 80; "Energy conservation and the electric motor," Science News, 21 Jul. 79 (energy statistics in colored figure appear to be incorrect); Exxon puts a tiger in your electric motor," Economist (London), 26 May 79; "Motor makers seek energy saving devices," Busin. Week, 21 Jun. 79.]