TORQUE

Not long ago, I described how Amory Lovins' campaign for fluorescent lights was wasting energy [AtE Sep 91]. His other litany is about electric motors, which I used to dismiss as nonsense be-cause the little motors used for shavers and coffee grinders use so little energy that it doesn't matter what their efficiency is, and the large industrial motors have such high efficiencies that their owners and operators do not need the advice of a Harvard drop-out who paid 5 British pounds to buy himself an unearned degree at a British university, and now preaches a brand of economics ac-cording to which it is cheaper to dismantle a nuclear plant than to generate power with it.

But it turns out that Lovins' scheme once again goes deeper than that; it is an energy waster of far bigger dimensions.

Torque may be described in fuzzy English as the effort used to turn something, or more accurately as the moment about the pivot point, i.e., the product of force and the perpendicular distance to the pivot (see figure). A motor has a torque which it applies to turns things, such as the wheels of a motorcar. In fact, a car makes the explanation of how torque depends on velocity (in the case of a car) very easy. At zero speed, when the car starts from standstill, it needs maximum torque, or effort to turn the drive shaft. Driving along the freeway at 55 mph needs very little torque, the car just sails along. But a gasoline engine is a very inflexible motor, which runs within a limited range of velocities (revolutions per minute). To make it develop a wide range of torques for a limited range of rpm, you need a gearbox or "transmission," which in the lowest gear develops very high torque, and in the highest gear very low torque; it does that, in effect (though not in reality), by changing the distance from the pivot point (left figure) while working within a comparatively small range of forces.

An electric motor is much more flexible and does not usually need a gearbox. There are, however, very different types of motors with different torque vs. rpm characteristics, and you choose the one best fitted to the application.

For example, an electric motor used for traction (to stay close to the automobile example) needs a high torque for low velocities and vice versa. One of the motors that has this characteristic is a series DC motor, which used to be popular in the days of electric streetcars and is again widely used with today's noiseless and effi-cient electronic AC to DC convertors. Its torque/velocity charac-teristic is indicated in the figure on the right, and as you can see is just what we need. (Almost. If no precautions are taken, a DC motor running with no load will run away into high velocities and can become dangerous, but that's another story.)

But of course traction is not the only application of an industrial electric motor, and there are many other torque/velocity depend-encies which one can choose by choosing the right motor for the application.