Motor/ generator action: Electric motors at the microscopic level

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.The unbelievably tiny motor sketched in FIG. 23 is fabricated by an etching technique like that used to produce semiconductor devices. The rotor spins freely on its bearing and is just 100 micrometers in diameter. The gap between rotor and stator poles is 2 micrometers. When appropriate, sequenced 80-volt pulses are applied to the twelve stator poles, rotational speeds up to 2500 RPM have been observed. Because the operation is dependent on the programmed excitation from a control IC, the performance is suggestive of a synchronous motor. Not shown, is an electric shield underneath the structure. Notice that there are twelve-stator poles, but only eight-rotor poles.

This device has been called a variable capacitance motor. The coulomb forces that generate the torque identify it as a true electric motor. (Conventional motors that we are familiar with are actually magnetic or electromagnetic motors). At this point, it is interesting to recall the macro-sized electrostatic voltmeter, which also translates coulomb force into mechanical motion, although not in the form of continuous rotation.

Stator poles; Micro-motor Material: Polycrystalline silicon; Rotor diameter: 100 uM; Rotor-stator gap: uM


FIG. 23 Mechanical motion in miniature—the micro-motor. Electrostatic force turns the eight-pole rotor at speeds up to 2500 RPM when the stator poles are appropriately sequenced with 80-volt pulses.

It is not easy to postulate practical applications for this diminutive device. The fact of its accomplishment is, in itself, rewarding to the creative spirit. No doubt, how ever, unique uses will be found in medicine, instrumentation, and in various surprising ways that elude our logic. In science, as well as in invention, sometimes the discovery or innovation occurs first, to be followed by stumbling upon practical applications. A situation that quickly comes to mind is the identification of helium in the spectra of the sun before the gas was found in the earth; here an academic achievement with little apparent practical value was later put to many useful purposes.

Unfortunately, the basic motor action principles of this micromotor cannot be scaled up to produce a macro-sized motor with useful torque. The frustrating difficulty is that impractically high kilo-voltages would be required. Yet, as hinted previously, it is not inconceivable that a way to harness coulomb forces for useful mechanical work might some day be developed. One can, for example, entertain such ideas as housing a high-voltage electrostatic motor in an evacuated vessel, with mechanical motion coupled to an external shaft via permanent magnets.

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