Important Facts About Electric Motors

In 1956 an article appeared in an American magazine which predicted that there could be an energy crisis roughly 15 years from that time. They weren’t far out, the first energy crisis made its appearance in 1973. They predict the eventual downfall of the internal combustion engine and considered choices for its replacement.

The electric engine, they said, wouldn’t be eligible, as it was too thick. In these days, the conventional DC traction motor was all that was available. In that regard they were correct, the DC motor was too thick to provide the sort of performance needed to meet drivers used to gas powered cars.

The three-phase induction engine had a superior power-to-mass ratio but was basically a constant speed device, and so wasn’t considered suitable.

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The decade of the 60’s brought power semiconductor devices which made the good state Variable Speed Drive (VSD) possible. The rate of a three-phase induction engine could subsequently be varied by varying the frequency and the voltage.

In America, the regular source frequency is 60Hz, while in the rest of the world the frequency is 50Hz. A 60Hz two-pole engine rotates at about 3420 r/m on optimal loading. The corresponding rate for a four-pole engine is 1710 r/m.

To get a 50Hz engine the corresponding rates are 2850 r/m and 1425 r/m. The power-to-mass ratios of these motors are like those of a high-speed diesel engine.

To obtain greater power-to-mass ratios you can go to higher frequencies. The rate of rotation of a motor using a given number of rods is proportional to the frequency. High-frequency motors rotate at high speeds and for that reason have enhanced power-to-mass ratios.

The electrical equipment in aircraft must have triple redundancy. It follows that three motors have to be provided for the exact same particular undertaking, any one of which must be capable of taking over the responsibilities of the other two.

With such a strict requirement, volume and mass become very critical. Thus the motors and alternators used on aircraft generally run on a frequency of 400Hz but some usage 1200Hz. Space and mass requirements are also important considerations on boats. Ships normally have electric systems operating on 400Hz.

This information provides an idea of the advantages of using high-frequency motors:

A 60Hz engine with two pole pairs clocked in 2840r/m and contains a power-to-mass proportion of 250W/kg.

A 400Hz engine with two pole pairs clocked in 22 720r/m and contains a power-to-mass proportion 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.

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