More on 60 Cycles

By R. Allen
From OTB Vol. 20, No. 3 (December, 1979)

So you think our 60 cycle electrical system was originally determined by our 60 seconds/minutes time standard?
Not exactly.

When Westinghouse and others were determining the frequency for alternating current back in 1889 and 1890, several frequencies were developed. One of the first to be used was 133. The choice of this odd frequency was based on their generating unit which ran at 2000 rpm, had 8 poles and gave 16,000 alternations per minute or 133 1/3 cycles (16, 000 divided by complete alternation or 60 plus 60 = 133 1/3).

Other frequencies were tried depending on the power source: steam engines and water power. The cylinder type steam engine ran at a relatively low speed. At one time some thought was given to 16 2/3 cycles since an 8-pole generator at a lesser driving speed gave 2000 alternations or 16 2/3 cycles.

The lower frequencies worked great for large low rpm electric motors but were impractical for lighting purposes because of the pronounced lamp flicker.

A strong contender and one used for many years, particularly in heavy industry, was 25 cycles. This frequency originated at the Niagara Falls hydro power plant in the 1890's. After several compromises they settled on a 12 pole, 250 rpm machine which gave 3000 alternations or 25 cycles. It is only in recent years that 25 cycle has been phased out in most industry.

High speed turbo generators did the trick for soon six-pole, 1800 rpm generators became standard giving 60 cycles which was a compromise for drive speed and machine design.

So you see, our 60 cycle system was not necessarily decided by 'time' but by source of motor speed and generator design.

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