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I am running an alternator with an electric motor (variable speed), but I do not know the shaft speed. I would like to calculate it from voltage frequency.

The rotor of the alternator has 12 poles (6 south + 6 north >> http://www.tb-training.co.uk/images/poles.gif). The stator winding is connected as Delta. If a connect a multimeter (Fluke 87V) across one of the three windings, I get 100Hz. Following formula:

Nshaft = 120*frequency/poles

I get rotation speed of 1000 rpm. Is my calculation correct?

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You are correct. The number of poles is the sum of the north plus south poles. Speed(RPM) = Frequency(Hz) X 120 / Poles = 100 X 120 / 12 = 1000 RPM. I think Andy must have read your question too fast.

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  • \$\begingroup\$ Oops, my math aint what it used to be. I'll delete my stupid answer. \$\endgroup\$ – Andy aka Oct 17 '15 at 15:53

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