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?


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.

| improve this answer | |
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.