2
\$\begingroup\$

I was able to buy a cheap used permanent magnet alternator, however the seller was not able to tell me much about the specifications. I could determine that it has a built in voltage regulator, however I would have to disassemble it more to check how many poles it has and was wondering if there was a way to determine it from the outside.

By manually turning it slowly it takes 16 "steps" (stable positions) to rotate it one full turn. This seems to point towards 16 poles, since there seem to be 16 magnets. However when looking at some images and reading some documentation about other alternators I am now suddenly no longer sure if it is maybe 8 poles?

\$\endgroup\$
1
\$\begingroup\$

The sketch below shows a permanent magnet rotor with 4 poles and windings on salient stator poles. You can see that if you turn the rotor, the keyway will stop in four positions. To the degree that the stator and rotor poles have saliency, the method that you used should correctly determine the number of poles. If the stator windings are distributed in slots to provide a more sinusoidal waveform and to make a three-phase machine, the stator will have less saliency and the saliency will tend to indicate the number of slots rather than the number of poles. You can probably not confidently determine the number of poles without examining the construction or performing a test. You may be able to determine the number of poles by slowly turning the rotor while observing the stator voltage with an oscilloscope or zero-center DC voltmeter.

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ I've decided to open it up and there are 16 magnets on the rotor and based on the wiring it only consumes a single phase unless they got some very weird wiring in the stator, so I assume 16 poles it is \$\endgroup\$
    – Cromon
    Sep 11 '20 at 17:09
  • \$\begingroup\$ Yes that would indicate 16 poles. \$\endgroup\$ Sep 11 '20 at 18:45
1
\$\begingroup\$

Since f = RPM * P/120 you can test it and measure f and RPM to find P, number of poles. Ahead of the rectifier/regulator so you are measuring AC (if possible).

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Indeed, that is of course the most accurate option, but I am still waiting for the right belt to get here (its not for a car) and if its 16 poles I can use that time to look for another device \$\endgroup\$
    – Cromon
    Sep 11 '20 at 13:02

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.