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Author Topic:   New Article on Alternators Is Great
bsmotril posted 05-18-2006 08:51 AM ET (US)   Profile for bsmotril   Send Email to bsmotril  
Thanks for the excellent article on alternators. Looking at the pictures of the stator coil, would that be an "8 pole" alternator? Does the number of poles equate to the number of coils, or do multiple coils make up a "pole" ?
jimh posted 05-18-2006 09:23 AM ET (US)     Profile for jimh  Send Email to jimh     
Bill is referring to

Permanent Magnet Alternators

First, thanks for the kind words about the article. I have been studying these devices and wanted to share what I have learned. I had a chance to take some pictures to illustrate the article, and they provided the spark to get it started. I plan to add some more illustrations and drawings to it, as I get time to expand it.

Your question about the classification of number of poles is a good one. It is a bit arbitrary, so I am not sure if Evinrude and the tachometer manufacturers would call this a 6-pole alternator or a 12-pole alternator.

The alternator produces 6 cycles of alternating current for each rotation of the flywheel. This is applied to the full wave bridge rectifier and becomes 12-pulses of direct current per rotation. Is that a 6-pole or a 12-pole device? We will have to wait for an OMC expert to let us know which description is used with this type of alternator.

Even though the coil is wound onto eight bobbins, there is just one coil electrically, as all the segments are connected in series.

bsmotril posted 05-18-2006 12:07 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
ok, that makes sense. I have always interpreted "Pole" to be the number of pulses generated per one rotation of the flywheel. Now you've got me re-thinking that. BillS
Riptide23WA posted 05-18-2006 12:44 PM ET (US)     Profile for Riptide23WA  Send Email to Riptide23WA     
I thought the nuber of poles referred to the number of north/south poles were on the rotor. i.e, a generator with a simple bar magnet for a rotor would be considered a "2-pole" rotor. The number of poles determines, in part, the frequency of the alternating output, assuming a known speed of the rotor. Voltage would be determined by the magnetic field strength...

I think you're on the mark, Bill...

jimh posted 05-18-2006 02:18 PM ET (US)     Profile for jimh  Send Email to jimh     
Bill--yes, as I mention, do you count the AC pulses or the DC pulses? It seems somewhat arbitrary.
bsmotril posted 05-18-2006 06:29 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I guess that depends on whether or not you sample the input or the output to the rectifiers. I'm not sure which one a tachometer takes it's feed from. BillS
Chuck Tribolet posted 05-18-2006 06:46 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The tach has to sample the input (alternator) side of the
rectifier. There's AC there. On the output side, the
battery smooths things out into DC and there aren't any
pulses to count (at least not without some expensive test


jimh posted 05-18-2006 08:23 PM ET (US)     Profile for jimh  Send Email to jimh     
The tachometer signal is a half-wave rectification of the output signal, so it has only half as many pulses as the full-wave rectifier output feeding the battery. The number of DC pulses in the tachometer signal equals the number of AC pulses in the stator output. This corresponds to the number of poles. In the rectifier described in my article, the arrangement is a six-pole alternator and would use a six-pole tachometer calibration.
jimh posted 05-19-2006 09:59 PM ET (US)     Profile for jimh  Send Email to jimh     
I just added a schematic showing how the full-wave bridge rectifier, the stator coil, and the battery are connected. If you click on the drawing you can see an animated view showing which diodes conduct and how the current flows and the polarity of the stator coil changes back and forth with the engine rotation.
jimh posted 06-02-2006 12:29 PM ET (US)     Profile for jimh  Send Email to jimh     
Riptide23WA writes:

"I thought the [number] of poles referred to the number of north/south poles were on the rotor."

Not quite. The combination of magnets and coil pole pieces determines the number of pulses per revoltion, which is what is generally understood to be meant by the "poles" designation. An alternator could have only one magnet, but could produce many pulses per revolution if there were many coil pole pieces over which it was rotating. Generally the number of magnets and coil pole pieces is the same in order to optimize the output.

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