3 Phase Alternators, Dynamos & Motors: Why 14 Com. Contacts Which In No Way Divides into 360 ?

[Kevenergy]

I've become Very interested in 3 Phase ever since Learning, after40 Years, that All Vehicle Alternators are 3 Phase AC Power Producers with Full Wave Bridge Rectifiers. I have Two THIRTY Year Commercial Union Electrician Friends Who, WITH ALL OF THEIR Combined, VERY SUBSTANTIAL Experience as Commercial Electricians Didn't Know that Either !!!

Question: Why do all 3 Phase Alternators seem to have 14 Contacts or Poles on the Armature, which in no way Divides into 360 Degrees Evenly, even if divided in two for 7 Pairs N/S Poles each ??? Are the 2 Extras for Staggering or Lead / Lag Time ? I know they're not to Energize the Rotor or Field Windings, because the ones I've seen have 2 Separate Slip Rings Constant to Two Poles to The Rotor for DC to Energize the Electromagnetic Field and Controlled by The Voltage Regulator and is separate & independent of going through the Armature. Please Advise, and Thank You Very Much !!! -Kevin.

 

[4pyros]

I believe it has to do with efficiency.

You can read more here;
**broken link removed**

 

[crutschow]

I don't think there is anything special about 7-poles. Some vehicle alternators have 6 or 8 poles. Whether it divides equally into 360 degrees or not is of no particular significance. The output frequency is still the RPM/60 * poles/2. The only oddity about a 7-pole alternator is that the frequency is not an even multiple of the RPM but that's of no consequence if you are rectifying the AC to get DC.

 

[tcmtech]

Question: Why do all 3 Phase Alternators seem to have 14 Contacts or Poles on the Armature, which in no way Divides into 360 Degrees Evenly

My math says that each pole would be about 25.714 degrees in width. That works just fine for me. Also as a alternator is designed all that is relevant is that the total number of slots in the stator is divisible by three. As long as there are an even number of slots and poles the total count is not relevant, sort of.

Ford likes to make most of their alternators in 12 pole rotor configurations with 11 pole stator windings.

 

[dr pepper]

I think its down to cost and compromise, minumum amount of copper windings and magnetic material in the rotor, plus a higher output freq means smoother dc when rectified.

The reason why the armature field is controlled to regulate the o/p is that the current in the armature is only a few amps instead of the o/p which is usually around 80 amps these days, the circuit for a few amps is easier and cheaper.

Knowledge has become very divided, a domestic or industrial electrician wouldnt have experience of auto electrics, in fact an auto electrician these days might not know much about a vehicle alty, they just need to know when its not working and how to replace it.

Black box syndrome.