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Alternator poles


Peregrine

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Following the discussion I started last autumn about a new tachometer (http://canalworld.net/forums/index.php?/topic/87213-replacing-a-tachometer/#comment-1882715) I bought a VDO Viewline. The calculation to calibrate it for the pulses from the alternator is straightforward, but means knowing the number of poles that the alternator has. The part number label shows it's a Prestolite AS123 80 amp, though the Aqualine's manual says it's a Lucas 70 amp.

I can't find any official info on the web about the number of poles, but one forum suggested that the Lucas A127 type typically has 12, so that's what I used in the calculation. However, that's producing a reading of about 1000 rpm at tickover, whereas the old tacho read about 600, and I don't know which one is correct. Without a strobe hand-held I'm a bit stuck!

Please does anyone know how many poles the AS123 has, or where I could find that info?

 

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I have an "ebay special" optical tachometer which cost about £15. It is a small handheld device which works by projecting light onto a supplied reflective sticky paper disc and it reads the pulses from the reflections on a digital display. It seems quite accurate on the few things I have checked... a few percent..

It would be a fair check on what you discover

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12 poles is apparently correct for an A127:

Hopefully Snib will be along in a while to confirm the correct number for the AS123. 

I think Nick's idea of using a cheap strobe tachometer to confirm and/or trim the reading is a good one but they've gone up a bit since he bought one:

http://m.ebay.co.uk/itm/Digital-LCD-Non-Contact-Stroboscope-Tachometer-Tester-19999RPM-with-Backlit-/391553601330?hash=item5b2a69cf32%3Ag%3A93oAAOSwFe5Xz88-&_trkparms=pageci%3A7a612ef5-3357-11e7-bdf5-74dbd18011be%7Cparentrq%3Ae449159415b0ac8036086660fffdab71%7Ciid%3A1

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I am tempted to say find a boat with a rev counter and the same engine that sounds similar to yours. Note the idle revs and set your counter to the same revs when idling. If I recall on the VDO it is done with a little push button thing. I am sure it would be accurate enough for boat in use, probably not more than 50 rpm out.

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You can get Android apps to turn your phone into a stroboscope.  You need a white mark on the flywheel and shade from ambient light.  Worth a try.  The idle spped should be in the manual somewhere and is often preset on the fuel pump.  600 sounds a bit low to me.  What is the engine?  Car engines idle at about 900.

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Many thanks to all who responded. It resulted in:

  • Getting a very speedy email response from Prestolite confirming that the alternator is 12-pole
  • Downloading a strobe/tachometer app for my best beloved's rather old iPhone

In the end it was a "Doh!" moment. As I was fiddling with the app, I realised that I'd not measured the crankshaft pulley correctly. That is 5'', and the alternator pulley is 3", so the correct calculation is 5/3 * 12/2 = 10 pulses per revolution. I'd entered 8 first time around. Once that was in, the idling speed was shown as 800 rpm, which is a little higher than that shown by the old tachometer, but perfectly reasonable.

(Of course, though now OT, that means that the engine alternator is just starting to provide charging current at tickover, while the domestic alternator with a 6" pulley is spinning at 1600 rpm, which the spec chart indicates should yield about 15 amps. Not great, but a bit better than what I'd assumed under the old regime.)

Edited by Peregrine
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16 hours ago, mross said:

It would be interesting to know the engine you have.

It's an Isuzu 42, dating from 2007. The boat's manual said it would be a Lucas 70 amp alternator, but the alternator label didn't identify the manufacturer, and was marked "80 Amp". Googling its HMI (Isuzu) part number led to the Prestolite website page for the 66021126 alternator, which gave lots of detail, but not the number of poles. However, their Contact Us page has an email address for their European operation, so I sent off an ROI and got a reply first thing Monday morning!

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4 hours ago, mross said:

does your engine manual give the idling speed?

No. I've checked through both the standard Isuzu manual and the additional one for canal boat users, and there's nothing.

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