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harvey4(at)earthlink.net
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 Posted: Tue Oct 21, 2008 4:17 pm Post subject: Alternator questions |
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When I bought a Honda auto alternator for my Cozy many moons ago,
it's spec said it puts out 70 amps at 10,000 revs.
I don't need anything like 70 amps (nor the associated heat) and I
didn't like the idea of anything turning at 10,000 revs under my
cowling, so I turned up a larger pulley to step down the alternator
speed and it's output.
I think I went too far. The original pulley was about 2" diameter and
used a flat belt. My new pulley is about 5 1/2" diameter with a
machined "V" belt groove to match the Lycoming flywheel. My voltmeter
barely indicates 12v at 2000 engine revs (and nothing like the 13.6
volts I think it should be) and my ammeter is showing a slight
discharge when the engine is running, suggesting the ammeter isn't
charging the battery. Up till now, I put the readings down to poor
gauge quality.
So my questions are;
1) Would I see a "low" voltage reading if the alternator wasn't
turning fast enough?
2) Is there a low-end threshold rpm at which the alternator must run
to sustain battery charge? What happens on a car at idle RPM?
3) Was my logic flawed about stepping down the alternator RPM?
Thanks for comments.
Neil
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mprather(at)spro.net
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| Posted: Tue Oct 21, 2008 4:56 pm Post subject: Alternator questions |
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It sounds like your alternator or wiring is kaput. The bare battery
should have about 12.5V by itself. If the alternator is turning at all,
and the bus loads are low, the bus should at least maintain 12.5V. Even
with the larger pulley, the alternator is going to turn several times the
engine RPM. I'd guess that the flywheel is probably at least 18 inches in
diameter which would be between three and four times the diameter of the
alternator pulley. That means that at 2000 engine RPM, the alternator is
turning between 6000 and 8000 RPM. Plenty for normal output.
I'd say check the wiring, and if all that looks good, pull the alternator
and take it to Autozone or someplace with an alternator test stand have it
checked out.
Matt-
| Quote: |
<harvey4(at)earthlink.net>
When I bought a Honda auto alternator for my Cozy many moons ago,
it's spec said it puts out 70 amps at 10,000 revs.
I don't need anything like 70 amps (nor the associated heat) and I
didn't like the idea of anything turning at 10,000 revs under my
cowling, so I turned up a larger pulley to step down the alternator
speed and it's output.
I think I went too far. The original pulley was about 2" diameter and
used a flat belt. My new pulley is about 5 1/2" diameter with a
machined "V" belt groove to match the Lycoming flywheel. My voltmeter
barely indicates 12v at 2000 engine revs (and nothing like the 13.6
volts I think it should be) and my ammeter is showing a slight
discharge when the engine is running, suggesting the ammeter isn't
charging the battery. Up till now, I put the readings down to poor
gauge quality.
So my questions are;
1) Would I see a "low" voltage reading if the alternator wasn't
turning fast enough?
2) Is there a low-end threshold rpm at which the alternator must run
to sustain battery charge? What happens on a car at idle RPM?
3) Was my logic flawed about stepping down the alternator RPM?
Thanks for comments.
Neil
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nuckolls.bob(at)cox.net
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| Posted: Tue Oct 21, 2008 5:56 pm Post subject: Alternator questions |
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At 06:55 PM 10/21/2008 -0600, you wrote:
| Quote: |
It sounds like your alternator or wiring is kaput. The bare battery
should have about 12.5V by itself. If the alternator is turning at all,
and the bus loads are low, the bus should at least maintain 12.5V. Even
with the larger pulley, the alternator is going to turn several times the
engine RPM. I'd guess that the flywheel is probably at least 18 inches in
diameter which would be between three and four times the diameter of the
alternator pulley. That means that at 2000 engine RPM, the alternator is
turning between 6000 and 8000 RPM. Plenty for normal output.
I'd say check the wiring, and if all that looks good, pull the alternator
and take it to Autozone or someplace with an alternator test stand have it
checked out.
|
That's a good first start. Automotive alternators
tend to develop full output in the 4000 to 5000 RPM
range. A typical plot looks like this:
http://www.aeroelectric.com/Pictures/Curves/80A_OutCurve.gif
If your alternator pulley is smaller than engine
pulley, then at 2,000 rpm your alternator should
be capable of full or nearly full output. I agree
with Matt that your particular specimen is either
not hooked up right or kaput.
On Lycoming installations, it's not uncommon to leave
the stock small pulleys on modern alternators. 10K+
RPM is not especially hard on them. Field current is
lower, fans run more air, and you get full output at
taxi rpms to recharge the battery after start-up.
See:
http://aeroelectric.com/Pictures/Alternators/60A_Nipon_Denso.jpg
This particular item has been installed on thousands
of Lycomings as has it's smaller stable mate, both of
which run the small pulley. Service record has been
excellent.
Check your wiring . . . then have the alternator
checked.
Bob . . .
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jim jewell
Joined: 09 Jan 2006
Posts: 82
Location: Kelowna B.C. Canada
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| Posted: Tue Oct 21, 2008 6:13 pm Post subject: Alternator questions |
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Hi Neil,
Assuming the alternator and it's support system (wiring) are in good form, a
pulley something close to three inch DIA. should offer good idle output
without over revving the unit at 2700 RPM. Modern alternators can
comfortably handle revs in excess of 12000 RPM.
Conservatively speaking the Honda that your unit was designed for ran a
pulley to pulley ratio of something like three to four to one with 5000 +
engine RPM survival expected by the designer.
A five inch + pulley seems to me to be well into overkill as far as rev
limiting is concerned. I would not be surprised to see low power output at
idle.
An awful lot of racers run these units at much higher RPM. than you will
ever reach with your engine RPM limited at something under 3000 RPM.
Happy building,
Jim in Kelowna - RV6-A for sale {[
---
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