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Alternator Noise
I know that this is another frequent topic, but I'm hoping that someone
can help narrow this down for me. I have a rebuilt Chrysler alternator (from Electrosystems) on my Cherokee that was installed about 400 hours ago. However, I am experiencing alternator whine that varies with engine speed, but which disappears completely when I turn off the alternator side of the master switch. In addition, increasing electrical load seems to make it worse. I have replaced the filter capacitor on the alternator output with a new one (15uF) and it doesn't seem to have made much of a difference (the filter is grounded to the alternator). My mechanic took a look at the alternator at the last annual inspection and thought that the cooling fins were backwards (auto style, not aviation style). Not sure if that's normal for the Cherokees or not, but I can't believe that Electrosystems would do it wrong. How long should I expect this alternator to last? Does this sound like a rectifier problem that would warrant replacement of the alternator after just 400 hours? Could it be a regulator problem? Any suggestions? Thanks, JKG |
#2
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On Sun, 05 Dec 2004 22:42:57 -0500, Jonathan Goodish
wrote: My mechanic took a look at the alternator at the last annual inspection and thought that the cooling fins were backwards (auto style, not aviation style). Not sure if that's normal for the Cherokees or not, but I can't believe that Electrosystems would do it wrong. I also have a Cherokee with a Chrysler alternator. They should be "aircraft type" as your mechanic puts it. If they are installed wrong, you will not get good cooling and could have burned something out prematurely. How long should I expect this alternator to last? Does this sound like a rectifier problem that would warrant replacement of the alternator after just 400 hours? Could it be a regulator problem? Any suggestions? Alternators life depends on many things, but if your re-build was done right it should last more than 400 hours. If the fins aren't pushing air the right way it could warrant another re-build. From just what you've described, it does not sound like a regulator to me, but I'm no expert, just someone who got a re-build a couple months ago because of a whine, and variable voltage output. If you have a descent rectifier repair shop (alternators, generators, starters, etc.) in your area, take the alternator to them and have them bench test it. In about 10 minutes they can tell you anything and everything that is wrong with it. HTH and Good luck. z |
#3
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Find or purchase a clamp on amp meter that will read both AC and DC
amps. Turn on enough loads so that the alternator is generating at least 50% of it's rated load. Measure the DC amps and the AC amps of the alternator output. The AC current should not exceed 15% of the DC amps. If it exceeds 15% then you have a bad diode or stator winding. In many cases you can not get 50% of rated output if you have a bad diode. John On Sun, 05 Dec 2004 22:42:57 -0500, Jonathan Goodish wrote: I know that this is another frequent topic, but I'm hoping that someone can help narrow this down for me. I have a rebuilt Chrysler alternator (from Electrosystems) on my Cherokee that was installed about 400 hours ago. However, I am experiencing alternator whine that varies with engine speed, but which disappears completely when I turn off the alternator side of the master switch. In addition, increasing electrical load seems to make it worse. I have replaced the filter capacitor on the alternator output with a new one (15uF) and it doesn't seem to have made much of a difference (the filter is grounded to the alternator). My mechanic took a look at the alternator at the last annual inspection and thought that the cooling fins were backwards (auto style, not aviation style). Not sure if that's normal for the Cherokees or not, but I can't believe that Electrosystems would do it wrong. How long should I expect this alternator to last? Does this sound like a rectifier problem that would warrant replacement of the alternator after just 400 hours? Could it be a regulator problem? Any suggestions? Thanks, JKG |
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#5
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The clamp on amp meters that read DC and AC current use a Hall effect
IC. I bought one on Ebay for $60 over a year ago. Most run of the mill clamp on amp meters use a transformer which will only read AC. On Tue, 7 Dec 2004 06:16:02 +0000 (UTC), David Lesher wrote: (John_F) writes: Find or purchase a clamp on amp meter that will read both AC and DC amps. Turn on enough loads so that the alternator is generating at least 50% of it's rated load. Measure the DC amps and the AC amps of the alternator output. The AC current should not exceed 15% of the DC amps. If it exceeds 15% then you have a bad diode or stator winding. In many cases you can not get 50% of rated output if you have a bad diode. Interesting approach. I'm not sure how well it will work with the tools a GA pilot has. A real clamp-on DC ammeter is a rather specialized tool. The usual ones are AC -- the clamp makes a 1-turn transformer out of the wire you are around. It may or may not read on dc-dominant paths; I can see the trasformer getting saturated and/or only reading accurately on balanced waveforms. Hmm, it should be possible to use a DVM in AC mode on the alternator output. Fair-good DVM's are really inexpensive. A True RMS one would tell you in a second. But it would take some experimenting to say what AC voltages would mean "yes, you DO have a bad diode."... |
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#7
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"David Lesher" wrote Hmm, it should be possible to use a DVM in AC mode on the alternator output. Fair-good DVM's are really inexpensive. A True RMS one would tell you in a second. But it would take some experimenting to say what AC voltages would mean "yes, you DO have a bad diode."... Take the alternator off, and take it down to auto zone, and they will put it on a machine to test the output and the diodes, for free. Another possibility is that there is a ground loop in the audio system. (using the frame as the ground to carry the audio signal) -- Jim in NC |
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Jonathan Goodish writes:
I know that this is another frequent topic, but I'm hoping that someone can help narrow this down for me. I have a rebuilt Chrysler alternator (from Electrosystems) on my Cherokee that was installed about 400 hours ago. However, I am experiencing alternator whine that varies with engine speed, but which disappears completely when I turn off the alternator side of the master switch. In addition, increasing electrical load seems to make it worse. You could have lost a diode. There are 6 main diodes, usually molded into a unit. If you have a friend with a portable oscilloscope, it's an easy test: load the alternator and look at the waveform. You'll see the chunk missing as compared to http://www.medar.com/images/3phase5.gif Or pull the alternator [disconnect the battery ground FIRST, please..] and take it to an alternator shop. [NOT a chain auto parts store who has an alleged tester.] They should be able to spin-test it. AFAIK, having a test done by a non-FAA-anointed shop would be legal. (Having them fix it, of course will get you sent to Gitmo..) http://www.windstuffnow.com/main/3_phase_basics.htm -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |
#9
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On Mon, 6 Dec 2004 06:15:07 +0000 (UTC), David Lesher
wrote: Or pull the alternator [disconnect the battery ground FIRST, please..] Why is this important? What will happen if you don't disconnect the ground first? Also thanks for the link on understanding the alternator! z |
#10
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Well lets say you are a hot shot mechanic and disconnect the hot side
of the battery first with your handy dandy metallic wrench. Some sweet young thing walks by and distracts you for an instant and the wrench handle contacts the battery box or some other part of the airframe. There will be a big flash as a couple of thousand amps flows in the wrench handle. A hole will appear in the aluminum airframe or battery box where the wrench contacted it . This is bad but what happens next is worse. The flash is next to a battery vent cap and the O2 and H2 gas inside the battery ignite and blow the top off of the battery spraying battery acid in your eyes and all over the airplane. Need I say more? John On Mon, 06 Dec 2004 16:48:03 GMT, zatatime wrote: On Mon, 6 Dec 2004 06:15:07 +0000 (UTC), David Lesher wrote: Or pull the alternator [disconnect the battery ground FIRST, please..] Why is this important? What will happen if you don't disconnect the ground first? Also thanks for the link on understanding the alternator! z |
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