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Alternator field fault

Acrophil

Active Member
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I am getting a field fault on the alternator occasionally. The wire has been checked from end to end. The alternator has been bench checked twice. I can reset it on the VPX and it never recurs on the same flight. Airpower 60 amp. Output is normal when the field is not shorted.

It was doing this before the panel upgrade and VPX.

Any ideas?
 
It's virtually impossible to diagnose electrical problems on internet forums, but I'll try.

Does your alternator have an overvoltage crowbar built in? If so, it may be tripping and shorting the field circuit.

You said you have an Airpower alternator, but is this actual brand or just the distributor?
 
Alternator fault

Whoops
AL12-EI60/B PlanePower alternator.

Yes I know it is nearly impossible to do over internet. I can't do it and I am standing here looking at it!

Just want to perform obvious checks before sending back to PlanePower for a real bench check.
 
What do you have the field current set at? The PP alternator will draw a full 5A (and maybe a little more) when asked.
 
Field fault

Walt
Field amps are usually around 4.5 . I don't know that it is adjustable.

Vern
Very interesting thread. My fault generally occurs during run up. I reset and field is usually around 4.5 and output is 14.5 volts. I have not tested for over voltage protection. Thanks for info

Phil
 
Phil,

How is your field switch wired? I just finished a project here at SteinAir and Marc at Vertical Power told me to use the dedicated alternator field C/B in the VP, and to make sure to tie it to a switch input (not "always on"). The field switch on your panel then only provides a ground to close that VP switch. If you do it this way a field trip will annunciate "overvoltage" instead of "short circuit". It also insures full functionality of the VP with respect to the charging system. This may not solve the tripping problem but since your VP shows "short circuit" after a trip it's probably not wired the way Marc recommends.
 
Little knowledge

Thanks Mike. A little knowledge is dangerous snd I didn't
even have that much! Will check and fix it.

Phil
 
Since this was happening pre VPX I would suggest that the over voltage crowbar is kicking in to soon and tripping the over current protection in the VPX. Sounds like the alternator is bad. The PP alternators have a built in over voltage crowbar. The crowbar shorts the field to ground when an over voltage condition exists. Thus tripping the field breaker.
 
Since this was happening pre VPX I would suggest that the over voltage crowbar is kicking in to soon and tripping the over current protection in the VPX. Sounds like the alternator is bad. The PP alternators have a built in over voltage crowbar. The crowbar shorts the field to ground when an over voltage condition exists. Thus tripping the field breaker.

It's critical to make sure every connection in the field circuit is solid. Turn on the master, measure the voltage at the master bus then measure the voltage at the input to the field connector on the alternator. This is a bit tricky... you'll need to push the voltmeter probe into the back of the connector to intercept the terminal.

You should have less than 0.25 volt difference. Any more and you need to start looking for problems.

If this measures OK, then start looking into the alternator. It's possible to remove the alternator and build a bench tester like I did, but it's probably easier to remove the brush/regulator/ovp assembly and test it on the bench.

You'll need a current limited variable voltage power supply. Set the current limit to 1 amp and vary the voltage upwards starting at 12V. The OVP trip point should be around 17V. My failed OVP tripped at 13.8V.

This takes some electrical skills, so find the guy on the field named 'Sparky' and ask for help.
 
The Plane Power alternators use the field voltage, as measured at the alternator field terminal, as the sense voltage. As Vern describes, if your field voltage is to low, the alternator will be fooled into thinking your main buss voltage is also low. When this situation exists, the internal PP regulator will command the alternator to output a higher voltage to the buss. This will then trip the over-voltage crowbar, which is internal to the alternator, when the alternator output exceeds the per-determined value. The crowbar does this by SHORTING the field terminal to ground thus tripping your breaker.
One thing to note, the Vertical Power creates a tiny voltage drop through the unit due to the way it switches. You will never see the field voltage at the same level as the buss voltage when you use the VPX system. This small voltage drop at the field output terminal will cause the PP alternator to regulate . 2-.3 volts higher then if you connected the field directly to the buss. If the (Ask me how I know) This in no way is a bad thing, just something to note.
 
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Great Resource

Thanks guys. I have printed this out and will spend tomorrow trouble shooting. Will probably get the guys at PP on the line as well.

One other question. The VPX has over voltage protection as well as the PP. Are they working together, in series, independent of each other or does PP need to be disabled or should I just be grateful for dual protection?
 
Interesting thread...my PlanePower alternator was doing something similar - random breaker trips for no apparent reason. The alternator finally died after 210 hours in service, and I sent it in a little over a month ago. Plane Power was great to work with, and covered my repairs under warranty even though I was well outside of their coverage.
 
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