PA-28-151 Altenator pegs at 20A

[airheadpenguin]

I have a PA-28-151 where the alternator pegs at 20A, after that point it doesn't matter what I turn on it stays at 20A on the ammeter. This is suspiciously 1/3 capacity, so I'm wondering if this is a 3 phase alternator where 2 of the phases have died but can't find the specs on the number of phases its outputting.

We're also getting increased whine in the intercom which we believe is brush related. On other alternators I've worked on (BMW R series bikes) the brushes are not connected to individual phases, are they here?

 

[Graueradler]

The brushes are connected to slip rings, not a commutator like in a generator, and provide the field current to the rotor. The output comes off of the stator with no brushes. It does have 3 diodes and if one fails, it will continue to have an output but with increased ripple and less capacity. The ripple can be heard as an engine RPM dependent whine on the radios.

 

[airheadpenguin]

The sllip rings and rotor I'm familiar with, the BMW R bikes which have 2 piece alternators (rotor friction fit to the crank) and a stationary stator have a 3 phase stator. So I guess the real question is, how many phases does the stator of a PA-28-151 alternator have and would 2 of the 3 fail simultaneously?

 

[Silvaire]

...So I guess the real question is, how many phases does the stator of a PA-28-151 alternator have and would 2 of the 3 fail simultaneously?

It's a standard Chrysler alternator and it has three. Could two possibly fail? Sure but even if that's the case I don't think you have any conclusive proof that they did it simultaneously. Check the diodes in the rectifier if you want to know for sure. That would be the most likely failure point.

 

[Dr. O]

Take it to an alternator repair shop for testing. Don't tell them it is for a plane. Say it is for your air boat or dune buggy. Get it fixed and put it back on.
Don't ask don't tell.

 

[Hiperbiper]

I believe the PA-28 series has a Loadmeter (meter in series with the alternator and showing how much power is required by the buss) versus an Ammeter (meter in series with the battery and showing charging or drawdown from the battery).

I would expect to see a loadmeter zing up to 20% to charge a battery after starting but it should settle back down to 5-10 without anything else turned on...

Use a voltmeter to check your voltage; with the engine running and everything turned on you should still have 13.8 to 14.2 off the buss. If so then everything would seem to be working. As for the whine, with the engine running switch the alternator off and see if the whine goes away...if it does you might have dirty pickup brushes, funk on the slip rings or a poor engine to airframe ground. This last one is easy to find; take the cowling off and clamp one end of a pair of jumper cables as close as you can get to the alternator and the other end (of the same cable of course) to the airframe/engine mount junction. If the whine is gone when you run the engine you need to look for crud on the engine/airframe ground strap.

Hope this helps
Chris

 

[Dan Thomas]

Whine in the headset (if its frequency changes with RPM) comes from the rectification of the alternating current generated in the stator of the alternator. The diodes turn it into pulsating DC, which we might hear for several reasons. If filters fail, noise can get through. There's often a filter on the firewall that the alternator's output runs through,and if it's not well-grounded (corrosion, maybe) it can't do the job. Or if its internal capacitor fails it can't filter. Or there might be a so-called "ground loop" where there's a bad ground somewhere between the alternator and the airframe, or along the airframe somewhere, and that pulsating current is finding its way through other circuits into the intercom or radios.

Or, most likely in your case, there's a bad diode in the alternator. That makes a really rough pulsating DC and also reduces the alternator's output big time. An oscilloscope will detect it.

Dan