Debugging alternator issues

[Michael Noel]

The Chrysler alternator on my Archer is no longer working and I'm trying to understand the failure mode. I checked the brush and it still has over 1/4 inch of material. The brush has good force against the slip ring. After cleaning the slip ring with q tips and contact cleaner while rotating the prop through multiple rotations, the measured resistance through the brush to aircraft ground is abought 8 ohms through half of a rotation of the alternator and into the K ohms through the other half rotation. Does this difference in resistance on half a rotation point to a particular type of alternator failure?

 

[Michael Noel]

After further reading, I think I have a possible explanation for the variation in field resistance while rotating the alternator shaft. This alternator has only one brush and so probably only one slip ring. That would make the alternator bearings the path for return current from the rotor and also make the bearings wear faster. And that could make the field circuit resistance dependent on the shaft orientation. If that's what Chrysler did back then, it was a poor design choice for them and for Piper.

 

[cowman]

It’s not unusual for these things to test out good on the ground and fail in the air. Heat, vibration, and g-forces can reveal an open or short circuit that you just won’t catch in ground tests.

As an example I was given a remanufactured alternator that worked great on the ground but shorted and took out my voltage regulator when I did a steep turn. Tested fine on the ground after. Eventually we figured out one needle in the needle bearing was left perpendicular in the shaft allowing it to wobble just enough to intermittently short. Got another reman that the brush holders were loose on right out of the box. So tip #2 if you get a reman unit don’t trust it either.

 

[Dan Thomas]

Your alternator brushes are shot. I have never seen any alternator with only one brush, and they would never use the bearings to carry ground current. That would fry the bearings in a few hours at most, and probably in a few minutes. And 1/4" of brush is a badly worn brush. If one of the brushes has popped out of its holder, the spring has likely chewed up its slip ring and the alternator is shot.

The field resistance (rotor resistance) should be three to five ohms or so, and it should be steady as you turn the pulley, through full rotation, in either direction.

Aircraft manufacturers publish service manuals, and in those manuals are inspection procedures and schedules. Alternators are normall a 500-hour inspection item, because those brushes wear much faster than they do in your car. That alternator is turning near its redline most of the time, while your car's alternator is running at maybe a third of its redline in cruise on the highway.

That alternator wasn't a poor design choice by Piper. It hasn't been maintained, an owner's choice.

 

[Clip4]

After further reading, I think I have a possible explanation for the variation in field resistance while rotating the alternator shaft. This alternator has only one brush and so probably only one slip ring. That would make the alternator bearings the path for return current from the rotor and also make the bearings wear faster. And that could make the field circuit resistance dependent on the shaft orientation. If that's what Chrysler did back then, it was a poor design choice for them and for Piper.

The alternator was cheaper to produce. The bean counters thought it was a great design because they saved $2 an alternator.

 

[Michael Noel]

If you look at the alternator itself and at the parts manual, this alternator only has one brush assembly. The only alternative I see is for the bearings to serve as the rotor ground. The rotor current is very small compared to the stator current.

 

[Capt. Geoffrey Thorpe]

https://www.hemmings.com/stories/article/chrysler-alternators

 

[Dan Thomas]

https://www.hemmings.com/stories/article/chrysler-alternators

Yup. Thanks for that. From that article:

Identification of these units is relatively easy; early alternators had a rounded back and only one field connection on one of the two brush holders, marked "FLD." The other field connection is grounded through the screw that holds in the brush holder, and is marked "GRD." The later units have a square section on the back of the casing where the rectifier assembly is mounted, and both brush holders are marked "FLD," for dual field connections.

Two brush holders means two brushes. Running any current, however low, through ball bearings is a really, really stupid engineering idea, and if they actually did that those alternators would have disappeared six months after they were introduced. Expecting electron flow through greased balls and races? Arcing that would pit them and destroy the whole bearing real quick. Even a dry bearing would soon fail. Alternators use graphite brushes and copper slip rings for good reasons.

I think this might be the alternator the OP has. How many slip rings do we see there? I see two: one radial, one axial.



More detail:



Edit: One more:



From https://www.cessnaflyer.org/maintenance-tech/item/1306-alternators-electrical-systems.html

 

[Michael Noel]

Yes, you're right. The second brush was underneath the heavy duty ground cable and probably hadn't been changed in about 1k hours. The other had probably been changed at least once since alternator rebuild...

 

[Michael Noel]

...and the local alternator shop charged about $4 total and required 10 minutes to rebuild them both. I'll need to examine the vertical slip ring to see if any damage was done.

 

[Capt. Geoffrey Thorpe]

View attachment 108218

I think you got your money's worth out of that one.

 

[Dan Thomas]

This is why we don't send electrical currents through bearings: https://www.pumpsandsystems.com/how-prevent-electrical-erosion-bearings

The problems addressed in the article relate to stray currents generated in the metal rotor itself as a result of magnetic forces within a motor. Variable-frequency drives are particularly prone to this, with their powerful AC signals. The currents may be quite small, but they're enough to start the pitting and erosion. The solution in many cases is to use insulated bearings.

 

[Dan Thomas]

...and the local alternator shop charged about $4 total and required 10 minutes to rebuild them both. I'll need to examine the vertical slip ring to see if any damage was done.

It won't be pretty. That copper lead in the brush will have arced and burned the slip ring.

The Ford alternators commonly used on Cessnas are more robust. They use larger, rectangular-section brushes that have more contact area, and the slip rings are both radial and their diameter is smaller than the large axial ring on the Chrysler unit. That allows a lower surface speed and lower brush wear.



Newer alternators are using small-diameter slip rings, probably to increase life by reducing surface speed on the rings and brushes. Plane Power likely has something like that. I recently opened up the alternator off my Hyundai Sonata to see what the brushes were up to, and they were 3/4 gone after 240,000 km (149,000 miles). Dusty roads contribute to that by adding grit to the unsealed brushes. The new alternator had a sealed brush affair that should reduce that. I couldn't buy brushes for the old one, and the slip rings were deeply grooved anyway.

 

[Michael Noel]

...and the local alternator shop charged about $4 total and required 10 minutes to rebuild them both. I'll need to examine the vertical slip ring to see if any damage was done.

The alternator with the new brushes is working noticeably better than before the failure. Before the failure, I had to be around 800 RPM before the alt warning light stuttered fully off. Now it stays off even when idling around 600 RPM.

I looked at the vertical slip ring with a borescope and saw a little discoloration and roughness in one particular area on the slip ring. Turning the prop through with a narrowed, squared off tongue depressor stick against the slip ring showed slight roughness in that area. I wrapped a narrow strip of 2500 grit Emory cloth around the end of the stick and moved the prop around and back and forth until the slip ring felt smooth. I'll revisit that brush in a few hours to make sure it's still OK.
Thanks for the help.

It won't be pretty. That copper lead in the brush will have arced and burned the slip ring.

The Ford alternators commonly used on Cessnas are more robust. They use larger, rectangular-section brushes that have more contact area, and the slip rings are both radial and their diameter is smaller than the large axial ring on the Chrysler unit. That allows a lower surface speed and lower brush wear.

View attachment 108233

Newer alternators are using small-diameter slip rings, probably to increase life by reducing surface speed on the rings and brushes. Plane Power likely has something like that. I recently opened up the alternator off my Hyundai Sonata to see what the brushes were up to, and they were 3/4 gone after 240,000 km (149,000 miles). Dusty roads contribute to that by adding grit to the unsealed brushes. The new alternator had a sealed brush affair that should reduce that. I couldn't buy brushes for the old one, and the slip rings were deeply grooved anyway.

View attachment 108234