Alternator failure modes

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
I have a 28V 20A B&C standby alternator system which is only putting out 20 volts at 1200rpm.
B&C took the troubleshooting report of ohms and volts from various components which I sent them, and say it's the alternator. (its got 800hrs on it) Adjusting the voltage regulator up does nothing. They say the field voltage is normal.
Just wondering what failure modes could cause this.
One brush failed? A diode?
Thanks for any ideas.
 
Check the field resistance. Contaminated or worn brushes are the single biggest cause of alternator troubles, and that by a wide margin. The field voltage at the field terminal only means that the regulator is working; it says nothing about the actual current flow through the field.

Resistance should be around 6-8 ohms. If it's up around 15 or 30 ohms, you have a problem. It might even jump around when you move the prop.
 
You are right, they called for a field resistance on the alternator of 3-10ohms and I measured zero. You are saying the brushes could fail to result in this?
 
Let'sgoflying! said:
You are right, they called for a field resistance on the alternator of 3-10ohms and I measured zero. You are saying the brushes could fail to result in this?
Click to expand...

"Zero" ohms means a dead short. Are you sure it's not an infinite reading? A shorted field would fry the regulator, and even if it wasn't cooked you'd still see no voltage at the terminal if it's at ground potential, as a short would be.

A broken brush could open the circuit. More commonly you'd find a brush worn right out and not making good contact, and even more commonly you find the brushes contaminated with grease from an overlubricated rear bearing, and the brushes start to dissolve and form a sludge on themselves and the slip rings that has a high resistance, inhibiting field current and weakening the output.
 
The very worst diagnostic problem I've seen is a rotor where the winding opens only at higher speed.o_O:confused:
 
nrpetersen said:
The very worst diagnostic problem I've seen is a rotor where the winding opens only at higher speed.o_O:confused:
Click to expand...

That does sound challenging, centrifugal force at work? Using hind sight, I suppose an ammeter measuring the field current would show this up nicely. How did you figure it out?
 
nrpetersen said:
The very worst diagnostic problem I've seen is a rotor where the winding opens only at higher speed.o_O:confused:
Click to expand...
That would be a stinker to find. The weirdest I found was a burned-out field winding. Someone, maybe an instructor with a student, had probably turned the Master/Alt switches on to demonstrate some nav procedure on the radios or something. When the alternator isn't turning, the voltage is at battery voltage and the regulator sees that and applies full current to the field to try to bring the voltage up. And when the alternator isn't turning, there's no cooling air going through it, so that field winding gradually heats up and the insulating varnish burns off the wires and they short out and get really hot.

If you turn the master on for a preflight or something, turn both the Master and Alt switches on and see that the Overvolt/Low volt light is OFF. Then turn the Alt switch off and see that the light comes ON. Do your other preflight electrical checks (flaps and lights, whatever) with the Alt still off. If the airplane has an electronic ACU, that light likely won't come on; it actually measures low voltage, not just a dead regulator like most do.
 
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