Multiple alternator failures

Kurt Zierhut

Kurt Zierhut

Filing Flight Plan
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kzierhut
I have a 2000 Piper Saratoga that is now on its third alternator failure in three months.
The folks making the alternator (Hartzel) cannot give us a solution.
The local FBO keeps replacing the alternator.
The failures usually starts with a buzz or whine heard in the headsets, then progresses to a flickering "Alt Fail" light, then to a complete failure and a buss under-voltage.
Analysis of the failed alternators shows failed diodes inside.
Who might have better suggestions?
Thanks.
 
It'ds not likely the alternator. Diodes usually die because you overheat them. Have you checked the voltage regulator and the connections back to the battery? Have you had any battery issues? Does this plane have the aluminum battery cables?
 
Sounds like you've got a short in the wiring. See if it's abrading on anything, take it all apart and do a detailed inspection. It sounds like it's intermittent so using a meter may not show you anything.
 
Which regulator do you have installed? What do you see for amp draw prior to failure?
 
How is the voltage regulator connected? Does it regulate the field to ground? I had a spell of alternator failures in my Viking. The voltage regulator in my plane is connected between the field coil and ground, so if the field is shorted to ground the alternator will put out maximum voltage. What was going on with my three failed Hartzel alternators is that the field coil on the rotor was expanding centrifually under rotation and rubbing against the inside of the alternator casing, thus shorting it to ground and causing the alternator to put out max voltage. The over voltage relay would trip and cause the alternator to drop out of the system. I went through three hartzel alternators with this manufacturing defect until, hopefully, getting a good one. The bad thing about this failure mode was that it would only happen above a certain RPM in order to generate enough force to cause the rotor coil to expand and rub against the inside of the case. Hartzel tested at low RPMs, so it took a lot of work and back and forth to get them to admit that the problem was a theirs with some bad batches during manufacturing.
 
Thanks for the inputs. There is a engine/electrical monitor system installed that shows voltage and currents.
When the Alt light is solid on, the indicated buss voltage is still about 28.5V, battery current is +2 A, and alternator current is 22 A.
This seems a contradiction but the monitor system only shows average voltage and current over a second or two.
The Alt Fail annunciator light shows that there is a dip in voltage. The dip in voltage is due to a failed diode or two in alternator.
We are still working out things but the decision is to review ALL aircraft wiring. A warning from another source was that aluminum oxidation is making poor contacts and an intermittent battery connection will result in voltage spikes that damage diodes.
 
Tom-D said:
replace it with.
https://www.aircraftspruce.com/catalog/eppages/jascoaltkit2o.php?clickkey=609251
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Tom- That's a link to an alternator; did you read this part?:
Kurt Zierhut said:
I have a 2000 Piper Saratoga that is now on its third alternator failure in three months.
Click to expand...
Could you please explain how yet another alternator will solve the problem?

It seems to me a better option is the understand what part failed on the previous alternators, and then solve the problem of whatever caused the multiple failures.
 
Checking the wiring is always the place to start. Especially airframe grounds. Don't overlook the master switch. I came across this article as I was troubleshooting my own regulator. Found it very helpful. In my case it turned out to be the overvolt relay being heat sensitive.

http://ronkilber.tripod.com/alternat/alternat.htm

Not sure I understand the high output at the alternator, but low draw at the battery. if you pull all the breakers, and turn them on one by one, do you see any of one them really spike the ammeter?
 
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I have been doing a lot of testing for the last two weeks.
The alternator seems to be putting out about the right voltage and current for the system.
But I just tested the diodes inside and 4 of 6 are open. Only one pair of diodes are ever conducting.
The result is high ripple current and a whine in the avionics.
This is the second Hartzell alternator to be pulled for open diodes.
I cannot believe the plane or wiring is causing it as the remaining diodes are working.
Either the diodes being used are out of spec or they are loose and not getting conduction or cooling.
 
Kurt Zierhut said:
I have a 2000 Piper Saratoga that is now on its third alternator failure in three months.
The folks making the alternator (Hartzel) cannot give us a solution.
The local FBO keeps replacing the alternator.
The failures usually starts with a buzz or whine heard in the headsets, then progresses to a flickering "Alt Fail" light, then to a complete failure and a buss under-voltage.
Analysis of the failed alternators shows failed diodes inside.
Who might have better suggestions?
Thanks.
Click to expand...

Ever had your prop dynamically balanced? Prior to the first alternator going out, was any work done on the prop or was the spinner removed and replaced?
 
If the alternator is a direct drive, have you checked the torque slip clutch?
 
Kurt Zierhut said:
No prop work has been done in the last year; since before this issue started.



This isn’t the clutch. Diodes inside are damaged.
Click to expand...
Exactly. If the clutch was shot you wouldn't be seeing 28 volts and a 22 amp output.
An intermittent short in the alternator-to-output breaker cable could zap the diodes. Worth looking for. But I've had plenty of hassles with Hartzell's alternators, and Kelly's before them. They put too much grease (WAY too much grease) in the rear bearing, and it gets squeezed out when they assemble the alternator and the rotor shaft goes into that bearing. The grease gets flung around and ends up on the slip rings and brushes, creating a resistive sludge that limits field current, which limits the output current, sometimes to almost nothing. They've been doing this for 15 or 20 years. SDRs didn't help.
 
We have been over the aircraft wiring now twice.
We do not see any intermittent source of high current/short.
The resistance thru slip rings to rotor is correct and constant.
If an short kills diodes when 6 are working, we would see last 2 fail even faster but last two are working now for a month.
Partial rectification is obvious from a bench test. See below from a bench test of BATT voltage terminal.
Yesterday we sent the alternator to another shop to first diagnose, then disassemble, then repair. Hartzel is out of the loop.
upload_2020-11-18_8-34-55.png
 
This is why I prefer my GENERATOR. Yes, it's heavier but a metal commutator is more rugged than diodes. It's called more thermal mass.
 
Snowmass said:
This is why I prefer my GENERATOR. Yes, it's heavier but a metal commutator is more rugged than diodes. It's called more thermal mass.
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Generators have shorter lives than alternators. Period. And they don't produce anything at idle RPM, a real drawback for those who fly at night.

Your metal commutator carries the ENTIRE output current of the generator, so it uses massive brushes. The alternator's brushes are very small and carry only the field current, typically two or three amps. The generator's commutator wears, and once it's down to minimum, it's junk. The alternator's slip rings last much longer. In the flight school I had alternators with six thousand or more hours on them and all they ever needed was periodic brush replacement and maybe new bearings after several thousand hours. Cessna's recomendation for generator brush inspection was every 200 hours. For alternators it was 500 hours.

An alternator weighs half what a generator does, and produces twice the current. Carmakers went to alternators nearly 60 years ago, and never looked back. I have never heard anyone wish they still had a generator. The only advantage a generator has is its ability to bootstrap itself; if the battery is dead and you can get the engine running somehow, it will pick up and start charging. The alternator won't do that.

In all my years of maintaining airplanes, almost all with alternators, I NEVER had a diode failure. If the OP has repeated diode failures and they're not due to a short causing an overcurrent, it's a quality-control issue, not a design issue.
 
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Are the individual diodes of the three phase bridge failing open, which would suggest excess current or a serious current spike, or are they failing shorted which would suggest an inverse voltage spike? If I interpret the digital oscilloscope trace above correctly, they are failing open. Is that correct? Are they maybe loose in their heat sink afterwards? Does the solder show signs of being melted?
 
nrpetersen said:
Are the individual diodes of the three phase bridge failing open, which would suggest excess current or a serious current spike, or are they failing shorted which would suggest an inverse voltage spike? If I interpret the digital oscilloscope trace above correctly, they are failing open. Is that correct? Are they maybe loose in their heat sink afterwards? Does the solder show signs of being melted?
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If they were failing shorted the OP wouldn't be seeing 28 volts or a 22-amp charge. The battery would be discharging rapidly through shorted diodes, probably tripping the breaker.
 
FWIW, our club plane also has had repeat alternator failures. I have noticed that the low voltage light flickers when the airplane has vibration, mostly when it is rolling down the runway for takeoff. The mechanic was never able to reproduce the problem, but I am thinking that there is something loose and shorting to ground. Large current surges can induce high voltages in inductors or wiring and can impact the regulator.
 
If a diode in a three-phase-bridge shorts, it immediately gets an unlimited current thru the opposite diode. The result is that both diodes are burned open. That current is from the alternator in parallel with the battery so it burns open REALLY fast.

The cause of the problem I am seeing is one of three: (1) below spec diodes, (2) diodes mounted loose without heat-sinking, (3) other wiring short in plane. But we have been over the plane twice.
 
sarangan said:
FWIW, our club plane also has had repeat alternator failures. I have noticed that the low voltage light flickers when the airplane has vibration, mostly when it is rolling down the runway for takeoff. The mechanic was never able to reproduce the problem, but I am thinking that there is something loose and shorting to ground.
Click to expand...
Check the overvolt sensor's ground connection. If it's loose it's not happy. Check the regulator's case for proper airframe grounding; if it's loose the regulator thinks the voltage is low and boost the field current to raise the voltage. Check the "A" wire connection at the regulator and at its other end, which might be the alternator output or the bus.

Vibration will aggravate such poor connections. Makes them intermittent. Hard to duplicate on the ground.
 
Kurt Zierhut said:
If a diode in a three-phase-bridge shorts, it immediately gets an unlimited current thru the opposite diode. The result is that both diodes are burned open. That current is from the alternator in parallel with the battery so it burns open REALLY fast.

The cause of the problem I am seeing is one of three: (1) below spec diodes, (2) diodes mounted loose without heat-sinking, (3) other wiring short in plane. But we have been over the plane twice.
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Kurt, when you say you have been over the plane twice, did this include tracing each wire in the circuit from beginning to end? I'm thinking either a bad run of diodes, or a chafed wire that intermittently grounds out.
 
Dan Thomas said:
Generators have shorter lives than alternators. Period. And they don't produce anything at idle RPM, a real drawback for those who fly at night.

Your metal commutator carries the ENTIRE output current of the generator, so it uses massive brushes. The alternator's brushes are very small and carry only the field current, typically two or three amps. The generator's commutator wears, and once it's down to minimum, it's junk. The alternator's slip rings last much longer. In the flight school I had alternators with six thousand or more hours on them and all they ever needed was periodic brush replacement and maybe new bearings after several thousand hours. Cessna's recomendation for generator brush inspection was every 200 hours. For alternators it was 500 hours.

An alternator weighs half what a generator does, and produces twice the current. Carmakers went to alternators nearly 60 years ago, and never looked back. I have never heard anyone wish they still had a generator. The only advantage a generator has is its ability to bootstrap itself; if the battery is dead and you can get the engine running somehow, it will pick up and start charging. The alternator won't do that.

In all my years of maintaining airplanes, almost all with alternators, I NEVER had a diode failure. If the OP has repeated diode failures and they're not due to a short causing an overcurrent, it's a quality-control issue, not a design issue.
Click to expand...
And in my 68 years and 8000 hours of flying I never had a generator failure; however, I have brush replacement requirements which would show up ahead of failure with flickering out put. At 1000 hours I now just replace bearings and super cheap brushes. I have had a couple of alternator failures in automobiles in the same time. All your arguments about weight and power generation at low RPM are true but totally irrelevant to my type of flying .And alternators are ipso facto much harder on drive belt slipping. Who worries about weight in a 1964 C-210 and I now almost never fly into airports where there is a long wait to TO because I am allergic to ramp fees so low RPM output is totally irrelevant.

And BTW fuses are much superior to CBs which you may also disagree with.
 
Snowmass said:
And BTW fuses are much superior to CBs which you may also disagree with.
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I've had fuse issues. They suffer corrosion, both inside and out, and I've found fuses that look perfectly good but corrosion has opened them inside the end cap.

And if you fly IFR or night and a fuse blows, it gets interesting. Give me breakers any day. I've found bad breakers too; their contacts oxidize after 40 years and the mechanism wears out as well if it's actuated too often, but fuses don't go for 40 years either without problems.
 
I already knew how you answers and I just made my statement to provoke you. I also prefer wet vacuum pumps and ventruri backups. In fact so much that I decided to measure super venturi drag. However, I will not waste time arguing with you.venturi & truck.jpg
 
Snowmass said:
I already knew how you answers and I just made my statement to provoke you.
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Do you still have vacuum-tube radios in your airplane too? And incandescent landing lights? How about a rotating beacon?
 
Dan Thomas said:
I've had fuse issues. They suffer corrosion, both inside and out, and I've found fuses that look perfectly good but corrosion has opened them inside the end cap.

And if you fly IFR or night and a fuse blows, it gets interesting. Give me breakers any day. I've found bad breakers too; their contacts oxidize after 40 years and the mechanism wears out as well if it's actuated too often, but fuses don't go for 40 years either without problems.
Click to expand...
It is hard enough to get the old 4AG fuses for Cessna fuse holders. They are either expensive or not available. I tend to buy boxes here and there when I see them on EBay. Cheap automotive fuses work in a pinch, but they are the wrong diameter.
When I got my plane there was a bunch of mud matched fuses and it took at least $75 to get a stock of all the sizes required, including the 15 amp slow blow for the flaps.
 
Dan Thomas said:
I've had fuse issues. They suffer corrosion, both inside and out, and I've found fuses that look perfectly good but corrosion has opened them inside the end cap.

And if you fly IFR or night and a fuse blows, it gets interesting. Give me breakers any day. I've found bad breakers too; their contacts oxidize after 40 years and the mechanism wears out as well if it's actuated too often, but fuses don't go for 40 years either without problems.
Click to expand...
I had a night time failure of a circuit breaker, Las Vegas to Phoenix, but never a fuse failure in 68 years and 8000 hours.
 
Here is (near) the end of the long alternator story.
We had a repair shop look at the near-new-but-failed alternator.
They immediately found shorted windings on the stator.
They saw loose and poorly completed windings.
This repeated (3 times) failure of an alternator is due to poor construction from Hartzel.
 
Kurt Zierhut said:
Here is (near) the end of the long alternator story.
We had a repair shop look at the near-new-but-failed alternator.
They immediately found shorted windings on the stator.
They saw loose and poorly completed windings.
This repeated (3 times) failure of an alternator is due to poor construction from Hartzel.
Click to expand...

Wow, just wow.
 
Kurt Zierhut said:
Here is (near) the end of the long alternator story.
We had a repair shop look at the near-new-but-failed alternator.
They immediately found shorted windings on the stator.
They saw loose and poorly completed windings.
This repeated (3 times) failure of an alternator is due to poor construction from Hartzel.
Click to expand...

Yikes. That’s a QC issue that needs to be stomped out quickly.
 
Somewhere I have seen alternator systems in which a seventh "master diode" is added between the bridge output and the rest of the system. IOW battery isolation is not just guaranteed by the bridge diodes, but also the master diode (or whatever it is called). I'm not sure why that was done, but is this one of those situations? If so, a shorted bridge diode could confine its mayhem to one of the alternator phases, and not short the battery to ground.
 
That seventh diode may be in place but I never saw a massive battery short.

There is also a principal in most aircraft systems that cars rarely do. That is a "battery disconnect relay" and an "alternator disconnect relay". That's why those system have a switch for each. Shutting off one or the other can isolate the problem source.

But... more very recent data...
The shop doing the work just discovered the idler for the belt driving the alternator has failed many hours ago. The pulley had wobbled for so many hours that the press-in fit was opened up by 0.1 inch diameter. The vibration this would cause is almost certainly the single common factor to four alternator failures. (Also kind of scary that it flew this way.)
 
Kurt Zierhut said:
That seventh diode may be in place but I never saw a massive battery short.

There is also a principal in most aircraft systems that cars rarely do. That is a "battery disconnect relay" and an "alternator disconnect relay". That's why those system have a switch for each. Shutting off one or the other can isolate the problem source.

But... more very recent data...
The shop doing the work just discovered the idler for the belt driving the alternator has failed many hours ago. The pulley had wobbled for so many hours that the press-in fit was opened up by 0.1 inch diameter. The vibration this would cause is almost certainly the single common factor to four alternator failures. (Also kind of scary that it flew this way.)
Click to expand...
Most older airplanes didn't have an alternator contator, just the voltage regulator shutoff. Killing the regulator kills the alternator. When Cessna went back into singles production in '96 they included an alternator contactor.

A failed idler! You need to find better inspectors.
 
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