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Dual Alternator "Failure"

 
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art(at)zemon.name
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PostPosted: Tue Aug 13, 2019 4:50 pm    Post subject: Dual Alternator "Failure" Reply with quote

Folks,
I had a weird "failure" this afternoon: neither alternator was doing anything and neither alternator field circuit breaker had tripped. I put "failure" in quotes because I don't believe that the alternators actually failed; that is just too unlikely. Here are the details:
First, the wiring diagram:
[img]https://drive-thirdparty.googleusercontent.com/16/type/application/pdf[/img] engine.pdf
I had taxied to the fuel pump, and back, and flown three legs. Five cycles of engine start, operation, and shut down. All with alternators operating correctly. As I started the takeoff roll for the fourth flight, the EFIS alerted that bus voltage was low. I continued the takeoff; it was only a 15 minute flight to home base.
I have two B&C alternators with B&C voltage regulators, one primary belt driven and one backup gear driven.
Once airborne, I cycled the alternator field switches. No joy from either alternator. I pulled and reset both field circuit breakers. No joy. I even power cycled the master, on the hope that the EFIS voltage regulator was at fault. No joy.
During the third flight, the primary alternator field breaker had popped. I reset it and the alternator returned to normal operation. It was stupidly hot in the cockpit and the sun was shining on my black glare shield and I know that I need ventilation behind the instrument panel. I chalked this up to heat and vibration. In 87 hours of operation, this is only the second time that that breaker has popped. After resetting the breaker, I checked both alternators independently (turning off each field switch in turn) and confirmed that each was operating normally.
I had 12.9 volts during the takeoff roll. It was down to 12.6 volts on landing. so definitely no output from the alternators. The voltage regulators are set for 14.5 volts and 13.0 volts. I typically see 14.8 or 14.9 volts from the primary and 13.1 from the backup alternator.
Do you have any ideas what could cause this? I did a tiny amount of debugging but I was hot and tired so I did not get into the weeds, yet. I confirmed that both current limiters between the alternators and the battery contactor are OK. I tugged on the cable between the current limiters and the battery contactor and it seems solid, no loose nuts. I removed the glare shield and checked the voltage regulators. Both were warm but not hot. All of the wires connected to them are tight.
    -- Art Z.
P.S. Yes, the engine was turning Smile so I assume that both alternators were turning.
--
https://CheerfulCurmudgeon.com/Love the stranger for you yourselves were strangers in Egypt. Deut. 10:19


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ceengland7(at)gmail.com
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PostPosted: Tue Aug 13, 2019 7:15 pm    Post subject: Dual Alternator "Failure" Reply with quote

On 8/13/2019 7:48 PM, Art Zemon wrote:

Quote:
Folks,


I had a weird "failure" this afternoon: neither alternator was doing anything and neither alternator field circuit breaker had tripped. I put "failure" in quotes because I don't believe that the alternators actually failed; that is just too unlikely. Here are the details:


First, the wiring diagram:
[img]https://drive-thirdparty.googleusercontent.com/16/type/application/pdf[/img] engine.pdf


I had taxied to the fuel pump, and back, and flown three legs. Five cycles of engine start, operation, and shut down. All with alternators operating correctly. As I started the takeoff roll for the fourth flight, the EFIS alerted that bus voltage was low. I continued the takeoff; it was only a 15 minute flight to home base.


I have two B&C alternators with B&C voltage regulators, one primary belt driven and one backup gear driven.


Once airborne, I cycled the alternator field switches. No joy from either alternator. I pulled and reset both field circuit breakers. No joy. I even power cycled the master, on the hope that the EFIS voltage regulator was at fault. No joy.


During the third flight, the primary alternator field breaker had popped. I reset it and the alternator returned to normal operation. It was stupidly hot in the cockpit and the sun was shining on my black glare shield and I know that I need ventilation behind the instrument panel. I chalked this up to heat and vibration. In 87 hours of operation, this is only the second time that that breaker has popped. After resetting the breaker, I checked both alternators independently (turning off each field switch in turn) and confirmed that each was operating normally.
I had 12.9 volts during the takeoff roll. It was down to 12.6 volts on landing. so definitely no output from the alternators. The voltage regulators are set for 14.5 volts and 13.0 volts. I typically see 14.8 or 14.9 volts from the primary and 13.1 from the backup alternator.


Do you have any ideas what could cause this? I did a tiny amount of debugging but I was hot and tired so I did not get into the weeds, yet. I confirmed that both current limiters between the alternators and the battery contactor are OK. I tugged on the cable between the current limiters and the battery contactor and it seems solid, no loose nuts. I removed the glare shield and checked the voltage regulators. Both were warm but not hot. All of the wires connected to them are tight.


    -- Art Z.


P.S. Yes, the engine was turning Smile so I assume that both alternators were turning.


--
https://CheerfulCurmudgeon.com/ Love the stranger for you yourselves were strangers in Egypt. Deut. 10:19















Hi Art,

Looking at your pdf, the only thing in common is the 'fat' wire from starter to master contactor, then to your main bus. Any circuit protections anywhere along that path?

Regardless of where your current failure is located, that's something to think about; any issue along that path can take out both alternators.

Charlie
Virus-free. www.avast.com [url=#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2] [/url]


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user9253



Joined: 28 Mar 2008
Posts: 1436
Location: Riley TWP Michigan

PostPosted: Wed Aug 14, 2019 4:59 am    Post subject: Re: Dual Alternator Reply with quote

A very heavy electrical load could cause those symptoms.
Of course that is highly unlikely also.


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argoldman(at)aol.com
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PostPosted: Wed Aug 14, 2019 8:15 am    Post subject: Dual Alternator "Failure" Reply with quote

Greetings Art,


An interesting problem.


Hopefully you will have solved it by now, however if not here are some diagnostic tests that you can easily do. The wring diagram seems like it is adequate, although I wonder why you used a hall sensor to the auxiliary alternator connected to the B&C regulator 7,1,&2.


A charging system of an aircraft (car etc) is a relatively simple deal. The alternator spins and under the right field input voltage sends regulated VOLTAGE out to the ship. The voltage regulator takes bus power (12V or so) and controls the VOLTAGE output of the alternator via turning the alternator on and off to maintain the appropriate set voltage.


That being the case, there are a couple of places that, using a simple VOM, you can check non-destructively.


1. With master and other switches on check the voltage at regulator pin 6 (bus voltage in) on each regulator.
If you get no voltage here the fault is somewhere between the battery and pin 6 ie switches, contactors, breakers, wiring etc.


2 if you are getting the appropriate voltage at these pins (screw terminals) then check the continuity between screw 4 and the field input wire at each alternator. if there is no continuity, it is probably the wire(s). If the continuity is there it may be the regulators (where are they located -- is it a hot place?



Dual failures probably mitigate the fact that the regulators, themselves are at fault, however I would suspect the bus wiring leading up to the regulators (screw 6) to be at fault.


Although you have a good wiring diagram, many times we (I) make modifications that seem minor which have consequences.


I would suspect your switches or circuit breakers.


When you tugged on the wires that confirmed physical continuity but not electrical continuity. Check continuity with VOM (multimeter) while pulling. Another source of non-conductivity is corrosion in the terminals somewhere along the line. You can use the multimeter to check resistance of these leads.



The breakers that we use are generally thermal and the high temperature behind the IP could have been the reason for the trip-- or what?? (did the first trip of the breaker happen with high heat in the cabin?")



Good luck in hunting---Let us know what you found.


Rich









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