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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Tue Jul 04, 2006 7:00 am Post subject: Alternator failure thoughts

There has been a lot of heat of late in the great alternator debate  

(GAB). I thought I would throw in some thoughts and experience I have  

had.



I have seen a number of alternator failures on cars, boats, and  

airplanes. Some were pretty simple, no-brainer failures and some were  

"HOLY S--T" failures. I thought I would toss in here the ways I can  

see alternators failing (and have failed) and then address the failures.



Here are the failure modes I have see so far (feel free to add to  

this list):



1. Straight electrical failures:



	a. regulator failed -- no output

	b. brushes failed -- no output

	c. output diode failure -- whine and reduced output

	d. diode trio failure -- no output

	e. regulator failed -- runaway output (destroys battery and anything  

else attached to the bus)



2. Mechanical failures:



	a. bearing failure -- armature chews up stator -- no output

	b. broken mounting brackets -- no output

	c. broken wires, usually field -- no output



3. Miscellaneous failures:



	a. burned up stator -- overload -- no output



First thing to think about is what to do when the alternator fails.  

If you are on this list it probably means you are building your  

electrical system with an eye toward safely completing the flight  

with the lights still burning in the panel. I have come to the  

conclusion that alternator failure is something that demands a  

landing ASAP. Why? Because I have experienced too many failed-bearing  

failures. The alternator stops producing output so you tend to think  

in terms of electrical failure and just keep on going. I wonder what  

would happen if the bearing were to fail so completely that the  

armature would jam against the stator or one of the bell housings.  

Instant stoppage. Nothing good is going to come from that much mass  

stopping suddenly. I can certainly imagine mounting ears and mounting  

brackets snapping and bits of alternator taking the grand tour of  

your cowling.



So, I hold that an alternator failure should require a "let's land  

and check it out now" response unless you can tell for sure it is an  

electrical-only failure. And I don't know of a good way to tell if it  

is an electrical-only failure.



Now as to causes, some have come up with vibration as a potential  

source of failure. I agree. Lycoming engines are not particularly  

smooth (but can be made a lot smoother by balancing, matching  

mixtures in all the cylinders, and doing a dynamic prop balance). I  

see extra-beefy mounting hardware as a plus here. The other form of  

imbalance is the armature rotating balance. If the armature is  

balanced the instantaneous vibrational loads on the bearings will be  

a LOT less. I would expect this to result in reduced incidence of  

mechanical failure of alternator bearings.



Heat is another killer. Insufficient cooling can result in burned  

stator windings and/or failure of the lubricant in the bearings  

leading to bearing failure. It seems to me that ensuring excellent  

cooling is very important too. Many alternators have external or  

internal fans but since most alternators are mounted on the hot side  

of the baffling the fan just circulates hot air through the  

alternator. The alternator really needs its own source of cool air  

from before it passes through the cylinders. I would probably try to  

find a source of cooling air that won't be as likely to contain water  

if you run through the rain. I can't imagine that circulating water  

through the alternator is likely to improve its longevity.



Hopefully this will spark some discussion about installation  

practices that may improve alternator longevity over and above  

anything we can do electrically.



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

paulm(at)olypen.com
Guest

Posted: Tue Jul 04, 2006 8:17 am Post subject: Alternator failure thoughts

Good post.



I would add I have had an alternator on a Excavator suddenly lock up due to 

a bearing seize and it was on the opposite end from the belt. It was a 

sealed ball bearing and the result was a burned up belt 2300 hours on the 

unit. A different failure than the too tight belt bearing damage often seen.



I also agree immediate landing is called for.



Paul



---

chaztuna(at)adelphia.net
Guest

Posted: Tue Jul 04, 2006 9:32 am Post subject: Alternator failure thoughts

 	  Quote: 	
		  snipped

 I have seen a number of alternator failures on cars, boats, and  

 airplanes. Some were pretty simple, no-brainer failures and some were  

 "HOLY S--T" failures. I thought I would toss in here the ways I can  

 see alternators failing (and have failed) and then address the failures.



 Here are the failure modes I have see so far (feel free to add to  

 this list):



 1. Straight electrical failures:



         a. regulator failed -- no output

         b. brushes failed -- no output

         c. output diode failure -- whine and reduced output

         d. diode trio failure -- no output

         e. regulator failed -- runaway output (destroys battery and anything  

 else attached to the bus) 	
snipped



 Brian,

  I'd like to expand on item C of the list above



 Output diodes comprise the rectifier. (converts AC current to DC)  For the purposes of the alternators we use, the rectifier is comprised of 6 diodes. These diodes can fail in two modes, open or short.  Failing open (diode blows apart) is the more common failure mode. In this mode, each diode that fails will reduce the alternator's output by 1/6th.

  Diodes that fail shorted are more insidious. Often, the pilot will not notice a decrease in performance. However, he/she will generally notice an increase in noise (generally on the radios) This is because 1/6th of the alternator's output is now bleeding through as AC current. The ship's electrical system is designed for DC current. The one item which really can not tolerate AC current is the battery. Any time an alternator fails, it is best to do a postmortem, to determine the cause.

  Shorted rectifier diodes which are not repaired promptly, will soon destroy the battery's storage capacity. This damage to the battery may not be noticed, without conducting a battery output test. This is important if you subscribe to Bob N's ideas regarding battery only emergency operations.

 Charlie Kuss

Dale Ensing

Joined: 11 Jan 2006
Posts: 571
Location: Aero Plantation Weddington NC

Posted: Tue Jul 04, 2006 10:05 am Post subject: Alternator failure thoughts

Output diodes comprise the rectifier. (converts AC current to DC)  For  the purposes of the alternators we use, the rectifier is comprised of 6 diodes.  These diodes can fail in two modes, open or short.  Failing open (diode  blows apart) is the more common failure mode. In this mode, each diode that  fails will reduce the alternator's output by 1/6th.



 Charlie,

 Does the reduce output show as lower voltage  and/or reduced maximum amps?

 Dale Ensing

_________________
Dale Ensing
RV-6A
Aero Plantation
Weddington NC

chaztuna(at)adelphia.net
Guest

Posted: Tue Jul 04, 2006 11:20 am Post subject: Alternator failure thoughts

At 02:02 PM 7/4/2006, you wrote:

  	  Quote: 	
		  Output diodes comprise the rectifier. (converts AC current to DC)  For the purposes of the alternators we use, the rectifier is comprised of 6 diodes. These diodes can fail in two modes, open or short.  Failing open (diode blows apart) is the more common failure mode. In this mode, each diode that fails will reduce the alternator's output by 1/6th.

 Charlie,Does the reduce output show as lower voltage and/or reduced maximum amps?

 Dale Ensing 	


 Dale,

  The reduced output can show itself as both a reduction in voltage and amperage. If the load on the electrical system is below the crippled alternator's ability to produce power, you may not notice any difference. Generally though,  reduced charging voltage will be noticed first. This is why Bob N says that a good voltmeter is more important to have than an ammeter.

 An analogy could be made to an aircraft's ability to climb. An aircraft can only climb as long as the engine can produce power in excess of what is needed to maintain level flight at a given altitude.

  An alternator's ability to increase the system voltage (assuming we had no voltage regulator to stop a voltage climb) is limited by it's ability to produce current (amps) in excess of the load on the system.



 Charlie Kuss

klehman(at)albedo.net
Guest

Posted: Tue Jul 04, 2006 12:55 pm Post subject: Alternator failure thoughts

We've discussed rotation speed a number of times but I still used larger 

pulleys to keep the speed more in line with what my alternators would 

see in their oem application. Automotive demonstrated reliability means 

nothing to me when we run a component faster, hotter, at higher load, or 

in a tight cowl, compared to an automotive engine compartment. Bearings 

generally run cooler and last longer at slower speeds and vibration 

loads are lower. I also like short dedicated belts that don't flop 

around as much as long belts driving several accessories that have to be 

fairly tight. I don't trust any alternator bracket that is less than 

about 3/16" steel or 1/8" if truly 3 way triangulated. I hadn't really 

considered it necessary to land soon following a alternator failure as 

while I've discovered poor bearings during the electrical repair 

(usually not on the pulley end), I've never had a bearing as the direct 

cause of the failure --  yet   Maybe time to rethink a bit!

Ken



Brian Lloyd wrote:



 	  Quote: 	
		   

 <brian-yak(at)lloyd.com>



 There has been a lot of heat of late in the great alternator debate  

 (GAB). I thought I would throw in some thoughts and experience I have  

 had.



 I have seen a number of alternator failures on cars, boats, and  

 airplanes. Some were pretty simple, no-brainer failures and some were  

 "HOLY S--T" failures. I thought I would toss in here the ways I can  

 see alternators failing (and have failed) and then address the failures.



 Here are the failure modes I have see so far (feel free to add to  

 this list):



 1. Straight electrical failures:



     a. regulator failed -- no output

     b. brushes failed -- no output

     c. output diode failure -- whine and reduced output

     d. diode trio failure -- no output

     e. regulator failed -- runaway output (destroys battery and 

 anything  else attached to the bus)



 2. Mechanical failures:



     a. bearing failure -- armature chews up stator -- no output

     b. broken mounting brackets -- no output

     c. broken wires, usually field -- no output



 3. Miscellaneous failures:



     a. burned up stator -- overload -- no output



 First thing to think about is what to do when the alternator fails.  

 If you are on this list it probably means you are building your  

 electrical system with an eye toward safely completing the flight  

 with the lights still burning in the panel. I have come to the  

 conclusion that alternator failure is something that demands a  

 landing ASAP. Why? Because I have experienced too many failed-bearing  

 failures. The alternator stops producing output so you tend to think  

 in terms of electrical failure and just keep on going. I wonder what  

 would happen if the bearing were to fail so completely that the  

 armature would jam against the stator or one of the bell housings.  

 Instant stoppage. Nothing good is going to come from that much mass  

 stopping suddenly. I can certainly imagine mounting ears and mounting  

 brackets snapping and bits of alternator taking the grand tour of  

 your cowling.



 So, I hold that an alternator failure should require a "let's land  

 and check it out now" response unless you can tell for sure it is an  

 electrical-only failure. And I don't know of a good way to tell if it  

 is an electrical-only failure.



 Now as to causes, some have come up with vibration as a potential  

 source of failure. I agree. Lycoming engines are not particularly  

 smooth (but can be made a lot smoother by balancing, matching  

 mixtures in all the cylinders, and doing a dynamic prop balance). I  

 see extra-beefy mounting hardware as a plus here. The other form of  

 imbalance is the armature rotating balance. If the armature is  

 balanced the instantaneous vibrational loads on the bearings will be  

 a LOT less. I would expect this to result in reduced incidence of  

 mechanical failure of alternator bearings.



 Heat is another killer. Insufficient cooling can result in burned  

 stator windings and/or failure of the lubricant in the bearings  

 leading to bearing failure. It seems to me that ensuring excellent  

 cooling is very important too. Many alternators have external or  

 internal fans but since most alternators are mounted on the hot side  

 of the baffling the fan just circulates hot air through the  

 alternator. The alternator really needs its own source of cool air  

 from before it passes through the cylinders. I would probably try to  

 find a source of cooling air that won't be as likely to contain water  

 if you run through the rain. I can't imagine that circulating water  

 through the alternator is likely to improve its longevity.



 Hopefully this will spark some discussion about installation  

 practices that may improve alternator longevity over and above  

 anything we can do electrically.



 Brian Lloyd                         361 Catterline Way

 brian-yak AT lloyd DOT com          Folsom, CA 95630

 +1.916.367.2131 (voice)             +1.270.912.0788 (fax)

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Tue Jul 04, 2006 1:06 pm Post subject: Alternator failure thoughts

On Jul 4, 2006, at 1:07 PM, Charlie Kuss wrote:

 	  Quote: 	
		  >         c. output diode failure -- whine and reduced output

>

 	  Quote: 	

 Brian,

  I'd like to expand on item C of the list above

 Output diodes comprise the rectifier. (converts AC current to DC)   

 For the purposes of the alternators we use, the rectifier is  

 comprised of 6 diodes. These diodes can fail in two modes, open or  

 short.  Failing open (diode blows apart) is the more common failure  

 mode. In this mode, each diode that fails will reduce the  

 alternator's output by 1/6th.

Actually, I am pretty sure it is still 1/3 of the output. (Warning:  

technical BS follows.) Each diode is part of a full-wave bridge for  

two of the three phases. Therefore when one diode opens you lose a  

half-wave of output from two of the three phases. Since there are six  

half-wave outputs, two from each of the three phases, you still lose  

1/3 of your potential output.

And then there is the whole root mean square (RMS) thing. Without  

doing the math I am going to guess that the loss of one diode (open)  

is going to reduce the available power (amps probably) by 1/3. Ripple  

will also be up substantially.

 	  Quote: 	
		    Diodes that fail shorted are more insidious. Often, the pilot will  

 not notice a decrease in performance. However, he/she will  

 generally notice an increase in noise (generally on the radios)  

 This is because 1/6th of the alternator's output is now bleeding  

 through as AC current.

Well, actually it is not. With one diode shorted it will show up as a  

dead short on two of the phases for half a wave. The current from the  

affected phases will go through the good diode but return to the  

winding through the shorted diode so two of the phases will see a  

short through one diode drop. So for half a wave two of the phases  

will be producing maximum current and dissipating it in the  

resistance of those two stator windings. To the outside world it will  

look like and ope diode (I think -- I am guessing here) but I suspect  

that the affected phases of the stator will very quickly burn  

themselves up unless the alternator has superb cooling.

 	  Quote: 	
		   The ship's electrical system is designed for DC current. The one  

 item which really can not tolerate AC current is the battery. Any  

 time an alternator fails, it is best to do a postmortem, to  

 determine the cause.

  Shorted rectifier diodes which are not repaired promptly, will  

 soon destroy the battery's storage capacity. This damage to the  

 battery may not be noticed, without conducting a battery output  

 test. This is important if you subscribe to Bob N's ideas regarding  

 battery only emergency operations.

I understand your logic Charlie but, as I indicated above, I don't  

think it will work the way you have outlined.

But this was just an off-the-top-of-my-head analysis and very will  

could be wrong.

 	  Quote: 	
		   Charlie Kuss

Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Tue Jul 04, 2006 1:30 pm Post subject: Alternator failure thoughts

On Jul 4, 2006, at 3:16 PM, Charlie Kuss wrote:



 	  Quote: 	
		   At 02:02 PM 7/4/2006, you wrote:

> Output diodes comprise the rectifier. (converts AC current to DC)   

> For the purposes of the alternators we use, the rectifier is  

> comprised of 6 diodes. These diodes can fail in two modes, open or  

> short.  Failing open (diode blows apart) is the more common  

> failure mode. In this mode, each diode that fails will reduce the  

> alternator's output by 1/6th.

> Charlie,



> Does the reduce output show as lower voltage and/or reduced  

> maximum amps?

> Dale Ensing



 Dale,

  The reduced output can show itself as both a reduction in voltage  

 and amperage.


	


The peak voltage available will remain the same as each winding of  

the stator is capable of that peak voltage. But the ripple will be up  

and the available current will be down. With one diode gone (open) I  

would bet that a 60A alternator becomes a 40A alternator. If one  

diode shorts the alternator becomes a 40A alternator and then shortly  

thereafter a 20A alternator as two of the windings in the stator burn  

out.



 	  Quote: 	
		   If the load on the electrical system is below the crippled  

 alternator's ability to produce power, you may not notice any  

 difference.


	


I agree with that but you will start to hear one heck of an  

alternator whine.



 	  Quote: 	
		   Generally though,  reduced charging voltage will be noticed first.  

 This is why Bob N says that a good voltmeter is more important to  

 have than an ammeter.


	


But the VR is going to increase the field current to keep the voltage  

where it is supposed to be. As long as you are drawing less than 1/3  

of the alternator's rated output you won't notice anything but the  

whine. What will be reduced is the charging current.



 	  Quote: 	
		   An analogy could be made to an aircraft's ability to climb. An  

 aircraft can only climb as long as the engine can produce power in  

 excess of what is needed to maintain level flight at a given altitude.

  An alternator's ability to increase the system voltage (assuming  

 we had no voltage regulator to stop a voltage climb) is limited by  

 it's ability to produce current (amps) in excess of the load on the  

 system.


	


True.



 	  Quote: 	
		  

 Charlie Kuss




	


Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

chaztuna(at)adelphia.net
Guest

Posted: Tue Jul 04, 2006 2:56 pm Post subject: Alternator failure thoughts

 	  Quote: 	
		    	  Quote: 	
		    	  Quote: 	
		  snippedDoes the reduce output show as lower voltage and/or reduced  

 maximum amps?

 Dale Ensing 	


 Dale,

  The reduced output can show itself as both a reduction in voltage  

 and amperage. 	


 The peak voltage available will remain the same as each winding of  

 the stator is capable of that peak voltage. But the ripple will be up  

 and the available current will be down. With one diode gone (open) I  

 would bet that a 60A alternator becomes a 40A alternator. If one  

 diode shorts the alternator becomes a 40A alternator and then shortly  

 thereafter a 20A alternator as two of the windings in the stator burn  

 out.



  	  Quote: 	
		  If the load on the electrical system is below the crippled  

 alternator's ability to produce power, you may not notice any  

 difference. 	


 I agree with that but you will start to hear one heck of an  

 alternator whine.



  	  Quote: 	
		  Generally though,  reduced charging voltage will be noticed first.  

 This is why Bob N says that a good voltmeter is more important to  

 have than an ammeter. 	


 But the VR is going to increase the field current to keep the voltage  

 where it is supposed to be. As long as you are drawing less than 1/3  

 of the alternator's rated output you won't notice anything but the  

 whine. What will be reduced is the charging current.



  	  Quote: 	
		  An analogy could be made to an aircraft's ability to climb. An  

 aircraft can only climb as long as the engine can produce power in  

 excess of what is needed to maintain level flight at a given altitude.

  An alternator's ability to increase the system voltage (assuming  

 we had no voltage regulator to stop a voltage climb) is limited by  

 it's ability to produce current (amps) in excess of the load on the  

 system. 	


 True.



 Brian Lloyd                         361 Catterline Way 	


 Brian,

  We both agree that voltage drop is caused by the alternator's inability to produce enough current (amps) to meet system demands.  What I was trying to express, is that it would be more likely for the pilot (Dale) to notice a drop in voltage first. This is why Bob N recommends a low voltage warning system. When alternator output drops below what is being consumed by the ship's systems, voltage will always sag. 

 Charlie

chaztuna(at)adelphia.net
Guest

Posted: Tue Jul 04, 2006 2:56 pm Post subject: Alternator failure thoughts

 	  Quote: 	
		  On Jul 4, 2006, at 1:07 PM, Charlie Kuss wrote:



  	  Quote: 	
		    	  Quote: 	
		           c. output diode failure -- whine and reduced output

 	


 Brian,

  I'd like to expand on item C of the list above



 Output diodes comprise the rectifier. (converts AC current to DC)   

 For the purposes of the alternators we use, the rectifier is  

 comprised of 6 diodes. These diodes can fail in two modes, open or  

 short.  Failing open (diode blows apart) is the more common failure  

 mode. In this mode, each diode that fails will reduce the  

 alternator's output by 1/6th. 	


 Actually, I am pretty sure it is still 1/3 of the output. (Warning:  

 technical BS follows.) Each diode is part of a full-wave bridge for  

 two of the three phases. Therefore when one diode opens you lose a  

 half-wave of output from two of the three phases. Since there are six  

 half-wave outputs, two from each of the three phases, you still lose  

 1/3 of your potential output. 	


 Brian,

  Your suggestion of a 33% loss of output is closer to real world losses. I have tested charging systems which showed a 20 to 35% loss of output. Tear down of the alternator later for repair showed that one diode had failed in the rectifier.



 

  	  Quote: 	
		  And then there is the whole root mean square (RMS) thing. Without  

 doing the math I am going to guess that the loss of one diode (open)  

 is going to reduce the available power (amps probably) by 1/3. Ripple  

 will also be up substantially.



  	  Quote: 	
		   snipped 	


 Well, actually it is not. With one diode shorted it will show up as a  

 dead short on two of the phases for half a wave. The current from the  

 affected phases will go through the good diode but return to the  

 winding through the shorted diode so two of the phases will see a  

 short through one diode drop. So for half a wave two of the phases  

 will be producing maximum current and dissipating it in the  

 resistance of those two stator windings. To the outside world it will  

 look like and ope diode (I think -- I am guessing here) but I suspect  

 that the affected phases of the stator will very quickly burn  

 themselves up unless the alternator has superb cooling. 	


 I've tested alternators which would meet the output spec (for a limited time) Then one (or more) of the rectifier diodes would overheat and short. My point here is that not all failures are "hard" failures. When diagnosing a charging system you can't always rely on what "theory" says should happen.  



 

  	  Quote: 	
		    	  Quote: 	
		  The ship's electrical system is designed for DC current. The one  

 item which really can not tolerate AC current is the battery. Any  

 time an alternator fails, it is best to do a postmortem, to  

 determine the cause.

  Shorted rectifier diodes which are not repaired promptly, will  

 soon destroy the battery's storage capacity. This damage to the  

 battery may not be noticed, without conducting a battery output  

 test. This is important if you subscribe to Bob N's ideas regarding  

 battery only emergency operations. 	


 I understand your logic Charlie but, as I indicated above, I don't  

 think it will work the way you have outlined.



 But this was just an off-the-top-of-my-head analysis and very will  

 could be wrong.



 Brian Lloyd                         361 Catterline Way 	


 My experience here is anecdotal, as it simply relates to what I've seen in my experiences.



 Charlie

chaztuna(at)adelphia.net
Guest

Posted: Tue Jul 04, 2006 3:14 pm Post subject: Alternator failure thoughts

At 05:08 PM 7/4/2006, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Brian Lloyd <brian-yak(at)lloyd.com>



 

 On Jul 4, 2006, at 3:16 PM, Charlie Kuss wrote:



  	  Quote: 	
		  At 02:02 PM 7/4/2006, you wrote:

  	  Quote: 	
		  Output diodes comprise the rectifier. (converts AC current to DC)   

 For the purposes of the alternators we use, the rectifier is  

 comprised of 6 diodes. These diodes can fail in two modes, open or  

 short.  Failing open (diode blows apart) is the more common  

 failure mode. In this mode, each diode that fails will reduce the  

 alternator's output by 1/6th.

 Charlie,Does the reduce output show as lower voltage and/or reduced  

 maximum amps?

 Dale Ensing 	


 Dale,

 snippedIf the load on the electrical system is below the crippled  

 alternator's ability to produce power, you may not notice any  

 difference. 	


 I agree with that but you will start to hear one heck of an  

 alternator whine. 	


 In most situations you are correct. Unfortunately, the absence of whine doesn't mean that everything is rosy. Like they say, one test is worth a thousand expert opinions. Another is: Test, don't guess.



 snipped

 Charlie

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Tue Jul 04, 2006 6:24 pm Post subject: Alternator failure thoughts

On Jul 4, 2006, at 6:55 PM, Charlie Kuss wrote:

 	  Quote: 	
		  

 Brian,

  We both agree that voltage drop is caused by the alternator's  

 inability to produce enough current (amps) to meet system demands.   

 What I was trying to express, is that it would be more likely for  

 the pilot (Dale) to notice a drop in voltage first. This is why Bob  

 N recommends a low voltage warning system. When alternator output  

 drops below what is being consumed by the ship's systems, voltage  

 will always sag.


	


Very true. But until you reach the point where the alternator can't  

carry the load, system voltage will appear normal.



You know, Bob used to sell a loadmeter/voltmeter that had the ability  

to measure field voltage (voltage across the field windings which is  

proportional to field current). What would be apparent is a sudden  

increase in field voltage for the same RPM and load. Another  

advantage of externally regulated alternators.

Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Tue Jul 04, 2006 6:24 pm Post subject: Alternator failure thoughts

On Jul 4, 2006, at 6:47 PM, Charlie Kuss wrote:

 	  Quote: 	

 I've tested alternators which would meet the output spec (for a  

 limited time) Then one (or more) of the rectifier diodes would  

 overheat and short. My point here is that not all failures are  

 "hard" failures. When diagnosing a charging system you can't always  

 rely on what "theory" says should happen. 

Ah, you are telling me that the difference between theory and  

practice in practice is greater than the difference between theory  

and practice in theory? OK, I agree with that. 

 	  Quote: 	
		  > I understand your logic Charlie but, as I indicated above, I don't

> think it will work the way you have outlined.

>

> But this was just an off-the-top-of-my-head analysis and very will

> could be wrong.

>

> Brian Lloyd                         361 Catterline Way

 My experience here is anecdotal, as it simply relates to what I've  

 seen in my experiences.

Ah, real-life trumps theory. Yeah, I can go with that too.

Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

FLYaDIVE(at)aol.com
Guest

Posted: Wed Jul 05, 2006 3:09 am Post subject: Alternator failure thoughts

In a message dated 7/4/2006 9:25:44 PM Eastern Standard Time,  chaztuna(at)adelphia.net writes:

  	  Quote: 	
		  For the    purposes of the alternators we use, the rectifier is  

comprised of 6    diodes. These diodes can fail in two modes, open or  

short.     Failing open (diode blows apart) is the more common  

failure mode. In    this mode, each diode that fails will reduce the  

alternator's output    by 1/6th.

Charlie, 	


 

 Charlie:

  

 Why 6 diodes?  Half wave rectification only requires 2 diodes.   And Full wave reverification only requires 4 diodes.  So why 6 and how are  they hooked up?

  

 Barry

 "Chop'd Liver"

Gilles.Thesee(at)ac-greno
Guest

Posted: Wed Jul 05, 2006 3:22 am Post subject: Alternator failure thoughts

 	  Quote: 	
		   Why 6 diodes?  Half wave rectification only requires 2 diodes.  And 

 Full wave reverification only requires 4 diodes.  So why 6 and how are 

 they hooked up?

  

 Barry

 "Chop'd Liver"

Hi Barry,
	




Those alternators are 3 phase.



Regards,

Gilles Thesee

Grenoble, France

http://contrails.free.fr

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Wed Jul 05, 2006 4:37 am Post subject: Alternator failure thoughts

On Jul 5, 2006, at 6:57 AM, FLYaDIVE(at)aol.com wrote:



 	  Quote: 	
		   In a message dated 7/4/2006 9:25:44 PM Eastern Standard Time,  

 chaztuna(at)adelphia.net writes:

 For the purposes of the alternators we use, the rectifier is

 comprised of 6 diodes. These diodes can fail in two modes, open or

 short.  Failing open (diode blows apart) is the more common

 failure mode. In this mode, each diode that fails will reduce the

 alternator's output by 1/6th.

 Charlie,

 Charlie:



 Why 6 diodes?  Half wave rectification only requires 2 diodes.


	


For a single winding, half-wave rectification only needs one diode.



 	  Quote: 	
		   And Full wave reverification only requires 4 diodes.  So why 6 and  

 how are they hooked up?


	


It would be possible to write a whole dissertation on this subject.



The alternator is three phase. There are three windings in the stator  

of the alternator. Each one by itself produces a full AC wave but  

each one has its phase starting 120 degrees after the phase of the  

previous winding. Imagine a sine wave starting at zero, rising to a  

positive peak at 90 degrees, crossing zero again at 180 degrees,  

rising (falling?) to a negative peak at 270 degrees, and then coming  

back to zero at 360 degrees to start the whole cycle over again. Now  

imagine that the next winding starts its wave at zero just as the  

waveform from the first winding is on its downward slope between 90  

and 180 degrees. Do this again for the third winding. (This stuff is  

hard to describe in words without pictures.)



If you think of how a full-wave bridge rectifier is connected to a  

single winding (as in a transformer) You see that each leg of the  

transformer has two diodes connected to it -- one going from ground  

to the winding and another going from the winding to output. So each  

leg of the transformer requires two diodes, one to conduct the  

positive half of the cycle to the positive output and the other to  

connect the negative half of the cycle to ground.



Since we have a three-phase device we have three windings, not just  

one. It turns out that you can save diodes by hooking the three  

windings together so you end up with only three leads total. They are  

hooked up in either delta configuration or wye ('Y') configuration.  

Here, let me try to draw a picture. The letter 'W' will denote a  

winding and 'T' will denote a connection terminal:



Delta:



           T

          / \

         W   W

        /     \

       T---W---T



Wye:



           T

           |

           W

           |

           t

          / \

         W   W

        /     \

       T       T



(The center-tap terminal in the wye configuration is often not used  

so I made it a small 't'.)



Most alternators are wired in wye configuration. Each of the three  

main terminals gets two diodes, one each for the positive and  

negative half-cycle of the waveform available at that terminal.



BTW, if your alternator has a 'stator' terminal (the Delco  

alternators do) it is usually the center-tap terminal. On boats the  

AC at this terminal is used to drive the tachometer. The frequency is  

proportional to engine speed. Having your tach drop to zero while the  

engine is running is a good indication you have lost your alternator.  

(This is how I usually find out I have burned up another stator.)



BTW, if you are wondering about how you can 'short' all the windings  

together in the delta configuration without hurting anything you will  

find that the voltage at the free ends of any two windings connected  

together (vector sum) will be equal and opposite in phase to the  

voltage on the other winding so no current will flow.

 	  Quote: 	
		  

 Barry

 "Chop'd Liver"


	


Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

pwmac(at)sisna.com
Guest

Posted: Wed Jul 05, 2006 6:29 am Post subject: Alternator failure thoughts

Its interesting about the number of diodes used 

in alternators. The Ford alternator used in 

recent vehicles has 8 diodes and a regulator 

bolted to the back of the case. I have not been 

able to determine if the field wire is externally 

accessible. Anyhow, the 95130/160/200amp versions 

of the 3G alternator are physically pretty big 

for a plane. A schematic and other details can be 

found at: 

http://fullsizebronco.com/forum/showthread.php?t=55169&highlight=alternator

I would appreciate an explanation of for all the diodes.

The guy who sells the upgraded alternators also 

sells various pulleys if anybody has interest. 

His link is buried in the above site.

Regards, Paul

==========================

At 06:35 AM 7/5/2006, you wrote:

 	  Quote: 	

On Jul 5, 2006, at 6:57 AM, FLYaDIVE(at)aol.com wrote:

>In a message dated 7/4/2006 9:25:44 PM Eastern Standard Time,

>chaztuna(at)adelphia.net writes:

>For the purposes of the alternators we use, the rectifier is

>comprised of 6 diodes. These diodes can fail in two modes, open or

>short.  Failing open (diode blows apart) is the more common

>failure mode. In this mode, each diode that fails will reduce the

>alternator's output by 1/6th.

>Charlie,

>Charlie:

>

>Why 6 diodes?  Half wave rectification only requires 2 diodes.

For a single winding, half-wave rectification only needs one diode.

>And Full wave reverification only requires 4 diodes.  So why 6 and

>how are they hooked up?

It would be possible to write a whole dissertation on this subject.

The alternator is three phase. There are three windings in the stator

of the alternator. Each one by itself produces a full AC wave but

each one has its phase starting 120 degrees after the phase of the

previous winding. Imagine a sine wave starting at zero, rising to a

positive peak at 90 degrees, crossing zero again at 180 degrees,

rising (falling?) to a negative peak at 270 degrees, and then coming

back to zero at 360 degrees to start the whole cycle over again. Now

imagine that the next winding starts its wave at zero just as the

waveform from the first winding is on its downward slope between 90

and 180 degrees. Do this again for the third winding. (This stuff is

hard to describe in words without pictures.)

If you think of how a full-wave bridge rectifier is connected to a

single winding (as in a transformer) You see that each leg of the

transformer has two diodes connected to it -- one going from ground

to the winding and another going from the winding to output. So each

leg of the transformer requires two diodes, one to conduct the

positive half of the cycle to the positive output and the other to

connect the negative half of the cycle to ground.

Since we have a three-phase device we have three windings, not just

one. It turns out that you can save diodes by hooking the three

windings together so you end up with only three leads total. They are

hooked up in either delta configuration or wye ('Y') configuration.

Here, let me try to draw a picture. The letter 'W' will denote a

winding and 'T' will denote a connection terminal:

Delta:

           T

          / \

         W   W

        /     \

       T---W---T

Wye:

           T

           |

           W

           |

           t

          / \

         W   W

        /     \

       T       T

(The center-tap terminal in the wye configuration is often not used

so I made it a small 't'.)

Most alternators are wired in wye configuration. Each of the three

main terminals gets two diodes, one each for the positive and

negative half-cycle of the waveform available at that terminal.

BTW, if your alternator has a 'stator' terminal (the Delco

alternators do) it is usually the center-tap terminal. On boats the

AC at this terminal is used to drive the tachometer. The frequency is

proportional to engine speed. Having your tach drop to zero while the

engine is running is a good indication you have lost your alternator.

(This is how I usually find out I have burned up another stator.)

BTW, if you are wondering about how you can 'short' all the windings

together in the delta configuration without hurting anything you will

find that the voltage at the free ends of any two windings connected

together (vector sum) will be equal and opposite in phase to the

voltage on the other winding so no current will flow.

>

>Barry

>"Chop'd Liver"

Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

Jim Baker

Joined: 30 Mar 2006
Posts: 181
Location: Sayre, PA

Posted: Wed Jul 05, 2006 7:13 am Post subject: Alternator failure thoughts

 	  Quote: 	
		   Its interesting about the number of diodes used 

 in alternators. The Ford alternator used in 

 recent vehicles has 8 


	


Recall seeing an article that said Mercedes or BMW (forget 

which) uses 12......



Jim Baker

580.788.2779

Elmore City, OK

jonlaury(at)impulse.net
Guest

Posted: Wed Jul 05, 2006 8:32 am Post subject: Alternator failure thoughts

I'm not flying yet, but my 60 A ND ER  alternator (of undetermined auto application) has been modified by removing  the stamped steel vented rear cover and replacing it with a fiberglass  closed cover with a stub to accept a blast tube to force air  over the diode plate, as an attempt to compensate for a backward turning alt  fan. Aside from buying a B&C or Plane Power unit, is there a better way? Is  this likely to have a positive effect?

  

 John

frank.hinde(at)hp.com
Guest

Posted: Wed Jul 05, 2006 9:11 am Post subject: Alternator failure thoughts

Not sure i understand this "backwards turning" deal. Frome  what i can see  the internal fan alternators appear to have simply radial  fan blades that simply throw air out thru the sides of the alt. Therby  presumably sucking air in from both front and back.

  

 Adding a cool air supply to the regulator should therefore  augment the existing fan. I am thinking of doing a similar thing by riveting  something similar to the stamped rear cover of my Autozone Toyota Camry  special.

  

 Cetainly couldn't hurt

  

 Frank



   From:  owner-aeroelectric-list-server(at)matronics.com  [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of John  Burnaby

Sent: Wednesday, July 05, 2006 8:55 AM

To:  aeroelectric-list(at)matronics.com

Subject: Re: AeroElectric-List:  Alternator failure thoughts

 

 I'm not flying yet, but my 60 A ND ER  alternator (of undetermined auto application) has been modified by removing  the stamped steel vented rear cover and replacing it with a fiberglass  closed cover with a stub to accept a blast tube to force air  over the diode plate, as an attempt to compensate for a backward turning alt  fan. Aside from buying a B&C or Plane Power unit, is there a better way? Is  this likely to have a positive effect?

  

 John

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