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Alternator stops working at low idle RPM

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Argonaut36

Joined: 19 May 2019
Posts: 35

Posted: Tue Dec 10, 2019 4:13 pm Post subject: Alternator stops working at low idle RPM

In my airplane I have a refurbished Hartzell (formerly Kelly Aerospace) ALY-8520R alternator and a Lamar VR371S  voltage regulator.

The idle is adjusted a little low.  When I land, I read 500 RPM with the throttle all the way back and the alternator goes off by itself.   The only way to re-set alternator is to turn the master off (and also the engine off, actually I am not completely sure about the engine off).  

Is this normal?  If it is not normal and the alternator should not go off with the engine at 500 RPM, what should I read on the voltmeter when the RPM is 500?  Battery voltage? Or something in between battery voltage and the 14.3-14.4 volts that I read in flight or at 1000 RPM taxiing?

Thanks

user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Tue Dec 10, 2019 5:23 pm Post subject: Re: Alternator stops working at low idle RPM

No it is not normal for the alternator to quit working.

When it does quit, the aircraft system voltage will be at battery voltage.

I suspect that the voltage regulator is at fault.

Call Lamar to get their opinion.

According to Lamar's Documentation, http://www.lamartech.com/files/71855887.pdf[

the voltage regulator will shut off in case of high voltage or ground-faulted field wire.

The problem is obviously not over voltage at 500 RPM.

The voltage regulator might think that the field wire is grounded because the field current is high at low RPM.

As an experiment, try putting a diode in series with the field wire with the banded end towards the alternator.

  The diode should be rated at 5 amps or more.

_________________
Joe Gores

Argonaut36

Joined: 19 May 2019
Posts: 35

Posted: Wed Dec 11, 2019 8:31 am Post subject: Re: Alternator stops working at low idle RPM

Thanks for your reply.  I agree that if the alternator is not working, the voltmeter will show battery voltage.  But my question was about what voltage the voltmeter should display, if the alternator is working and the RPM is low; for instance, what should be the voltage shown on the voltmeter at 500 RPM, 600 RPM and 700 RPM?

With reference to what is causing the problem, on the voltage regulator there is a light that illuminates if the voltage regulator shuts off due to a ground-faulted field wire. That light never illuminated.  So, unless the light bulb is not working, there should be no ground-faulted field wire issue.

nuckollsr

Joined: 24 Mar 2009
Posts: 95
Location: Medicine Lodge, KS

Posted: Wed Dec 11, 2019 2:17 pm Post subject: Re: Alternator stops working at low idle RPM

As the rpm drops on a normally functioning alternator, field voltage

goes up and available output current goes down. At some point,

the regulator saturates and the field voltage can go no higher.

This rpm is called the 'minimum speed for regulation'.



Below this rpm, available output current goes to zero and the

bus voltage falls to what ever the battery is capable of delivering

at the moment.



So the answer to your question is, when RPM is below MSR, the

voltage will be determined by battery condition and loads. As you

exceed MSR the alternator will begin to deliver energy to the bus

and the voltage will being to rise. At some point, RPM will be high

enough to carry the present loads whereupon the regulator will

begin to reduce field voltage to maintain the bus at the regulator

set-point.



Joe is correct, the alternator is expected to drop off line below

MSR but it should come right back as rpm is increased.



A good confirming experiment is to buy a generic 'ford' regulator

at Smiley Jack's Autoparts Emporium or off eBay (they're cheap).

See https://tinyurl.com/u6tzu7u



Make a test setup on your airplane as suggested here:



https://tinyurl.com/vn2awd7



You should find that the alternator comes on line at some

RPM and picks up full loads at rated bus voltage above

that speed. You should be able to reduce rpm to any idle

speed with the alternator dropping out at MSR . . . but should

come right back if you increase rpm.



Joe's hypothesis has merit. The alternator may be latching

OFF due to a maximized field current that is expected with

speeds at or below MSR. As long as you're aware of it, it

shouldn't be a big deal as long as the regulator is working

okay otherwise . . . just shut the alternator off and then

back on when rpm is greater than MSR and it should come right

back on  line.

Argonaut36

Joined: 19 May 2019
Posts: 35

Posted: Thu Dec 12, 2019 6:44 pm Post subject: Re: Alternator stops working at low idle RPM

Thanks for all the explanations and recommendations.  I come back only now because I wanted to fly one more time to do a few additional tests.

This is what I observed today:



Tachometer reads about 60-70 rpm slow  (I checked it with a Trutach tachometer that is supposed to be much more accurate than the airplane tachometer).  So the 500 RPM idle that I previously reported is in reality 560-570.

When I throttle back to idle during engine warm-up, the voltmeter was reading 14.4 volts and not battery voltage, which I found disconcerting.

The voltmeter reads 14.4-14.5 V on the ground and 14.2 V in flight with the same electrical loads

The alternator stopped operating on the runway after landing as in the recent previous flights.  Landing was intentionally as smooth as I could in order not to cause a bad contact (if existing) to open.

After getting off the runway I tried (twice) to reset the alternator by turning off all the electrical equipment including the master and then turning everything on again.  The alternator did not come online.

I contacted the maker of the voltage regulator.  They said that alternator is supposed to go offline at a very low RPM, but it should come back on, when the RPM is increased.   This is not the case with my alternator!



This alternator was installed recently.  It is a factory refurbished alternator.  The previous alternator was also going offline, but not just after landing.  I replaced it, because it had already 800 hours on it, hoping that the new one would not go offline. 

The recommended tests are a bit complicated for me to perform.  Access is difficult in my airplane and it takes long time to disassemble and re-assemble. 

I plan on bumping up the idle to 650-700 RPM and doing another test flight.

Thanks in advance for any additional suggestions and comments

user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Thu Dec 12, 2019 7:39 pm Post subject: Re: Alternator stops working at low idle RPM

A good test is to measure the alternator field voltage, preferably at the alternator when it is offline and while engine is running.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Thu Dec 12, 2019 8:00 pm Post subject: Re: Alternator stops working at low idle RPM

Also monitor the voltage regulator input voltage.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan

Posted: Fri Dec 13, 2019 2:42 am Post subject: Re: Alternator stops working at low idle RPM

Intermittent problems are difficult to troubleshoot.  It is easy to jump to false conclusions.  The problem could be a loose connection.

One likely suspect is the alternator on-off switch, which in this case might be one half of the master switch.

_________________
Joe Gores

echristley(at)att.net
Guest

Posted: Fri Dec 13, 2019 7:19 am Post subject: Alternator stops working at low idle RPM

Sounds to me like a heat soak could be affecting solid state components in the ALT or regulator.  Could you heat soak both with a heat gun or hair dryer before starting and doing a static run-up?



                  

                                                                    On Thursday, December 12, 2019, 9:46:48 PM EST, Argonaut36 <fmlibrino(at)msn.com> wrote:                 

                 



                 



                 --> AeroElectric-List message posted by: "Argonaut36" <fmlibrino(at)msn.com (fmlibrino(at)msn.com)>



Thanks for all the explanations and recommendations.  I come back only now because I wanted to fly one more time to do a few additional tests.



This is what I observed today:



Tachometer reads about 60-70 rpm slow  (I checked it with a Trutach tachometer that is supposed to be much more accurate than the airplane tachometer).  So the 500 RPM idle that I previously reported is in reality 560-570.



When I throttle back to idle during engine warm-up, the voltmeter was reading 14.4 volts and not battery voltage, which I found disconcerting.



The voltmeter reads 14.4-14.5 V on the ground and 14.2 V in flight with the same electrical loads



The alternator stopped operating on the runway after landing as in the recent previous flights.  Landing was intentionally as smooth as I could in order not to cause a bad contact (if existing) to open.



After getting off the runway I tried (twice) to reset the alternator by turning off all the electrical equipment including the master and then turning everything on again.  The alternator did not come online.



I contacted the maker of the voltage regulator.  They said that alternator is supposed to go offline at a very low RPM, but it should come back on, when the RPM is increased.   This is not the case with my alternator!



This alternator was installed recently.  It is a factory refurbished alternator.  The previous alternator was also going offline, but not just after landing.  I replaced it, because it had already 800 hours on it, hoping that the new one would not go offline. 



The recommended tests are a bit complicated for me to perform.  Access is difficult in my airplane and it takes long time to disassemble and re-assemble. 



I plan on bumping up the idle to 650-700 RPM and doing another test flight.



Thanks in advance for any additional suggestions and comments



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skywagon185(at)gmail.com
Guest

Posted: Fri Dec 13, 2019 1:04 pm Post subject: Alternator stops working at low idle RPM

This is a comment only about your tach error that you mentioned.

I am assuming that the tach is the original Cessna tach that is drive by a cable like a speedometer.

Those are usually factoryÂ calibrated to be "accurate" in mid to high rpm range.

Do to age, meter movement drag, etc. they loose accuracy. 

Interestingly, if you want, the tach can be adjusted to be accurate in the range you pick.Â  The tach mechanism is regulated by a coiled spring, much like a watch movement, and the spring setting can be adjusted.

The "fix" is... remove the tach from the panel.Â  Connect it is a portable drill motor rotating in the "correct" direction.

And, you need an accurate rpm calibrated strobe light. You probably have a friend with one.

Trick is to run the portable drill at the desired rpm verified by the accurate strobe, next viewing the tach error and carefully adjusting the tach spring tension such that is reads accurately in the range you want.Â  Must likely, it will not be fully accurate over all ranges of rpm.



On Thu, Dec 12, 2019 at 6:51 PM Argonaut36 <fmlibrino(at)msn.com (fmlibrino(at)msn.com)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "Argonaut36" <fmlibrino(at)msn.com (fmlibrino(at)msn.com)>

 

 Thanks for all the explanations and recommendations.Â  I come back only now because I wanted to fly one more time to do a few additional tests.

 This is what I observed today:

 

 Tachometer reads about 60-70 rpm slowÂ  (I checked it with a Trutach tachometer that is supposed to be much more accurate than the airplane tachometer).Â  So the 500 RPM idle that I previously reported is in reality 560-570.

 When I throttle back to idle during engine warm-up, the voltmeter was reading 14.4 volts and not battery voltage, which I found disconcerting.

 The voltmeter reads 14.4-14.5 V on the ground and 14.2 V in flight with the same electrical loads

 The alternator stopped operating on the runway after landing as in the recent previous flights.Â  Landing was intentionally as smooth as I could in order not to cause a bad contact (if existing) to open.

 After getting off the runway I tried (twice) to reset the alternator by turning off all the electrical equipment including the master and then turning everything on again.Â  The alternator did not come online.

 I contacted the maker of the voltage regulator.Â  They said that alternator is supposed to go offline at a very low RPM, but it should come back on, when the RPM is increased.Â  Â This is not the case with my alternator!

 

 This alternator was installed recently.Â  It is a factory refurbished alternator.Â  The previous alternator was also going offline, but not just after landing.Â  I replaced it, because it had already 800 hours on it, hoping that the new one would not go offline. 

 The recommended tests are a bit complicated for me to perform.Â  Access is difficult in my airplane and it takes long time to disassemble and re-assemble. 

 I plan on bumping up the idle to 650-700 RPM and doing another test flight.

 Thanks in advance for any additional suggestions and comments

 

 

 

 

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Argonaut36

Joined: 19 May 2019
Posts: 35

Posted: Sat Dec 14, 2019 8:37 pm Post subject: Re: Alternator stops working at low idle RPM

Today the alternator went offline again after landing, despite a raised idle and I was not able to reset it.  I turned the engine off and I re-started it after a few minutes.  The alternator did not come online. So heat seems to be causing the voltage regulator to cut off.  My regulator is a VR371S.  The S at the end means ‘sealed’ and, because it is sealed, it can be installed in the engine compartment on the firewall.  As pointed out by echristley, mine does not seem to tolerate heat well.

racerjerry

Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY

Posted: Sun Dec 15, 2019 4:38 am Post subject: Re: Alternator stops working at low idle RPM

How is regulator grounded?  The voltage regulator must be solidly grounded in order to function.  You might want to install an external ground wire from the voltage regulator case directly to the alternator housing.  Also, remove and clean each wire terminal connection.  



If problem is still present, run a temporary "test" wire from the alternator field terminal into the cockpit allowing field voltage measurement.  If alternator shuts down with voltage present at the alternator field terminal, the alternator is definitely at fault. 

 

If there is no field voltage, the regulator is at fault OR as previously suggested, no power is getting to the regulator.  Poor contacts on the alternator side of the master switch may be the culprit and it is not at all uncommon.

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Jerry King

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