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Alternator Putting out Max Current

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Fri Aug 30, 2013 5:01 pm Post subject: Alternator Putting out Max Current

I have a 35 Amp B&C alternator on an experimental aircraft.  Also installed is the B&C regulator.  I went thru the troubleshooting guide and all voltages are in spec.  But, when running the engine at takeoff power, the alternator is putting out 35 amps.  Bus voltage is regulated at 28.8 but the current is maxed out.  This has never happened before.  Any insight into where to look for the problem,



Thanks!!

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nuckolls.bob(at)aeroelect
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Posted: Fri Aug 30, 2013 6:17 pm Post subject: Alternator Putting out Max Current

At 08:01 PM 8/30/2013, you wrote:

 	  Quote: 	
		  



I have a 35 Amp B&C alternator on an experimental aircraft.  Also 

installed is the B&C regulator.  I went thru the troubleshooting 

guide and all voltages are in spec.  But, when running the engine at 

takeoff power, the alternator is putting out 35 amps.  Bus voltage 

is regulated at 28.8 but the current is maxed out.  This has never 

happened before.  Any insight into where to look for the problem,


	


  If the voltage is normal (I presume this is a 28v airplane)

  and the alternator is maxed out after starting the engine

  then your battery is probably dead and the alternator is

  running full bore to get it recharged.

   Bob . . .

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sat Aug 31, 2013 2:11 am Post subject: Re: Alternator Putting out Max Current

Bob:



Thanks.  Actually the battery is new.  28 v system.  The alternator (SK 35) is putting out no current at idle.  As RPM increases, the current increases.  At approx. 1200 rpm 10 amps, 1500 rpm 15 amps, 2000 rpm 22 amps, etc up to maxing out at t/o rpm or 2500 rpm (radial engine).  



Craig

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 31, 2013 6:24 am Post subject: Alternator Putting out Max Current

At 05:12 AM 8/31/2013, you wrote:

 	  Quote: 	
		  



Bob:



Thanks.  Actually the battery is new.  28 v system.  The alternator 

(SK 35) is putting out no current at idle.  As RPM increases, the 

current increases.  At approx. 1200 rpm 10 amps, 1500 rpm 15 amps, 

2000 rpm 22 amps, etc up to maxing out at t/o rpm or 2500 rpm 

(radial engine).



Craig




	


   Okay. If your reading the proper bus voltage under

   load, then the regulator is working. The alternator's

   output is RPM limited so as you increase rpm, the

   AVAILABLE output current will rise to a maximum of

   the alternator's nameplate rating. If the bus voltage

   is correct and alternator is delivering a lot of current,

   then something is LOADING the alternator . . . either

   a discharged battery, operating accessories or a

   combination of both.



   Do you have a battery charger/maintainer that you

   can put on the battery overnight so that we can

   start with a KNOWN topped-off battery to do some

   tests?



   Is this a 'new' condition . . . in other words,

   has this airplane flown with a normally operating

   system?



   Bob . . .

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sun Sep 01, 2013 5:11 am Post subject: Re: Alternator Putting out Max Current

Bob:



Well I went thru the B&C troubleshooting guide and the entire system checked out.  I then thought thru how a charging system works and that since the bus voltage was being held at 28.8 volts (the B&C setpoint) that it had to be something else.  While the batteries were new, that does not mean fully charged.  



So, I ran the engine at 1200 rpm which was 12 amps of output.  Ran it for 15 minutes and watched the output drop to 9 amps.  This confirmed it had to be the batteries (all radios, etc off).  I then ran the engine to max RPM and 24 amps was max seen.  So I went and flew it for an hour and it now indicates 8 amps with radios on, etc.



SO, the moral of the story was that new batteries did not imply fully charged batteries (I have 2 12V SLA batteries in series to make a 24 VDC system).



Thanks for the advice!!



Craig

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Sep 01, 2013 7:22 am Post subject: Alternator Putting out Max Current

At 08:11 AM 9/1/2013, you wrote:




Bob:



Well I went thru the B&C troubleshooting guide and the entire system 

checked out.  I then thought thru how a charging system works and 

that since the bus voltage was being held at 28.8 volts (the B&C 

setpoint) that it had to be something else.  While the batteries were 

new, that does not mean fully charged.



So, I ran the engine at 1200 rpm which was 12 amps of output.  Ran it 

for 15 minutes and watched the output drop to 9 amps.  This confirmed 

it had to be the batteries (all radios, etc off).  I then ran the 

engine to max RPM and 24 amps was max seen.  So I went and flew it 

for an hour and it now indicates 8 amps with radios on, etc.



SO, the moral of the story was that new batteries did not imply fully 

charged batteries (I have 2 12V SLA batteries in series to make a 24 

VDC system).



Thanks for the advice!!



   What you have observed is a demonstration of the idea

   that the regulator's primary function is to take care

   of the battery . . . and the battery is a bucket for

   the storage of watt-seconds of energy. A NEW battery

   can be in a discharged state and present a significant

   opportunity for the regulator/alternator team to square

   off and break a sweat while topping off the bucket.



   Had a roofer in my driveway in Wichita a few weeks ago

   cursing his battery after his pickup wouldn't start.

   The battery's installation date WAS several years back

   so there was no reason to discount the possibility that

   the battery was bad.



   I put a charger on it and put 20+ amps in to the battery

    . . . the terminal voltage didn't even get to 14.0

   volts at 20A charge rate . . . a very good sign. A soggy

   battery's terminal voltage will pop right up and acceptance

   current will rapidly decline.



   After 15 minutes, I load tested the battery and it

   readily delivered 300+ amps for 10 seconds while

   loaded down to 9v. I.e. the battery was healthy,

   just depleted. We started the truck and the battery's

   terminal voltage stayed down around 12.2 volts . . .

   i.e. the alternator was not coming on line. If the

   alternator HAD come on line, the terminal voltage

   would have almost immediately risen to 14+ volts.

   Had the alternator been fitted with an ammeter,

   it might have 'pegged' at the name-plate rating

   for the alternator until the battery had accepted

   a substantial replenishment whereupon the current

   would begin to taper off.



   What you witnessed was the good and proper operation

   of your ship's electrical system.

   Bob . . .

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