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Erratic Alternator Control with LR3C-28

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jrichard(at)ccser.com
Guest

Posted: Thu Dec 06, 2007 1:11 pm Post subject: Erratic Alternator Control with LR3C-28

I wonder if anyone else using the LR3C-28 and the Kelly Aerospace 100 A gear driven alternator (common on new TCM IO-550N) has experienced erratic overcharging when electrical load is low?  Not enough overcharge to trip the over voltage circuit, but enough to cause the bus voltage to reach 30.5 V in spikes from the normal 28.5 V.   This erratic voltage surging only occurs at engine RPM over about 2100 and when electrical load is below about 10 A.    

   

Here’s just a brief run-down of the couple things I’ve looked at.    

   

I’ve replaced ground wires, circuit breakers, re-crimped all ring terminals and measured voltages on the regulator during flight.  In looking at the “field” terminal of the LR3C (pin 4) with an oscilloscope in flight, I see an average voltage of about 5 to 6 volts but I also see 25 volt spikes occurring almost all the time with spacing anywhere from about 10 milliseconds to about 100 milliseconds.  These spikes go away when electrical load is increased and field voltage raises above 10 V.    

   

At low engine RPM, less than 1200,  the charging system is stable independent of load.    

   

If this subtle electrical problem is familiar to you and you have an idea what might be going on, I’d appreciate a reply.    

   

Thanks  

   

John Richardson  

Lancair Legacy, 30 hours flying    

        [quote][b]

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nuckolls.bob(at)cox.net
Guest

Posted: Sat Dec 08, 2007 2:46 pm Post subject: Erratic Alternator Control with LR3C-28

At 12:51 PM 12/6/2007 -0800, you wrote:



 	  Quote: 	
		  I wonder if anyone else using the LR3C-28 and the Kelly Aerospace 100 A 

gear driven alternator (common on new TCM IO-550N) has experienced erratic 

overcharging when electrical load is low?  Not enough overcharge to trip 

the over voltage circuit, but enough to cause the bus voltage to reach 

30.5 V in spikes from the normal 28.5 V.   This erratic voltage surging 

only occurs at engine RPM over about 2100 and when electrical load is 

below about 10 A.



Here s just a brief run-down of the couple things I ve looked at.



I ve replaced ground wires, circuit breakers, re-crimped all ring 

terminals and measured voltages on the regulator during flight.  In 

looking at the field terminal of the LR3C (pin 4) with an oscilloscope in 

flight, I see an average voltage of about 5 to 6 volts but I also see 25 

volt spikes occurring almost all the time with spacing anywhere from about 

10 milliseconds to about 100 milliseconds.  These spikes go away when 

electrical load is increased and field voltage raises above 10 V.



At low engine RPM, less than 1200,  the charging system is stable 

independent of load.



If this subtle electrical problem is familiar to you and you have an idea 

what might be going on, I d appreciate a reply.


	


   I've never had occasion to run this regulator/alternator

   combination. In fact, beyond the original bench testing we

   did for 28V systems on MUCH smaller alternators back about

   15 years ago, I don't think there's been any further IR&D

   investigation of this regulator's dynamic performance.



   I pretty sure B&C still runs each regulator on the test

   stand with a real alternator and battery but it's a cursory

   acceptance test. Being a linear regulator with some unforgiving

   requirements for close loop voltage stability, it would

   not surprise me if your particular combination exhibits

   the characteristic you've observed. Sounds to me like some

   "tweaking" of internal component values may be necessary

   to bring this situation to heel.



   Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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