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Alternator Aux

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rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Wed Apr 15, 2009 6:14 pm Post subject: Alternator Aux

I am considering the use of the PlanePower regulator to control a SD-20 alternator.  The regulator provides an Aux input that monitors for low voltage (or no voltage due to crowbar trip), but I am told by PlanePower that the expected input for low voltage monitoring is a half voltage (0-7 volt) output from an alternator.



I don't see any such output on the SD-20.  Would it be reasonable to produce this half voltage with a simple voltage divider made from a pair of identical resistors between the bus voltage and ground or is there more to this than I am seeing?

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rck

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Apr 16, 2009 3:29 am Post subject: Alternator Aux

At 09:14 PM 4/15/2009, you wrote:

 	  Quote: 	
		  



I am considering the use of the PlanePower regulator to control a 

SD-20 alternator.  The regulator provides an Aux input that monitors 

for low voltage (or no voltage due to crowbar trip), but I am told 

by PlanePower that the expected input for low voltage monitoring is 

a half voltage (0-7 volt) output from an alternator.



I don't see any such output on the SD-20.  Would it be reasonable to 

produce this half voltage with a simple voltage divider made from a 

pair of identical resistors between the bus voltage and ground or is 

there more to this than I am seeing?


	


   You're going to have to has the designer of

   the regulator about that. It sounds like their

   regulator is designed to exploit the old

   "S" terminal that used to grace the back of

   many alternators.



   The "S" lead was connected to the center

   junction of a wye-connected stator winding

   and under normal operations this lead produced

   an output about 1/2 of system voltage when

   the alternator was being turned by the engine.



   This output was often used to pull in a relay

   that closed the field supply circuit to the

   regulator. This feature provided automatic

   disconnect of the field when the engine was

   stopped. This was a automatic disabling of

   the field supply that would prevent an unnecessary

   drain on the battery.



   I'm mystified as to why they would offer such

   a regulator. A few alternator designs on

   Wichita piston aircraft had "S" terminals

   but to my knowledge, that terminal was never

   used as a source for powering an excitation

   control relay. I used to build a voltage

   sensor that would annunciate a failed

   alternator by monitoring the "S" terminal

   bit I don't recall it being used any

   other way.



   I'm thinking the "S" terminal has all

   but disappeared from modern alternator

   offerings.



   Assuming that my hypothesis for their

   inclusion of this connection is correct . . .

   and assuming further that the regulator

   is needing to see this voltage just

   to "come alive" then your voltage divider

   suggestion may be fine. Or perhaps it just

   needs to be jumpered to the field supply

   input terminal.



   You'll need to ask them about that. Certainly

   their STC list speaks to a ton of aircraft

   that will not have an alternator fitted with

   "S" terminals so they'll have to do something

   with that input to the alternator.

        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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rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Thu Apr 16, 2009 7:35 am Post subject: Re: Alternator Aux

Thanks Bob,



From what I was told, the Aux terminal does nothing but monitor voltage and control the low voltage/overvoltage lamp.   So it sounds like it works similar to your voltage sensor that you mention.



This regulator is intended to replace whole catalogs of other regulators, so it probably something provided just for those that use it, although the function has apparently changed somewhat.



If there is no Aux output on the alternator and a voltage warning light is used, the instructions are to jumper the Aux terminal to the field control terminal.  In this case, it will only signal an overvoltage condition (after the field breaker is crowbarred).  Since the voltage it is looking for is in the neighborhood of 6-7 volts, it is cannot provide a low voltage indication in this configuration (until it is way too late).



I will check with them.

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_________________
rck

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Apr 16, 2009 4:54 pm Post subject: Alternator Aux

At 10:35 AM 4/16/2009, you wrote:

 	  Quote: 	
		  



Thanks Bob,



 >From what I was told, the Aux terminal does nothing but monitor 

 voltage and control the low voltage/overvoltage lamp.   So it 

 sounds like it works similar to your voltage sensor that you mention.



This regulator is intended to replace whole catalogs of other 

regulators, so it probably something provided just for those that 

use it, although the function has apparently changed somewhat.



If there is no Aux output on the alternator and a voltage warning 

light is used, the instructions are to jumper the Aux terminal to 

the field control terminal.


	


   Okay, that makes sense.



 	  Quote: 	
		    In this case, it will only signal an overvoltage condition (after 

 the field breaker is crowbarred).  Since the voltage it is looking 

 for is in the neighborhood of 6-7 volts, it is cannot provide a low 

 voltage indication in this configuration (until it is way too late).


	


   Don't know about the "too late" idea. Using the

   aux terminal on an alternator for ANY purpose other

   than the original intent of controlling field

   excitation is a stretch of design goals.



   LV warning should be independent of the regulator

   function and have nothing to do with connections

   to the alternator. This is how it works in the B&C

   regulators. If the regulator you're considering

   is NOT fitted with a 13.0 volt threshold LV sensor

   then you'll need an external lv monitoring device.

   The feature you're wrestling with was never intended

   to provide that functionality with precision.



        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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