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Dual Voltage Operation of Ford-style Alternator

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nuckollsr(at)cox.net
Guest

Posted: Thu May 10, 2007 7:37 pm Post subject: Dual Voltage Operation of Ford-style Alternator

At 03:15 PM 5/9/2007 -0700, you wrote:



 	  Quote: 	
		  

<paulmillner(at)COMPUSERVE.COM>



Hi Bob,



I'm considering a slightly unusual configuration on a Lycoming IO360A 

installation... 12 volt alternator on a vacuum pad on the back of the 

engine... powers the "essential" bus (there's a word you prefer to 

essential, but it's late and I can't recall... well, I'm in Holland, too, 

standing up in an internet cafe, and the wind is howling... enough excuses)



The "standard" Ford-style alternator on the front will attach to the 

airconditioning (HVAC) bus... in the back of the plane, a variable voltage 

air conditioining unit (Kelly Aerospace Thermawing or similar) will be 

powered by the HVAC bus.



Under normal, non-cooling operation, the HVAC  bus will be connected to 

the main bus.  The regulator will recognize this fact and regulate for 

nominal 14 VDC.  Both alternators will power the 14 volt systems.



When cooling is wanted (and of course loads allow for the rear mounted 

alternator to carry the requirements), the breaker or contactor will be 

opened between the HVAC bus and the main bus.  This will also enable the 

regulabor to assume 28 VDC mode.



The air conditioning unit will put out some cool air at 14 volts, but 

twice as much cool air at 28 volts.  The unit draws 50 amps over a voltage 

range of 12 volts to 48 volts, and cooling output varies proportionately.



Any fatal flaw to this plan?



Assuming I use a 28 volt alternator (assume for the moment the 

Cessna-style Ford Aeromotive once), should it regulate "happily" at 14 

volts?  I can think of schemes to "fool" the regulator with a voltage 

divider connected to the A lead... switched as required to make a 14 volt 

regulator think that 28 volts meets its needs, I'm sure you have the picture.



Thoughts?  I've already tried to talk the better half out of retiring to a 

hot place... and am now preparing contingency plans to deal with the 

eventuality!  



Paul, Berkeley CA (soon to be Palm Springs CA)


	




    Is this the same Paul Millner with whom I shared many an

    intriguing conversation back in the Compuserve AVSIG days?



    What you propose has merit. I think I'd go for a more

    controlled approach. Wire per Z-14 but with the main bus

    split into a 14v ops and 28V HVAC busses. When in 14V ops

    mode, both busses are tied together and the alternator

    is controlled by a 14V regulator. When the higher performance

    HVAC ops are desired, split the busses and control the

    alternator with a B&C SR-1 regulator which is specifically

    designed to operate a 14V alternator in 28V system. Under

    this mode, goodies on the 14v portion of the main bus would

    be allowed to go dark but could be brought up quickly by

    closing the crossfeed contactor to the opposite side which

    is always up and running at 14v.



    Maybe we need an new Z-figure . . . perhaps a Z-14PM?



    Bob  . . .

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paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Thu May 10, 2007 10:41 pm Post subject: Re: Dual Voltage Operation of Ford-style Alternator

Hi Bob!



Indeed, it's the same me.    I wasn't aware of that functionality of the SR-1 regulator... do folks operate 14 V alternators in 28 volt systems simply because 14 V alternators are less expensive?



Any other ampacity limits to a 14 volt alternator in that service?



I was curious how far one could push a 14 volt alternator before reliability becomes an issue... making 48 volts at 50 amps seems like a stretch for a 14 volt / 60 amp device... I'm thinking of brush current/arcing, and I squared R heating of both the rotor and the stator.



The good news is that *usually* the HVAC usage should be transient... but there are times, especially with an inversion and ATC weirdness, when one would bump along in warm conditions quite some time... though the output could likely be reduced, say, to 28 volts.



How do 14 volt alternators hold up in 28 volt systems at, say, 80% rated load?



Paul, remote from Amsterdam

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_________________
Paul Millner, Berkeley CA [OAK]

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