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paulmillner
Joined: 09 May 2007
Posts: 21
Location: Berkeley, California
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 Posted: Wed May 09, 2007 2:15 pm Post subject: Dual Voltage Operation of Ford-style Alternator |
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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)
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Paul Millner, Berkeley CA [OAK] |
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jmfpublic(at)comcast.net
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| Posted: Thu May 10, 2007 8:03 am Post subject: Dual Voltage Operation of Ford-style Alternator |
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Hello Paul,
Your idea sounds good, and something similar is being done by Lancair (or the commercial variant, Columbia) with their electrically heated de-ice system. They run an alternator at very high voltage. I forget the exact number, but it may by 48 volts.
If you cooling unit will take 48 volts, why not use it? Hard to believe that you will get too much cooling. Since you will not have a battery on line for either 28 or higher voltage use, the choice of regulator will need some care, but I suspect your cooling unit will be less sensitive to over voltage than most avionics systems. Make sure that your high voltage regulator can be fed by a diode from the 12v battery just to get started, and the diode will prevent back feed into the battery.
I recall in the 1960's when alternators came out, that the back of Popular Mechanics would advertise plans for using your car alternator as a welding supply, putting in a connection to the alternator that would cut out the regulator and let the voltage soar. I never tried it...
Jim Foerster, J400, wiring....
[quote][b]
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paulmillner
Joined: 09 May 2007
Posts: 21
Location: Berkeley, California
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| Posted: Thu May 10, 2007 10:47 pm Post subject: Re: Dual Voltage Operation of Ford-style Alternator |
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Hi Jim, thanks for your comments.
>> something similar is being done by Lancair (or the commercial variant, Columbia) with their electrically heated de-ice system. They run an alternator at very high voltage. I forget the exact number, but it may by 48 volts.
That's the same group of folks, Kelly Aerospace Thermowing, that make this HVAC unit. It's a 135 amp, 70 volt alternator... but of course it's designed for that service, and costs just under $3,000.
>> If your cooling unit will take 48 volts, why not use it?
I'm concerned about i-squared-r heating of the rotor and stator windings, leading to insulation breakdown, as well as significant reduction of brush life due to arcing at the signficantly higher field current/voltage. Don't want to smoke the alternator regularly!
>> Hard to believe that you will get too much cooling.
That's likely not a concern, true!
>> Since you will not have a battery on line for either 28 or higher voltage use, the choice of regulator will need some care, but I suspect your cooling unit will be less sensitive to over voltage than most avionics systems. Make sure that your high voltage regulator can be fed by a diode from the 12v battery just to get started, and the diode will prevent back feed into the battery.
Good thought.
>> I recall in the 1960's when alternators came out, that the back of Popular Mechanics would advertise plans for using your car alternator as a welding supply, putting in a connection to the alternator that would cut out the regulator and let the voltage soar. I never tried it...
Well, I don't know about welding, but of course, that would be transient loading, not hours of operation in that mode... typically. In fact, I installed one of those adapters on my '72 Ford Galaxie, and operated AC/DC power tools at my tiedown quite well.
Paul, remote from Amsterdam
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