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JohnInReno
Joined: 08 Sep 2007
Posts: 150
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 Posted: Wed Jul 09, 2008 11:34 am Post subject: Do Alternators fight each other on Z-13/8 |
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I am wiring an RV9A per Z-13/8 and the certified A/P that is helping with my panel believes that the two alternators in Z-13/8 will fight each other if they are ever on at the same time. He also believes that the smaller one will be fried. I admit that I am not knowledgeable enough to address his concerns. I did find a reference in the archives to set the voltage lower on the SD-8 if you want to leave it on all of the time, but that is not my intent.
Can someone help me to address his concern if the 2 alternators are accidentally both on?
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John Morgensen
RV-9A - Born on July 3, 2013
RV4 - for sale |
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frank.hinde(at)hp.com
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| Posted: Wed Jul 09, 2008 12:24 pm Post subject: Do Alternators fight each other on Z-13/8 |
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Alternators are simply voltage generating devices...They aim to put the regulated voltage on the buss....If the voltage is less than the required voltage (just over 14V) it will crank up the current to jack up the volts.
If you have two alternators running its almost certain that will not be exactly the same regulated voltage so the lower voltage alternator will simply start backing off on the output current. Eventually you will have one alternator running the entire load and the other will be backed off to zero amps.
They won't "fight each other" but the non-outputting alternator will still consume some current in it's field windings so it represents an additional electrical load.
The SD8 being a permanent magnet device presumably does not consume any current because it does not have a field winding.
I guee if you have two alternators with exactly the same regulated voltage I guess it is possible to have each alt taking up the load and then flip flopping but I would highly doubt that would happen in real life. So no big deal in my estimation.
Why would you accidently leave them on anyway?..I never switch my SD8 on because I don't want to risk damaging my Plane power alt by turning it on and off to test the SD8...So I simply test it at the annual.
Frank
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mprather(at)spro.net
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| Posted: Wed Jul 09, 2008 2:06 pm Post subject: Do Alternators fight each other on Z-13/8 |
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Two comments:
The field current on the unloaded alternator should go to zero - the
regulator will clamp it off because the sensed voltage is above the set
point. There will be a very small amount of current consumed by the
regulator itself - I'd think much less than 1A.
I believe there have been some problems when two alternators share a bus.
It seems I remember hearing that it's related to the time-constant of the
regulator circuit, reactance on the bus, and voltage is sensed. On a
light twin, I think. The bus voltage was unstable if I recall correctly.
I'd guess that two could share a bus safely if the set point of one of the
regulators was a good amount higher than the other - meaning that one of
the alternators is off until bus loads go way-up, or if the primary
alternator fails off.
Regards,
Matt-
[quote]
<frank.hinde(at)hp.com>
Alternators are simply voltage generating devices...They aim to put the
regulated voltage on the buss....If the voltage is less than the required
voltage (just over 14V) it will crank up the current to jack up the volts.
If you have two alternators running its almost certain that will not be
exactly the same regulated voltage so the lower voltage alternator will
simply start backing off on the output current. Eventually you will have
one alternator running the entire load and the other will be backed off to
zero amps.
They won't "fight each other" but the non-outputting alternator will
still consume some current in it's field windings so it represents an
additional electrical load.
The SD8 being a permanent magnet device presumably does not consume any
current because it does not have a field winding.
I guee if you have two alternators with exactly the same regulated voltage
I guess it is possible to have each alt taking up the load and then flip
flopping but I would highly doubt that would happen in real life. So no
big deal in my estimation.
Why would you accidently leave them on anyway?..I never switch my SD8 on
because I don't want to risk damaging my Plane power alt by turning it on
and off to test the SD8...So I simply test it at the annual.
Frank
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