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andrew.d.zachar(at)GMAIL.
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 Posted: Tue Sep 27, 2011 10:55 am Post subject: A refresher question about switching for the battery and "al |
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Good afternoon, everyone.
I need a little refresher on the latest guidance on DC Power Master switches. (Quoting from http://www.aeroelectric.com/articles/Alternators/Know_Your_Charging_System.pdf, which quotes another article...)
"The alternator switch turns the voltage regulator on and off, not the alternator output. The voltage to the alternator switch comes from the alternator field circuit breaker, which is tied directly to the aircraft battery through the master switch." (I think I'm correct in pointing out that the field circuit is from the bus, and whether the bus is getting it's electrons from the battery or alternator is another matter...)
One of the larger discussions is with reference to the split rocker: "1) Leave the alternator switch OFF BEFORE starting the engine," and "2) Turn the alternator switch ON AFTER the engine is running." Mr. Nuckolls responds to this by pointing out there is "there is no demonstration in the physics of how alternators work that will support this notion...[that starting the engine with both sides powered somehow overloads the alternator, causing failure.]" Okay. I understand.
Mr. Nuckolls also points out that even if you WANTED to sit on the ground without the engine running listening to the radio, why would one turn on both sides of the DC power master switch?) Also, "Yes, any truly qualified mechanic will conduct ground tests of electrical system components with only the battery side of the switch closed.")
Okay, so while I won't hurt the alternator by cranking with both sides powered, you shouldn't sit around (with the engine not running) with them both on for fun...
Here's the actual question. I see in Figure 11-20 of AEC (page 8-9 , the example DC Power master is a DPST switch (same with Z-10/8 and Z-13/, but some of the other diagrams show a DPDT with OFF-BATT-BATT/ALT. Why the difference?
I'd rather use a DPST, but is there a good reason to use the DPDT? If I have a DPST, will pulling the ALT FLD breaker be the same as leaving the ALT side of the split rocker OFF in the few cases I'll be "sitting around listening to the radio?" I think the answer is no (removing field circuit power instead of voltage regulator power), but then how do I mitigate against the scenarios above?
As always, thanks in advance.
--
Andrew Zachar
andrew.d.zachar(at)gmail.com (andrew.d.zachar(at)gmail.com)
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tomcostanza
Joined: 19 Oct 2008
Posts: 49
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| Posted: Wed Sep 28, 2011 2:49 am Post subject: Re: A refresher question about switching for the battery and |
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I would have thought that starting the engine with the alternator field powered would load the engine, not the alternator. And with a marginal battery, in cold weather, it might make a difference as to whether the engine started or not.
| Quote: | | One of the larger discussions is with reference to the split rocker: �"1) Leave the alternator switch OFF BEFORE starting the engine," and "2) Turn the alternator switch ON AFTER the engine is running." Mr. Nuckolls responds to this by pointing out there is "there is no demonstration in the physics of how alternators work�that will support this notion...[that starting the engine with both sides powered somehow overloads the alternator, causing failure.]" Okay. I understand. |
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_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project |
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