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#11
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#12
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Alternator should run everything... Not completely true.
The FARs require that the NORMAL loads do not exceed the 80% of the output of the alternator (or generator) unless there is a loadmeter. Landing lights and landing gear are the kinds of loads that are excluded. So it's not required that the alternator be able to run all those things at once. Lesse, on my BE33, it's a 70 amp alternator = 56 amps continuous. I'm very close to that. The landing light is 20A all by itself = 56+20 = 76 amps = 70A. It works, but barely. Low voltage indication comes on while taxiing at low rpm after a landing. But unless they are 20% of the alternator output, it SHOULD be able to handle it. Makes me nervous when people keep adding landing lights. |
#13
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"Alternator should run everything... Not completely true."
That's right. In fact, without a battery to supply a field voltage to the alternator, the alternator will not work at all. "Certified airplanes with alternators are supposed to be able to handle ALL constant loads at the same time. Adding up all the radio and light and pitot heat and whatever else current daws should come to something less than alternator capacity. A drop from 14.5 to 13 volts is a BIG drop and shouldn't be there, especially if the RPM is above 1000 or so." Just because the voltage drops doesn't mean the alternator isn't supporting the load. As long as it is charging the battery, it's doing it's job. |
#14
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Mike W. wrote:
: Just because the voltage drops doesn't mean the alternator isn't supporting : the load. As long as it is charging the battery, it's doing it's job. At idle, it would be more appropriate to say that as long as the battery isn't discharging, it's doing its job. There's a difference of a volt or so between appreciable charging and appreciable discharging of a lead-acid battery (like 11.5-12.5). A battery that is full won't charge no matter what. If it draws 5A at 16V, it's not charging the battery... it's splitting the electrolyte. If it draws 100A at 15V, *THEN* it's charging. Again, 13V on the bus is a perfectly acceptable idle voltage under load. The battery is *NOT* discharging, so the alternator *is* doing its job. -Cory -- ************************************************** *********************** * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * ************************************************** *********************** |
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