Altnerator and landing light

wrote:
: 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. Sounds like maybe the alternator is getting weak or has a
: bad diode or worn brushes, maybe.

That's true, but only in cruise. You aren't going to be the air long enough to have
the battery go dead at only 1000 RPM. At cruise RPM you're required to limit the maximum
load to 80%, and/or instrument the system properly to indicate when the load has been
exceeded. Automotive (and thus aviation) alternators do *NOT* put out full load at idle, nor
should they be expected to. The entire design of the alternator would have to be compromised
to do so, and it would be even *more* inefficient than it already is (50-70% typically).

Pulling 30A out of the alternator at 800RPM would definately be acceptable to have
the voltage output drop to 13v. Remember... the battery isn't discharging until the bus
voltage is below 12-12.5.

-Cory
--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

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.

"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.

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 *
************************************************** ***********************