Altnerator and landing light

[Robert M. Gary]

I have a 70amp alternator in my Mooney. I find that when I turn on the
landing light, the voltage drops down to around 13.0 volts or less.
During off the landing light brings it back to 14.5 volts. The Mooney
Factory Service Center is telling me this is normal. They said Mooney
didn't design the plane to fly around under the continuous load of the
landing light.

Opinions??

-Robert

[[email protected]]

On 3-Mar-2005, "Robert M. Gary" > wrote:

> I have a 70amp alternator in my Mooney. I find that when I turn on the
> landing light, the voltage drops down to around 13.0 volts or less.
> During off the landing light brings it back to 14.5 volts.



Where are you measuring the voltage?
--
-Elliott Drucker

[[email protected]]

Robert M. Gary wrote:
> I have a 70amp alternator in my Mooney. I find that when I turn on
the
> landing light, the voltage drops down to around 13.0 volts or less.
> During off the landing light brings it back to 14.5 volts. The Mooney
> Factory Service Center is telling me this is normal. They said Mooney
> didn't design the plane to fly around under the continuous load of
the
> landing light.
>
> Opinions??
>
> -Robert

Mooney should be ashamed of such a statement. I will wait for your
answer to "where did you check the voltage" but for a thorough reply,
where did you measure it, what else was on (what was the load on the
alternator) and what lamp (s) are we talking about? 4509 or Q4509
type require about 9 amps each. If you are measuring at the lamp, I
would expect the drop you saw.

Neal

[[email protected]]

Robert M. Gary > wrote:
: I have a 70amp alternator in my Mooney. I find that when I turn on the
: landing light, the voltage drops down to around 13.0 volts or less.
: During off the landing light brings it back to 14.5 volts. The Mooney
: Factory Service Center is telling me this is normal. They said Mooney
: didn't design the plane to fly around under the continuous load of the
: landing light.

: Opinions??

That isn't unusual to have reduced voltage output under load at low RPM.
What RPM brings it back up to 14v or so? Remember that even at 13v, you aren't
discharging the battery... it's not discharging until you're less than 12-12.5 or so.
It's just that the alternator isn't able to put out it's full voltage under that
current load. Depending on the state of charge of the battery, there's a 1-2v
difference between "appreciable current into battery" (i.e. charging), and
"appreciable current out of battery" (i.e. discharging).

-Cory

--

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

[Nathan Young]

On 3 Mar 2005 16:18:46 -0800, "Robert M. Gary" >
wrote:

>I have a 70amp alternator in my Mooney. I find that when I turn on the
>landing light, the voltage drops down to around 13.0 volts or less.
>During off the landing light brings it back to 14.5 volts. The Mooney
>Factory Service Center is telling me this is normal. They said Mooney
>didn't design the plane to fly around under the continuous load of the
>landing light.

I would be very surprised if that is normal behavior. What kind of
loads does the ammeter indicate? (With the landing light on/off).

-Nathan

[[email protected]]

wrote:
> Robert M. Gary > wrote:
> : I have a 70amp alternator in my Mooney. I find that when I turn on
the
> : landing light, the voltage drops down to around 13.0 volts or less.
> : During off the landing light brings it back to 14.5 volts. The
Mooney
> : Factory Service Center is telling me this is normal. They said
Mooney
> : didn't design the plane to fly around under the continuous load of
the
> : landing light.
>
> : Opinions??
>
> That isn't unusual to have reduced voltage output under load at low
RPM.
> What RPM brings it back up to 14v or so?

<snip>

Excellent point. The original post is kind of lean on details. If
this is happening at idle RPM, it's perfectly normal. If it's
happening at cruise RPM I'd be a little concerned that the landing
light is dragging the bus voltage down so much. That is, unless the OP
forgot to mention that his "landing light" is actually 3 - 5 lights
spread across the wings and nose :-)

John Galban=====>N4BQ (PA28-180)

[[email protected]]

Nathan Young > wrote:
: I would be very surprised if that is normal behavior. What kind of
: loads does the ammeter indicate? (With the landing light on/off).

If there's a voltmeter (built-in), chances are there *isn't* an ammeter.
Also, alternators do not put out their full rated current capacity until somewhere
above idle. The will put out *something*, but won't be able to regulate beyond full
field current. Also, the regulators in alternators are typically proportional
control, so by definition they have a steady-state error that's proportional to the
load being drawn.

-Cory

--

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

[jsmith]

I don't know what the trip voltage is, but the under voltage light on
the panel of the C182 I fly comes on below 800 RPM with only the
avionics and rotating beacon.

> Nathan Young > wrote:
> : I would be very surprised if that is normal behavior. What kind of
> : loads does the ammeter indicate? (With the landing light on/off).

wrote:
> If there's a voltmeter (built-in), chances are there *isn't* an ammeter.
> Also, alternators do not put out their full rated current capacity until somewhere
> above idle. The will put out *something*, but won't be able to regulate beyond full
> field current. Also, the regulators in alternators are typically proportional
> control, so by definition they have a steady-state error that's proportional to the
> load being drawn.

[Nathan Young]

On Fri, 4 Mar 2005 20:05:17 +0000 (UTC),
wrote:

>Nathan Young > wrote:
>: I would be very surprised if that is normal behavior. What kind of
>: loads does the ammeter indicate? (With the landing light on/off).
>
> If there's a voltmeter (built-in), chances are there *isn't* an ammeter.

That is unfortunate, but not surprising. My PA28-180 is the exact
opposite. It has an ammeter which indicates alternator output, but no
voltmeter.

When you couple the lack of information provided by the plane's
instrumentation, and the lack of electrical knowledge by most A&Ps, it
is quite obvious why electrical gremlins are a major haunt to GA.

[[email protected]]

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.

Dan

[[email protected]]

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

[[email protected]]

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.

[Mike W.]

"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.

[[email protected]]

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