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Update: Strange alternator behavior at Startup

 
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nuckollsr(at)cox.net
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PostPosted: Mon Feb 27, 2006 10:31 am    Post subject: Update: Strange alternator behavior at Startup Reply with quote

At 07:08 AM 2/27/2006 -0500, you wrote:

Quote:

<mstewart(at)iss.net>

Update:
Was up north this weekend and was able to do a good test in the cold.
Temp was 10F and I cranked her up.
Set immediate idle at 800rpm. No charge. Sat ther for 10 seconds or so.
Went to 1100rpm for about 5 seconds. And Wallah, charging.
Back to 700rpm for 10 seconds. Still Charging.
Down to 350rpm for 10 seconds and still charging.

This MAY be an on-purpose feature built into your
alternator. My little romp through the data sheet on the
MC33092A regulator chip offered some insights into
the thinking of at least these chip designers. Don't
know that the MC33092A found its way into your alternator
but it's a sure bet that any designer of a new chip who
is worth his salt has studied the market. The time delay
function offered in the MC33092A combined with the low/high
RPM transition switch may well have been included in your
alternator.

Don't jerk it out yet. It just might be performing
exactly as the designers intended.

Bob . . .


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mstewart(at)iss.net
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PostPosted: Mon Feb 27, 2006 12:27 pm    Post subject: Update: Strange alternator behavior at Startup Reply with quote

Just as a reminder,
This behavior does not exist when OAT is above 40F OR is it is an
already warm engine.

On that same cold day Saturday, I started the warm engine at 400rpm and
it was charging immediately. There was no need to raise the rpm. Its
definitely temperature related.
Mike
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nuckollsr(at)cox.net
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PostPosted: Mon Feb 27, 2006 2:50 pm    Post subject: Update: Strange alternator behavior at Startup Reply with quote

At 01:46 PM 2/27/2006 -0700, you wrote:

Quote:


Hey everybody (especially Bob),

I've been sitting around thinking about how to make circuits more
complicated than they need to be (grin)... Here's an idea for turning off
an IR alternator with the end goal of not causing damage due to load dump.
It definitely adds parts count, and doesn't address over voltage nuisance
trips. The idea depends on the alternator using the sense lead as a
reference voltage input for the regulator. One of the schematics that I
looked at for an IR chip supported such behavior...

So, here's the idea. Actuating the 'off' position of the alternator
switch would actually connect a pumped ic regulator (LM1577 or similar)
output to the sense lead of the IR alternator at a voltage higher than
alternator output voltage. Say bump it up by 2Volts or so. This would
signal to the IR that the alternator output current needs to be reduced in
order to return to regulated voltage (in effect re-regulating, or fooling
the IR). After 1 sec or so, the alternator disconnect relay is then
opened, safely disconnecting the unit from the bus - no load-dump.
Turning the alternator switch back 'on' boots the circuit, closes the
alternator disconnect relay, and disconnects the pump regulator from the
sense lead circuit.

I can work up a schematic if this sounds interesting..

Your solution pre-supposes that we have access to an
internal sense lead and that the builder is willing/
able to add that lead at installation. Your premise is
sound. Anything you can do to fool a normally working
regulator into an orderly shut down would do the trick.

The approach I have in mind requires no modifications
to an alternator . . . it would even work with the so-called
one-wire alternators that don't have a control-wire.

Bob . . .

< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Mon Feb 27, 2006 5:19 pm    Post subject: Update: Strange alternator behavior at Startup Reply with quote

Robert L. Nuckolls, III Startup wrote:

Quote:
> Set immediate idle at 800rpm. No charge. Sat ther for 10 seconds or so.
> Went to 1100rpm for about 5 seconds. And Wallah, charging.
> Back to 700rpm for 10 seconds. Still Charging.
> Down to 350rpm for 10 seconds and still charging.
...

Don't jerk it out yet. It just might be performing
exactly as the designers intended.

This is actually normal behavior for a self-exciting alternator if you
don't hook up the ignition terminal or the idiot light. There is no
source of power to provide any field excitation so excitation comes from
the residual magnetic field in the armature. Eventually you get it
spinning fast enough that there is enough output from the stator to
begin to excite the regulator and the field. The result is that the
alternator begins to bootstrap, i.e. a little current gets to the field
which slightly increases the output which causes a little more current
to get to the field, until the alternator has enough output to fully
turn on the regulator and the field.

--
Brian Lloyd 361 Catterline Way
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery


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mstewart(at)iss.net
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PostPosted: Tue Feb 28, 2006 4:07 am    Post subject: Update: Strange alternator behavior at Startup Reply with quote

Oh.
Well that is very interesting Brian. Could it be that my IGN wire is
dead?
I have the IGN with voltage applied before cranking (I think. I have not
checked it since initial install) and I have an idiot light installed
which does work as expected. I will check that IGN wire next time I have
the cowl off to verify it is supplying power before cranking. Based on
your note below, it is quite possible the IGN wire is doing nothing.
Wonder why temperature is at play though.

Ill check it and report back.
Thanks
Mike
Do not archive

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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Tue Feb 28, 2006 7:24 am    Post subject: Update: Strange alternator behavior at Startup Reply with quote

Stewart, Michael (ISS Atlanta) wrote:
Quote:


Oh.
Well that is very interesting Brian. Could it be that my IGN wire is
dead?

Could be.

Quote:
I have the IGN with voltage applied before cranking (I think. I have not
checked it since initial install) and I have an idiot light installed
which does work as expected. I will check that IGN wire next time I have
the cowl off to verify it is supplying power before cranking. Based on
your note below, it is quite possible the IGN wire is doing nothing.

Some older regulators get their initial excitation through the idiot
light circuit. If the alternator has no output the regulator draws
current through the idiot light causing most of the current to flow
through the field which starts the bootstrap ball rolling. If the
alternator has an ignition terminal then that terminal is used to
provide the initial field current to bootstrap the alternator.

And every IR alternator is different. If you want the details, you need
to find out how your particular alternator works. It is just that I have
worked with a fair number (cars, boats, gen-sets, and airplanes) and
what I wrote above has been generally true for these devices.

The Delco truck alternator I had on my CJ6A had nothing connected to the
idiot light circuit or the IGN lead and used to require that I get up to
around 2000 RPM (2300 RPM was red-line) before it would self-excite.
After that it would be fine even at idle. I ended up selling the
airplane before I got around to fixing that problem.

Quote:
Wonder why temperature is at play though.

Just a wild guess but most silicon devices have a negative temperature
coefficient. The drop across a cold diode is greater than the drop
across a warm diode. This could change the required voltage from the
alternator requiring it to turn faster to self-excite.

But this latter comment is pure speculation on my part with nothing to
back it up so is not worth much.

[quote] Ill check it and report back.
Thanks
Mike
Do not archive



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PostPosted: Tue Feb 28, 2006 1:19 pm    Post subject: Update: Strange alternator behavior at Startup Reply with quote

Michael
If it makes you feel better I have some IGN wire sleuthing to do as well
although it is not near the top of my to do list at the moment.
My brand new mini ND IR alternator (100211-1680) draws 3.3 amp field
current through the B-lead any time the OV contactor is closed with the
engine not running. The IGN terminal is simultaneously energised so my
next test is to see if this still happens with the IGN wire
disconnected. If so then it is shopping time for another regulator but
of course on mine it is difficult to get at that wire to test it. I
thought most of these things were smart enough to not energise the field
until the rotor was turning but maybe not. Or perhaps it is just annoyed
because I'm not allowing it to draw its normal 4 ma of standby current
that it would draw from the battery if I wasn't using an OV contactor Wink
It charges and regulates perfectly when the engine is running.
Ken

Brian Lloyd wrote:

Quote:


Stewart, Michael (ISS Atlanta) wrote:


>
>
>Oh.
>Well that is very interesting Brian. Could it be that my IGN wire is
>dead?
>
>

Could be.




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PostPosted: Wed Mar 01, 2006 6:35 am    Post subject: Update: Strange alternator behavior at Startup Reply with quote

At 10:49 PM 2/28/2006 -0500, you wrote:

Quote:


In a message dated 2/28/2006 8:02:43 P.M. Central Standard Time,
mstewart(at)iss.net writes:

So it would seem that it needs that initial rpm to get the juices
flowing in the cold.
I can live with that.
Thanks to whom ever it was that suggested an initial burst of RPM to get
her going. Worked
Best,
Mike
Do not archive

Good Evening Mike,

Do you really want to give it that short burst of power?

Personally, I like to warm up the engine at as low an RPM as possible
consistent with adequate oil pressure and any need for the crankshaft to
throw the
oil against the innards of the engine. I do not know what engine you have,
but I would not give it a burst above any RPM that I wanted to use for
warm up
purposes.

Isn't that the whole idea behind doing a warm up?

Absolutely. The behavior noted is NOT what we would
generally consider acceptable . . . the "work around"
described is functional but it's almost a certainty
that this alternator is either deprived of some accommodation
that allows it to come up normally at ramp idle -OR-
has a design feature that makes it a poor choice for
use on aircraft.

Bob . . .


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