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gregcampbellusa(at)GMAIL.
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 Posted: Sat Aug 04, 2007 8:17 am Post subject: Intermittent vs. Continuous Duty Contactors (was batteries). |
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Bob wrote:
This is why we SWITCH starter loads with STARTER CONTACTORS.
These have exceedingly thin and light contacts compared
to continuous duty contactors . . . these contacts are
driven together with much greater force so as to minimize
the bouncing and contact resistance. See:
http://www.aeroelectric.com/Pictures/Contactors/S702-1b.jpg
---- To which I'd like to add this little anecdote ---
I recently read where a friend of mine smelled electrical problems
while testing things on the ground and it came from using an
intermittent duty "starter" contactor where a
continuous duty "master switch" type contactor was called for.
http://groups.yahoo.com/group/Lancair_ES/message/8615
To his credit - the box he bought said it contained a continuous duty contactor.
Unfortunately, the little metal can inside the box was an intermittent duty contactor.
When he smelled things getting hot on the ground and saw smoke, he shut everything off
and discovered a cooked contactor in the back and later figured out why.
(The box had contained the wrong part!)
LESSON: Check the contents of those boxes - someone may have switched them around!
Greg
Lesson #1) - check the contents of the box to make sure it's the correct item inside.
Lesson #2) - the "starter contactors", as Bob said, are driven together with more force.
Typically this means their coils draw about 3 or 4 times as much current
as their "continuous duty" brothers. This means more heat in the same area.
This means that the "starter" contactors will get hot and that's why they need
to cool off between prolonged uses.
It's all about heat dissipation - and that boils down to power over surface area.
Most of us can relate to a 60 Watt lightbulb.
A "regular" light bulb would be the A-style bulb. You can grab one
and unscrew it after it's been on for awhile. You probably can't hold it -
but you can grab it and spin it out
Just as an example:
Intermittent Duty "Starter" Contactors - may draw 5 or 6 amps.
This lets their coils authoritatively slam the contacts together and
allows for more "spring tension" to break the connection reliably & quickly.
On a 12v system - this translates to about 60 to 70 Watts.
Continuous Duty "Master" Contactors - may draw 1 or 2 amps.
This is enough to switch an infrequent and lower current than a starter.
On a 12v system - this translates to about 12 to 24 Watts.
This comes down to a matter of dissipating the heat. That's a matter of
surface area and wattage. The contactors are typically in the same size
metal "can", so the surface area is the same.
60 Watts spread out over a large volume won't get too hot.
60 Watts spread out over a
Here's a simple way for most folks to visualize the heat dissipation.
It's mostly related to Power divided by Surface Area.
Put a 60 Watt heater inside a toaster oven and it's not much of a toaster.
Put a 60 Watt bulb inside a smaller metal box and you have an Easy Bake oven.
Put a 60 Watt bulb inside:
- a toaster oven - and it's not much of an oven
- a small metal box and it's an Easy Bake oven
- inside a candelabra size bulb
So for anyone who's not an engineer - here's a simple way to relate
this to how hot things can get:
Imagine grabbing a common 60 Watt or 75 Watt incandescent light bulb
that's been on for an hour with your bare hand. Ouch...
Since the starter contactor is smaller than a regular "Type A" light bulb,
imagine grabbing a 60 Watt candelabra sized light bulb that's been on
for an hour. Ouch + fried skin!
Now imagine grabbing our "continous duty" starter by grabbing
a 15 Watt candelabra bulb. Warm, but not burning hot.
Mount it in a metal can with a little metal standoff bracket -
and no problem letting it run all day long.
Anyway - that's the way I visualize the power and heat dissipation issues
between an intermittent and continuous starter contactor.
It's also the way I judge how hot something should "normally" be.
For example - if you have an EFIS drawing 5 Amps (at) 12V,
then you can imagine a 60 Watt light bulb inside a metal box of similar size
to the EFIS - and have enough experience to estimate how warm it might
be after an hour or two of continuous use.
But in the meantime - make sure you double check the part numbers
on your "master switch" contactors and make sure it's not an intermittent duty!
Fly safe,
Greg
[quote][b]
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blackoaks(at)gmail.com
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| Posted: Mon Aug 06, 2007 7:33 am Post subject: Intermittent vs. Continuous Duty Contactors (was batteries). |
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Greg or Bob , I know that there is an electrical way of telling the two apart in the Connection, but do you know if there is a way to tell them apart visually? All mine as they came with the firewall from Lancair seem to look alike, even to the stenciled numbers on them?
--
John McMahon
Lancair Super ES, S/N 170, N9637M (Reserved) [quote][b]
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