Master contactor question

jan olieslagers wrote:
schreef:
The large ST35 and ST36 are the only continuous duty units rated at
14-16 ohms. All the smaller ones are 16-25 ohms.

Isn't there a difference in resistance betwen the active state and the
passive? I feel some people have determined coil resistance by measuring
with a cheapo multimemer - this uses a small battery, the ensueing
measurement will reflect the inactive state. Better would be to activate
with a battery (or better still, a stable 12V reference) and measure the
current.


Yes there is a difference in resistance with temperature for most metals.

But I certainly hope you couldn't spot the difference with any ease.
This is not a filament whose resistance can double (or more) at
operating temp.

Brian W

On Oct 29, 10:35 am, brian whatcott wrote:
jan olieslagers wrote:
schreef:
The large ST35 and ST36 are the only continuous duty units rated at
14-16 ohms. All the smaller ones are 16-25 ohms.

Isn't there a difference in resistance betwen the active state and the
passive? I feel some people have determined coil resistance by measuring
with a cheapo multimemer - this uses a small battery, the ensueing
measurement will reflect the inactive state. Better would be to activate
with a battery (or better still, a stable 12V reference) and measure the
current.

Yes there is a difference in resistance with temperature for most metals.

But I certainly hope you couldn't spot the difference with any ease.
This is not a filament whose resistance can double (or more) at
operating temp.

Brian W

And it isn't operating on AC, where inductive resistance would reduce
the current flow. If the ohmmeter says 14 ohms, then it'll draw one
amp at 14 volts. That might decrease a small amount as it gets hot,
but not much.

Dan

On Wed, 28 Oct 2009 16:28:04 -0400, wrote:

On Wed, 28 Oct 2009 15:30:30 -0500, rich
wrote:

My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?


Check the coil resistance. Constant duty contactors are higher
resistance, so they draw less wower and heat up less than starter
solenoids.

Does the coil resistance change much as the contactor wears out?
Somehow, the contactor coil seems to get weak over time, or the
contacts inside are dirty or both. My master contactor would click,
but not turn everyhing on until I cycled it a couple times.
One thing I've discovered is, on new contactors, the coil resistance
on continuous duty vs. intermittent duty isn't much different. Just
barely enough to make a determination of what type they are.
I've got one new intermittent one reading 14.7 ohms, and a continuous
one brand new from ACS reading 15 ohms, then another continuous one
that reads 17.3 ohms. I wish there was a more difinitive reading
between the two types. They are so close, it's hard to make the call
using that method.

On Oct 29, 10:18 pm, rich wrote:

Does the coil resistance change much as the contactor wears out?
Somehow, the contactor coil seems to get weak over time, or the
contacts inside are dirty or both. My master contactor would click,
but not turn everyhing on until I cycled it a couple times.

It's the contacts inside that get oxidized or burned, not a
weakening coil. They're usually copper, and copper's oxide isn't a
good conductor so they will heat up and oxidize even faster. The
opening of the contacts creates a spark and burns them, adding more
resistance. Clicking it on and off will sometimes result in contact,
but the contactor is basically shot.

Dan

One trick worth trying is to put a pretty good sized (say, 100 amps or so)
load across the contactor when in the closed position and then open the
contactor. The arc thus formed will be enough to melt any oxidation on the
contacts and you will be left with a fresh metal surface nearly as good as
the original.

Do NOT put the load across the line and then turn the contactor on. You
have every chance of welding the contact surfaces together.

Jim

"rich" wrote in message
...
My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?

RST Engineering - JIm wrote:
One trick worth trying is to put a pretty good sized (say, 100 amps or so)
load across the contactor when in the closed position and then open the
contactor. The arc thus formed will be enough to melt any oxidation on the
contacts and you will be left with a fresh metal surface nearly as good as
the original.

Do NOT put the load across the line and then turn the contactor on. You
have every chance of welding the contact surfaces together.

Jim




If it's not a sealed unit one can use contact a burnisher.

Dan, U.S. Air Force, retired

On Sat, 31 Oct 2009 18:38:56 -0500, Dan wrote:

RST Engineering - JIm wrote:
One trick worth trying is to put a pretty good sized (say, 100 amps or so)
load across the contactor when in the closed position and then open the
contactor. The arc thus formed will be enough to melt any oxidation on the
contacts and you will be left with a fresh metal surface nearly as good as
the original.

Do NOT put the load across the line and then turn the contactor on. You
have every chance of welding the contact surfaces together.

Jim




If it's not a sealed unit one can use contact a burnisher.

Dan, U.S. Air Force, retired
But the VAST majority are sealed.

wrote:
On Sat, 31 Oct 2009 18:38:56 -0500, Dan wrote:

RST Engineering - JIm wrote:
One trick worth trying is to put a pretty good sized (say, 100 amps or so)
load across the contactor when in the closed position and then open the
contactor. The arc thus formed will be enough to melt any oxidation on the
contacts and you will be left with a fresh metal surface nearly as good as
the original.

Do NOT put the load across the line and then turn the contactor on. You
have every chance of welding the contact surfaces together.

Jim

If it's not a sealed unit one can use contact a burnisher.

Dan, U.S. Air Force, retired
But the VAST majority are sealed.

I know. It's a pity since the old, unsealed ones, were at least
repairable.

Dan, U.S. Air Force, retired

"rich" wrote in message
...
My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?

I've been reading this thread with modest interest and a little amusement.

At the moment, I am not entirely sure why a "typical" homebuilt would use a
master contactor and I suggest that you take a look at what the professional
designers may have done. For example, to the best of my recollection, the
Cessna 150 and 152 and also the Piper Tomahawk had starter contactors (a/k/a
solenoids) and a had master breakers that also functioned as switches; but
did not have master contactors--and I really have difficulty understanding
why a well designed aircraft in that size and weight range would need one.

I suggest that you determine whether your battery is located in an unusual
way and then ask a mechanic what was used in reasonably similar factory
built aircraft. For example: I would certainly expect a master contactor
in a Piper Cheyene; but I would not extpect to find one in a Cherokee 140.

Of course, as always, YMMV.

Peter

On Nov 1, 7:37 am, "Peter Dohm" wrote:

At the moment, I am not entirely sure why a "typical" homebuilt would use a
master contactor and I suggest that you take a look at what the professional
designers may have done. For example, to the best of my recollection, the
Cessna 150 and 152 and also the Piper Tomahawk had starter contactors (a/k/a
solenoids) and a had master breakers that also functioned as switches; but
did not have master contactors--and I really have difficulty understanding
why a well designed aircraft in that size and weight range would need one.

I suggest that you determine whether your battery is located in an unusual
way and then ask a mechanic what was used in reasonably similar factory
built aircraft. For example: I would certainly expect a master contactor
in a Piper Cheyene; but I would not extpect to find one in a Cherokee 140.

It's there so the battery can be totally isolated from the
electrical system in case of electrical fire or forced landing. The
Cessna 150, I can tell you, did have a master contactor, as did all
the 172s including the nice new 172S G1000 one in our fleet. Every
airplane I ever worked on has had a master contactor except for my
1946 Auster, which had a huge toggle switch on the panel, and my
Jodel, whose tiny battery powers a handheld radio. You DO want to be
able to cut off all electrical power in flight if it becomes
necessary, because getting out and running is hardly an option when
the smoke and flames start up, or if a forced landing is going to tear
up metal and start shorting stuff and making sparks around spilled
fuel.

At the same time, I don't understand why we don't have a solid-state
device for this application by now.

Dan