Hello all...

I recently dreamed up a way to add thermal control to a window air
conditioner that I'd picked up off the curb some time back. It worked,
outside of the fact that whenver the thermostat was connected, the fan motor
would eventually grind down to a halt. It made no sense to me, seeing as the
compressor is the only thing switched by the thermostat. So I did the quick
and dirty thing, and I bypassed it so the compressor is running whenever the
unit is on.

My first means of control was a heavy duty timer that could turn the unit on
for a while and then turn it off. This was cumbersome at best, especially as
the timer had no way to "know" when it was too cold for A/C.

So I got a programmable thermostat, a 24 volt AC transformer and a heavy
duty contactor with a 24 volt coil. This setup works great to cycle power to
an outlet placed especially for the air conditioner, and the parts were all
freebies. But I've noticed that the coil on the contactor gets hot, and I'm
wondering if this is normal. I've looked around for wiring diagrams and even
looked inside some furnaces like the one where the the parts came from to
see if I'd need a current limiting resistor or something. But I don't see
anything like that in use. It looks just like the wiring comes from the
transformer, with one end going to the contactor and the other going through
the thermostat.

Am I going to ruin the coil, or do they just run hot when energized?

William

William R. Walsh wrote:

Hello all...

I recently dreamed up a way to add thermal control to a window air
conditioner that I'd picked up off the curb some time back. It worked,
outside of the fact that whenver the thermostat was connected, the fan motor
would eventually grind down to a halt. It made no sense to me, seeing as the
compressor is the only thing switched by the thermostat. So I did the quick
and dirty thing, and I bypassed it so the compressor is running whenever the
unit is on.

My first means of control was a heavy duty timer that could turn the unit on
for a while and then turn it off. This was cumbersome at best, especially as
the timer had no way to "know" when it was too cold for A/C.

So I got a programmable thermostat, a 24 volt AC transformer and a heavy
duty contactor with a 24 volt coil. This setup works great to cycle power to
an outlet placed especially for the air conditioner, and the parts were all
freebies. But I've noticed that the coil on the contactor gets hot, and I'm
wondering if this is normal. I've looked around for wiring diagrams and even
looked inside some furnaces like the one where the the parts came from to
see if I'd need a current limiting resistor or something. But I don't see
anything like that in use. It looks just like the wiring comes from the
transformer, with one end going to the contactor and the other going through
the thermostat.

Am I going to ruin the coil, or do they just run hot when energized?

William


I hope your relay is designed for AC, and not DC. Is it quiet?

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

Hi!

I hope your relay is designed for AC, and not DC. Is it quiet?

The relay is a 24 volt AC relay. Both items were salvaged from the same
defunct cooling system. It is quiet, and seems to operate normally. Output
from the transformer is reasonably stable when loaded, and ranges from 23-26
volts.

William

On Fri, 29 May 2009 03:32:43 GMT, "William R. Walsh"
m wrote:

Hi!

I hope your relay is designed for AC, and not DC. Is it quiet?

The relay is a 24 volt AC relay. Both items were salvaged from the same
defunct cooling system. It is quiet, and seems to operate normally. Output
from the transformer is reasonably stable when loaded, and ranges from 23-26
volts.

William


Also, define 'hot'... Have you measured the temp, and what was it?

Hi!

Also, define 'hot'... Have you measured the temp, and what was it?

Not being able to find my noncontact thermometer, I can only guess.

After the coil has been running, and with the power *off*, I noticed
heat emanating from it. So I touched the coil body and found it was
nearly hot enough to make continued contact undesirable.

William

William R. Walsh wrote:
Hi!

I hope your relay is designed for AC, and not DC. Is it quiet?

The relay is a 24 volt AC relay. Both items were salvaged from the same
defunct cooling system. It is quiet, and seems to operate normally. Output
from the transformer is reasonably stable when loaded, and ranges from 23-26
volts.

Then don't worry about it, if it's only getting warm, rather than hot.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

(posting from a "real" news server this time, in hopes of attracting
more eyes!)

Not being able to find my noncontact thermometer, I can only guess.

After the coil has been running, and with the power *off*, I noticed
heat emanating from it. So I touched the coil body and found it was
nearly hot enough to make continued contact undesirable.

William

Hi!

Then don't worry about it, if it's only getting warm, rather
than hot.

That's the thing. The outer surface of the coil does get hot after it's
been on for a while. It's too hot to stand touching it for more than a
few seconds. (It isn't so hot that you immediately pull back--there is
more than enough time to think "gee, this is somewhat hot".)

On the other hand, it has been running trouble free for some time now,
and I can't find any evidence from the original unit it was in, that the
power was in any way limited to the relay, such as by a resistor.

William

On Fri, 29 May 2009 23:19:09 GMT, "William R. Walsh"
m wrote:

(posting from a "real" news server this time, in hopes of attracting
more eyes!)

Not being able to find my noncontact thermometer, I can only guess.

After the coil has been running, and with the power *off*, I noticed
heat emanating from it. So I touched the coil body and found it was
nearly hot enough to make continued contact undesirable.

William

My suspicion is that they contactor has a short in the coil perhaps.
If it is too hot to maintain continued contact, then the rule of thumb
(sorry, bad pun...) is that it is too hot.

On Fri, 29 May 2009 23:21:33 +0000, William R. Walsh wrote:

Hi!

Then don't worry about it, if it's only getting warm, rather
than hot.

That's the thing. The outer surface of the coil does get hot after it's
been on for a while. It's too hot to stand touching it for more than a
few seconds. (It isn't so hot that you immediately pull back--there is
more than enough time to think "gee, this is somewhat hot".)

On the other hand, it has been running trouble free for some time now,
and I can't find any evidence from the original unit it was in, that the
power was in any way limited to the relay, such as by a resistor.

William

If it's that hot on the outside, think of how hot it is at the core. My
advice, replace it.

Al

William R. Walsh wrote:

Hi!

Then don't worry about it, if it's only getting warm, rather
than hot.

That's the thing. The outer surface of the coil does get hot after
it's been on for a while. It's too hot to stand touching it for more
than a few seconds. (It isn't so hot that you immediately pull
back--there is more than enough time to think "gee, this is somewhat
hot".)

On the other hand, it has been running trouble free for some time now,
and I can't find any evidence from the original unit it was in, that
the power was in any way limited to the relay, such as by a resistor.

William

Its quite possibly normal for the coil to get quite hot ! AC contactors
have a shorting loop on them to stop the armature chattering due to the
alternating flux. If it bothers you that much reduce the voltage
across the coil a little. Just make sure that it pulls in solidly if
you do.

--
Best Regards:
Baron.

Baron wrote:

William R. Walsh wrote:


Hi!


Then don't worry about it, if it's only getting warm, rather

than hot.

That's the thing. The outer surface of the coil does get hot after
it's been on for a while. It's too hot to stand touching it for more
than a few seconds. (It isn't so hot that you immediately pull
back--there is more than enough time to think "gee, this is somewhat
hot".)

On the other hand, it has been running trouble free for some time now,
and I can't find any evidence from the original unit it was in, that
the power was in any way limited to the relay, such as by a resistor.

William


Its quite possibly normal for the coil to get quite hot ! AC contactors
have a shorting loop on them to stop the armature chattering due to the
alternating flux. If it bothers you that much reduce the voltage
across the coil a little. Just make sure that it pulls in solidly if
you do.

Or, just put a single diode in series with the coil to remove one half of
the cycle so that a AC coil can operate normal at %50 duty with DC.

On 5/29/2009 4:21 PM William R. Walsh spake thus:

Then don't worry about it, if it's only getting warm, rather
than hot.

That's the thing. The outer surface of the coil does get hot after it's
been on for a while. It's too hot to stand touching it for more than a
few seconds. (It isn't so hot that you immediately pull back--there is
more than enough time to think "gee, this is somewhat hot".)

On the other hand, it has been running trouble free for some time now,
and I can't find any evidence from the original unit it was in, that the
power was in any way limited to the relay, such as by a resistor.

Not sure how helpful this is, but this reminds me a lot of a repair I
did to a house I lived in about 10 years ago. House had an old old gas
furnace whose gas control finally gave up the ghost. The control was a
standard one, it turned out, operated through the thermostat with a
24-volt transformer.

Seemed simple enough: I installed the new control, wired it up. It
worked, but as soon as the thermostat called for heat, it made a really
horrible buzzing noise (60-cycle) that could be heard through the entire
house.

I carefully checked the installation instructions and the existing
transformer, which was emitting as close to 24 volts a.c. as no never
mind. The control was clearly rated for 24 v.a.c. Called the place I
bought the control, who were as puzzled as I was. They finally gave me
another unit as a replacement. But after installing it, got the same
result: BZZZZZZZZZZZZZZZZZZZZZZ!

I even contacted the manufacturer of the control, who tried but couldn't
offer me any fixes.

I finally did what any reasonable tinkerer would do: inserted a resistor
in series with the control, which reduced the buzzing to a soft,
tolerable level. I had to determine the value by trial and error, high
enough to reduce the sound but low enough so that the control's solenoid
would engage reliably. After that, the furnace worked fine, for several
years.


--
Found--the gene that causes belief in genetic determinism

David Nebenzahl wrote:

On 5/29/2009 4:21 PM William R. Walsh spake thus:

Then don't worry about it, if it's only getting warm, rather
than hot.

That's the thing. The outer surface of the coil does get hot after
it's been on for a while. It's too hot to stand touching it for more
than a few seconds. (It isn't so hot that you immediately pull
back--there is more than enough time to think "gee, this is somewhat
hot".)

On the other hand, it has been running trouble free for some time
now, and I can't find any evidence from the original unit it was in,
that the power was in any way limited to the relay, such as by a
resistor.

Not sure how helpful this is, but this reminds me a lot of a repair I
did to a house I lived in about 10 years ago. House had an old old gas
furnace whose gas control finally gave up the ghost. The control was a
standard one, it turned out, operated through the thermostat with a
24-volt transformer.

Seemed simple enough: I installed the new control, wired it up. It
worked, but as soon as the thermostat called for heat, it made a
really horrible buzzing noise (60-cycle) that could be heard through
the entire house.

I carefully checked the installation instructions and the existing
transformer, which was emitting as close to 24 volts a.c. as no never
mind. The control was clearly rated for 24 v.a.c. Called the place I
bought the control, who were as puzzled as I was. They finally gave me
another unit as a replacement. But after installing it, got the same
result: BZZZZZZZZZZZZZZZZZZZZZZ!

I even contacted the manufacturer of the control, who tried but
couldn't offer me any fixes.

I finally did what any reasonable tinkerer would do: inserted a
resistor in series with the control, which reduced the buzzing to a
soft, tolerable level. I had to determine the value by trial and
error, high enough to reduce the sound but low enough so that the
control's solenoid would engage reliably. After that, the furnace
worked fine, for several years.


I've seen AC relays where the shading ring has not been fastened
properly buzz like that ! Often a tap with a hammer and punch in the
right place solves the noise problem.

--
Best Regards:
Baron.

Hi!

Its quite possibly normal for the coil to get quite hot !

I'm beginning to think that it must be. It is definitely a coil rated for
use with AC power.

I talked to an HVAC guy and asked what he thought the other day in the
grocery store. He wasn't sure how hot the contactors ran, but he did raise a
good point. In this thing's normal application, it would have received some
"relief" (if you will) from the circulating fan in the equipment. I hadn't
thought of that, but it makes sense.

In any event, I put an inline meter on the power wiring going into the
transformer. With the contactor closed, the transformer is pulling about 7.2
watts at 125 volts AC.

What I've finally decided to do is to add a fuse to the transformer. It's a
3 amp slow-blow fuse (hey, it was handy!) from my parts box, but it should
stop any gross meltdowns before they happen.

The coil runs silently when powered, with no humming or chattering at all.

William

William R. Walsh wrote:

Hi!

Its quite possibly normal for the coil to get quite hot !

I'm beginning to think that it must be. It is definitely a coil rated
for use with AC power.

I talked to an HVAC guy and asked what he thought the other day in the
grocery store. He wasn't sure how hot the contactors ran, but he did
raise a good point. In this thing's normal application, it would have
received some "relief" (if you will) from the circulating fan in the
equipment. I hadn't thought of that, but it makes sense.

Now you mention it, it hadn't occurred to me either. There is usually
one or more extractor fans in the top of the system cabinets to remove
heat generated by the multitude of contactors and inverters !

In any event, I put an inline meter on the power wiring going into the
transformer. With the contactor closed, the transformer is pulling
about 7.2 watts at 125 volts AC.

That seems about right for a small single phase contactor.

What I've finally decided to do is to add a fuse to the transformer.
It's a 3 amp slow-blow fuse (hey, it was handy!) from my parts box,
but it should stop any gross meltdowns before they happen.

You could always use a salvaged PC PSU fan to provide an airflow !

The coil runs silently when powered, with no humming or chattering at
all.

William

--
Best Regards:
Baron.

Hi!

Now you mention it, it hadn't occurred to me either. There is usually
one or more extractor fans in the top of the system cabinets to remove
heat generated by the multitude of contactors and inverters !

This was set up in such a way that the contactor was mounted inside the part
where the interior air circulation fan goes. It would benefit from the air
flowing by to meet the fan and get pushed through the registers in the
house.

It was easy to do, so I also went out and had a look at the contactor used
on my central air conditioner. Couple of screws and one small metal panel
later, I saw pretty much the same unit as the one I'm using for this other
project. And it can't get much, if any cooling air where it is situated. The
makers chose to put it in a relatively weather-but-also-air tight box. It's
run for 14 air conditioning seasons now...so the designers must not have
done too badly.

That seems about right for a small single phase contactor.

Small, but with two sets of switched contacts. It could be used to switch
both the hot and neutral portions of the line, but I suspect that it
actually switched two 120 volt hot lines in the original application. (The
original unit is no longer around now for me to check.)

You could always use a salvaged PC PSU fan to provide an airflow !

I thought about doing that, but I don't want to precipitate a fire if one
should ever occur. Nor do I want the dust buildup in the box that all this
stuff is going to be put inside. I have noticed that when bolted to a metal
plate, that the contactor sheds heat through it. It would be easy to work
that into what I'm doing.

Thank you for all your help and information so far.

William

William R. Walsh Inscribed thus:

Hi!

Now you mention it, it hadn't occurred to me either. There is
usually one or more extractor fans in the top of the system cabinets
to remove heat generated by the multitude of contactors and inverters
!

This was set up in such a way that the contactor was mounted inside
the part where the interior air circulation fan goes. It would benefit
from the air flowing by to meet the fan and get pushed through the
registers in the house.

It was easy to do, so I also went out and had a look at the contactor
used on my central air conditioner. Couple of screws and one small
metal panel later, I saw pretty much the same unit as the one I'm
using for this other project. And it can't get much, if any cooling
air where it is situated. The makers chose to put it in a relatively
weather-but-also-air tight box. It's run for 14 air conditioning
seasons now...so the designers must not have done too badly.

That seems about right for a small single phase contactor.

Small, but with two sets of switched contacts. It could be used to
switch both the hot and neutral portions of the line, but I suspect
that it actually switched two 120 volt hot lines in the original
application. (The original unit is no longer around now for me to
check.)

You could always use a salvaged PC PSU fan to provide an airflow !

I thought about doing that, but I don't want to precipitate a fire if
one should ever occur. Nor do I want the dust buildup in the box that
all this stuff is going to be put inside. I have noticed that when
bolted to a metal plate, that the contactor sheds heat through it. It
would be easy to work that into what I'm doing.

Thank you for all your help and information so far.

William

You're welcome.

I agree you don't want to encourage a build up of dust around it ! It
wouldn't do the contacts any good. Using the metal box as a heatsink
is probably how any excess heat is got rid of, particularly if there is
air flow around that.

I reckon that you have got it well taped ! ;-)

--
Best Regards:
Baron.