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24 Volt Contactor/Relay Wiring

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

Reply to
William R. Walsh
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I hope your relay is designed for AC, and not DC. Is it quiet?

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Reply to
CJT

Hi!

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

Reply to
William R. Walsh

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

Reply to
PeterD

Hi!

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

Reply to
William R. Walsh

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

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Reply to
Bob Larter

(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

Reply to
William R. Walsh

Hi!

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

Reply to
William R. Walsh

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.

Reply to
PeterD

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

Al

Reply to
alchazz

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.

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Best Regards:
                     Baron.
Reply to
Baron

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.

Reply to
Jamie

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

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.

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Reply to
David Nebenzahl

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.

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Best Regards:
                     Baron.
Reply to
Baron

Hi!

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

Reply to
William R. Walsh

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 !

That seems about right for a small single phase contactor.

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

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Best Regards:
                     Baron.
Reply to
Baron

Hi!

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.

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.)

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

Reply to
William R. Walsh

William R. Walsh Inscribed thus:

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 ! ;-)

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Best Regards:
                        Baron.
Reply to
Baron

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