PDA

View Full Version : electrical ? about contactor coils

Tommy1010
09-21-2011, 11:04 PM
Every 24v contactor coil I ohmed out ranges between 8-11 ohms...new or atleast fairly new.

I have one now thats showing 4 ohms.

Im not going into the issue cuz I just wanna know if you agree wit an electrician on this one..

He said .."its better to have a lower resistance cuz it gives higher amps..which creates a stronger magnetic field to pull contactor in stronger...."

His reasoning is the p.i.e. formula....

T or F

Ford3517
09-21-2011, 11:42 PM
I would want to know if the contactor in question is different than anything else you have seen. For example is this an exact same contactor as another one reading different resistance or is it a new contactor all together to you? Also there is a little more to resistance in A/C than just what you read with a meter. In a coil you get Inductive reactance which will cause current to lag voltage. This effects the Impedence which is defined by the total opposition to A/C current. This is also effected by the Plunger ( I hope that is the correct technical term) or the part inside that moves because of the magnetic field produced by the coil. An iron rod effects reactance and will cause a drop in current, for example: a solenoid valve that is stuck will cause the coil to burn up because since the valve didn't move the magnetic field doesn't have the stability it would if the valve was working properly.

So I can't really answer your question but you would need to put the coil in a circuit and measure the current to calculate the resistance the circuit is actually seeing. I would compare them that way because I believe it would be more accurate. Although I am looking forward to someone with more knowledge to explain this.

timebuilder
09-22-2011, 08:26 AM
Stronger field? Yes.

More current? Yes.

More heat? Yes.

Shorter life? Yes.

Possible short in the windings? Almost certainly.

Coils are designed with a certain effective wire length and reactance. There is no benefit for a coil in an identical application to have a lower resistance, as the 8 ohm coil (to use an example value) was already designed to have a field strength that is capable of pulling in the armature.

A coil with a 4 ohm resistance will potentially tax the control breaker that feeds the unit, causing nuisance trips.

If this resistance value is not the same as similar contactor coils, I'd change it out.

Redwood650
09-22-2011, 09:35 AM
So at about what ohm reading do you say, ok it is time to replace it?

timebuilder
09-22-2011, 10:14 AM
I'd try to compare pull-in and holding current to a typical unit and look for a 10-15% difference.

john.0522
09-22-2011, 11:14 AM
If you’re averaging between 8-11 ohms on similar contactors then 4 ohms is 50% of you lowest average I’d change it. It has been my experience 10 ohms is what I average.

SwampTromper
09-22-2011, 04:54 PM
Check its rating to make sure its the same coil as others and if its rated for 8 ohms but only has a resistance of 4 then i'd call it defective and trash it. It's not like its an expensive part and if your talking 3 phase, you dont want to chance anything with a faulty contactor.

09-22-2011, 05:50 PM
Dont mess with electricity bud ... when in doubt replace it

Stephen260B
09-28-2011, 12:05 AM
If the contactor Ohm reading should be closer to the OEM contactor for best operation.

Redwood650
09-28-2011, 12:27 AM
What?

10-06-2011, 11:41 AM
What?
If the question is still at what ohm reading to replace, there's at least one experienced instructor who says replace every year at the annual inspection.
Thus, if it isn't close to the reading of a new one, then as Timebuilder said, it's probably shorted. A short is going to WEAKEN the magnetic field, not strengthen it. I'd replace it.

second opinion
10-06-2011, 12:13 PM
If the question is still at what ohm reading to replace, there's at least one experienced instructor who says replace every year at the annual inspection.
Thus, if it isn't close to the reading of a new one, then as Timebuilder said, it's probably shorted. A short is going to WEAKEN the magnetic field, not strengthen it. I'd replace it.

this exsperienced instructor be?

10-06-2011, 12:46 PM
this exsperienced instructor be?

Henry Barclay, one of my instructors when I went to PTEC, who used to work at a big university (in the HVAC department). Their PM called for new contactors every year.

And, yes, I know that they may work for a decade without failure; however, they do become pitted. They're cheap and easy to replace.

Keith

garyed
10-06-2011, 02:47 PM
I've seen quite a few contactors in the last five or so years blow transformers because of shorted coils so if the resistance has dropped below half of normal I would replace it.
Its going to draw more than twice as much amperage as the other ones so its much more load on the transformer. This is assuming your matching these numbers with the same type contactors.

dan wong
10-06-2011, 03:24 PM
I often put a fuse holder in series with the low voltage transformer ( output- common), and I usually tape 2-3 spare fuse in a plastic bag attach to the fuse holder.

http://www.ebay.com/itm/20X-In-line-Fuse-Holder-ATC-Motorcycle-Waterpoof-16GA-/120769661338?pt=LH_DefaultDomain_0&hash=item1c1e6ec99a#ht_500wt_969

Tip: Many walk-in cooler have several evaporator motors, occasionally one will blow fuse intermittently (say-once a month), I put a fuse holder in line with each motor. Very easily identify intermittent motor. Work good for me.

You maybe able to identify a bad contactor the same way.

timebuilder
10-06-2011, 06:08 PM
If the question is still at what ohm reading to replace, there's at least one experienced instructor who says replace every year at the annual inspection.
Thus, if it isn't close to the reading of a new one, then as Timebuilder said, it's probably shorted. A short is going to WEAKEN the magnetic field, not strengthen it. I'd replace it.

Possibly.

If the current can increase more than the decrease of wire length that is lost in the short, field strength can increase. That depends on how current is affected in the AC environment of the coil, and I don't have my books available to look at the likelihood of that.

Thomasg
10-06-2011, 07:36 PM
Reset fuse so no need for fuses.

10-08-2011, 02:36 PM
Possibly.

If the current can increase more than the decrease of wire length that is lost in the short, field strength can increase. That depends on how current is affected in the AC environment of the coil, and I don't have my books available to look at the likelihood of that.

You might be right... what I was thinking was that IF it would work okay, then the manufacturers of contactors would be using shorter coils and saving \$0.001 worth of copper. That they aren't was a "thinkin'-about-it" shortcut to reach a conclusion that the shorter coil isn't being used because it's not quite enough for reliability. Doesn't mean I came to a good conclusion for a right reason. :grin2: The P-Nut relays, though, have a lot more resistance. They're about equally reliable as far as I know. So you may be right. Now I'm curious.

Reset fuse so no need for fuses.

Yes!!!

Those 5A breakers are readily available, BTW, at Grainger. I had two supply houses give me squinty looks when I was in search of one, and the only thing in stock was part of a kit that cost about \$20. I found 'em at Grainger for about \$2. It's a Carling Technologies 4VA66 Carling Switch P/N CTB-B-B-05-XG.

I've put in some UV germicidal lights and I have used a dedicated transformer that's kicked on by a separate relay off the green power. That way I put the smallest additional load on the original transformer and run the light only when the fan is moving air through the A/H. I wanted protection, but I didn't want a fuse. Thus the cheap 5A breakers. That way I'm SURE to not overload the original transformer. The breakers come with the males for push-on connectors.

There's the part number and source if it helps anyone.

Keith

timebuilder
10-08-2011, 02:47 PM
In the case of p-nut relays...they have a higher impedance, and draw less power. They can close their low-current contacts reliably without a strong field, using less current for a given voltage.

Stronger fields are needed to operate the larger (higher current) relays we call contactors. BUT, they have to do this without taking up too much power from the transformer secondary, because a larger transformer for the unit costs more to build. So, a shorted coil can sometimes remain functional and have the field strength needed to operate the contacts and armature, but the additional current draw is not optimal for most applications. Note that really large contactors use line voltages controlled by a pilot relay, because an increase in either I or E will give a larger VA value, and the needed field strength to operate the coil.