Coil Voltage

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I think I am confused by the facts. Our engineer hit me with more motors which means I have to order more MS. Problem- I am trying to maintain uniformity with my control cabinet and I have 24 VAC from I/O Modules. The distro can't get me the 24 VAC coil on the contactors that I need but he has 24VDC in stock. There should be no reason why it would would matter whether I am applying VDC or VAC to the coil. The only difference should be that the coil can handle higher current given the lower voltage. Am I looking at this the wrong way?

I either get what I can without the lead time or rectify the voltage to the coil with a XFMR. I just don't see that I would have to.
 

SG-1

Senior Member
The 24VAC & 24VDC coils are not interchangable, smoke will result. It may be possible to use a rectifier, keep in mind that you will get a 10% reduction in voltage out of a full wave bridge rectifier. A power supply is probably a better solution.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Using 4 $0.20 diodes will work fine. I have done it several times. The slight voltage drop will not keep the relay from working properly.
Radio Shack has a full wave bridge rectifier on a mountable block for a couple of bucks too, easier to use because it has flag terminals that accepts female disconnects. The Op will have to be carefull not to overload the exsisting power supply though, the wattage adds up quick operating on such a low voltage.
 
Radio Shack has a full wave bridge rectifier on a mountable block for a couple of bucks too, easier to use because it has flag terminals that accepts female disconnects. The Op will have to be carefull not to overload the exsisting power supply though, the wattage adds up quick operating on such a low voltage.
I've used radio shack recifiers item a couple of times. I prefer the diodes because I can solder them and wrap them in a neat bundle that looks as small as a splice once taped up.
 

SG-1

Senior Member
So the difference would probably lie in the resistance of the coil?
If you change the word resistance to impedance you have it.

The DC coil impedance you can measure directly with an ohm meter & calculate the current it will draw provided DC voltage is applied.

The AC coil impedance you cannot measure with an ohm meter. It should be specified on the data sheet. You will measure something very low with an ohm meter. When AC voltage is applied the actual current will be much lower, than the one calculated with the measurement that was taken.
 

zog

Senior Member
Location
Charlotte, NC
SG is 100% correct, but to state the obvious, why not just get the right coil? Perhaps your supply house just sucks. Give some details like model or part #'s and I am sure myself or someone can help you find the right coil.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
We needed to do some soldering at a nuke plant in a bad location, humped over between cable trays, many tiny terms and I as one of the older guys just could not pass the test! I could have but my back is shot and my eyesite is going, my production would have been low. All the young guys just made fun of me; untill they found out where the work was! The old "Too old to ..." trick worked good on the young guns!
 
SG is 100% correct, but to state the obvious, why not just get the right coil? Perhaps your supply house just sucks. Give some details like model or part #'s and I am sure myself or someone can help you find the right coil.

Well a lot of what I am doing is trying to save money. The IEC contactors I have found were very reasonably priced and I am working on a literal science project. So I am trying to keep costs down. The brand I am using is WEG. I wanted to use Allen Bradley but I was sent off and asked to do better. I did with WEG as they are half the cost of Allen Bradley. Whether they are as reliable remains to be seen.

I am having a difficult time finding the 24VAC coils for some reason and most of what I have found is online. I have three supply houses in my area to use and I do for simpler items. I did find an alternate brand that will suffice since the distro I ordered the WEG products from has a 4 week lead on the 24VAC coil.

Thanks for the info and answers.
 
The biggest problem I have is the fact that this is a science project and things have been turned around and upside down from the original process control specs I have had since last year. Since needing additional Motor starters, I had to send the control cabinet purchased back to get a bigger one. I could not fit the MCCB, distro blocks, and additional MS's in the one I had.
 

sii

Senior Member
Location
Nebraska
The biggest problem I have is the fact that this is a science project and things have been turned around and upside down from the original process control specs I have had since last year. Since needing additional Motor starters, I had to send the control cabinet purchased back to get a bigger one. I could not fit the MCCB, distro blocks, and additional MS's in the one I had.
Gotta love how helpful engineers can be.:happysad:
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
SG is 100% correct, but to state the obvious, why not just get the right coil? Perhaps your supply house just sucks. Give some details like model or part #'s and I am sure myself or someone can help you find the right coil.
A lot of places do not stock 24VAC coils in anything. They are very uncommon outside of the HVAC industry. 24VDC are very common.
 

gar

Senior Member
110816-0908 EDT

MrArachnid:

You need to understand how relays work, change in inductance as a function of armature position, and what is unique in an AC relay to make it an AC relay.

What determines the maximum voltage that can be applied to a coil?
Some factors are temperature rise, ambient temperature, thermal characteristics of the device, maximum insulation temperature for a desired life.

Will an AC relay work with DC applied?
Will a DC relay work with AC applied?

Get a small AC relay and a DC of comparable size. Disassemble and see what the difference is. Do some Internet search on the subject.

Somewhere on this forum I discussed some of these factors and provided some experimental data. I can not presently find the thread.

.
 
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