# Weak coils



## TOOL_5150 (Aug 27, 2007)

Ill take a swing...

Given the correct voltage and amperage is supplied:
I believe that a coil that was wound years ago has the same "power" it has today as it did years ago when it left the mfg line.

A weak coil can only be led to insufficiant current or voltage.

~Matt


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## drsparky (Nov 13, 2008)

I have seen it. I was upgrading a large machine and found a few bad solenoids. Over the years the maintenance department upsized the fuses when they blew instead of finding the problem. If the coil gets hot the winding insulation can melt, this shorts out some of the windings. The resistance drops and current rises. Get a new coil and get an accurate resistance measurement, yes you can use a digital if you can manually adjust the range. Now check the “weak” coil, do the math, next check the fuse sizes I bet they have been “upsized“. In my case the coils were 120vac, the bad coils were down to about 20 ohms, a good coil would read about 90 ohms. They had put 10 amp fuses to replace 2 amp fuses. When I put in a new PLC with the correct fuses they would blow after a few cycles. When I went to the maintenance department for new solenoids their response was “it worked before the shut down, the problem must be your new equipment”. They had just been checking resistance with an auto ranging meter and the difference between bad and good were not apparent.


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## alimaj (Jun 15, 2009)

Hi this is my first post .I do not share the view that there are 'weak coils'. There would be bad coils or shorted coils, which in my mind is something different . My thinking is that the coil resistance does not change nor does its inductance. A solenoid not operating properly could be friction . A bad or shorted coil is one in which the insulation breaks down. A METER THAT MEASURES THE 'Q' of the coil will detect any short.


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## mutabi (Jun 2, 2009)

Ok, how bout this.....Lets say for arguments sake, the coil is shorted inside. Not completely, but lets say its resistance is now half what it was when it was brand new. What is going to happen when it is energised? 

Take a motor for instance. If you single phase a motor, its current will increase, as it is gonna still try to do the same work. It will trip OLS, blow fuses or burn itself up, but it will ultimately pull more current. 

So wouldnt a coil be the same? If its resistance was half what it was when it was new, would it simply pull more current to get the same work done (and risk burning up), or would it simply be a weak coil? If you take the plunger out of a coil and energise the coil, it will pull more and more current until it burns itself up.


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## mutabi (Jun 2, 2009)

How about an equation:

And i may be WAY wide off the mark here so feel free to correct me. 

P = v^2/R 

If you decrease R, as in my earlier example(shorted coil), the resultant power will go up according to that power equation. Of course it may simply result in a lot of heat, but it interesting nonetheless.


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## drsparky (Nov 13, 2008)

They can and do partially short. Do a little detective work with your ohm meter.:detective:


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## mutabi (Jun 2, 2009)

Oh i totally agree that they can short. The question is, will it have the same power as before?


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## John Valdes (May 17, 2007)

I would say 99.9% of coil failures are mechanical failures. In my opinion the coils either are good or bad. There is no in between. Mechanical failures cause heat and heat causes coil burnout. The smallest piece of trash will jam the plunger or diaphragm. I have cleaned hundreds of them and most every time they work just fine afterwards. Like anything else, they will need replacement on occasion.
Like any other wound apparatus there will be insulation breakdown. This is bad coil not a weak coil.


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## Bob Badger (Apr 19, 2009)

mutabi said:


> Oh i totally agree that they can short. The question is, will it have the same power as before?


Without a doubt if there are enough coils shorted that you can read a difference then the strength will also be effected.


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## drsparky (Nov 13, 2008)

lower number windings = lower inductance = weaker coil
higher number windings = higher inductance = stronger coil
:wallbash:


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## alimaj (Jun 15, 2009)

Hi for the sake of clarity if I can repeat a coil that has shorted turns should not be termed a'weak coil' it is simply a BAD coil. It cannot be used in the application in which it is in. The inductance and perhaps the resistance would change depending how many turns are shorted out.


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## drsparky (Nov 13, 2008)

bad=not work at all
weak=not work well
:wallbash::wallbash:


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## Richard Rowe (May 25, 2009)

I have replaced coils that did not operate properly, they would still work but sometimes did not open a valve fully or occasionly not open at all. Most of the time they make a buzzing sound.... the new coil fixed it so it must have been the coil.


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## MDShunk (Jan 7, 2007)

Related to coils... I've seen a heck of a lot of motor starter coils replaced and blamed as "bad" when the real culprit was lower voltage control circuits run way too far for the gauge of wire used; voltage drop. I once witnessed a factory maintenance man measure the control circuit voltage, with starter coil disconnected, to "prove" that a full 24 volts was present. I said, "Okay, hook the coil up and show me what you have". 17 volts is not enough.


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## MDShunk (Jan 7, 2007)

alimaj said:


> Hi for the sake of clarity if I can repeat a coil that has shorted turns should not be termed a'weak coil' it is simply a BAD coil. .


No matter if you call it bad or weak, it's weak just the same. I sometimes leave other people to sort out what exactly to call it... if you need me, I'll be over here fixing the problem. :laughing:


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## TOOL_5150 (Aug 27, 2007)

MDShunk said:


> Related to coils... I've seen a heck of a lot of motor starter coils replaced and blamed as "bad" when the real culprit was lower voltage control circuits run way too far for the gauge of wire used; voltage drop. I once witnessed a factory maintenance man measure the control circuit voltage, with starter coil disconnected, to "prove" that a full 24 volts was present. I said, "Okay, hook the coil up and show me what you have". 17 volts is not enough.


This better explains what I thought in post #2. I suppose my judgement was correct.

~Matt


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## mutabi (Jun 2, 2009)

Thanks guy for all of the replies. It has been most thought provoking. I believe I have a reach my own personal conclusion as a result of everyones contributions. 

Here it is:

If a coil is properly fused, the coil has no chance of being weak. It will either work or it wont. If it doesnt, then there is a mechanical problem somewhere, junk in the valve etc. 

If a coil is not properly fused, and is allowed to pull too much current trying to pull in a plunger, it risks breaking down the insulation, and shorting the windings, resulting in lower resistance. This will in turn, lower the inductance, and decrease the coils ability to pull in the plunger. It has now become a weak coil. 

However with the proper voltage at the coil (under load), and pulling the proper current, the coil can never be called "weak". 

Seem like a reasonable conclusion?


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## MDShunk (Jan 7, 2007)

mutabi said:


> ....Seem like a reasonable conclusion?


Sounds like you've got it straight.


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## drsparky (Nov 13, 2008)

mutabi said:


> Thanks guy for all of the replies. It has been most thought provoking. I believe I have a reach my own personal conclusion as a result of everyones contributions.
> 
> Here it is:
> 
> ...


Perfect!:thumbsup:


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## alimaj (Jun 15, 2009)

MDShunk said:


> No matter if you call it bad or weak, it's weak just the same. I sometimes leave other people to sort out what exactly to call it... if you need me, I'll be over here fixing the problem. :laughing:


 Can you say if a transformer has shorted turns it could be described as WEAK and it's weak just the same. I responded to mutabi's second post where he referenced to a shorted coil.


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## MDShunk (Jan 7, 2007)

alimaj said:


> Can you say if a transformer has shorted turns it could be described as WEAK and it's weak just the same. I responded to mutabi's second post where he referenced to a shorted coil.


If we used transformers to do some sort of mechanical work, like we do solenoid coils, I might. Since we don't use transformers that way, there would be better terms to use when talking about transformers.


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