OT,But still... AC v DC

geniusintraining said:

Thank you very much Ron…This is one reason that when I see you have replied to a thread I never pass it up. I always look forward to your explanations (past and present)

The "past" part is in reference too, I often search your name (and a few others) and read your responses to other threads…thank you for what you have done, it helps us all.

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Yea what he said. I have read several of your post since I started using this sight and your post seem to always explain the "why".

A follow-up question.

What is the relationship between AC and DC coils in terms of voltage. For example if I wanted to replace a 125VDC coil with an AC coil, what would the voltage rating of the AC coil need to be? This is not a theoretical question. We have a feeder breaker with 125VDC control that has a history of burning up closing coils. This is an intermittant problem and we're not sure what is causing it but the breakers are old some of the coils that have been purchased and used were marked AC with no other additional information. These coils were purchased specifically for these breakers and work for one or two closing operations, sometimes 3 or 4, and then burn up. Usually this breaker stays energized so the length of time elapsed between operations can be many months. The DC resistance of these coils is about 14 ohms. If DC coils have many more wraps than AC coils it seems that you could use the resistance to determine whether or not the coil would work in a DC circuit but I think there's probably more to it than that and I'll need one of you brainy theory guys to help me out. Thanks

Charley,

Welcome to the forum. Under normal circumstances etiquette would require you to start another thread for your question. But I see you have just joined and this is your first post.

Charley Higgins said:

We have a feeder breaker with 125VDC control that has a history of burning up closing coils.

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You say this is a closing coil. Does it need to stay energised or the breaker will drop out?

Charley Higgins said:

the breakers are old some of the coils that have been purchased and used were marked AC with no other additional information.

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When I read this it sounds like you have the wrong coils for the application. If this antique gear and all that is available anymore is the AC coils, then perhaps this approach will work.

Many large DC powered contactor coils use a resistor in series with the coil to limit the continuous current. But they also use a contact to short across the resistor when the coil is first energised. This way the coil has the power to "seal" in but is current limited during "run". Most DC powered coils use a "late breaking" aux contact for this purpose. If you don't have one of these available, then perhaps a DC powered "delay on make" relay will work for you.

As far as the amount of resistance to use, I'm afraid it's experimentation time. How large and heavy is the coil? Do you have any simular sized and or weighted coils that are AC powered on site? If you do then see if you can get

Edit: This test is for the 120v AC powered simular sized coil.

1. Ohm reading.

2. Amp reading using Ron Beaufort's test with the armature not sealed. Don't do this for very long!

3. Amp reading with the armature sealed.

This will give you a starting point for the amount of resistance needed. The size and weight of the coil determine the amount of energy it can dissapate before burnout occurs. Of course the final determination would be: Will the coil remain sealed with the resistance in line?

The wattage can be figured by Ohm's law

The coil isn't sealed in. It pulses, energizing a solenoid that trips a mechanical latch and a charged up spring closes the breaker. Thanks for your response, looks like I've got some experimenting to do.

Does it have a surge suppressor across it? may be hard to find at that voltage but that may help

Like this http://web1.automationdirect.com/adc/Shopping/Catalog/Motor_Controls/Fuji_Contactors_-z-_Overloads/65_to_80_Amp/SZ-Z37

I thought 125vdc would be harder to find, but this may help

Charley Higgins said:

The coil isn't sealed in. It pulses, energizing a solenoid that trips a mechanical latch and a charged up spring closes the breaker.

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Ok. Now I'm wondering "How does a pulse burn up a coil?"

Are you positive it's not getting power for long periods when it's not supposed to?

Sounds like its a tie breaker for a large switch gear...

Like the manual style that you jack into place but its electrical.. so it turns on and off, I would look into a solid state not mechanical

Charley Higgins said:

The coil isn't sealed in. It pulses, energizing a solenoid that trips a mechanical latch

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Charley,

Are you sure it doesn't seal in untill it trips the latch? If the mechanisim was balky and the coil was sealed untill the point where the spring discharges. The coil might be energised for along time.

Charlie

Having worked on large circuit breakers in the past I can think of a couple things. Since this is old gear
I would pull the breaker and put it on the bench THEN
check the latching mechanism to ensure it will move freely. A lot of times these coils are wound for a set operation time - any longer and they fry. Friction in the mechanism will increase the time of operation of the coil and the coil may be pulling its little heart out to overcome the friction which adds to the problem.

I would also double check to make sure there is absolutely no voltage and current thru the coil when the breaker is shut.

I think I would check with the factory (IF they are still alive) and see what they recommend for replacing DC coil with an AC coil.

Dan Bentler

So, if I can summarize, you cannot use AC on a DC rated coil at any voltage. You can, however, use DC on an AC coil but the required DC voltage will be less than the rated AC voltage.

I would think that the easiest way to find the lowest DC voltage for good activation without burning up the coil would be to measure the ohmic resistance of the AC coil, multiply by 10, and add a series resistor of that ohmic value between the power supply and the relay coil (in this case, a 24VDC power supply. It is unlikely that the coil will operate with this much resistance in series with it. If it does operate, increase the resistance until it no longer pulls in. Reduce the resistance slightly and that is the series resistance needed to safely operate the AC coil on DC current.

If the coil does not pull in with the 10x resistor, gradually reduce the resistance until it does pull in. Reduce it slightly further and that is also the required resistor for the job.

The resistor wattage could vary from 2 watts to 50 watts but I would do my testing around 10 watts. If the resistor gets too hot, increase the wattage. If the resistor does not get too hot, leave the wattage at 10watts. They are cheap enough!

I have used a 24 Volt Dc coil on a 48 Volt Ac solenoid when I did not have the neccessary spare. I was lucky I knew what the resistance should be and they came out as a very close match, it ran in continuous operation like that for several days with no problems.

So using that rational try 230 Volt Ac coils on 115 Volt Dc.
Even better is measure the value of a known good 115 Volt Dc and try matching it to an Ac coil

A very rough rule of thumb I use is 4 Ohms per Ac volt on average sized relays and solenoids.
If its a large coil or motor contactor be prepared to halve the ohms.
If its a tiny coil be prepared to double the ohms value.
If you have seen my thumb its rough.

Ron Beaufort
Nicely put..
So a DV coil will never fail if stuck open?

So a DC coil will never fail if stuck open?

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well ... "never" is a tricky word - but in theory at least, your statement is correct ...

(I can't believe that it's been 12 years since I wrote all of that up ... time flies when you're having fun) ...

Ron Beaufort said:

well ... "never" is a tricky word - but in theory at least, your statement is correct ...

(I can't believe that it's been 12 years since I wrote all of that up ... time flies when you're having fun) ...

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That is a great post. I’ve always struggled with explaining impedance. It’s one of those things that really has to click in your head, you can try to explain it all day long but until you find a way to get it click in someone’s head, it’s just static.