Induced voltage (I think)

[hurk27]

I want to add that even if this is not a PLC control system, any electronic control is very suspectable to ghost voltage and if encountered will need a resistor placed between the output contact and the neutral to drain off this ghost voltage to get the "0" the control is looking for, just had to do this on an electronic timer relay that controled a pump. a 10kΩ/2w carbon resistor (not wire wound) worked just right.

10kΩ@120 volts is 1.44 watts if any want to know why the size.

 

[Rick Christopherson]

Rick...

An AC-Coil may become polarity sensitive, i.e., reverse force direction, with DC applied, if the armature material is not that normally found in a DC Solenoid!

Phil

Phil, you have that backward. If the plunger becomes magnetically polarized, it will be in the same direction as the DC pull-in current, not opposing it.

Moreover, an AC solenoid plunger cannot afford to be easily magnetized, or the plunger will lose all pull-in power due to the magnetic field constantly changing and the plunger constantly switching from being attracted to repelled. In the worst case scenario with a fully magnetized plunger on an AC coil, it will never move, but just hum a lot.

That is why you can't feed a DC solenoid with AC current, but you can feed an AC solenoid with DC current (as long as you do not exceed amperage on the coil).

 

[gar]

121214-2334 EST

drbond24:

With your new description of the circuit layout I think it is perfectly clear where the problem is located. A different schematic could have made this clear from the beginning.

The shielded cable probably serves no useful purpose.

You have internal to the cable created a capacitor in parallel with your switching contact. I don't quickly see the length of the cable. I am going to guess 200 ft and 80 pfd per ft capacitance between the two conductors in the cable. 200 * 80 = 16,000 pfd, or 0.016 mfd. At 60 Hz this equals about 170,000 ohms. Assume the coil impedance is 10,000 ohms instead of the 5000 previous estimate I made.

120 * 1/17 = 7 V across the coil. Since you read 50 V across the coil it means the capacitance per foot is higher than my guess and/or the cable length is longer. But I also don't really know a good value for the equivalent coil resistance.

Several solutions:

1. Find another spare wire in the bundle that is not inside the shielded cable, and use it to replace wire a-f. Then connect wire a-f to point b or c or both.

Or

2. Put a 0.22 ufd 400 V rating Mylar capacitor in parallel with your coil (between points d and c). This is about 10,000 ohms at 60 Hz. Add some series resistance in wire d-e at e. Experimentally pick a resistance that produces about 50 V across the coil (d-c) with your contact closed. Select an adequate power level. An initial suggestion might be 10,000 at 2 W.

For those thinking this was a conventional electromechanical coil all you needed to do was look at the possible pull-in and drop-out power levels to see the unlikeliness, and conclude that it was some sort solid-state interface. Probably an optical coupler.


.

 

[drbond24]

Our technicians fixed this tonight before I tried any these ideas. They installed an IDEC relay in the circuit that does not pick up due to the 50 VAC. It is preventing this voltage from reaching the valve positioner contact input where I didn't want it.

gar: Cable length is probably shorter than 200 ft but not by a great deal. 150 ft perhaps. Coil impedance is 12,000 ohms per the manual.

 

[Besoeker]

Gee. Instead of being a jackwaggon, why not explain it, like I suggested you openly could.

I don't think it is wrong for a solenoid, but I am open to hearing reasons why it could be, if you could stop being snarky for a minute.

We are tood in the opening post that the coil is AC. Puting a diode in series with it would subject it to DC and probably saturate it. Then there's the possible problem that the contacts in series with it might not like to break DC.

 

[gar]

121215-0744 EST

drbond24:

There are some bits of information that you provided that had me confused. I now think I have a reason.

You read about 50 V from wire d-e to ground (I assume ground means neutral) with this wire d-e disconnected at both ends. I believe wire a-f was still connected to 120. This provides a capacitive voltage divider consisting of the capacitance from the 120 V wire (a-f) to the d-e wire, and another capacitor from d-e to the shield (ground and neutral). It is reasonable to assume approximately equal capacitance values for both, and thus, the approximate 50% voltage drop. The Fluke is about a 10 M input resistance with about 100 pfd shunt capacitance.

The circuit changes substantially when wire d-e is reconnected. With your latest information my guess is that the cable capacitance is about what I estimated or a little less. This won't produce 50 V across the 12000 ohm load resistance. Thus, I suspect that your contact (the 220 V rated thing) is a solid-state relay with a shunt snubber. The snubber is the likely source of the leakage current to produce an off state 50 V. This is a common problem in PLC circuits.

Back in 71 I first used a 5 k resistor as current limiting to a 4N35 (maybe it was 4N25 then) optical coupler for a 120 V AC interface. This was in a system using the new Modicon controller for a machine going to Buick. I had this leakage problem from the PLC output contact and its leakage. Our solution was a 1 k 20 W shunt resistor at my input.

.

 

[jim dungar]

The separation between the contacts and the coil is a very important detail. Things turn on but don't turn off.

Try this link for a little more information on 'distance control'.
http://www.newark.com/pdfs/techarticles/squareD/RemoteContactorControl.pdf

 

[kwired]

I say either use another load in parallel with the coil to drop the induced/coupled voltage when the switch is open (which if you have a coil I find it hard to believe it doesn't drop that voltage better than a bleed resistor would) or use existing contact to control a relay located closer to the controlled equipment and control the equipment with a contact from this relay.

I get the feeling this circuit is more complex than what you submitted a drawing of.

You sure you don't have some solid state relay serving as the contact, maybe even a PLC output that is not a dry contact?

 

[Rick Christopherson]

First off, the Original Poster has resolved his problem (and it is also likely that he is not discussing a typical solenoid). So I am only addressing Besoeker's comments as a side topic.

We are tood {told} in the opening post that the coil is AC. Puting a diode in series with it would subject it to DC and probably saturate it. Then there's the possible problem that the contacts in series with it might not like to break DC.

Besoeker, I respectfully submit that you are confusing a solenoid for a transformer. You wouldn't want a transformer in saturation, but a solenoid is intended to be in saturation. Preferably you want to keep it at the knee-saturation point, because any additional increase in current will not appreciably increase the flux, but will increase the heat. That's where it achieves its maximum pull-in power for the least amount of wasted heat.

As I said previously, you can place an AC solenoid in a DC circuit, but you cannot place a DC solenoid on an AC circuit. There is one caveat to this which the discussion hadn't previously gotten to yet, but I did mention it parenthetically. You wouldn't want to put 120VDC into a 120VAC coil because the amperage would be too high. I can't remember what the impedance would be for a half-rectified signal, but it wouldn't be zero like a full DC signal would. However, even without adding a current limiting resistor, the V_rms would be reduced by an additional 1/sqrt-2 {V_rms = V_p/2 versus V_p/(sqrt-2)}. V_rms = 85V

So in all likelihood, the diode by itself will keep the coil amperage within a safe operating range. This would have been the first thing I would have asked the OP to confirm if he had chosen this approach.

The reason why an AC solenoid can take DC power (within current limits) is because the AC solenoid coil/core has additional features that its DC counterpart does not require. That's why you can't do the reverse. The AC core is laminated and also contains a shading ring. These features are not needed when DC power is present, but they do not impair the function if they are present.

As for the second comment, the upstream contacts are not breaking a DC signal. They are breaking a have-wave rectified signal. Even though the signal doesn't invert, it still reaches zero volts, or close to it depending on the filter effect of the solenoid coil.

 

[Besoeker]

First off, the Original Poster has resolved his problem (and it is also likely that he is not discussing a typical solenoid).

Other than by you, I don't recall that the coil was termed a solenoid. Typical or otherwise.

Besoeker, I respectfully submit that you are confusing a solenoid for a transformer.

And I respectfully tell you that you are totally wrong on that point.

You wouldn't want a transformer in saturation, but a solenoid is intended to be in saturation.
Preferably you want to keep it at the knee-saturation point, because any additional increase in current will not appreciably increase the flux, but will increase the heat. That's where it achieves its maximum pull-in power for the least amount of wasted heat.

For least material cost transformers are operated near to saturation. I thought you might have known that.

As I said previously, you can place an AC solenoid in a DC circuit, but you cannot place a DC solenoid on an AC circuit.

Care to cite sources that support that contention?

 

[jim dungar]

I had this leakage problem from the PLC output contact and its leakage. Our solution was a 1 k 20 W shunt resistor at my input.

Eventually I gave up on the custom calculations after finding just about any off-the-shelf 120VAC RC 'coil suppressor' worked. All of the major industrial control maufacturers offer some type of packaged coil suppressor. The one I used were from a company called RK Electronics.

 

[Rick Christopherson]

Besoeker, contrary to our past, I am trying my hardest to not be antagonistic, but it is difficult to do when you are trying your hardest to be antagonistic. Could you please try to keep this as a civil discussion without the needless jabs?

As I said previously, you can place an AC solenoid in a DC circuit, but you cannot place a DC solenoid on an AC circuit.

Care to cite sources that support that contention?

Simply perform a Google search for "AC versus DC Solenoid" and you will have hundreds of references stating exactly what I said. Many of these come from the manufacturers of these solenoids.

You've made two arguments for your assertions, but I countered those arguments. If you wish to stand behind those two arguments, please support them. Simply telling someone they are wrong is not supporting an argument.

 

[G._S._Ohm]

. . .Simply telling someone they are wrong. . .

It's probably ad hominem.

I don't know the history of you two gentlemen, but my experience is that the jabs will come and they will not much be related to the content of your posts.

There is a member on the another forum that I seriously considered suing for defamation and libel, but to do so I would have had to prove damages, damages in the legal sense.
Ironically, he used his real name and I didn't use mine, so if I defamed him he would possibly have had damages.

The laws have not caught up with the Internet.
http://www.amazon.com/Offensive-Internet-Speech-Privacy-Reputation/dp/0674064313

 

[Besoeker]

Besoeker, contrary to our past, I am trying my hardest to not be antagonistic, but it is difficult to do when you are trying your hardest to be antagonistic. Could you please try to keep this as a civil discussion without the needless jabs?

Simply perform a Google search for "AC versus DC Solenoid" and you will have hundreds of references stating exactly what I said. Many of these come from the manufacturers of these solenoids.

I'm not being antagonistic. Nor even trying to be.
All I'm asking is for you to provide support for your opinion that:

you can place an AC solenoid in a DC circuit, but you cannot place a DC solenoid on an AC circuit.

Your opinion. and that's fine. I think it neither unreasonable nor antagonistic to request that you provide supporting information.

 

[Rick Christopherson]

I think it neither unreasonable nor antagonistic to request that you provide supporting information.

It is not an obscure concept. Why have you chosen not to Google it? :blink: If you had simply done that, you would already know that it is not an obscure concept and is cited hundreds of times.

If all you are going to do is keep bouncing back and forth asking for commonly found references, then forget it. It is not that important of a topic, and I will just walk away. :dunce:

 

[kwired]

It's probably ad hominem.

I don't know the history of you two gentlemen, but my experience is that the jabs will come and they will not much be related to the content of your posts.

There is a member on the another forum that I seriously considered suing for defamation and libel, but to do so I would have had to prove damages, damages in the legal sense.
Ironically, he used his real name and I didn't use mine, so if I defamed him he would possibly have had damages.

The laws have not caught up with the Internet.
http://www.amazon.com/Offensive-Internet-Speech-Privacy-Reputation/dp/0674064313

Those two have never disagreed on anything before:happyno: I have always supported 110% anything that either one of them has said myself:happyyes:.

 

[G._S._Ohm]

Those two have never disagreed on anything before:happyno: I have always supported 110% anything that either one of them has said myself:happyyes:.

I thought in a recent post one guy zinged the other.

Who knows? Life situations, stress levels, all change. And there may be some detail that makes them both right at the same time.

 

[Rick Christopherson]

Those two have never disagreed on anything before:happyno: I have always supported 110% anything that either one of them has said myself:happyyes:.

You owe me a new keyboard, Mister!!! :lol::lol:

 

[Besoeker]

It is not an obscure concept.

Then you should have absolutely no difficulty in providing supporting information for your opinion rather making it incumbent on me to search for it.
I have given you my technical reasons for suggesting that it might not be such a good idea.
I haven't said yours were wrong.

 

[kwired]

I thought in a recent post one guy zinged the other.

Who knows? Life situations, stress levels, all change. And there may be some detail that makes them both right at the same time.

Your thought was probably correct, Rick owes me a keyboard as much as I owe him one so I feel we are even on that one.