capacitive coupling

[jcole]

Hello guys and gals.

Was troubleshooting an actuator today that had 130V on the close input but would not close. To keep a long story short, I went and bought a solenoid tester to make sure it was not ghost voltage caused by capacitve coupling. The solenoid tester showed no voltage.

Questions.

Have any of you ever seen ghost voltage this high? In my experience I have not.

What would cause a voltage reading this high caused by ghost voltage? In my experience about 45v is as high I have ever seen this phenomenom.

Should I be concerned about saftey or the operation of my equipment with capacitive coupling this high?

This is a new actuator with up to date techology.

Thanks for replies.

 

[jim dungar]

I have seen 'ghost' voltages higher than lie voltages (these were on ungrounded hospital systems). For solid state control devices 'ghost' voltages of nominal L-G are not uncommon.

 

[TxEngr]

I suspect Jim nailed it on this one. You stated that the actuator showed 130V on the close input - I assume you mean that the close limit switch showed this voltage on the input to the control system. You also stated that this is a very new 'modern' actuator so I suspect that the limit switch is a proximity type solid state device. If the control system input card has a very high impedence, the 'bleed through' on the solid state proximity switch can cause line voltage to be read on the input card but will quickly dissipate if a load is applied across the input such as a wiggy or light bulb.

If this is the problem, you'll need to install an interposing relay or other device that will properly load the proximity switch. In an emergency situation, I've seen people use a 120V light bulb to provide this load. You might check with your control system vendor to see if he makes a differenct input card that provides a higher load.

TxEngr

 

[jcole]

I suspect Jim nailed it on this one. You stated that the actuator showed 130V on the close input - I assume you mean that the close limit switch showed this voltage on the input to the control system. You also stated that this is a very new 'modern' actuator so I suspect that the limit switch is a proximity type solid state device. If the control system input card has a very high impedence, the 'bleed through' on the solid state proximity switch can cause line voltage to be read on the input card but will quickly dissipate if a load is applied across the input such as a wiggy or light bulb.

If this is the problem, you'll need to install an interposing relay or other device that will properly load the proximity switch. In an emergency situation, I've seen people use a 120V light bulb to provide this load. You might check with your control system vendor to see if he makes a differenct input card that provides a higher load.

TxEngr

No. This is the input to the actuator to tell it to close from a relay that is controlled by an output from a plc.

The problem I am having is that the actuator sometimes will not close when I have a true 130V applied to the close input on the actuator.
Before I was reading 130v on both the open and close input when the output from plc for each direction was false. This voltage was not present when I used a solenoid tester instead of electronic tester.

I just noticed that when It messes up (does not close) I have 130V on the close input on the actuator but I also have about 11v on the open input on the actuator.

Any ideas where this 11v is coming from? Maybe the actuator sees this voltage as a true signal and doesnt move because it thinks it is being told to go both directions.

I been beating my head against the wall on this for about a couple of weeks. Brand new actuator. It only actuates every 2 hours and most of the time works great.

 

[steve66]

It seems to me that a solenoid should provide a low resistance path to your reference (which I assume would be a neutral or ground wire).

Unless there is something more to this actuator than a simple solenoid, you should not have a 130 V ghost voltage on either solenoid lead.

A ghost voltage would indicate an open circuit. So it seems like you have an intermitent open either in your solenoid, or the wiring to it.

Steve

 

[gar]

090814-1547 EST

jcole:

A much better definition of this actuator is needed.

Apparently there are two control inputs to this device. One is OPEN and the other CLOSE. Nominally a 0 or 120 V signal is applied to each input. 120 V is most likely used to assert the condition assuming positive logic where 120 = logic 1 and 0 = logic 0. There must be a common reference point at the actuator for these signals. Your measurements must be referenced to this common. Obviously you can not have two asserted logic 1 conditions at once because this would produce a conflict.

Your 11 V on the input that should be 0 may be the problem. But make sure the voltages you are measuring are relative to the common terminal for those inputs, and this measurement should be at the actuator..

Is there a separate power supply to the actuator to supply the energy to operate the actuator?

Or is the power to operate the actuator derived from the logic signal?

.

 

[ELA]

Maybe there are "End of Travel" sensors in series with each input?
If the EoT sensor is not adjusted correctly (still open) then the input could be a higher impedance than when the EoT is closed.

This could explain 120V (ghost) on a completely open circuit vs. 11V (ghost) on a continuous input circuit.

Measure the input impedance when the unit is at true end of travel vs somewhere in-between if possible to confirm this.

 

[TxEngr]

Thanks for the clarification. A couple more questions - is this a motorized actuator or a double acting cylinder? You stated that there is an interposing relay on the output of the PLC - is this for both outputs and are they on the PLC output card or separate?

If it's a motorized actuator, then check the manufacturers info on the max and min voltages to actuate the device. I suspect that 11V is not enough to give a command to the device. It sounds like you have some amount of induced voltage on the line, possibly due to a long run with small wire not sufficiently shielded. One other option that I'm not particularly fond of to get bleed off induced voltage is using a capacitor on the signal to bleed off the voltage. Like I said, I don't like this solution, but I've used it in a pinch.

Since the problem is intermittent, take a look at the internal wiring of the actuator (if it's motor driven) since a loose or shorted wire on one of the internal limit switches could create a similar problem.

If this is a double acting cylinder with true air solenoids on the actuator that would provide sufficient load, I don't have any good suggestions.

TxEngr

 

[jcole]

Hello again.

I apprecitate all you guys input. I found the problem though.

What was happening was a timing issue like one of you guys mentioned. The open input goes false at exactly the same time (or extremely close to the the same time after you consider scan time and time it take for contacts to close on interposing relay) the close input goes true.

So I put in a small delay and the problem hasnt reoccurred.

The wierd voltages are still present though.

Again thanks for all your inputs.


Jonathan