Induced voltage on control wires 2

[rockman7892]

We are building a new plant in which we are using smart MCC's throughout the plant. Each starter in the smart MCC contains an Allen Bradley E3+ solid state overload relay.

All of the starter buckets are for 480V motors and use 120V AC for the field and internal bucket controls.

We recently have been seeing a number of cases where we have been getting some kind of induced voltage (most likely capacitively coupled)on our control wires giving us false inputs to the solid state relay. This induced voltage is in the range of about 18-28V and is just on the border of the turn on voltage of 20V required for the inputs to the solid state relay. These occasional false inputs are causing problems with the operation of these starters and their control.

After doing some testing I found that the open circuit voltge on these control wires when removed from the input on the relay is about 65V. I am trying to determine where this coupled voltage is coming from. All contors for each starter are supplied from a control transformer within the bucket so when bucket breaker is off, control power is off.

When I shut the power off to the bucket all of this induced voltage dissapears. This tells me that the induced voltage is a function of the wires origonating from this bucket and not from some other circuit elsewhere in the plant. I can even isolate it to when a particular wire in the control wire is removed the induced voltage drops significantly When looking further, I noticed that in the several cases where this has appeared, the contorl cable has been a multi-conductor cable.

Now I know long distances can be a major factor in this type of coupling, and that a typical solution is to use DC controls. However in the 8 or so cases that I have seen there have been some long and some short distances and they have been in different areas of the plant. I am able to bleed the induced voltge off and eliminate the false input by connecting a capacitor between the input and ground.

I am concerned that I will continue to see this problem. Although a capacitor has been a sort term solution, I do not want to have to go sticking capacitors in every single starter. Has anyone ever seen this on happen this abundantly thoughout a plant? Are solid state relay devices specifically suseptiable to this kind of voltage? Am I missing something or maybe overlooking something that could be a source of the problem?

 

[Thedroid]

What type of inputs are you talking about? I've used the E3+ in an environment with more than 30V or induced voltage with no problem. How is the solid state relay wired in?

 

[rockman7892]

The solid state relay is powered by an external 24V DC supply. The control inputs from the field are all landed on the relay input terminals and are 120V inputs as mentioned. The relay then also has a neutral connected back to the neutral of the control transformer where the 120V for the control voltage origonates. The relay is also grouned to a ground in the bucket.

 

[dbaird]

Square-D has a Bulletin that discusses this issue:

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[rockman7892]

dbaird

Very informative paper thanks for sending. It looks like from the tables that distances as short as 500ft can cause issues with this distributed capacitance.

The article suggests two solutions which are using a DC voltage for the controls or using a spare contact for shorting the capacitance. We do not have the DC avaliable so this would not be a solution. We may be able to implement the grouding of this capacitance via an extra contract on a control station. For now I have just inserted a capacitor across the input to shunt this induced voltage.

The thing that I find strange is that we have an existing plant on the same site which identical to the plant we are not building. Therefore all the control wire distances are roughly the same, yet we have never had a problem. The existing plant does not have these solid state relays but rather has conventional mechanical overloads and starters. I am thinking that maybe this problem may exist on the existing plant, but this 28V is not enough to cause an problems with these conventional starters.

Does a multiconductor cable increase this capacitance?

Could there be another underlying issue such as a grounding issue that is also contributing to the problem?

 

[Pikoplatt]

I have experienced this problem with Cutler-Hammer Westinghouse Advantage Contactors, and have used the following methods:

1.Wired the 120VAC 'RED' RUN Pilot lamp across the contactor coil (Run input terminal and Neutral Terminal for the Allen-Bradley starter in your case). This is a quick fix, because if the pilot lamp bulb is blown you are back to square one.

2.Used the interposing relay as described in the Squared D paper referenced earlier in this thread.

3.Used the ground connection on the stop button additonal contact block, as described in the Square D paper. The pitfall here may be if during replacement of a stop button or start-stop station, the grounding jumper is inadvertently left out.

Thanks to 'dbaird' for posting the Square D paper, I had not seen anything written on this problem before, although I had encountered it on more than one occasion.

 

[rockman7892]

There are several other circuits that have long runs however do not use a multi-conductor cable but rather stranded cable. Seeing this I have to ask, does the fact that the conductors in a multiconducotr cable are closely bunched together increase the capacitance coupling effect? Are these multi-conductor cables more prone to this type of problem?

Aso the thing that I find strange, is that whe the wire in question is lifted from its input terminal it always seems to have the same open circuit voltage of about 65V. I would think that this OC voltage or even the voltage when landed on the input would vary depending on the length of the cable run. However they seem to be roughly the same for drastically different distances. Could this be an indication there thre is something else at hand here?

 

[Pikoplatt]

I would think that if a single conductor (Core # 1) in a 3-core multicore cable is energized at 120VAC, 60 Hz, then there exists multiple capacitance paths between the energized core and the core (#3) carrying the start signal back to the contactor. Core # 2 will also be energized at 120VAC if Cores #1 and #2 are connected across the STOP button in a 3-wire START-STOP control station. The two(2) capacitance paths will be Core # 1 to Core #3, and Core # 2 to Core #3. These two(2) capacitances will be in parallel and would add, thereby lowering the overall Capacitive Reactance between source (Cores #1, #2) & Core #3. This would mean increased leakage voltage appearing on Core #3 which is connected to the START input of the contactor.
Increased voltage also appears via induction from Core #1 and Core #2 onto Core #3 due to the long parallel run in close proximity to each other, as is the case for a multi-conductor cable. For conductors with wider separation between them, the leakage via capacitance effect and leakage due to inductive pickup are both reduced, compared to the case of a multiconductor cable. This is because of the wider separation between conductors.

What is the input impedance between the 'START' terminal of your contactor and neutral? If this is very high, then the measured value of 65V may not change significantly when you lift the wire.

 

[rockman7892]

I spoke with an engineer at Rockwell regarding this issue with the E3 module and he mentioned that there is another type of adapter module that can be added that will only accept inputs in the frequency range of 57hz to 63hz. He said that the current adapter module that I am using has a frequency tolerance between 47Hz and 60Hz.

Is it possible that this coupled voltage is at some lower frequency than 60HZ and I can possibly ignore it by adding an adapter module with a tighter frequency tolerence?

 

[ScottyUK]

Have you tried hanging a burden resistor of (say) 10k[Ω] across the affected input? The capacitive coupling which is the source of the interference must be weak because there simply isn't enough inter-core capacitance within a starter cubicle for it to be otherwise: a low resistive burden should quash any pickup to a negligible level. The input impedance of the device must be pretty high too.


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