surge suppression for AC pneumatic valves

[Skiroy]

It has come to my attention that the old PLC-2 system was Tric outputs so it did not matter. We have a Micrologix 1400 PLC with add on I/O modules which do not come in TRIACs and only have Relay outputs for 120ACv. Anyways I had to come up with a solution to protect the life of the outputs due to high inrush currents and backwards current, due to the pneumatic valve inductive loads. I had looked into AB solidstate relays with surge suppressors built in which would be perfect unfortunately it would cost $1600.00 to do this and there is no way Im am going to convince my boss to shell this amount of money out.

Now I know this can be done with running MOVs in parralel with the load(pneumatic valves).But I was told that you should not wire these at the modules or PLC,but that they should be wired at the load. But at the load ,would mean splicing a MOV into wires in the conduit cable that runs into each Pneumatic valve manifold. This seems odd,sloppy and unsafe to do and I would think that it can be done in the panel, in a clean way. And MOVs are a heck of a lot cheaper than the AB interpolasing relays which I was earlier refering to, as the solid state relays with the surge protection.

1. Any suggestions on this?

2. Do MOVs cover both backwards current(collapsing inductive load) and high inrush current issues or just one or the other?

3. How would I figure out the rating of the MOV I would need for a load(In this case a .36A 120ACV valve)?

Thankx

 

[iant]

1. at the end of the day consider the replacement cost of a PLC with blown relay contacts,
Interposing relays - 6mm wide range may be ok.
the MOV - for an old bgr like me ?
As far as surge suppression or relay capacity
- The type of contact material may help you
- Also the number of operations each contact does
- consider the purchase the suppressors

 

[sthompson]

2. Do MOVs cover both backwards current(collapsing inductive load) and high inrush current issues or just one or the other?

MOVs, are Voltage Devices. They will deal with Voltage Spikes, but will not deal with Current Inrush.

3. How would I figure out the rating of the MOV I would need for a load(In this case a .36A 120ACV valve)?

For a 120VAC Circuit, you would normally use 130VAC MOVs. The other Parameter is Energy absorption, this is rated in Joules or a fraction of Joules. Without knowing exactly what kind of Inductive Device you are switching, so I can't advise you on what rating you need.

Contacts can normally Make greater Current than they can Break because the Arcing. The MOV will help you out with that part, and increase the life of the Contacts.

For more information, do a Google Search on Metal Oxide Varistors.
You can start here;

http://en.wikipedia.org/wiki/Varistor

Stu....

 

[BryanG]

In Europe solenoid valves tend to have standard solenoid valve connectors DIN 43650, you can get add on modules for these plugs that give suppression. I haven't used any for a long time but did find these:

http://www.majordin43650.com/

Go down to 'Specialty Valve Products' and 'Circuit Modules'.

 

[keithkyll]

You can use MOV or RC. I prefer RC - seems to supress spark better.
MOV works like back-to-back Zener. Will conduct (go low resistance) when it crosses voltage threshold. Size 14 should be enough.
I like a prepackaged module that has both MOV and RC. Connect this across valve, or across the relay contacts as you prefer. It really doesn't matter that much.
See this link.

 

[mendonsy]

I always install interposing relays on all solenoid valves. If something goes wrong you destroy a $6 relay not the PLC output and the 781 relays are less expensive to replace than fuses.

 

[Skiroy]

I always install interposing relays on all solenoid valves. If something goes wrong you destroy a $6 relay not the PLC output and the 781 relays are less expensive to replace than fuses.

Well whenever I thought I am getting a handle on things I am humbled to the fact that I have so much more to learn. Im looking at the 781 icecube relays. Now I talked to a tech and I asked him that how can adding a seperate relay help protect the PLC relay out puts life? My question was, if the little(.36A)max current draw from switching the 120VAC pneumatic valves create a high inrush current on the PLC relays why wouldnt an seperate relay do the same thing? I told him I understood that the ice cube relay coil requires alot less current, but asked him if a high inrush current would still take place on the PLC relays from the icecube relay coil load? He said I was correct and that a supression device would still be needed. I do get the advantage of just changing a single bad realy vs the whole PLC,but the point is to eliminate any damage to the PLC outs with these relay,. So does your 781 relays have surge suppressors built in or do you add MOVs,RCs ect in paralell to the Icecube coil?

 

[Skiroy]

In Europe solenoid valves tend to have standard solenoid valve connectors DIN 43650, you can get add on modules for these plugs that give suppression. I haven't used any for a long time but did find these:

http://www.majordin43650.com/

Go down to 'Specialty Valve Products' and 'Circuit Modules'.

These are a great idea but unfortunately these are older Parker valves and there are no external connectors.

http://www.numatics.com/applications/products/valves/mk55.aspx


All solenoid connections are done via pins within the Air Valve manifold.

But this is great idea for future projects.

 

[Skiroy]

There is obviously several issues going on here. Voltage spike protect,inrush current and backwards current cause from collapsing 120VAC pneumatic valve coil(.36A). There also several devices such as RCs,Varistors,MOVs,Resistors. Im not sure which are considered surge suppressors. I am also unsure about which of these devices are appropriate for each issue or if a combination of these are required. Now I do understand the cause of high inrush current and the cause of a backwards current.What I do not understand and need clarification on is:

1. What is the difference and causes of a Volatge spike vs high inrush current and what are the options to solve it?

2. Can someone break down a little about each of these issues and talk about why you would choose one device over another and also ultimately which device or combination of devices are needed to cover all issues(Voltage spikes,inrush current and backwards current)?

It seems a little like Im running in circles. Like one device can solve one problem, like high inrush current, but but leave a backwards current problem unsolved? Is surge suppression a general term that covers all these issues? Someone made a comment that made me think that using a MOVs in not the solution but will only help with Voltage spikes. I am a little unclear as what a "surge suppressor" is actually capable of or if its a term that can cover many devices that do different things.

 

[mendonsy]

The 781 relays don't have a built in MOV but they also produce a minimal spike (if any) because the are a physically much smaller and lower current coil. You should locate an MOV as close as physically possible to the solenoid valves to get rid of the spikes they generate.
The big problem with solenoid valves is that they can mechanically hang up from contaminants in the air supply. When that happens your 360 ma rated current can easily turn into several amps continuous draw which will cause all sorts of problems.

 

[keithkyll]

What is the difference and causes of a Voltage spike vs high inrush current and what are the options to solve it?

Inrush - high current when powering on. Caused by energy needed to magnetize the core.
Spike - Caused by collapsing magnetic field generating a high voltage in the coil.

Is surge suppression a general term that covers all these issues?

Yes.

Don't worry about the inrush. That will occur when the relay contacts are closed.
The problem is when you de-energize. With an open coil, the voltage can rise to infinity, causing an arc across the relay contacts.
The idea is to short the coil during this event. When shorted, the energy remains in the coil. The solenoid will take longer to close as the magnetic field decays.

An MOV (AKA Varistor) will start conducting above 200 Volts or so, depending on which type you select. Doesn't matter too much - you need much higher voltage to generate a spark.
What I've seen in practice is a smaller spark with an MOV, but it doesn't eliminate it.
Before MOV's, RC networks were used as spike suppressors. The cap conducts, and dissipates the energy in the resistor.
I like both. The MOV shorts the high voltage, and the RC gets the rest.

Connecting across the coil is preferred. This keeps all of the "noise" local to the solenoid. Connecting across the relay contacts does the same thing, but the wires can transmit the noise. That can be a concern at 24 DC, because other DC electronics can be close by, or connected to the 24DC supply. At 120 VAC, we're not to worried about noise. AC is noisy to begin with. Sensitive digital circuits shouldn't be in this area.

Be aware of one thing. The RC network is always conducting. With it connected across the relay contacts, there will be a bit of leakage current. Won't be a problem with your solenoids, but may take you by surprise when troubleshooting.

This document says RC will also limit inrush current. RC network from Red Lion.

With DC it gets easier. All you need is a diode. Some have suggested 1N4007. Proper device is 1N4937. It's faster, and the standard.

 

[Skiroy]

Thankx,do these RC snubbers go bad and how so? Would they be burn or blow up? Im asking because If I installed them in the manifolds I would never see them again likely and never know if they go bad or not. It would be nice if they had LEDs and I could install them in the panel. I will take pics of what Im working with for further suggestions as far as install options.

And what rating do I need?

 

[iant]

I have an idea in the back of my mind that there are Phoenix or similar RC terminals available - i will have a look

 

[iant]

http://catalog.phoenixcontact.net/p...52611858&parentUID=852611857&reloadFrame=true
this is it

EDIT

no try this one
http://catalog.phoenixcontact.net/p...reloadFrame=true&UID=2905417&parentUID=205032

 

[keithkyll]

That looks good Ian. A SSR with MOV and RC built in.
Failures are not typical. If hit too hard, MOV's go shorted and explode.
For RC, a cap can go shorted and burn the resistor.
Rating needed is 120 VAC or higher.

Use the device offered by Ian above. Interposing relay, with all the snubber, etc. issues handled. Even has an LED.