Confirming/maintaining control relay switching current

[Seansie]

Pathetically beginner question, I'm sure.. (see last paragraph)

The problem: Lighting relays oxidize and get "stuck" after a short time, becoming unresponsive.

- 20A mechanical relays switching 120V lighting circuits w/ 24VDC signal
- No more than 15A on any relay
- Control signal is reliable 24VDC; no voltage loss/weak signal causing near-contact and "chattering", burning contacts

I should mention, not every relay is having this problem. Maybe a dozen out of a hundred, and the amperage never comes close to 20. Other forums mention using gold-flashed relays, mercury-wetted relays, or reed relays to solve this problem, but the gentleman from tech support at Schneider with whom I spoke sounded like he had heard the problem a million times before, and said to make sure our switching current was never below 100mA.

The problem is, this is where my knowledge/experience gets real fuzzy. I don't know where to begin!
- What could possibly be causing the switching current to drop that low? The power supply is more than 100mA..
- Is one method and/or location for measuring amperage better practice than another in this instance? (i.e. amp clamp vs. probe in series)
- If so, what/where? I second guess myself, and any measurements I take.
- How do I determine what I'll need to fix it, and, if it's a power supply issue, how do I calculate what I'll need to fill our needs?

I apologize if my ignorance combined with the abundance of my questions bothers anyone. I'm not an electrical engineer (no matter how much I pretend). However, I'm tasked with troubleshooting/maintaining an existing controls system as much as possible before we have to bring in engineers to solve problems at our facility. Sound a little funny? It is. But what can ya do? Even with a few hundred hours OTJ with controls and PLCs, I still find myself in over my head more than I'd like, and so was hoping it wouldn't be too presumptuous to look for some help here. I'm only sorry I don't have more to contribute. I'm just trying to solve an issue that's beyond me, and learn something in the process. Thank you for taking the time to consider this.


- Sean

 

[iceworm]

said to make sure our switching current was never below 100mA.

Just curious - Are the lighting circuits tungsten?

By "switching current" do you mean the 24VDC control voltage current to the relay coil? If so, yes, that can be a problem. Control voltage drops far enough the coil doesn't pull in cleanly.

I have had this issue only once before. I changed out a bank of relays cause they were a bit too small. They all operated from the same control xfm - and all pulled in together. The coils on the next size up drew enough more current it pulled the control voltage down to where the contacts chattered just a bit. Had to use a power analyzer to see it. Bumped the control xfm from a 500VA to a 750VA and the chatter stopped.

However, if you don't mean the control voltage - then forget this.

Other option is the current through the main contacts is really low.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[Seansie]

Are the lighting circuits tungsten?

If you're referring to the lamps in the fixtures, they differ throughout the facility. Problematic circuits have been incandescent (tungsten) only, fluorescent only, or a combination of both.
If you're referring to the relay contacts, they're silver alloy.

Schneider Electric/Magnecraft 781XAXM4L-24D

http://www.alliedelec.com/search/productdetail.aspx?SKU=70185275

By "switching current" do you mean the 24VDC control voltage current to the relay coil? If so, yes, that can be a problem. Control voltage drops far enough the coil doesn't pull in cleanly.

I have had this issue only once before. I changed out a bank of relays cause they were a bit too small. They all operated from the same control xfm - and all pulled in together. The coils on the next size up drew enough more current it pulled the control voltage down to where the contacts chattered just a bit. Had to use a power analyzer to see it. Bumped the control xfm from a 500VA to a 750VA and the chatter stopped.

Yes, I'm referring to the control voltage - or at least that's what the guy from tech support was referring to, as far as I can tell. However, in terms of voltage, it seems to be fine. It's the amperage that is apparently insufficient, though I haven't observed that so far. Not positive how I can conclusively measure it..

Correct me if I'm wrong, but wouldn't VA only be relevant in AC applications? Or am I oversimplifying? My control voltage is DC.

Thank you so much for the input.

 

[Seansie]

Sorry for the double post..

As I research more, I'm wondering if it may be an issue of inrush current.
If that turns out to be the case (I'm not on-site right now to investigate), does anyone have any experience with different options to protect relays from inrush? Or would I need to switch them out for something like reed relays?

Thanks

 

[waross]

Inrush is caused by the changing inductive reactance of an AC coil as the moving element is pulled in and the air gap is reduced.
There is no inrush with a DC coil.
To measure 100 mA DC I suggest using a multi-meter in series.
Yes, VA applies to AC circuits. However a transformer or a power supply has an internal impedance or an internal resistance. When a load is applied, the terminal voltage will drop. In the instance of the AC circuit a larger transformer with less internal impedance solved the voltage drop issue.
In you case, you may have a high resistance issue in the wiring.
Do the relays with issues have longer than average control conductors?
Are you operating more than one relay simultaneously when the problem develops?
You may need a larger power supply to hold up the voltage (and as a result the current).
You may need heavier conductors to correct a voltage drop issue in the control leads.
Possibly both.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I agree. If you are only assessing the health of the control circuit by measuring the power supply voltage, then you are not seeing the whole picture. It most likely is that the more relays you turn on, the more current is drawn by the power supply. The relays in your link use 3ma of coil current. Not much, but then you mention "hundreds" of them, and you mention a 100ma power supply? Your math is way off somewhere!

A lot of 24VDC power supplies, when overloaded, have a crowbar circuit that just shuts them down. But cheaper ones sometimes do not, so what happens is that the voltage drops in order to maintain a current limit. When the voltage drops, the relays chatter, and chattering destroys the power contacts in a hurry.

"Will work for (the memory of) salami"

 

[DRWeig]

What are the complete ratings for your relays?

Tungsten = ??A
Ballast = ??A

There's no way you have less than 100 mA through a contact unless you're switching only a 12-watt load.
Sometimes, general-purpose relays might have ratings like:

General purpose 20A
Ballast 8A
Tungsten 2A

Lighting contactors have to be designed to handle the ballast and tungsten inrush currents.





Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Seansie]

Wow, thank you all so much. waross, jraef, DRWeig, I'll get back to you a bit later today. On-site now, but being pulled in a few different directions.

 

[iceworm]

Correct me if I'm wrong, but wouldn't VA only be relevant in AC applications? Or am I oversimplifying? My control voltage is DC.

My guess is you are feeding the DC power supply from a control transformer. Most of the DC control power supplies for simple contactor circuits, that I've seen, are just xfm/rectifier. And if the current draw is high, as in energizing too many relays at the same time, the voltage pulls down through the xfm impedance. However, if it is a switching type, then as jaref said.

"Tungsten" refers to the lamp type. They have a high inrush - tend to damage contacts easily.

As for control ckt inrush, some contactors have a closing coil and a holding coil - these have an inrush - sort of. This doesn't sound like anything you have.

does anyone have any experience with different options to protect relays from inrush? Or would I need to switch them out ...

No easy ways to protect from inrush. The load is what it is. You have to get relays with contacts suitable for the load. 20A of tungsten lamps takes a pretty healthy relay.

As for seeing dips in the control ckt voltage, or relay chatter/bounce, my recommendation is to use a scope or power analyzer - two channel input, one on the coil and one on the contacts. I've used both. I haven't had much luck with a DVM.

I haven't tried to measure transients in DC coil current. But I have access to a low current hall effect scope probe - I'd probably try that.

Definitely measure the control circuit dips out at the coils of the misbehaving relays. As warross said, you could easily have control circuit resistance issues - lead length, connections.

ice

Harmless flakes working together can unleash an avalanche of destruction