Arc Suppression for large discharge

[moregas]

Hi,
I was thinking to build arc suppression circuit, one for each of the three contacts, for a three phase motor contactor. I was thinking of a RC circuit. The motor (Refrigerant compressor) information, guessing, is about- 20 H.P., 480 ACV, starting current is three or four times running current, and current changes with load.
Please give me your thoughts on this idea. One or two seconds, RC delay should be ok? Also need some good formulas for capacitor and resistor sizes. Thank you.

moregas

 

[waross]

Why?
Millions of refrigeration compressors are running without R/C circuits on the main contactors. The arc chutes and de-ion plates found in many contactors seem to work well.

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

 

[moregas]

Hi Waross,
If the contact is pitted is it working well?

 

[waross]

Are the contacts constructed of silver oxide? They tend to pit and oxidize but silver oxide is also a good conductor.
Is the contactor smoking?
Bottom line: some pitting is normal on a starter for 20 HP at 480 V. If it is a good quality contactor it will have silver alloy contacts and will work well despite moderate pitting.
Large craters, large grapes and a lot of arc flash extending sideways from the contact surface are bad.

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

 

[Skogsgurra]

Severe pitting and reduced contact life is not caused by anything that you can suppress with an RC circuit.

Pitting is more a result of the contactor being too small for the job. What size is it?

It can also be a result of contact chatter, which in its turn is caused by voltage drop when starting, contactor being mounted on a weak support (or screws not tightened) - or simply a bad quality contactor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[moregas]

Thanks Waross and Skoqsqurra for your assistance,
Waross I am sorry, type of contacts, I not sure, but I can tell you it came with a 30 amp contactor. I knocked it up to a 40 amp contactor, then a 50 amp contactor. (furnas #42df35af) the voltages and currents are recorded and normal. Tag reads 460v, 25a. act- 478v, 479v, 481v, 15 to 20 amps. Also starting lock rotor is 178 amps. No there is not any smoke, but they are pitted to the point of replacement every four to six months, with the 50 amp. The problem is short cycling do to low load and poor engineering. I have this problem in a lot of chillers. Well anyways I have been working in this trade for 32 years, all it takes is a small pit to start a weld. Contactor don’t work well in my opinion do to that pitted contact ten to weld and single phase one winding to a crisp. And this isn’t a good thing for me because I am a lazy guy and don’t like to work!
Skoqsqurra is not the RC network place directly across the contact the most popular method of quenching an arc between separating contacts? Isn’t the arc shunted into the capacitor through the resistor, depriving the arc of some of its energy reducing heat greatly? There is no chatter, no voltage drops, mounted tightly with three screws, and furnas over sized contactor. As I said before bad engineering.

Ok so why not use a RC?

 

[itsmoked]

All you need is one botched closure and you can have blasted contacts. Chattering is almost guaranteed to destroy the contacts almost immediately.

You say they aren't chattering but I suspect that over several days you may be getting an occasional chatter. Whatever is controlling the contactor can be having problems. A temp or pressure switch? Something that is causing the contacts to close and before the motor has come down the LRA curve the contacts open for some reason. THAT, is how you get welded contacts and surprising wear. Otherwise your setup should not be having this problem. You shouldn't be needing to contemplate a snubber at all. Something nefarious is afoot.

Furthermore, a chatter damage like that would not be altered in anyway by a snubber since the problem is switching the LRA not switching a normal current.

Snubbers also dump energy all the time and occasionally confuse troubleshooters because you have leakage that shows up with modern meters in places people don't expect to see power when the contacts are open.

Find the chatter - it's there.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

RC is good when it comes to reducing HF interference from low power arcing. Not for high-energy arcs and fighting welding or pitting. If anyone told you that, he probably doesn't know what he is talking about.

There may be something else causing the pitting. If there are long cables or if there are power factor correcting capacitors, the capacitance makes a very short duration and high amplitude current flow. Do you have any of that?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[moregas]

Hi
All you need is one botched closure and you can have blasted contacts. why? Is it- LRA + capacitor discharge. Also can you explain why different rules from low power arcing to high power arcs?

moregas

 

[waross]

Low power arcs are often inductive discharges. The voltage may be very high and destroy electronic equipment.
Power circuit arcs from disconnecting motors are usually less than line voltage but they do melt things.
Is this he first set of contacts that you have seen? Newbies are often horrified by the look of normally operating silver oxide contacts. Yes there may be damage, but if you haven't seen normal contacts before, you don't know what is normal and what is bad.
A snubber circuit often is connected across the load and absorbs the energy and potential high voltages ascociated with inductive discharges. You don't have inductive discharges to speak of when disconnecting an integral HP motor.
You may be chasing a figment of your own imagination, trying to suppress arcs in a 20 HP motor starter. If it was neded it would already be part of the design.
What is often there on larger starters are de-ion plates and magnetic blowouts. Sometimes one sometimes the other depending on the design. Some designs use blowout coils to blow the arc into the de-ion plates.
Motor starters work well. Don't try to redesign the wheel into a stone boat.

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

 

[Skogsgurra]

OK. Bill bet me to it. And he said it better than I would have done.

You really have to distinguish between different situations. Contact pitting or welding is a whole different story than RFI. RFI can be fought with RC (or any suitable means) while pitting and welding has to be dealt with otherwise.

There is no room to deliver a complete lecture on the different phenomena here. But if you google 'pitting', 'welding' 'arc', 'contactor' etcetera, you may find that google is your friend.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Forgot: Are there power correcting capacitors that are switched together with the motor?

Inrush for capacitors can easily do what you are describing.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[moregas]

You said:
“There is no room to deliver a complete lecture on the different phenomena here. But if you google 'pitting', 'welding' 'arc', 'contactor' etcetera, you may find that google is your friend.”


Ok I get it. If you don’t understand it, you can’t explain it.

 

[Skogsgurra]

You are very, very wrong there.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

One thing to consider is that a contactor is designed to make locked rotor current and to break running current. If you frequently have a stalled motor then the contactor breaks locked rotor current, which is 6 to 8 tiems what it is designed to do on anything more than an occasional basis. Does your load ever stall?

Oh, and you might want to consider an apology to Gunnar. He's likely forgotten more about electrical engineering than you have ever known.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

moregas – I see this is your first post. Welcome to the forum! I hope you will stick around... there is a lot to learn here and I'm sure a lot you can contribute here as well. I am sure you are reading more into the google comment than was intended.

itsmoked: All you need is one botched closure and you can have blasted contacts.
moregas: why? Is it- LRA + capacitor discharge.

I believe itsmoked was describing how easy it is for contacts to get damaged appearance to begin with.... I don't think he was discussing the effects of your proposed capacitor (perhaps he will clarify).

There are a lot of different good angles on the discussion. As was mentioned, that there are thousands... maybe millions of low voltage starter/contactors in operation. The OEM's do not provide RC snubbers. And addition of RC network is not a standard or conventional approach. There are many things that can be inspected and adjusted within a contactor as well as evaluating suitability of the particular contactor for the particular application as well as determining what is really normal. All of this will come from the OEM literature as well as advice from the experienced folks here.

Perhaps if you start with a description of the problem you are trying to solve, rather than a recommended solution without description of the problem, you would get a different angle on the discussion.

With the proviso that any operation outside OEM and standard practices is not to be taken lightly, there are some academic questions related to your lingering imo:
1 – Would RC snubber potentially increase the life of the contacts?
2 – are there adverse consequences.

I don't know the answer to those. Some thoughts focusing mostly on #2 (potential adverse consequences)
· If 460 vac RC devices are not located properly they can be another failure point or personnel hazard.
· AC circuit interruption is a complicated phenomenon. For air contactors, I believe the design of the device is to allow formation of an arc and then orderly termination of the arc near a current zero. Successful interruption involves a timing race between recovery of dielectric strength of the circuit and recovery voltage of the circuit. If you have a capacitor added in parallel with the interruption it can introduce ringing which can cause oscillation that might interfere with operation and produced undesired restrikes which (if occurs) can worsen contact life and generate voltage spikes to the load. I'm not saying this will happen but I'm saying it's one concern that would need to be thought through very carefully before you embark on something like this.

Also can you explain why different rules from low power arcing to high power arcs?

For sure a very wide variety of devices are used for circuit interruption and they tend to get more complex as the voltage and current increase. The reasons that drive those designs are best known to OEM's, certainly not to me. I have heard of RC networks used in parallel across transformers. And surge arrestors or capacitors have variouis applications but these tend to be in parallel with the load or line to ground ... none of them directloy in parallel with the interrupting contact. I have heard of EHV breakers that have capacitors in them... don't recall the exact configuration.

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[electricpete]

none of them directly in parallel with the interrupting contact.

Of course that does not include control circuit and electronics where snubbers are widely used as you pointed out in the beginning.


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[moregas]

Moregas: Well anyways I have been working in this trade for 32 years, all it takes is a small pit to start a weld. (Six down, middle)

Waross: Is this he first set of contacts that you have seen? Newbies are often horrified by the look of normally operating silver oxide contacts. Yes there may be damage, but if you haven't seen normal contacts before, you don't know what is normal and what is bad.------------------------------ You may be chasing a figment of your own imagination, trying to suppress arcs in a 20 HP motor starter. If it was neded it would already be part of the design. (Nine down)

Skoqsqurra: OK. Bill bet me to it. And he said it better than I would have done.----------------------------- There is no room to deliver a complete lecture on the different phenomena here. But if you google 'pitting', 'welding' 'arc', 'contactor' etcetera, you may find that google is your friend.

Hi electricpete,
Very well said. Thank you. I have to think on that.

War- How can I be a newbie horrified at a look of a carbonized contact, and chasing dreams of making stone boats?

Skoq-He said it better then you could have said it?

Ok this is a figment of my imagination. So then sorry Skoq.

 

[waross]

If you have been in the industry that long you must be aware of "Economizers". The restricting orofices that may be installed ahead of the expansion valve. These are often overpriced and may be grossly oversold. However they do even out the duty cycle on an underloaded compressor. From a few years experience with motors, some of them compressors, I suspect that the short cycle is more of an issue than contact pitting.
A couple of suggestions;
1> Do what it takes to even out the duty cycle. Restricter, economizer, smaller expansion valve.
2> Those of us who work with industrial controls and refrigeration units are not impressed by the quality of OEM starters on unit condensers. That's an understatement. We sometimes wonder how such crappy contactors hold together at all.
Replace the OEM POS with a quality motor starter or contactor from a good quality manufacturer.
Note. Good contactors seldom weld with motor duty.
BUT, The short cycle can and will lead to burn out. Closing a starter into a shorted motor winding will often cause the contacts to weld.
The burn out may be what causes the contact welding rather than the welding causing the burnout.
Almost every set of welded contacts that I have seen have closed into a short circuit. I've seen a couple of sets over the years.

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

 

[moregas]

Hello War, I hope God who raised, has caught your eye today. And not the bunny from Hell.
Also in no way am I upset or trying to picking on you,if I Possibly could, it is just what I think I know, if I am wrong correct me. Ok enough said.
Well its funny you brought up economizer because at low load conditions, when O.S.air B.T.U. content is less then R. air B.T.U. content, that is, what is really my problem. It handles 70% to 90% of the load. Then the 100 ton chiller handles the rest, I know that’s dumb. Also note that out of four 25 ton pots with one hot-gas-by-pass at coil, maybe knocked down to 18 tons. But not much help lowering that 25 ton down, which would keep that compressor on, instead of on and off for 24HRS/day. Next problem lets say we have 27 ton load, now stage # 2 is over kill and that contactor gets beat to death to! Do you have any ideas?


The restricting orofices that may be installed ahead of the expansion valve. These are often overpriced and may be grossly oversold. However they do even out the duty cycle on an underloaded compressor.

I think that this is what you meant when you said: a figment of your own imagination. LOL, the only thing that I can think of after a T.X.V. is a refrigerant distributor. They use orifices in place of a T.X.V. not after; they are a cheap way to save money in small applications. An orifice is just a smaller hole, and once you seen one hole you seen them all? Wrong, maybe you are thinking of the distributor nozzle that increases the velocity of the flash gas and liquid so the distributor tubes are fed properly, so that all the evaporate coil runs are evenly filled, very important. A T.X.V and distributor job is only to keep the evaporator filled as much as possible with out sending liquid to the compressor. Just because much money is saved, and it does work doing it the “LEFT” way, don’t make it the “RIGHT” way!
No capacity devices. A compressor takes gas from the low side evaporator and pushes it into the high side condenser at a given rate, then the condenser turning it to liquid, then the T.X.V. Fills the evaporator and drops the pressure, then the evaporator turns that liquid into a gas. Then it repeats. Now if you make the condenser do its job better have a lower pressure great! The compressor current drops! Or if you get the evaporator filled with more liquid safely, great more heat is removed from the load and energy can be saved. But, and I don’t care what they have told you, in no way do you foul this system to try to control load. It is stupid. And I know the manufactures are doing exactly that, and I also know why they do it. Money! But any way I don’t agree with you, and I am sorry, but the only way to keep the compressor going is to unload the compressor and have more T.V.V.’s. But it is not going to happen. Why, money. I have been thinking of putting a drive on it. Of course the manufacture will disagree. All I have to deal with would be oil flows and super heat; hopefully I can get away with as low as 35hz. What do you think?

Moregas