Contactors welding together

[JRLAKE]

I have a problem with contactors welding together on an air compressor starter. It always happens on hot days so I suspect low voltage is the problem. Any ideas on an improvement that could be made to avoid this type of failure? Or am I venturing down the wrong path?

 

[itsmoked]

You are probably having a bit of chatter and they are welding on turn on. Can you run the coil from a different power source? Often the coil is factory hooked to one line of the contactor. Maybe you can change it to another line that dips less. Sounds like you are very marginal if hot days are what get you. A different brand contactor may be enough to fix the problem.

Would be nice if you could use a fast data logger to look at the coil voltage on a startup.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JRLAKE]

Thanks for the response. I know we have had issues trying other brands as far as making them fit. It's a cheap compressor and I'm sure the cheapest starter available was used. I will try changing the line and see if that helps. If it doesn't, is there anything I can do to the supply line to prevent this?

Thanks for the tip.

 

[jraef]

You should also check out your pressure switch. If the differential is set too low, the switch could be starting and stopping the compressor too fast. Or the switch is malfunctioning all together and causing the contactor to chatter.

If it is a supply line low voltage problem, the answer would be to install a line voltage monitor and wire it so that the compressor cannot run at all if the voltage is that low, rather than chatter the contactor. Contactor coils usually have a 70% drop out voltage, meaning they should stay in at 71% of line voltage. If it is chattering, you are probably hovering around that that low level and that is NOT good for the motor either.

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Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hi;
In support of jraef's comments, I would expect to see motors failing if the voltage was low enough to cause starter chatter.
When I see welding, it is almost always a short circuit.
Is this a single phase or three phase motor?
Is this a magnetic contactor or a direct acting pressure controlled contactor?
Either way, it is probably undersized.
Either type may be remote mounted, but if unloading is combimed in a direct acting starter, it may be best to leave it in place. A direct acting starter (multi pole pressure switch) may be left in place to provide unloading if required and used to control a remote mounted magnetic contactor of higher capacity.
respectfully

 

[itsmoked]

AH I have it! Could also be the head unloader is screwed up. This could make the compressor start real hard lowering the voltage exceptionally.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I would try wiring a small boost transformer in series with the contactor coil. Select one with secondary voltage around 10 percent of primary and wire so it boosts (not bucks) coil voltage. Of course, if your voltage changes wildly, this may mean that you get too high a coil voltage doring low load periods. So you have to bee careful.

There are also the new "electronic coil" contactors that are very voltage tolerant and never fail welding. They have the same foot-print as standard contactors. But not sure if they are available in small sizes.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Contactors with 24V DC coils are available which can switch up to at least 22kW AC3. There might be bigger ones. Add a cheap 24V power supply and the control circuit should be immune to voltage dips.

Contactors typically use special alloys for the pole faces which are resistant to welding and to erosion from arcing. Cheap contactors use copper rivets or something not much better, so a cheap contactor used at the limit of its rating is asking for problems. Either fit an oversize contactor, or find one from one of the reputable manufacturers.

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I don't suffer from insanity. I enjoy it...

 

[jraef]

Make sure you aren't chasing a problem by solving the wrong aspect. I still think that before you go trying to solve a coil voltage problem, you look carefully at the other issues such as the unloader valve as itsmoked mentioned (nice catch) or the pressure switch as I mentioned.

I once had a compressor with a soft starter where the unloader valve failed to open and the compressor needed almost locked rotor current for 20 seconds to start, which meant we were right up against the thermal overload curve. That would have been really hard on a contactor as well (had there been one in mine). I whacked the unloader valve with the back of my screwdriver and the starting current requirement dropped to 350% for 5 seconds.

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http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

I had a 5HP 1phase 240V compressor (6 cylinder!) If the head unloader didn't.. the smoke would start coming out of the motor in about 1-1/2 seconds..

Glad yours hung on jraef.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JRLAKE]

Thank you all for the posts. I will be at the site tomorrow and I will proceed as you have advised. We just welded another one this afternoon after one day of operation. Given the short MTBF, hopefully the problem will be obvious.

Thanks again!

 

[jraef]

LOL, mine was 350HP and in a sawmill full of combustible dust. Smoking electrical equipment = big problem!

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Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

I did a new control cabinet to control two compressors. If the old compressor was set as the base machine the new compressor would cycle as needed.
If the new compressor was the base machine, the old compressor would start if needed, but would then continue running until it was stopped manualy. The person who stopped it would also hit the unloader with a wrench so that it would be ready to start again when needed.
respectfully

 

[Hewlett]

Make sure contactor is not installed so that vibrations from compressors itself is transferred to the contactor. Have seen this problem a few times on hi-pressure washing units - which are often built very compact...

 

[Hewlett]

...oops that might not be relevant if it its "only on hot days"...anyway, it's another possibility...

 

[moira]

Hi,JRLAKE: From your brief report, I'm thinking the starter is a NEMA size 2 or 3 magnetic with a good bit of duty on it. A good check out of the entire control circuit is in order with particular attenion to short cycling issues. Check what voltage you are getting to the coil, its actual connections and physical appearance , condition, You don't want to have more than 3 to 5 % iblevoltage drop thru the motor running overloads . Actually you want near none! Check what the DC resistance is too. Check what the resistance above ground is. Suggest you disconnect the neutral side of the control transformer from ground ( if this is what you have) when you make resistance checks to ground. If possible; you should disconnect the motor legs and using a jumper pull in the starter. See how it operates in these different modes; get a idea of noise , rattle , hum. Open the main (disconnect section) ; don't want any motor power to the starter. Then check , by hand, the mechanical operation- up and down of the main moveable contact set. blow out any fine dust , dirt, rust. Drop the armature out of the coil center. Clean out any dirt, observe any corrosion. You can use a light amount of penetrating oil (say? WD-40) to remove any deep down corrosion to the armature and its guides. As you go thru check and note for better , easier performance. Inspect and lube-clean moveable contact set with (say?-CRC 2-26). When you pull in with ( an external ,other source of coil voltage) the jumper check the DC resistance across each switch closed! Tighten/check all mountings and connections. All brand name (A/B, etc.) starters can easily be totaly or partialy rebuilt. These checks and retrofits are big- buck savings over replacement

 

[powerjunx]

jrlake,

Your contactor is somewhat under-rated and it's never fit to its duty. In IEC, contactors were classified as " AC " that would never found in NEMA size contactor.

If it happens that it is IEC, perhaps you'll check its AC class and application.