Motor starter Failure why?

Wondering if anyone has some thoughts on why we have been experiencing issues with the motor starter on our in house compressor. Short story we had a NEMA 4 rated around 140amps that welded two legs on the motor starter after a relatively short time (~2 weeks) and significant pitting/welding on the third leg. Checked amp draw and seemed that we were a little undersized for the load. So we bumped up to a NEMA 5 rated around 240amps more than sufficient our max draw is around 150 when building up to the pressure we need.

Did the install and set pressure switch and timers for unload and run times, went to check the contacts condition after a short time running today and we are already seeing black around two of the legs and some pitting with the new NEMA 5 starter. Oddly enough the damage seems similar, the two legs that look the worst are the same two as before with the NEMA 4 that completely welded.


I am kind of at a lose to what could be causing this. Any thoughts?

check voltage and amperage of the motor. look for phase imbalance.
megger the motor and check for insulation breakdown.
Does the amp draw match the nameplate of the motor?
Check the amperage and voltage of the line side of the starter. Is there a difference between the line and load side?

An air compressor is a different type of load every time it cycle starts it at max load
with max inrush the started should be rated as a plugging starter.
Don't forget that the normal motor inrush at start can be as high as 10X the full load on the name plate this would normally only last for about a sec then drop down to normal run levels. With an air compressor the inrush can be much larger it could be 20X the motor full load and last for many seconds before dropping back to normal levels. And if you have a short cycle time on the compressor the starter contacts may not have time to cool down between the start cycles. All adding to the failure
Check the ratings on the starter and gat NEMA starter rated for plug duty.

The #1 killer of Across-the-Line starters is rapid re-starting or chattering of the contacts. Look at the control circuit for something that is making it bang on and off rapidly, i.e. your hysteresis (difference between cut-in and cut-out) for the pressure switch is too tight. One possibility for chattering is that your control power source is too small and the act of starting that big motor causes a line voltage drop, which drops the control voltage too, making the contactor chatter until the motor gets to full speed and the current drops. Just a couple of seconds of chattering every time it starts is enough to vaporize contacts.

Consider a soft start. We had an Atlas Copco 90 KW (120 Hp) air compressor that would eat contacts and smoke motors about every 18 months. We added a Fairfield QFE soft start about 6 years ago and it's still going strong.

I agree Automation Direct has a great line of "Soft Starters" that work very well with large industrial air compressors.

But make sure your pressure release valve is working. (Compressor Head Pressure)

Whoops, got the Fairford name wrong. The QFE model is discontinued but other models are availabe. Most of the major automation manufacturers have soft starters available so there's plenty of choices.
https://www.fairford.com/

Have you verified the unloader is properly de-pressurizing the compressor head on shutdown?

shawn_75 said:

Have you verified the unloader is properly de-pressurizing the compressor head on shutdown?

Click to expand...

Yes, the compressor seems to unload correctly. We have this setup for the solenoid to fire once pressure is reached and we run like 10min unloaded waiting for pressure drop before shutting down the compressor. If pressure drops during 10min the solenoid will drop out and we will load pressure until it hits the switch.

damica1 said:

I agree Automation Direct has a great line of "Soft Starters" that work very well with large industrial air compressors.

But make sure your pressure release valve is working. (Compressor Head Pressure)

Click to expand...

Never having used a soft starter how would it help with the compressor, ie what keeps it from smoking or burning up?

jraef said:

The #1 killer of Across-the-Line starters is rapid re-starting or chattering of the contacts. Look at the control circuit for something that is making it bang on and off rapidly, i.e. your hysteresis (difference between cut-in and cut-out) for the pressure switch is too tight. One possibility for chattering is that your control power source is too small and the act of starting that big motor causes a line voltage drop, which drops the control voltage too, making the contactor chatter until the motor gets to full speed and the current drops. Just a couple of seconds of chattering every time it starts is enough to vaporize contacts.

Click to expand...

I do not believe this is the case, I have not noticed any chattering and cycle times are decent with the unload timer being set around 10min before shutdown.

GaryS said:

An air compressor is a different type of load every time it cycle starts it at max load
with max inrush the started should be rated as a plugging starter.
Don't forget that the normal motor inrush at start can be as high as 10X the full load on the name plate this would normally only last for about a sec then drop down to normal run levels. With an air compressor the inrush can be much larger it could be 20X the motor full load and last for many seconds before dropping back to normal levels. And if you have a short cycle time on the compressor the starter contacts may not have time to cool down between the start cycles. All adding to the failure
Check the ratings on the starter and gat NEMA starter rated for plug duty.

Click to expand...

First I have heard of plugging but the NEMA 5 starter we have seems to be rated for it. With the setup though in normal operation we wouldn't cycle that quickly.

Snyd1 said:

Yes, the compressor seems to unload correctly. We have this setup for the solenoid to fire once pressure is reached and we run like 10min unloaded waiting for pressure drop before shutting down the compressor. If pressure drops during 10min the solenoid will drop out and we will load pressure until it hits the switch.

Click to expand...

Not to beat this one to death - but something changed. If you are burning up/welding your contacts, there is too much current going through. The most likely candidate is the unloader sticking. I am embarrased that in 2018 you can't easily get a solenoid valve with feedback! It's one of my pet peeves. A simple feedback that shows whether the solenoid moved as commanded would be very helpful is many situations. But particularly in this one.

I'd get your mechanic to sit beside the solenoid and verify .. with a hand on the valve if necessary .. that the unloader is dropping out. It *REALLY* sounds like your compressor is starting up loaded.

Never having used a soft starter how would it help with the compressor, ie what keeps it from smoking or burning up?

Click to expand...

We use VFDs on our Screw-type compressors. Even when we start unloaded, it is still a relatively torquey load. Since soft starts have much less starting torque (30 - 40% of rated, if memory serves), you may be paying the same for a soft start that is 3X the rated motor size versus a VFD that is sized for the motor. If you are starting up the compressor loaded, the soft start will fault out. Then you troubleshoot whether the soft start is broken or the compressor has something sticking.

BUT the VFD is a load on the air conditioning for your electrical room. It's a bit of a waste if you are running the compressor at full speed all of the time.

How do you control pressure right now? It sounds like you pump up a large pressure tank and cycle between limits. There may be cost savings to control the speed of the compressor instead and have more consistent pressure.

If you don't need the speed control, you can put a bypass contactor in, so the VFD runs the motor up to speed, synchronizes with the grid, then the contactor pulls in and the VFD shuts off. But that's the overkill method!

Snyd1 said:

Yes, the compressor seems to unload correctly. We have this setup for the solenoid to fire once pressure is reached and we run like 10min unloaded waiting for pressure drop before shutting down the compressor. If pressure drops during 10min the solenoid will drop out and we will load pressure until it hits the switch.

Click to expand...

Do you unload only the output or (if this is a multistage compressor) at the output of each stage? I have compressors that have an automatic unloader only on the output of the first stage. I'd also check for an obstruction in the unloader tubing/piping and verify it blows air when it's supposed to be unloading.

Shawn

Do you start the compressor unloaded and delay the loading sequence? I had some old Atlas Copco compressors that would trip breakers if it was told to load on startup or shutdown without unloading first, generally after a power outage.