# Ammonia compressor motor



## MadDawg (Jun 12, 2012)

So, last week I had an interesting service call.

INFO:
75HP 600V 3Ph motor belt coupled to 4cylinder ammonia compressor. 2 cylinders unload automatically on start (not electrically controlled)

Motor is started by a ABB PST soft start. currently using 5 other soft starts in same panel, same brand, same age.

Age of soft start in question is about 7 years old (30,000hrs 17,000 starts)
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First problem:

Starter fault reported by operator, "Open Phase"

Checked fuses (110Amp SCR fuse) fould L1 fuse open. Replaced.

Restarted plant, motor tried to rotate, but the compressor oil pressure differential switch opened. The switch should have a time delay to allow compressor to run up to speed. Motor also growled.

Reset switch and tried a restart.

Motor growled and fuse L1 opened, oil switch also opened. No rotation on motor.

Diagnosis:

Decided to replace oil pressure differential switch. Completed, and restarted plant. Back in service. Went home and had a beer.
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3 days later:

Over night, operator said compressor went down again. Soft start showed "open phase". Same fuse open, L1.

Replace fuse (also counting spare fuses to see how many times I can "try" things).

Tried to start ice plant, motor growled, no rotation. Shut down before fuse opened.

Decided to eliminate compressor, so removed drive belts from motor.

Tried to start motor, motor growled........... Sounds exactly like single phasing.

NO FAULT OR ERROR ON SOFT START...

Decided to replace motor.
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Next Day:

New motor installed, run motor without belts. Runs perfectly.
Run motor coupled to compressor, runs same.
Shut down to install guards, restart plant, motor growls, L1 fuse opens.......
Only fault on soft start is "Open Phase".........
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As it turned out, I replaced the soft start with a spare one I had, although not really designed for compressor duty, and has been running for a week.

I know this really isn't as much of a question, as it is a discussion on troubleshooting soft start controls.

Would anyone have done anything different?
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NOTE:
The soft start tested okay as per the ABB troubleshooting guide, the motor megged okay, connections were all checked, all on day one.

Motor was also from early '80's and had been overhauled at least twice over the years.


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## MechanicalDVR (Dec 29, 2007)

Did you try taking an output voltage reading from the starter when the motor "growled"?


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## MadDawg (Jun 12, 2012)

Didn't have a lot of time to set up data logger, but did throw on a clamp meter on each leg. Of course I wasn't watching them at the time of motor start. The sequence runs through re-start inhibit timers and other timers. I should have set the meter to read max.

Power logger would definitely show that the soft start was causing single phase.
I was expecting the soft start controller to know if it wasn't working and show a fault.


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## MechanicalDVR (Dec 29, 2007)

MadDawg said:


> Didn't have a lot of time to set up data logger, but did throw on a clamp meter on each leg. Of course I wasn't watching them at the time of motor start. The sequence runs through re-start inhibit timers and other timers. I should have set the meter to read max.
> 
> Power logger would definitely show that the soft start was causing single phase.
> *I was expecting the soft start controller to know if it wasn't working and show a fault.*


Sometimes that can be too much 'expecting'.

At least now they have a spare motor...


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## MadDawg (Jun 12, 2012)

Damn I miss the old school DOL starters!!!


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## RePhase277 (Feb 5, 2008)

Have you disconnected the motor and megger the winding?

Edit: Nevermind I skipped the part where you said you replaced the motor.


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## JRaef (Mar 23, 2009)

Sounds as though the firing board for your soft starter is beginning to fail, but has not completely failed, hence the lack of any fault codes, _*YET*_. The firing circuit for each of the 6 SCRs use what's called a "pulse transformer" and capacitor that boosts the PC board level voltage up to something that will trigger (fire) the SCR. As that circuit ages, the transformer over heats and a connection might get lose, or the capacitor degrades and takes too long to boost the voltage, so one SCR stops firing every time and when that happens, the motor is getting incomplete power. That's what causes the "growling" sound; it's very similar to a single phasing event from a motor viewpoint. You can probably buy a new main control board for that soft starter.


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## MechanicalDVR (Dec 29, 2007)

RePhase277 said:


> Have you disconnected the motor and megger the winding?
> 
> Edit: Nevermind I skipped the part where you said you replaced the motor.


But: "NOTE:
The soft start tested okay as per the ABB troubleshooting guide, the motor megged okay, connections were all checked, all on day one."


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## JRaef (Mar 23, 2009)

MechanicalDVR said:


> But: "NOTE:
> The soft start tested okay as per the ABB troubleshooting guide, the motor megged okay, connections were all checked, all on day one."


Most of the low-level test procedures they will give you in the owner's manual will only detect a fully failed component; a shorted SCR or a completely failed gate IN the SCR, not the gate firing CIRCUIT. But even if they did (I haven't looked at a manual for a PST), in that firing circuit board there are "levels" of intermittent failure that take place right before the full fledged "magic smoke expulsion" event. In that case you diagnose it based on symptoms. his list of symptoms fits the classic profile of a failing firing circuit (to people in the Soft Starter biz as I was for a couple of decades).

The control board isn't (likely) difficult to change; a few screws and plug-on connections. The ABB PST is their direct copy of the one I had designed for another company. We had brand-labeled it to them for a few years, then they copied it and built their own in-house once they proved it's viability (a common occurrence in the industry). They copied everything about it, so changing it should be as I described.

PS: Ok, I looked at their manual, it appears they have separated the control PCBs into a Low Voltage Board and a High Voltage Board (probably to make it smaller than ours was). The HV Board is where the firing circuit is, that will be the one with the problem. Their manual does not include any component test procedures at all, they just say "Call an authorized service organization".


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## paulengr (Oct 8, 2017)

Always ohm the soft start bridges first thing (L1 to T1, L2 to T2, L3 to T3) except those cheap ABB's don't have a bridge in L2. Looking for between about 10K and 100K ohms typically. On bigger ones you can also check the gate resistance but probably not on those little PST's. Low ohm bridge and megger motor or better if you got something to prove motor good. At that point it's a failed SCR driver or control board.


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## frenchelectrican (Mar 15, 2007)

paulengr said:


> Always ohm the soft start bridges first thing (L1 to T1, L2 to T2, L3 to T3) except those cheap ABB's don't have a bridge in L2. Looking for between about 10K and 100K ohms typically. On bigger ones you can also check the gate resistance but probably not on those little PST's. Low ohm bridge and megger motor or better if you got something to prove motor good. At that point it's a failed SCR driver or control board.


Paulengr.,

Can you clearify one part for me ? 

The soft start bridges that means like bypass concants ? if not can ya make a short quick pointer so I can get the picture what ya refering to that.


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## paulengr (Oct 8, 2017)

frenchelectrican said:


> Paulengr.,
> 
> 
> 
> ...




There are two SCRs per phase at a bare minimum (one for positive, one for negative) that BRIDGE from the line to load. Also known as phase stacks since in larger drives each phase is a stack of parts (phase rails/heat sinks, SCRs, snubbers, temperature sensors, firing circuits, and maybe some other stuff depending on voltage) that are part of each individual phase. At higher currents and voltages there will be multiple SCRs in parallel and/or series.

You can test for shorts or opens which happens when SCRs outright fail. Intermediate ohm readings (between single digit ohms and about 10k+) happens when the capacitors in the snubber circuit (basically an RC filter) fail after about 10+ years. You can test the gates too on soft starts where you can disconnect the SCR gate but when it comes to the gate circuit itself usually the best way I’ve found to spot those without knowing the circuit in detail (not that component level repairs are cost effective) is to stick an oscilloscope on it. Even if you only know the basics if it doesn’t look right, it isn’t.

I’m not a fan of the ABB PST drives skipping out on the center phase stack. It distorts the voltage balance. Granted it’s just starting but i just don’t like it. Square D prices are very close to ABB.


Sent from my iPhone using Tapatalk


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## frenchelectrican (Mar 15, 2007)

Paulengr.,

Thanks for clear it up on that part I used to the diffrent terms of words but I get the grist of the system now.

Ya I am not a fan of small ABB units either .,, I have few blew up from a bad motor windings. and one by installer error.


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## Cow (Jan 16, 2008)

In my opinion, equipment that needs to run should have spare components. One of those components being a soft start. 

At this point, I would try your spare soft start and see where that gets you.


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## JRaef (Mar 23, 2009)

paulengr said:


> ...
> I’m not a fan of the ABB PST drives skipping out on the center phase stack. It distorts the voltage balance. Granted it’s just starting but i just don’t like it. Square D prices are very close to ABB.
> 
> Sent from my iPhone using Tapatalk


The PST series doesn’t use the “2-phase” SCR design, it’s their cheaper product lines that do that. Almost all soft starter mfrs offer a version like that now, including Schneider, because it fills the market niche of people who want cheap and don’t care about the potential risks involved. I’ve always hated the concept, but like a lot of things, it happens anyway.


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## MechanicalDVR (Dec 29, 2007)

Cow said:


> In my opinion, equipment that needs to run should have spare components. One of those components being a soft start.
> 
> At this point, I would try your spare soft start and see where that gets you.


Especially when the motor megs good.


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## MadDawg (Jun 12, 2012)

The spare soft start that I installed until the correct one comes in is one of the 2 phase control type. Still ABB, but not rated for compressor duty, mostly fans and pumps. It was purchased large enough to be used on any other motor at any of the other facilities. This being the largest motor, the sooner it is replaced with the PSTX the better. The spare is a PSE normal duty.

This link is their very basic test procedure. https://search-ext.abb.com/library/...LanguageCode=en&DocumentPartId=&Action=Launch


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