ATS48 Soft Start

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Rocky58

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Location
Lexington, KY
We have a total of 9 ATS48C66Y Soft Starts that are operating 500HP Hammer Mill Motors (WEG) for wood chip processing. On 8/3/2015 the drives were locked out for servicing other areas of the plant. When we went to restart later on in the afternoon the drives would not start the motors. We have experienced either overcurrent fault on the soft start or it has tripped the circuit breaker (1000 A - set at 5x Is). 5 days before this occurred the hammers were replaced in the mill portion and the motors all ran since then. Our incoming voltage (unloaded) is in the 492 range and we have seen dips in the 450 range but only during initial startup. With all motors on line, along with associated motors we average in the 478 to 485 range. We have uncoupled the motors from the load and the soft starts will start and run the motors. When we recouple and attempt to start again, we see motor rotation and hear a very loud rumbling noise (hard to describe). It's not like a single phasing hum that we've all heard in the past. We do have Schneider involved but I'm trying to reach outside the box for some assistance. This all occurred at the same time. Two months earlier we experienced shorting of the SCR's on five units and we replaced the starters and everything has run fine for two months. We've meggered the motors and the feeder cables and we have over 100 megohms to ground at 1000V. Any suggestions would be greatly appreciated.
 

jim dungar

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Wisconsin
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Why, if you did mechanical work on the driven load, are you focusing on the electrical equipment?

Are your starter parameters, like ramp time, still valid for the new load?
 

Jraef

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Electrical Engineer
That loud rumbling noise with no rotation is a classic sign of shorted SCRs or at the very least, a gate circuit on one phase not working. Odd that it would happen on ALL 5 at the same time. Most likely while they were off, they were still connected to the line and someone made some sort of major fault in one phase, then covered it up thinking nobody would know. But the spike took out the SCRs or gate drivers in that phase on all of the starters.

Oh wait, you said they DID rotate... That then points to the gate firing circuit (usually a chip) being corrupted or possibly the Phase-Lock-Loop zero cross sensor malfunctioning*. It's firing the SCRs, but not at the right time. With no load, it creates enough torque fast enough to accelerate it, then goes across-the-line so you don't hear anything. but with a load, it can't finish accelerating and stays in the misfiring situation where you hear it. Likely still the same cause, a fault on the line while the machinery was being worked on.

* In order for a soft starter to know when to fire the SCRs, it must know the timing based on when the sine wave crosses zero. But you cannot measure a value of zero accurately, so you look at when two of the phases cross; one going up and the other going down, then calculate the zero-cross from there. this is called a "Phase Lock Loop". Some mfrs, Schneider for one, use only one PLL circuit, then ASSUME the other three phases follow suit accurately. If they don't, you are screwed. What can happen sometimes is that some other piece of equipment was added or is malfunctioning, causing a huge "notch" in the incoming sine wave, and it happens to be one of the two that the soft starters are using for the PLL circuit. That makes the timing for all of the SCR gates wrong. The usual suspects are DC drives and older VFDs that used GTO thyristors, or any other large power supply unit, usually older, that is not a Switch Mode Power Supply, like older welders, HID ballasts, Vibrating Feeders etc.
 
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Rocky58

Member
Location
Lexington, KY
ATS48 Soft Start

Upon returning to the site this morning I was informed by the plant that they had started and ran two of the motors that earlier in the day would not start. They said they did not change any of the settings. I had Schneider support on the phone and started 3 of the motors that would not start yesterday and they all ramped up fine and went across the line just like they should. We then went and started 3 more at the secondary hammer mill and same situation. All 3 started and went across the line fine. Of course Schneider says if it ain't broke, can't fix it. We had plant engineers there and the typical looker-on's but I've got no explanation at all on what happened or why, nor does anyone else. The common thread between these motors are the starters. We have 6 MCC's that are fed from 3 transformers (2 per transformer, one MCC at the primary hammer mill and one MCC at the secondary hammer mill per transformer). The 3 transformers are fed from it's own 34.5 Primary Switch in parallel. Another note is that the motor frames are magnetized - this was found out when the mill wrights were checking the alignment. Not sure how that may play into the grand scheme of what's happened but I don't recall seeing that in the past during my career. I don't believe that our problem is fixed and not sure now on how to proceed.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Upon returning to the site this morning I was informed by the plant that they had started and ran two of the motors that earlier in the day would not start. They said they did not change any of the settings. I had Schneider support on the phone and started 3 of the motors that would not start yesterday and they all ramped up fine and went across the line just like they should. We then went and started 3 more at the secondary hammer mill and same situation. All 3 started and went across the line fine. Of course Schneider says if it ain't broke, can't fix it. We had plant engineers there and the typical looker-on's but I've got no explanation at all on what happened or why, nor does anyone else. The common thread between these motors are the starters. We have 6 MCC's that are fed from 3 transformers (2 per transformer, one MCC at the primary hammer mill and one MCC at the secondary hammer mill per transformer). The 3 transformers are fed from it's own 34.5 Primary Switch in parallel. Another note is that the motor frames are magnetized - this was found out when the mill wrights were checking the alignment. Not sure how that may play into the grand scheme of what's happened but I don't recall seeing that in the past during my career. I don't believe that our problem is fixed and not sure now on how to proceed.
Then I’d suspect what I said about the PLL sensing issue. They likely had a serious line notching issue when you tried starting it before. If these are in different MCCs and fed from the same large service transformer, then the source may be inside of their own plant. If fed from different transformers, that would indicate the problem came from the utility side.

The fact that you previously lost a bunch of SCRs points to there being something serious going on nearby that is stressing these systems. A good power quality survey might be in order here.
 
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