# Motor's magnetic center



## 51360 (Jun 9, 2014)

glen1971 said:


> Is there anything that would cause this to change on a larger horsepower motor? There is a 1,000 hp motor that apparently has developed excessive thrust issues.. Apparently it is moving about 3/16" back and forth.. The question I was asking to their experts was "Is it the motor pushing, or is the compressor pulling the shafts?".
> 
> They have a flex coupling and the motor is controlled by a 1557 ALB VFD and is 1,000 hp.. We are going to be going through the drive and checking the SCRs and GTO boards tomorrow.. They are going through the compressor and adjusting the thrust pads and replacing some other parts.. *We're looking at uncoupling it and running it to determine the center and see if it thrusts unloaded. One person's hunch is that it was misaligned to the magnetic center of the motor...*
> 
> ...


With my limited experience with motors of this size and mechanical coupling, testing uncoupled makes sense to me.

I of course did a search on line, and this should be an interesting thread to follow. I am sure there are many members here with the knowledge to help you.

Borgi


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## xpertpc (Oct 11, 2012)

thrust washer or worn shaft walking through spinning bearings caused by sloppy end-bells.


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## glen1971 (Oct 10, 2012)

Thanks for the replies.. I'll post once we do some checking and testing today..


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## John Valdes (May 17, 2007)

Vertical or horizontal motor?
Misaligned rotor will cause heat and high current. Are those your symptoms?
Rotors can come loose from the shaft or it could be end play caused by bearing and end bell wear.


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## McClary’s Electrical (Feb 21, 2009)

Are these babbit bearings? When you said thrust , I thought you meant back and forth, then you mentioned magnetic center, so I'm confused. What is the problem, symptoms, and what kind of coupling?


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## JRaef (Mar 23, 2009)

mcclary's electrical said:


> Are these babbit bearings? When you said thrust , I thought you meant back and forth, then you mentioned magnetic center, so I'm confused. What is the problem, symptoms, and what kind of coupling?


Yeah, I think you may have misinterpreted the term "magnetic center". That refers to the AXIAL position of the rotor within the stator, because once the stator is energized and the magnetic flux starts getting tossed around, any play in the bearings will be exacerbated by the motor wanting to find the point of equilibrium on the magnetic forces, which might not be centered at the _mechanical _center of the shaft. "Thrust" is longitudinal movement of the shaft, and depending on whether the motor is vertical or horizontal, will be controlled by thrust bearings or thrust collars. If you are getting too much thrust, it's more likely that the system used to control it is worn out.

There is nothing that drive can do to the motor which would make it change it's magnetic center, or it's thrust for that matter. There are issues with some types of VFDs causing bearing EDM damage (Electric Discharge machining, like a welder), but that phenomenon is attributed to the ultra-fast rise time of high speed transistors used in Voltage Source Inverters. The 1557 was a Current Source Inverter using thyristors, which fire much more slowly and do not create that issue.

Side note: the 1557 is obsolete now and parts are getting to be harder to find, it might be time to start budgeting for a replacement.


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## glen1971 (Oct 10, 2012)

John Valdes said:


> Vertical or horizontal motor?
> Misaligned rotor will cause heat and high current. Are those your symptoms?
> Rotors can come loose from the shaft or it could be end play caused by bearing and end bell wear.


It's a horizontal motor.. The winding and bearing temps have been the same (well below any alarm) for months.. The currents and voltages are about right for the load on it...


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## glen1971 (Oct 10, 2012)

*A bit of an update with a few pics*

So we did some testing and maintenance today on the drive, the motor and had the mechanics finish up their repairs on the compressor..

We tore all the power supplies out of the drive and cleaned it out, and blew the dust out.. I think the 3 pounds of dead flies weren't helping.. When the tech we had down did the recalibrating the boards, there was only one that was out a fair amount - supposed to 22.75 vdc and it was around 27 vdc... The rest were fairly close.. The 5 vdc power supply was about 0.1 vdc low, so we tweaked that up.. When we ran the drive the commanded speed and the output were very close - within 0.1 Hz. All the waveforms on the drive looked fine.

The rest of the drive looked good and seems to run really well... JRaef - I've mentioned it to the client for about the last 7 years that these drives are obsolete and parts are getting less and less available.. The techs from Rockwell and now another company have mentioned it too.. 

The motor tested out fine. There is a bit of dirt on the rotor but they said it was still well within acceptable readings.. I wasn't too involved with them today, as I was on the drive with a tech for most of the day...

We ran it up and did see some thrust, but nothing like what was happening before.. Tomorrow the mechanics are going to pull the coupling and we'll run it up to 60 Hz and see if there is a thrusting issue.. 

Where they are leaning to now is that the motor and compressor may not have been properly lined up since the last service and the motor's magnetic center and the compressor's operating center may not be correctly spaced and the two may be fighting each other... 

I'll update again after tomorrow...

Here are some pics of the inside of the 1557... I didn't get any of the stacks behind the controllers, but figured I'd grab a couple before we buttoned it up...


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## five.five-six (Apr 9, 2013)

I am fascinated by this thread.


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## glen1971 (Oct 10, 2012)

*I think we found our issue...*

So the mechanics uncoupled the compressor and we ran the motor up to 60 Hz and made a mark for the actual magnetic center. We put the coupling back in and needed to bring the coupling slightly closer to the motor. We pulled the lid off the compressor crankcase and found that the 20 thousandth clearance was all on the opposite end and there was none on the motor end of the compressor. The mechanics are gonna be moving the bearing caps and the crankshaft closer to the mag center line, then reinstall with 12 thou on each end (as per the manufacturer's specs for clearance).. 

So in a nutshell, the compressor was too far away from the motor's mag center. When it was started up, the motor was trying to get to mag center and pulling the compressor crankshaft until it was hitting the thrust pads on the bearing caps on the motor and "bouncing" it back to the compressor's center. The two were fighting each other... 

Gonna give that a whirl and see what happens in the next couple hours...


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## McClary’s Electrical (Feb 21, 2009)

Just to play devils advocate, I think all you've done is given it more room to move. There's another problem. Again, what were the symptoms to get everybody's attention?


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## glen1971 (Oct 10, 2012)

mcclary's electrical said:


> Just to play devils advocate, I think all you've done is given it more room to move. There's another problem. Again, what were the symptoms to get everybody's attention?


I don't think so... With the motor's magnetic center determined and the compressor recoupled, it was aligned slightly to the compressor side - right side facing our mark of the electrical center (toward the compressor). When we opened up the compressor, it was resting against the DE thrust pads and had no way for it to reach the magnetic center of the motor... 

Our determination of what was going on was: During normal operation, the motor was trying to get to magnetic center, pulling the compressor which would hit the thrust pad and push it further away from magnetic center... The motor would keep trying to get to center and the compressor kept fighting it...

We loosened the bearing caps, and repositioned the crankshaft so that it lines up with the motor's magnetic center. Then the mechanics regapped the spacing from the ends of the crank to the thrust pads to 10 thou.. 

Even the compressor manual said that end thrust is pretty much "determined by the driver, whether it is an electric or gas motor.."

We had 3 mechanics, 1 rotating spe******t, 2 senior motor rewind technicians that have experience on larger horsepower motors, a tech to service the aging VFD and myself working on it... As we went along, everyone was in agreement of what was happening, where to have everything set-up after the magnetic center was determined, and how where the center of the compressor needed to be...

When we buttoned it up, and started it up there was no noticeable thrust at all.. We ran it up and monitored the thrust at both ends of the motor and no one could see it, unlike before.. The currents, temps and voltages were all bang on the money for the load we had..

The original symptom was that they noticed an abnormal amount of thrust at the NDE of the motor... Their vibration tech also noted some vibration, which we think can be attributed to a bad valve in the compressor, so they replaced all the suction valves.. 

One thought of a cause was a misalignment at a previous overhaul of the compressor, that set the thrust pads too close to the motor..


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