11kV, 3000kW motor - High vibration and axial float issues after overhauling

[krisys]

One of the 11kV, 3000kW motor was recently overhauled in an outside workshop which is not part of the plant. The workshop did not have the testing at 11kV. Hence they have tested the motor at a reduced voltage level of 3.3kV. The testing was found OK.

When the motor was sent to site, they have conducted the motor solo run. During the solo-run some abnormalities were noticed as below.

Motor total axial float showing 11mm

Carried out solo run test for one hour under observation found full axial movement of the motor to the pump side and we found magnetic center 10mm

Not getting proper magnetic center during solo run test (It is not in the center 5.5mm)

Slightly higher Vibrations, but within the acceptable limit.

Most of the problems are mechanical in nature. As I am an electrical person, I have some difficulties in understanding the exact problem. Can anyone throw some light on this issue or share their past experience of similar nature?

Also how the problem can be solved?

 

[edison123]

Axial float in sleeve bearing motors is normal. Run the motor decoupled at rated voltage, scribe the running center on the shaft near a bearing housing, stop, bring the rotor to the scribed position by hand turning the rotor, open both side bearing tops and see if sufficient axial clearances are there for both the bearing shoulders of each bearing.

Muthu

http://www.edison.co.in

 

[electricpete]

Motor total axial float showing 11mm...
...Not getting proper magnetic center during solo run test (It is not in the center 5.5mm)

It is unrealistic and unnecessary to expect magnetic center to coincide exactly with mechanical center). The two important aspects are:

1 - That the coupled-up running position has enough clearance to each limit of travel to avoid contact (I'd say 2mm would be more than enough). Otherwise you can end up with continuos running with the motor shaft shoulder up against the bearing. This surface is only designed for intermittent contact and continuous contact could result in overheating, and other problems.

2 - That the coupled-up running position coincides with magnetic center. This one is not as critical as the first. Conventional wisdom is that if the motor is not coupled on magnetic center, it will be more susceptible to axial "hunting". Personally, I have seen motors not coupled on magnetic center that ran fine, and I have seen axial shuttling of the motor shaft on motors that were coupled up on axial magnetic center (the hunting was not magnetic in origin... it was small axial shuttling of the double-suction pump within it's small clearance which translated to larger axial motions of the motor at the low resonant frequency of the motor rotor mass and the coupling axial spring).

Edison explained well the procedure to achieve these two objectives. I just figured I'd add my two cents of agreement.

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(2B)+(2B)' ?

 

[dArsonval]

"...motor was recently overhauled".

There are missing pieces in describing [the problem].

What was overhauled, rebuilt, repaired?

Were all of the pieces of the apparatus removed from service, or just portions of the motor overhauled?

John

 

[dArsonval]

I'm leaving my earlier questions intact. Yet another thought occurred to me which somewhat
ties in with what Edison and Pete are relating.

When a sleeve bearing type motor is taken out of service, and then reinstalled...
the nature of the repaired work can change/shift the electrical center to a different dimension.
Typically, a new line is scribed on the shaft of the rebuilt apparatus after it has settled its "hunting" to assist those in the field in re-installation.

If the line differs from original measurements "stamped" by the manufacturer, a new tag with
proper dimensions can be refastened to the machine in addition to the new scribed shaft line.

If the motor is continually trying to find its electrical center, (rotor hunting) we're talking about a whole other set of issues.

In attached photo, note the blank space made available to stamp a specific dimension
determined exactly by a machine brought up to operating temperature etc.

John

 

[edison123]

John

Unless something was mechanically changed to shift the rotor, I don't see how an overhaul or even a rewind can modify the motor magnetic center.

Muthu

http://www.edison.co.in

 

[electricpete]

We have some 2500hp horizontal sleeve bearing Siemens motors where the core mounting is adjustable relative to the frame (core is bolted to the frame via slotted holes) and the bearings are fixed with respect to the frame. You have to remove the core from frame for rewind. When you put it back, if not bolted the same place within the slotted holes, the magnetic center will change (the mechanical limits of travel established by the bearings do not change). Photos attached.

I've heard that some other large motors have stator core fixed relative to the frame but bearing housings are adjustable (which moves the mechanical limits of travel relative to the magnetic center).

Most of our motors have no obvious means to adjust magnetic center nor mechanical limits of travel.

Most likely imo the op question is simply a matter of misunderstanding since he is expecting magnetic center to be exactly in the center of mechanical limits (5.5mm through the 11 mm endfloat). This is simply not necessary (unless dictated by driven equipment / coupling axial positioning considerations).

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(2B)+(2B)' ?

 

[krisys]

I thank for the responses.

The motor overhauling involved standard overhauling jobs.

Additionally,
Some of the stator slot wedges were found damaged. These were repaired by inserting new slot wedge. Then varnishing of the stator winding overhang. General cleaning etc.

Please note that one side of the motor bearing is insulated.

 

[electricpete]

Op - I apologize that I said twice that you misunderstood and you were expecting motor exactly at magnetic center 5.5mm. It was me who misunderstood. I did not read close to see this piece:

Carried out solo run test for one hour under observation found full axial movement of the motor to the pump side and we found magnetic center 10mm

So I'll recap your choices, in case it's not obvious
1 - adjust magnetic center. Might be doable depending on the motor type. For the motor I discussed above, it can be done. But have to recheck airgaps each end after move the stator.

2 - Couple up on magnetic center at 10mm. That is only a problem IF the axial movement allowed by the coupling plus your driven machinery thrust clearance plus perhaps measurement error exceeds 1 mm. It does not seem like a good option to me…I'd go with option 3 before option 2.

3 - Couple the motor off a little off of magnetic center, maybe at a position of 9 or 8 mm so you have 2 or 3 mm margin to work with before contacting the shaft shoulder. I have gone down near 1 / 16" (1.5mm) margin one our motors that have Thomas Shim packs (no intentional endfloat other than stretch under the minimal axial load afforded by the magnetic centering force) and didn't have a problem in a situation where we had additional constraints from the driven equipment and coupling. But I was nervous about that one, would've preferred more margin if I could get it. It certainly depends on coupling type (if you have a "limited endlfoat" gear coupling the name is misleading, it's limited to some value maybe 1/8" and you'll need more margin to accommodate that type of coupling). The risk of having too little margin is continuously running against the motor axial thrust bumper (where shaft shoulder contacts bearing) which is bad news. To increase your margin you have to move further away from magnetic center, which has never been a problem for me (I personally haven't seen hunting problem from coupling off of magnetic center). My priority is to make sure it is far enough away from mechanical contact first and foremost, more important than getting close to magnetic center.

So obviously solution 1 is ideal. Solution 2 is not recommended. Solution 3 is what I'd recommend if you have no easy way to do solution 1. To get the best guess on solution 3 requires a little knowledge of your coupling type and its expected axial stretch. Driven equipment thrust clearances might also be a consideration if they are getting up near 0.5mm and you are trying to cut it close.

Bearing insulation is between the bearing and housing or between bearing housing and frame. I don't think it would affect anything related to axial centering.

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