Magnetic Center of 1MW Wound Rotor Motor 2

[MacMcMacmac]

thread237-507222
In reference to the above thread: The 1MW, 1780rpm wound rotor motor is experiencing excessive axial vibration after overhaul to replace D.E. bearing which failed due to a lack of lubrication.

A year on, and this issue has not been solved. The Hail Mary play was to take out .055" of soft foot and see if that was causing problems with distortion of the frame when the final bolt-down took place, even though the alignment system said everything was ok with our installation. Everyone is frustrated by now and I get the feeling the motor shop is chasing their tail on this.

I was wondering if this might be a case of multiple magnetic centers? It seems like a long shot but everything measures straight and true, air gap is consistent, new collector rings were machined and installed for the wound rotor to eliminate the porosity found on the old rings. The rotor was sent to another shop to be balanced at higher than usual RPM to ensure everything was ok. We are on the cusp of a major testing campaign and this machine is vital to achieve certain altitude test points.

There has been a theory advanced that changing out to double row ball bearings may make the arrangement more rigid, since the motor shop's engineers stated the current bearings have way more thrust capability than is necessary, but I'm skeptical at this point.

There is the second stage exhauster next to it which has the exact same motor. Management was suggesting installing that motor on the first stage to rule out any machine vibration being transferred to the motor, but I don't see the value in this since absolutely everything points to the motor being the problem, including the motor shop personnel who stated the motor shook excessively even on their testing rig.
Any input is gratefully received.

 

[edison123]

An electric motor has only one magnetic center.

For a 1 MW, 4 pole motor, plain ball bearing in ODE/NDE and cylindrical roller bearing on DE is more than enough.

Align the stator and rotor core.
Center both the bearings in their end caps/brackets/end shields.
Do the necessary corrections on the shaft and the end brackets to lock both the inner and the outer rings of both the bearings in place.
Measure the axial movement of shaft after assembly. It should be 4 to 6 thou.

A good motor repair shop should do it within a week.

Muthu

http://www.edison.co.in

 

[electricpete]

I think there can in theory be multiple magnetic center on a 4-pole motor because it's not just how the ends line up but also how stator vent ducts align with rotor vent ducts.

But I'm not on board to conclude magnetic centering is necessarily that cause of axial vibration.

What is the frequency of the vibration? Does it change between uncoupled, coupled, and loaded conditions?

 

[Gr8blu]

I Apologize for not reading the referenced thread first but ... is there any chance the rotor winding is skewed with respect to the stator winding? If so, there will be a noticeable axial force which MAY act (at specific rotor loading points) to push the rotor one way. If it then hits a hard stop (edge of a bearing clearance, for example), there may be a rebound effect, resulting in continuous hunting.

The magnetic center is the magnetic center - there is only one in each of the rotor and stator core lengths. The presence (or lack) of radial ducts does not affect this. However, the presence (or absence) of ducts - and how they line up with one another - may create enough unbalanced air flow to physically move the rotor in an axial direction when at specific speeds.

Also, having the rotor-mounted fans installed incorrectly may have the same effect (e.g., both "pulling" in one direction, when the intended design is to "push" against each other).

Converting energy to motion for more than half a century

 

[electricpete]

The magnetic center is the magnetic center - there is only one in each of the rotor and stator core lengths. The presence (or lack) of radial ducts does not affect this.

I disagree. Axial magnetic forces arise from aligning iron to iron vs air. In most cases vent duct contributions cancel when summed and are weak compared to end effects, but it is not always the case. See The Influence of Axial Magnetic Centering Forces on Sleeve Bearing Induction Motors presented at IEEE PCIC 2006

When non-aligned stator and rotor cooling vents are used, it is expected to have a magnetic center from medium to strong if the stator and rotor are manufactured within acceptable tolerances. However if the machine is manufactured outside of these tolerances, it is possible that the machine may experience a weak magnetic center or may have more than one magnetic center position. Figure 1 illustrates discrepancies in manufacturing that may position the rotor on one or two magnetic centers. .... A characteristic of a motor with more than one magnetic center is its failure to seek center. ... may oscilate. ... Another possibility is to axially push the shaft from its float in position to the float out position. The rotor may find another magnetic center within the endplay limits"

But I only post that to respond to previous posts. I don't necessarily think magnetic centering forces have anything to do with op's high axial vibration. And even less so multiple magnetic centers... because those tend to occur where magnetic centering forces are weak. AND any potential role of multiple magnetic centers seems even less relevant on rolling bearing motor where the rotor is held pretty closely in one axial position.

So some question for op
[ol 1]
[li]what is it that leads you to bring up multiple magnetic centers[/li]
[li]what is the frequency of this high axial vibration: 1x turning speed or 2x line frequency?[/li]
[li]do we know if the vibration magnitude changes between uncoupled / coupled?[/li]
[li]When the motor starts up, does the vibration jump high as soon as it gets up to speed... or it takes awhile to get there? (thermal bow)[/li]
[li]Can you characterize how fast vib decreases during powerdown? (one way to distinguish electrical from mechanical vibrations)[/li]
[/ol]

including the motor shop personnel who stated the motor shook excessively even on their testing rig.

What did they conclude was the cause? Were the able to try reducing voltage to see if that affected vibration (if it's caused by magnetic centering force as you suspect, then vibration should decrease as stator voltage decreases).

There has been a theory advanced that changing out to double row ball bearings may make the arrangement more rigid, since the motor shop's engineers stated the current bearings have way more thrust capability than is necessary, but I'm skeptical at this point.

We are too far away to draw any conclusions, but a datapoint which might point in this direction is that the motor originally had sleeve bearings and was modified (any modification is suspect). And if this observation came from an experienced motor shop I'd be inclined to try to understand exactly what he's saying.

 

[MacMcMacmac]

Hello again guys. Sorry to leave you hanging. The multiple mag centers are more of a (me) grasping at straws explanation for a problem that has bedeviled us for almost a year now. I have faith the shop doing the work is capable, since I visited back in late October and had a good tour. There was a 30,000hp motor in stock for a local mine, so I don't think our little motor is a big deal for them, but it is proving very difficult to remedy. The vibes grow as the motor ramps up during start up. It is a very gentle start via manually actuated liquid rheostats. We keep starting current under 200A and it take about a minute to bring both exhausters up to speed. Vibes peg suddenly once max speed is achieved. The motor shop offered up the bearing change out as a way to make the whole assembly more rigid compared to the current bearings which they say have more thrust capacity, but are somewhat lacking in rigidity compared to double ball bearings. Vibes coupled or uncoupled do not vary and we have undertaken several tests to make sure the exhauster is not causing the issue, and that there is no cross talk vibration from the second unit. Vibes die almost instantly when the power is cut. Vibration levels do drop somewhat during operation over a few hours, but this was observed even when the unit was running correctly. Current plans for the unit is to use it sparingly to achieve the high altitude test points when required and to use a second 5MW exhauster/compressor to do the lions share of the work. It is unfortunate this happened when it did, since after this very important test, it looks like we will have an extended down period while we go through a major facility expansion.

 

[edison123]

Post the vibration data in repair shop with and without power, on your bed decoupled with and without power and coupled as a bare minimum.

Muthu

http://www.edison.co.in

 

[MacMcMacmac]

Well, to close off this thread just out of courtesy to those that replied, let me say, it has all ended well.

The motor was installed on Monday. The shaved feet seemed to aid in a very rapid and satisfying alignment, and power up on Tuesday revealed reduced, but still high vibration. Virtually everything was in alarm. We tried loosening individual feet one by one but things only got worse. Then we decided to actually tune the vibrating mass dampers attached to the motor and lo and behold, everything went quiet(ish). These were installed back in the early aughts, but unfortunately, the poor guy who was installing them passed away mid-project. They have been largely forgotten since, but we decided we needed to try something to get things back to an acceptable level and I'll be damned if they didn't work. They are simple lengths of 3" X 3/4" flat bars with short sections of the same material clamped onto them near the upper ends with four heavy bolts, attached to the motor frame with piece of heavy c channel. Once you set them at the proper height, the equal but out of phase vibes cancel out the shakes like magic. During our investigation, we discovered that a 2ft x 2ft x 1/4" thick cover plate over the top air vent of the motor was buzzing quite energetically, so we pulled them off before a re-start. Vibes actually went up a LOT, so we replaced them. Odd that such relatively light components could have such a major effect on a six ton motor. This whole job has been quite an eye opened as to the nature, causes and mitigation of vibration in motors and turbomachinery.