High voltage motor magnetic center and the Axial drift of the rotor 2

[krisys]

One of the 11 kV, 3000 kW, 2900 rpm motor was recently overhauled.

When the motor was sent back to site, they have conducted the motor solo run. The magnetic center of the rotor was found to be drifted.

Total axial free float of the motor is 13 mm. The magnetic center is expected to be at 6.5 mm from either end (i.e the mid point of 13 mm). This is an ideal case, which may not be the case in the actual scenario.

During the solo run (de-coupled running at location) the shaft was found to be travelled fully towards the driving End (DE). However with the slight adjustment in the leveling the rotor has moved towards the center. As it was a two pole machine, the magnetic pull being weak, the rotor was oscillating slightly. From this exercise, we can make out that the axial trust is not very high. So can we couple the motor with the pump by manually adjusting the rotor to the mid point of total axial float?
The motor was only overhauled, without any mechanical modifications. Hence we don’t find any reason which might have drifted the magnetic center of the motor.

This is a 28 years old motor. The condition of the stator is not very good. Can a drift in the magnetic center happen over the period time during the service?

While removing the motor for overhauling, the magnetic center was not tested and marked. Hence it is now not possible to ascertain whether the drift has taken place during the overhauling or it was developed before taking the motor out for overhaul, but not noticed or recorded.

Can you share your experience in this regard?

Also I would appreciate if you can give some practical solution or mitigation.

 

[krisys]

I would also appreciate if you can share some write up or conceptual paper or case studies in the subject of motor magnetic center and its adjustments.

 

[sidpred]

In our plant there are electric motors of 6,6 KV, 2500 KW, 1480 rpm
Always check the magnetic center prior to assembly and alignment. As is done with the motor without load, and the current in this case is 3 to 5% of the rated current, the magnetic centering forces are very weak. We have seen that sometimes poor leveling of the motor's legs causes the magnetic center to move in one or the other direction.
Another cause of the displacement of the magnetic center is the axial thrust caused by the cooling air fan. For a correct check, you should cover the suction of the fan.
Regards

 

[krisys]

sidpred,
Thanks for your input.


Does it mean, when the load current increases, whether the axial thrust towards the direction of drift will be much higher?

We are proposing as below assuming a slow change in the stator core magnetic properties might have occurred during the service life of the motor. This might have resulted in the drifting of the magnetic center. But so far remained undetected. Now at the time of trial run only it was detected.

Aligning the motor by moving the motor to the desired float.

When the motor was tested in the workshop an additional trial run was taken by putting a 6 mm shim below one end of the motor feet. The rotor was found to be moved towards the desired magnetic center easily. This establishes that the axial drift of the rotor has a very low thrust. However the thrust exerted by the rotor if it were to be retained at the physically identified (desired) magnetic center.

So the rotor will be adjusted to the original magnetic center at rest and couple. During the running some axial thrust will be exerted on the pump o coupling. Can the system work safely in this way?

 

[edison123]

Induction motors are constant flux motors, there is no significant change in magnetic flux from no-load to full load.

No, the magnetic center does not change with aging.

Normal requirements for sleeve bearing motors - keep the motor as level as possible, maintain the bearing axial clearances (as explained in earlier thread) and the driven equipment/coupling must arrest axial thrust.

Muthu

http://www.edison.co.in

 

[krisys]

Thanks Muthu. This is a very important piece of information for me.

Whether the driven equipment/coupling will be able to absorb/arrest this kind of axial thrust?

Also I would like to know, whether there is any method to measure the axial thrust. What is the typical thrust value which we can expect in this case?

I am proposing to measure the axial thrust and pass the information to the mechanical person. He should confirm whether the driven equipment/coupling can absorb this thrust or atleast he should accept the motor with the measured thrust value.

 

[edison123]

krisys

As you have already noted, the axial thrust from the motor is pretty weak. And in case of a perfectly leveled motor, it is practically non-existent. Not sure how you can measure its force. It is up to the mechanical team to ensure any axial thrust from the driven equipment is not transmitted to the motor. Limited end play couplings are one way to do it.

Muthu

http://www.edison.co.in

 

[krisys]

Muthu,

Is there any affect on the motor by running it with slightly drifted magnetic center?

I guess with the drifted magnetic center, the flux linkage between the rotor and stator may not be the optimum. Hence a minor change in the power loss or the possibly power factor.
Any way the motors with antifriction bearings have no issue of float. So even if the if the magnetic center is slightly drifted, the antifriction bearings hold the rotor in position.

 

[edison123]

krisys

I have personally seen and commissioned a few sleeve bearing motors, both low & high speeds, whose rotors were not in ideal magnetic center, and still running without any issues.

As for motors with anti-friction bearings, a good OEM would have centered the rotor at proper magnetic center with proper dimensioning of the end shields, bearing houses, shaft steps etc.

Muthu

http://www.edison.co.in

 

[dArsonval]

Electrical Center, Magnetic Center does not drift over time as already mentioned.

The OP is suggesting, "...a slow change in the stator core magnetic properties might have occurred during the service life of the motor".

ehhh, No.

For what it's worth, the Electrical/Magnetic Center of a motor will eventually tear out or destroy any rigidly held Mechanical Center.

When running the machine, one should be able to push on the shaft end with a suitable implement (I.E. a piece of wood) observing its free return float to electrical center.

[Yes, I know. Don't try the above at home.]

I agree with Edison123 (again) in the other thread posted by the OP that, " 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."

Agreed, it indeed would not.

I'm curious as to whether enough time is being allocated for the ENTIRE machine to come up to full operating temperature during testing.
Measuring the temperature of each bearing during a test can reveal very useful information.

It's unfortunate that the repair facility did not have the means to fully power the motor
before it was returned to its service installation.

Reassembly of the machine is suspect.

One's best guess is... it's a mechanical issue. The motor likely needs to be dismantled
and reevaluated with a more careful attention to subtle details.

When taken apart, two pole motors can be a very frustrating beast to tame when they are reassembled.

As for the motor being 28-years-old? That's actually quite young.

John

 

[electricpete]

I agree with everything above except one piece:

I agree with Edison123 (again) in the other thread posted by the OP that, " 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."

I posted an attachment in that thread showing a motor whose core position is adjustable within the frame (bolts go through slotted holes). Removing the core for rewind poses risk that the stator core gets installed at a different position within the frame... i.e. change in magnetic center. I can't find the thread. I can repost the attachment if you're interested.


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