How important is an electrical center

[seakro]

On a 200 HP electrical motor driving a vertical centrifugal pump how important is determining an electrical center when spacing and setting the coupling?

Seems we are shearing alot of coupling bolts and we do not know why.....

 

[electricpete]

Magnetic centering is an important consideration for horizontal sleeve bearing motors.

I don't think it is a big concern for vertical motors, and I don't think it would be related to sheared coupling bolts.

Some things that come to mind as possible causes of sheared coupling bolts:
- torsional resonance or torsional oscillation
- momentary power interruptions/reclosures resulting in high transient torques.
- violent engagement of motor anti-reverse rotation ratchet
- misalignment, loose coupling bolts, misapplied coupling ?



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[edison123]

For your motor, I guess it is angular contact + roller bearings fixed on bearing brackets. Unless there was an original machining mistake or a later improper replacement of these brackets, the axial and radial magnetic centres of such motors are pre-fixed. You cannot change them just by dismantling and reassembly.

As pete, I would look for the dragons elsewhere.

 

[DougMSOE]

Under the assumption that this is a squirrel cage induction motor I would look for broken rotor bars as these can and have shered much more than coupling bolts. My personel experience was a 10 inch shaft, and an electrically un-balanced rotor. IEEE has had several good papers on this over the years.

 

[jbartos]

Comment: Certainly, a broken bar might be considered a form of electrical eccentricity, and depending how much is broken even the mechanical eccentricity.

 

[electricpete]

That is interesting Doug. I would have thought the magnitude of the torque oscillations would have been far less than full load torque. And I would think pole pass frequency would likely be far below any torsional resonance.

Is it a case of fatigue failure of the bolts from repeated cycling below yield?

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[seakro]

Thanks for all the interesting aspects.....

Here is what we know.. At least 2 or 3 sets of coupling bolts have broken over the course of about three years on a 186 kW motor driving a vertical centrifugal pump. After is was determined??? that that bolts were insufficeint, they were replaced with ones made of higher tensile strenght. After running for a few short months... the upper roller bearing of the pump, burnt up...

Again the bearings were replaced, the unit was started and ran for 3 weeks. Supposedly alignment was checked and when compared to other circ pumps in use, everything including amp load looked good....

I got to thinking if the motor shifted axially on start up perhaps it would load up the stress on the coupling bolts.
Unfortunately I am not at site to check this. Hence the question of electrical center.

Keith

 

[electricpete]

How does a vertical motor shift axially (vertically)?

I am trying to think through the scenario's and con't come up with much.

When you say everything checked ok, did they check vibration as an independent (imperfect) indicatio of alignment.

There can be a lot of problems where you think the machine is aligned but it is really not. Thermal growth or movement of machine due to system pressures pipe strain etc can change alignment.

It seems like themal growth could cause bearing damage if shaft is locked between two thrust bearings. One should be floating (probably the one in the pump), but if it bound could cause a problem. But this type of scenario would put the coupling in compression. Hard to think how that damages the coupling bolts. I'm assuming simple rigid coupling... is that correct?

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