Stator/rotor air gap measurement on large MV motor

[civlgrrl]

I ran across electricpete's post regarding how to measure stator/rotor air gap.

https://www.eng-tips.com/viewthread.cfm?qid=277915

He specifically references API 1068. Reviewing API, I could not find that standard - but like how it's requiring physical confirmation of the air gap measurements.

Then he referenced API 541-1995, Section 2.4.7.16.

Is the information on how to do static measure the gap located in API 541?

Have a 3500 HP GE motor for a vertical pump being witness tested in Brazil and they’re saying they can’t measure air gap and are suggesting using the method outlined in API 541-2014, Section 4.4.7.2.4 – see attached. I don’t see how this method truly measures air gap. Has anyone seen the calculated air gap method be used in the past?

We have had problems with a couple of past motors where the air gap was eccentric - that motor stator/rotor rubbed against each other and we've had to get it rebuilt after 5 years service - and sustaining damage to the stator windings/ribs. Thus we now require review of air gap prior to leaving the motor factory.

 

[edison123]

What epete and API 541 state is correct for measuring air-gaps for vertical machines. For salient pole machines, you measure top and bottom air gaps for each pole with feeler gauges. For round rotors, you measure 4x90 deg or 8x45 deg for large rotors at top and bottom. The air-gap variations should not exceed 10%. Our in-house standard is 5% variation.

If there was a stator-rotor rub in vertical motors with white metal bearings, it is always due to wrong assembly of the motor i.e. improper centering of the rotor.

Muthu

http://www.edison.co.in

 

[electricpete]

I'd say API 541 paragraph 4.4.7.2.4 is not a substitute for an airgap measurement for factory, it is an acceptance criterion for air gap measurements for factory(namely that max airgap measurement minus min airgap meas urement is less than10% of the average of all airgap measurements). They still need to measure the airgaps to see if they meet the criterion. But during any subsequent disassembly/reassembly there should be no need to repeat the measurement at any later time since the spec requires doweling to create repeatable reassembly.

Sounds like your vendor is suggesting not to perform the checks during initial factory assembly? In that case I'd ask them how do they plan to demonstrate that the requirement is met.

As edison mentioned this would be limited to wound or squirrel cage rotors, not salient pole motors with pole pieces (typically only used on slow speed synchronous machines).

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