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How do you measure the air gap? 1

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buller

buller

Electrical
Apr 8, 2001
3
We have a GE 32000 Hp synchronous motor and we would like to measure the rotor stator airgap. Is it best to use long feeler gauges or just do point measurements around the rotor?
 
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For induction motors, the typical procedure is measure at 5 locations 90 degrees apart, on both ends, using long feeler gages. Then rotate rotor by 90 degrees and repeat.

Is it salient pole or smooth rotor?




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It is a salient pole machine. 4 pole,1800 rpm 13,8kv
 
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We don't rotate the rotor here. We transfer to another location and measure the gap using feeler gauge;(90 degrees apart).
Respectfully.
 
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4 x 90 deg in your case. For salient poles, measure across each pole.

Also, for salient poles, you have to make sure that you are measuring the air-gap at the center of the pole, where it is minimum. No need to turn the rotor unless you're measuring the rotor or stator run-out.

Muthu
 
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The question on whether to rotate or not... I agree most (but not all) specifications do not require it.

There is at least one that does. API 1068;
API 1068 said:
Centering of the rotor within the stator should be checked, whenever permitted by the machine
construction, by both stationary gap and rotating gap feeler gage readings at both ends of the motor. Readings should be taken at not less than three points 90° apart around the rotor periphery. In the stationary check, feeler gages are inserted successively at the separate points and the values are recorded. In the rotating check, the gages are left at one location and the rotor is turned in 90°
steps, noting the reading at each step. This test can reveal an eccentric rotor that may go undetected
by the stationary test. Readings shall not exceed a 10% deviation from the average at each end.
(See API 541-1995, Section 2.4.7.16.)
What I described imo accomplishes the intent of both tests.

From their discussion you see as we all know there is stationary eccentricity (basically lack of centering) or rotating eccentricity 9typically runout).

If rotor iron runout is small for example < 0.002", then probably the stationary test is good enough.

If rotor iron runout is 0.005" (as is common for our very large motors), then you see the rotor TIR is on the same order as the 10% airgap acceptance criteria... tough to ignore it imo.

At any rate, it was a good clarification. Certainly rotating rotor is not required for smaller motors and many standards. Some standards and customers (like me) prefer it for reasons discussed above

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To clarify, one concern I would have is that combination of rotor runout and static eccentricity can combine to give good gap results in a certain orientation of the rotor, but when you rotate it you'd fail. As a customer, that is a little unsettling and I'd rather check both so at least I will be alerted to the condition and be able to talk about whether any correction is required... rather than leaving it up to random chance whether rotor will be in a position that determines pass/fail of my test.

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Actually rotor iron TIR can be quite a bit higher than what i mentioned... tough to separate the bumpiness from the runout and can be subjective. I used low TIR numbers because I didn't need high numbers to support my point... even 0.005" TIR would be significant compared to the airgap acceptance tolerance.

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