Hydropower Generator Airgap

[QCE]

Does anyone know of a standard for the % error in airgap, rotor roundness/concentricity and stator roundness/concentricity of a hydropower generator?

The only one I can find is:

CEA - Guide for erection tolerance and shaft system alignmnet.

The values seem a lot higher then what I see used in industry.

 

[electricpete]

IEEE 1095 (1989) - Vertical Hydro Machines:

10.11 Shaft-Plumb and Straightness Check.
This is a plumb-wire check of the straightness and plumbness of the combined generator and turbine shafts. The use of four plumb lines spaced 90 degrees apart around the shaft is recommended. The readings should be taken from the same points on each shaft that were used during the factory runout check. The shaft assembly shall be considered to be straight when no runout check point (correlated for diameter variations) deviates more than 0.003 in from a straight line joining the top and bottom points. The shaft assembly shall be considered to be plumb when the top and bottom points do not deviate from plumb by more than 1/4 mil/ft of shaft length.

They refer to the manufacturer guidelines regarding airgap uniformity and runout


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[QCE]

The 0.003 in number sounds strange since it should depend on the rotor diameter. If you have an 8m rotor or a 1m rotor will make a big difference and that is why CEA guideline is in %. Although it is in % of airgap which will also vary but mostly due to rotor size.

 

[edison123]

The industry standard for air-gap variation +/- 5% of the average value.

This implies that the stator and rotor roundness & concentricity should also be within 5% of the air-gap value.

* I would go green if only I were not yellow *

 

[QCE]

Thanks edison,

Is there actually a standard or is it like a mechanic using his finger to measure things. Is this just what is 'done' or is it really a standard published by some respected organization.

I disagree that +/- 5% implies 5% error in rotor and stator roundness. I would guess it should be more like 2% deviation for rotor and 8% for stator as it is harder to control.

Also if you had 5% error in concentricity of the stator and rotor and 5% error in stator and rotor roundness you could have a 20% error in the airgap.

 

[edison123]

These are values given by OEM's like GE, Alstom and Siemens with whom I have worked. USBR has also some procedures specifying the same.

We have built stator cores with diameters ranging from 3 to 12 meters. The ID has has always been within +/- 1 mm of the nominal dia. Right now, we are rebuilding a 50 MW, 10 pole machine core with a dia of 3000 mm and core length of 1600 mm. We got a maximum dia variation of - 0.3 mm to + 0.2 mm.

Could you tell me how 20% error in air-gap will be produced by following the 5% limit ?

* I would go green if only I were not yellow *

 

[QCE]

Sure +5% on rotor roundness and -5% on stator roundness with 5% error in the offcenter of the rotor compared to the center of rotation plus 5% error in offcenter of stator in the opposite direction. Total error 20%.

Do you have any information on the USBR procedures?

I would prefer an IEC standard but it looks like it is not possible.

I have seen the results from GE, Alstom and Voith Siemens generators also but in some cases they are not so good as you state. The problem is that some of the contracts state to follow this CEA specification which states:

http://vibrosystm.com/pdf/MECHTOL.PDF

1mm error on a machine with 20mm airgap might not meet the spec.