Rotor iron T.I.R. limit 1

[electricpete]

Does anyone know of thumbrule or standard limits for total indicated runout (T.I.R.) measured on the rotor iron (at ends and center of the rotor) ?

I have seen one document that mentions 0.002" T.I.R. limit on rotor iron but that seems very tight considering the inherent bumpiness of iron along with the fact I'm thinking about a big rotoron a 13.2kv 324rpm motor. Rotor O.D. is approx 78”. Distance between bearings approx 80”.

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[edison123]

0.05 mm is the TIR I allow in my shop. The rotor laminations are manufactured to 0.1 mm tolerance and before winding, the most of the rotors are turned down to the right diameter and to keep TIR within 5 microns.

Muthu

http://www.edison.co.in

 

[electricpete]

0.05 mm = 50 microns matches my 0.002", which I thought was a pretty tight limit.

5 microns would be 0.0002" ... was that a typo?

And the same limit applies regardless of size of the rotor?

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[edison123]

Yes, 50 microns of course. Regardless of size of rotor. Due to reasons of unbalance, uneven air-gap etc. When the laminations themselves are made to a tight tolerance 0.1 mm, I don't see any reason to allow TIR above that.

Muthu

http://www.edison.co.in

 

[electricpete]

Thanks Muthu.

Just to explore the discussion a little, I have some devil's advocate questions.

If a large rotor like the one described above came through your shop with 100 micron TIR, you would recommend bringing it down to 50 microns using a for skim cut, even though airgap readings with within spec (for example less than 10% deviation from average airgap, rotate 90 degrees and repeat) ?

Isn't that relatively expensive (most shops would need to send out for that)?

What are the expected consequences of not correcting it?

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[edison123]

Pete

Just confirmed with EASA that allowable TIR is 0.1 mm (though no standards to cite). They too say mechanical unbalance is one of the issues. Though the air-gap will not be greatly affected by such a TIR, the constant changing of air-gap (unlike permanent uneven air-gap)could produce electromagnetic vibrations.

I would be wary of doing any machining of rotor core since it could increase the no-load current. In my experience, the rotor TIR exceeds due to shaft problems - bend, non-circularity, worn-out journals etc and not due to rotor core itself. So I replace the shaft when required.

Muthu

http://www.edison.co.in

 

[electricpete]

Thanks again.

It’s hard to fathom mechanical balance as a basis for reworking the rotor, when we can achieve good balance using a balance machine.

If we rework the rotor, skim cut still seems to be a more traditional repair then shaft replacement based on cost. Smearing of rotor laminations by machining is much more tolerable than smearing of stator laminations, because the rotor sees such low frequency during operation. There will be small increase in airgap, which I guess is why you mentioned increase in no-load / magnetizing current (and presumably small drop in full load power factor).... but if we’re talking 0.1mm – 0.2mm TIR, ... it a small percentage of the total airgap.

We have previously done rotor skim cut on one of these rotors to bring 0.005” (0.15mm) TIR down to 0.002” (0.05mm) during the course of troubleshooting a motor with high vibration. We didn’t notice any adverse effects from that skim cut. But also I am not sure if it was necessary.


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[edison123]

I've gotten rotors in my shop after some other rewinder decided to turn the rotor (for whatever reason), with the complaint that the no-load current was as high as full load current. Can't do much in such a case unless the client accepts a derating of the motor with a stator rewind with lower flux density design.

You said

"We have previously done rotor skim cut on one of these rotors to bring 0.005" (0.15mm) TIR down to 0.002" (0.05mm) during the course of troubleshooting a motor with high vibration."

Did the vibration get reduced after bringing down the TIR ? If it did, then it proves the unbalance created by a high TIR, isn't it ?

Muthu

http://www.edison.co.in

 

[wolf39]

Smearing of rotor laminations by machining does increase rotor surface iron losses caused by air gap flux pulsations. Such flux pulsations are caused by the stator slots. The smaller the air gap the higher the iron loss increase.

Regards

Wolf

http://www.hydropower-consult.com

 

[electricpete]

Wolf – thanks. That is a good point. The slot harmonics which create a higher-frequency flux at the rotor surface. Also shorted rotor laminations create potential for rotor bow. So smearing of rotor iron laminations certainly is a concern. It is just not (imo) as much a concern as smearing of the stator laminations (I don’t think anyone would ever machine the stator core bore... would they?).

Interestingly, the EASA Mechanical Repair document describes rotor skim cut as one possible means for correcting rotor hot spots. But the technique/parameters of the cutting are critical.

METHODS FOR CORRECTING HOT SPOTS
When a core test indicates shorted rotor laminations,
there is the possibility of uneven thermal growth of the rotor.
In most designs, the shaft is stiff enough to prevent it from
actually bending. With many two-pole machines, the shaft
is long enough and flexible enough that rotor hot spots can
deflect the shaft. When this happens, vibration is inevitable.
As the hot spot causes the shaft to bend, the air gap
becomes non-symmetrical and magnetic forces bend the
shaft further.
To correct these hot spots, use one of the following
techniques:
Rotor skim cut
• Align the shaft journals true in the lathe.
• Take a light cut from the rotor body. It is recommended
that no more than .010” (.25 mm) cut be
made per pass.
If the air gap is increased too much, magnetizing
current will increase. The more poles a motor is designed
with, the closer the air gap. A 2-pole motor with a large air
gap has more latitude than a 10-pole motor.
Tooling for rotor skim cut
There are two schools of thought on this subject. One
is to use a very sharp tool at a slow rate of feed (400 to
450 surface feet per minute), so that the tool separates
the laminations. The theory is that this sharp tool
prevents any smearing of the laminations. The second
recommendation is to use a #4 radius insert at 840
surface feet per minute (250 m per minute).

Muthu – thanks.

I've gotten rotors in my shop after some other rewinder decided to turn the rotor (for whatever reason), with the complaint that the no-load current was as high as full load current.

Do you happen to know how much the airgap increased (10%... 20?%). And was it a fabricated copper bar rotor or something different. Just curious.

You said

"We have previously done rotor skim cut on one of these rotors to bring 0.005" (0.15mm) TIR down to 0.002" (0.05mm) during the course of troubleshooting a motor with high vibration."

Did the vibration get reduced after bringing down the TIR ? If it did, then it proves the unbalance created by a high TIR, isn't it ?

The short story – we changed so many things that we don’t know precisely what fixed the vibration.

The long story, it was a unique situation – a good motor was removed for proactive refurbishment, final uncoupled run at the shop was good... put the motor into the plant and vibrated like crazy at 1x during uncoupled run. Management was understandably vexed that we succeeded in creating a problem on a motor that was fine before we spent $100k on it. So I followed the motor back to the shop with a mandate that the motor absolutely must be sent back in good condition at almost any expense (did not want a repeat problem). We had people from 3 different shops involved to help sure we made it right. We did find the as-found vibration varied strongly with voltage (did not show up at 4kv, did show up at full voltage 13.8kv) which explains why we didn’t see it the first time when it was only tested at reduced voltage in the shop. We also found the upper bearing radial clearances which were supposed to be 5-7 mils were 25 mils radial in one direction. That is a pretty big error since average airgap is around 0.060 – 0.070” and imo explains the vibration. However given the situation we went the extra mile and fixed everything that looked remotely suspicious including the 0.005” TIR on the rotor.

Imo, the only concern for TIR on this rotor was the effect on unbalanced magnetic pull, not the effect of mechanical unbalance. As I mentioned the rotor was of course mechanically balanced on balancing machine. I am pretty sure the rotor operates well below any flexible rotor critical speed, so acts like a rigid rotor which means that we can correct for any unbalance using at most two correction planes. And that correction negates the effect of the unbalance. There may still be effects that cause 1x vibration after balancing the rotor (such as eccentricity of the bearing runner or carrier), but those are not attributable to mechanical unbalance.


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