132kw motor.

[rapparee]

I have a 132kW 1490rpm squirrel cage motor. It is electrically fine but the bearings are gone. It vibrates and when I do a clamp test on each of the phases, I have L1 at 69A and L2 and L3 both at 81A. This is when there is no load on the motor. We have ordered a replacement motor to go in on Monday.

I was just wondering what others thought of the imbalance across the phases. What would they tolerate as acceptable.

 

[Kurt]

If you can get at both ends of the windings you could always to a continuity to see if they are similar there, but I would be more concerned with imbalance on a loaded motor when there's some proper slip.

 

[Evans Electric]

That would be an ecumenical question  Rapparoony .     What about resitance & insulation readings ?    I would take a guess that one winding is failing  but I'm sure Sidey or Canoey can offer something more substantial . 

 

[rapparee]

TBH I'm not worried about continuity or ir. We haven't the time to stop the motor let alone test it or fix it. A new IE 3 motor only costs £5000. It's a drop in the ocean compared to the £100,000 that is going to be spent on the mechanical end of the press later on in the year. 

The press will run until Monday morning and then we take over. I have the new motor ordered and crane driver ready to go. The fitters have to do a patch job on the clutch. Maybe I'll do a postmortem on the motor if I have time.

I was more asking a question on imbalances on 3 phase motors. I have found this on smaller motors where say there is two phases at 12A and one at 14A. What is an acceptable imbalance across a 3 phase motor.

 

[Sidewinder]

As has been suggested, offload readings can be a bit misleading.

Winding inductance & impedance are a better indicator of winding condition than resistance.

I would expect everything to be within 10% as a rule of thumb.

Clamping phases one at a time is another thing that can be misleading too.

Better to use a 3 clamp power analyser type instrument, that way you get all 3 phase current readings in real time.

Curious as to why you didn't lob bearings in it?

 

[rapparee]

If you can get at both ends of the windings you could always to a continuity to see if they are similar there, but I would be more concerned with imbalance on a loaded motor when there's some proper slip.

I tested the motor on no load because the press it was driving has some broken clutch plates that are making me think I couldn't get a proper look at the problem. (I know I still didn't, but it was all I could do). 

 

[rapparee]

As has been suggested, offload readings can be a bit misleading.

Winding inductance & impedance are a better indicator of winding condition than resistance.

I would expect everything to be within 10% as a rule of thumb.

Clamping phases one at a time is another thing that can be misleading too.

Better to use a 3 clamp power analyser type instrument, that way you get all 3 phase current readings in real time.

Curious as to why you didn't lob bearings in it?

To lob bearings in it means too much downtime. Plain and simple. We're having severe mechanical problems with 4 big presses down at the minute. I was told by the production manager to stop a fifth I might as well stop the factory.

In fairness to the clamp on meter the readings are pretty constant. I was looking at it during a tool change. I know I'm doing things wrong and I appreciate all input to this thread but real world constraints are on me.

I am surprised at the 10% figure, I thought that seems a bit high. I'm not questioning, I listen to my elders lol.

It sometimes depends how its wound as to what the imbalance would be, its its wound layered with a lift all coils will be the same and there should be no imbalance, its its wound 2 or 3 layer where each layer has a different size of coils then there will be an imbalance, but its not normally much and certainly not as much as your 132kW which sounds more like it has a winding fault, possible shorted turn or similar.  A 3 phase analyser or logger will give you a better idea of the balance of the phase loadings rather than a clamp on. 

Would a worn out bearing that causes heavy vibrations in the motor throw it out of balance.

 

[Sidewinder]

Yes badly worn bearings will cause the rotor to rotate out of true, which would cause varying flux coupling as it were, this would cause varying current readings.

Off load is more often than not the only way to measure the current, but, as long as one is aware of the limitations then that's fine.

This is where a power analyser would be useful, lob it on the motor tails in the panel and monitor the load on the press when running, then download and look at in the office.

Not in my experience, that would cause all 3 motor leads (or 6) to have the same issue as it was rotating at speed, although a worn - collapsed - damaged bearing can chuck bits of metal into the windings and cause all sorts of issues

We have had the varying current thing with squirrel cage servo motors.

 

[poni]

can I have the old motor? (£400 per tonne don't you know)

 

[Kurt]

I tested the motor on no load because the press it was driving has some broken clutch plates that are making me think I couldn't get a proper look at the problem. (I know I still didn't, but it was all I could do). 

Was testing a mixer, brand new Chinese motor, was around 35kw, L1 0.7, L2 1.5, L3 1.6A on no load, as soon as I got 25% load on, the amps soon converged, although L1 never overtook the others. I was testing because the users were stalling the drive, so I was helping get the right volume of product in the mixer before it stalled.

Just an example but I often see quite different readings on no load, don't really take that much notice of them, unless its zero, lol

 

[Tony S]

From the readings given

Winding current L1→L2 = 24.2A

Winding current L1→L3 = 24.2A

Winding current L2→L3 = 28.4A

Like Canoe I think you have a shorted winding on L2→L3.