Basic induction motor question 5

[ccjersey]

I am trying to understand the failure of a standard 3 phase squirrel cage induction motor. This one is a Baldor
7.5 hp
1750 rpm
TEFC
213T frame
208/230-460 9 lead, connected low voltage per the motor diagram (2 parallel wye) Connections appeared to be secure, requiring the wire nuts and splices to be cut off with cutters to remove the cord at disassembly.

It's old as dirt and has recently begun slowing down under load and as part of the function of the machine is to blow material, the blower doesn't function adequately at lower rpm. As load cycle reduces, the motor speeds back up. this went on for several weeks and motor still runs though we have replaced it with another that does not show this variation in rpm which should eliminate the load as a primary cause of this problem.

Amperage was not over service factor amps, usually not over rated amps even while the motor was lugging

Voltage (at the motor plug, 1 foot cord) was 240 (good 3 phase present) while under load and lugging.

Can this be a rotor problem causing increased slip?

Rotor looks fine.

Windings (all 6 circuits)ohm good with equal resistance and coils appear undamaged (after cleaning)

Bearings are sealed ball though the motor has grease fittings installed, so as you might guess, the end bells were full of accumulated grease and some dust.

Never saw anything like this that didn't result in a burnout over the short term, this went on for several weeks running half hour out of every hour for several hours am and pm 7 days a week.

 

[waross]

How is the drive end bearing?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

both drive end and ode bearing are servicable, certainly no lack of lubrication, EVER, apparently.

The cyclic load which resulted in lugging of the motor was caused by an obvious hump in the surface of the silage the unleader was cutting and blowing out the silo door. The poor motor performance while lugging accentuated the hump over time and we ended up reducing the cutting depth of the machine to attempt to reduce the load, but this was not very successful. As the machine reached the hump, it's leveling mechanism increased the depth of cut to attempt to maintain a level surface and the motor began slowing down. As the machine began to move off the hump, the mechanism would reduce the depth of cut again and the motor would speed up.

The machine is belt drive, belts and pullies were good and also tensioned properly. My initial impression was that there was certainly a problem with the machine, since the motor couldn't possibly survive a repeated severe overload. This is an old motor, probably 10 years and is/has been missing the centrifugal fan cover that makes the air blow over the sides of the motor as well as several vanes on the external fan! It should have been overheating and pulling excessive amperage, but it didn't burn up and I couldn't measure excessive amperage.

There seems to have been no problem with the machine and it was disassembled into component sections and removed from one empty silo, greased and inspected and then reinstalled in another silo with out finding a problem. The failing motor would not pull the load in the soft silage surface which often exists at the top of a silo, so it was replaced by another which performed as expected.

 

[jraef]

You can over grease a bearing. Read the section in this article about over greasing.

http://www.mrotoday.com/mro/archives/MRO Coach/Geyer/GeyerON06.htm

A possible scenario is that even with sealed bearings, someone decided to add grease fittings and ended up over greasing; the bearing seals blew out, dust got in and they kept pumping in more grease to compensate. Eventually you get more dust than grease and new grease just lubes the sludge a little until eventually that fails to have an effect either.


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[ccjersey]

Certainly the motor was not maintained properly.

+inappropriate lubrication resulting in contamination of the windings with grease and dust

+no fan shroud and damaged fan impeller resulting in poor cooling

So it failed, but the failure is not a familiar mode.

I am wondering if the squirrel cage rotor might be the cause since it is pretty much a "black box" which I cannot inspect internally or test in any way I know of. I have another set of shaft, rotor and bearings I could install as a trial, but that rotor is not quite as long as the one in the failed motor, it's probably 20-25% shorter. The other dimensions appear to be identical.

 

[Skogsgurra]

There is one rather simple way of testing for rotor failures.

If you consider the standard transformer model (stator winding creates field that induces current in the rotor bars), then you can test those bars by connecting a low, say 24 V AC, voltage to two of the motor terminals. Put an ammeter in series.

Then turn the rotor slowly and check current variation as rotor turns. Most motors keep current constant withing a reasonable tolerance band, usually five - ten percent. Those with rotor problems show a greater variation because rotor bars are broken and do not short the "transformer secondary" as effectively as an OK rotor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Great tip Gunnar. LPS**

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

Thanks!

That seems simple enough. My only challenge will be to find an analog meter. I will attempt to measure the current with a clip-on digital, but from prior experience with a varying load, it is hard to get a good impression of what the variation is other than that the meter has a max function.

I suppose there is no need to turn the rotor at any certain speed and it could be held still to stabilize the amperage at any point of interest.

 

[electricpete]

You don't need any peak recording feature. Turn the rotor perhaps 1/2 turn in 1 minute and you will observe 2 peaks and 2 minimums during that minute (results for 4-pole motor will vary 4 times per shaft revolution).

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[Marke]

Another thing to do, is to watch the current as the motor is running under variable loading conditions. Note, this is best done with an analog meter!!
When the motor is at light load, if there is a significant current swing at a low frequency, and this current swing increases in frequency with load, that can be a sure sign of a rotor bar problem. The changing frequency of the current modulation is the slip frequency.
I have experienced current swings in the order of 20 - 30% with bar problems.

Best regards,

Mark Empson
L M Photonics Ltd