White smoke coming out of MV Motor 1

[alexandergr8]

Last month, we got our 1000 HP 6.6 KV motor cleaned after 6 years of operation. When the motor was run, it gave whitish smoke. Motor was stopped immediately (it ran for appr. less than a min). Megger results were showing the motor to be OK.

We called the service guys who have cleaned the motor and replaced its bearings. They asked to run the motor in their presence. In about 10-15 sec, white smoke appeared. Motor was stopped (ran for about 40 sec this time). Bearing temp and vibrations and current remained normal as before.

Both times the motor was run uncoupled. At stop position, motor is easily rotatable through hand.

Have anyone experienced white smoke coming out of motor? What could be the reasons?

 

[electricpete]

A few possibilities come to mind. I list them what I consider most likely to least likely:

Most likely - contaminant on the rotor or stator. Possibly some kind of cleaning compound, possibly water producing steam.

Possible - lubricant level improper in the bearing.

Possible - core surface was extremely damaged during handling causing intense local heating upon energization.

less likely - abnormal rubbing at a bearing/seal or - (less likely because it should show up on vibration.)

extremely unlikely - some kind of stator turn-to-turn shorting problem which has not yet progressed to phase-to-ground fault.

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[alexandergr8]

Motor winding heater was kept on 24 hours before starting till the winding temperatures became 3 C higher than ambient.

Bearing Oil level is still OK. It is at the mid level as recommended by manufacturer.

We have opened the side covers. No signs of burning are there. Winding resistances have been checked by DLRO. All three phase readings are similar which almost rules out a interturn phase short.

This motor is running a vertical centrifugal pump on cooling tower.

We are opening the motor currently to inspect rotor/stator for a possibility of rubbing.

I will post the observation.

Thanks.

 

[RalphChristie]

Maybe a little off-track...

We have some 3.3kV slipring-motors with vapourmatic (liquid)starters. Sometimes, especially on cold days, there is also some "white smoke" coming out of the starters when we start the motors. This "white smoke" is the liquid vapourising during starting, and many times the plant was stopped because someone thought a motor was burning.

I like the first idea of Pete - steam - maybe from water coming in contact with the heated winding during starting.

How did you clean this motor?




Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Regards
Ralph

 

[powerjunx]

I HAD ENCOUNTER SUCH INCIDENT ON SIEMENS AND G.E..
IN YOUR CASE AND SET UP OF THE MACHINE.. I THINK THE MACHINE IS "OPEN DRIP PROOF ENCLOSURE", WITH THIS, THE HEATER NEVER COVER ALL THE BACK IRON CORE SURFACE OF THE STATOR - PROBABLY A MOIST,OR A DEWS EMBODIED ON THE SURFACE WHILE HEAT UP (RUNNING THE MOTOR), IT VAPORIZES AS A WHITE SMOKE - CONSIDERING THAT THE TIME YOU TEST RUN THE UNIT HAS A HIGH RELATIVE HUMIDITY OR THE SITE IS WET.
As Electricpete had pointed out: "Most likely - contaminant on the rotor or stator. Possibly some kind of cleaning compound, possibly water producing steam...."

SOME WHITE SMOKE I'D EXPERIENCED ARE MOSTLY COMING FROM THE BEARINGS. OTHERS FROM SOLID PARTICLES SUCH AS HARDENED DUST, UNSCRIPPED VARNISH, AND STATOR-ROTOR RUBBING CONTACTS.

OTHER POSSIBILITY; PLEASE CHECK ITS LUBE OIL (PLACED ABOVE THE FREESIDE), IF THERE IS A LEAK GOING DOWN TO THE ROTOR.
THIS WOULD VAPORIZED. SEE TO IT THAT THE THRUST BEARING ARE WILL FITTED AND SEALED FROM LEAKING.

ALEX, MAY I KNOW THE MANUFACTURER OF YOUR MACHINE?

 

[alexandergr8]

We have opened the motor at site and found severe rotor/stator rubbing. It is amazing as before running the motor we have moved it by hand as a test of free movement of rotor and it did not take extra force. One electrician moved it. No abnormal noise was observed with stethoscope.

Now it appears that wedges have also got damaged. But the megger results are still good.

I would like to know what tests can be performed on this motor to ascertain the extent of damage to insulation, core due to overheating, if any are possible.

We have sent the motor to Siemens repair shop. I will be going there tomorrow for the inspection of motor and we will hold a meeting also. I will appreciate greatly any advice relating to tests that can be performed at their workshop / manufacturing facility.

 

[electricpete]

Do a core loss test to check for damage to stator core laminations. Possibly a rotor loop test to check for similar damage on rotor although that is not as critical. dc step voltage test for damage to windings (specify that max test voltage). Surge test for good measure. Recheck rotor balance. Most important I think you have to determine and correct the cause of the rub. Is there a defect in the bearings or bend in the rotor (possibly thermal)>

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[electricpete]

It may not be surprising that the motor would rotate by hand after the rub. The factors causing the rub may included unbalance, eccentricity, clearance, electrical force etc which are not present when the motor is deenergized.

It does seem surprising that this item did not show up on vibration. We had a "mild" rotor rub (is there such a thing) resulting from looseness of the upper bearing support plate (vertical motor) which in effect created excessive clearance.

http://www.reliability-magazine.com/pub/Im002319.jpg

The rub was evident from noise to anyone standing near the motor. Also it showed up heavily on vibration. Most heavily was the 1x component and also a lot of high frequency stuff. The only explanation I can come up with perhaps in our case the 1x was a result of the excessive clearance which creates an unbalance force. Also we had high frequency stuff which showed up in time waveform and acceleration... perhaps you guys were only monitoring the velocity overall?

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[alexandergr8]

Thanks for the reply. I have seen the motor today at the workshop. Rubbing is severe. They are recommending DC Hi Pot at 10.5 KV ( 80 % of 2 x rated voltage + 1).

Can you eloborate the rotor core loss? I will discuss the possibility with them.

 

[electricpete]

I would say a stator core loss test is a must-do. You may have smeared the surface of the laminations and this may result in unacceptable surface heating (core loss test passed after our rub) which could lead to near-term stator failure.

Smearing of stator laminations is more critical to detect/repair than smearing of rotor laminations because stator laminations see a higher frequency field during motor running (stator sees line frequency, rotor sees slip times line frequency).

Core loss test would involve using either a commercial core loss tester or a low-voltage, high current ac power supply through as many loops as required to get approximately 105% rated backiron flux. Repair shop should know how to do it. View with infrared. Keep the core excited until temperatures stabilized or at least 1/2 hour. Expect no localized hotspots more than 10C above average core temperatures. Also if core loss testser measures watts loss, expect less than approximately 4 watts per pound.

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[aolalde]

For vertical motors running uncoupled, the thrust bearing could get out of radial alignament due to the magnetic radial forces and allow rubbing the stator.The axial bearing needs the thrust load to keep it aligned.

 

[powerjunx]

alex, since the machine has been in operation for 6 years.
this time the problem emerges after it was subjected to preventive maintenance..i suggest that you check the ROTOR assembly through DYNAMIC BALANCING. In my past experience, we balance check the rotor everytime we conducted a preventive maintenance. there we find an unbalance and rough vibration as compared to the past data.

 

[aolalde]

I have a 700 HP, 255 rpm vertical circulation water pump induction motor. This motor has an upper “Kingsbury” bearing with titling shoes for down thrust with radial sleeve guide bearing. At the bottom it has a sleeve radial bearing. Both are oil lubricated.
The motor “smoked” suddenly after 8 years in operation. When it was disassembled we found the upper bearing off-set and the guide dowel pins were bent distorted. The stator inside diameter showed a localized small rubbing area (around 50 sqin) at the top edge of the core stack. The rotor shows a continuous rubbing band around 3 inches wide in the upper edge.
Our theory is that the holding bolts of the bearing got loose allowing the rotor hammering against the dowel pins due to the magnetic radial forces, until the movement of the bearing allowed the rotor dragging against the stator lamination.

 

[electricpete]

That scenario described by aolalde is remarkably similar to the scenario that caused the rub we described. Vertical 3500hp 324 rpm (slow speed) with 19" Kingsbury Thrust bearing and tilted pad radial on top. There is a component called the "upper bearing support ring" which holds all the upper bearing components. It is held onto the motor by 6 bolts and three dowels. The bolts have insulating washers and loose sleeves and the dowels have a tight hard plastic insulating sleeve. We found 4 of 6 hold-down bolts loose (could move with 60 ft-pound force for 1.25" bolt), 2 dowel pins loose and 1 dowel pin completely missing (not sure where it went). When we pulled out the loose dowel pins we saw the insulating sleeve was cracked which is what allowed them to become loose. Looseness of the hardware allowed the upper bearing support ring to move which shifted the center maintained by the radial tilt pads.

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