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Bearing Current Problems on non-VFD Induction Motors

Bearing Current Problems on non-VFD Induction Motors

Bearing Current Problems on non-VFD Induction Motors

(OP)
I deal with large 3-phase induction motors used on submerged cryogenic pumps.  The fluids are always dielectric materials and flow through the motor and bearings.  THese are large motors ranging from about 300Hp up to 3500Hp mostly 2 pole with relatively standard induction motor construction.  We have had major problems at one site with bearing arcing damage after our rotor design was changed by our supplier going from 40 rotor bars to 41 along with some end ring design changes.  These motors are all driven DOL (direct on line) so there should not be HF components causing these currents.  We have not had problems at other customers with these new rotors even on VFD driven units.  We are going to implement a grounding brush which should help.  Any feedback on where this current is coming from and would ceramic bearings help more than a grounding system?  We do not have access to the motor shafts to do any shaft voltage measurments as these are submerged in -160C fluid while operating.

RE: Bearing Current Problems on non-VFD Induction Motors

I have had this problem with pump motors sitting on the ocean floor, which were filled with synthetic oil to avoid salt water in the interior of the motor.

It was very clear that the rotor produced static electricity when running in the synthetic oil. The static electricity discharged through ceramic seals and destroyed them. I think that you may have a similar phenomenon. Use shaft grounding (either brushes or AEGIS rings) to avoid the problem.

I usually get paid a handsomly for a tip like this. But since you are a BigMotorGuy, it is free smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Bearing Current Problems on non-VFD Induction Motors

(OP)
Thanks Skoqsqurra,
 We have a couple of test sample of the Aegis ring and an Inpro CDR ring.  The Aegis has not been used in cold fluids before according to their techs, but we will see how it performs.  The CDR ring supposedly is good down to -240C.  We have installed our first CDR ring on site and will see how that works.  I don't understand why it has only happened with the new rotor.  The bar style is the same only slighly smaller in order to incorporate the additional bar in the same OD.  Is there something about the ratio of bars to stator slots (4o bars old design, 41 bars new design, 48 slots on stator) that is adding to the static build up?  Anyone have experience with this ratio and how it might affect this discharge?

RE: Bearing Current Problems on non-VFD Induction Motors

See here:
http://www.reliance.com/pdf/motors/whitepapers/raps868.pdf
Some thoughts:
Assymetries in stator iron – not expected if you simply changed the rotor.
Assymetries in rotor characterisics – seems a possibility.
Assymetric in airgap – could be due to assembly. Can create homopolar flux in 2-pole motor.

Of course changing the number of rotor bars also has a profound distrubtion on the space harmonic fluxes.  I don't think this woudl be a factor but space harmonics can have some strange effects.
 

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RE: Bearing Current Problems on non-VFD Induction Motors

I should have mentioned pages 5 and 6 of above document address non inverter motors.  

=====================================
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RE: Bearing Current Problems on non-VFD Induction Motors

(OP)
Update on the bearing arcing.....we tested 3 grounding systems (Aegis ring, Inpro CDR ring, and traditional carbon brush) with intial success.  I ran impedance measurements on each system from 1MHz to 10MHz ranges.  We found stable impedance levels for all 3 at ambient and liquid nitrogen immersed temps both static and dynamic.  We have installed a CDR ring into a pump and will evaluate the results after either several thousand hours or high vibration readings.

RE: Bearing Current Problems on non-VFD Induction Motors

Just out of curiosity, LNG pumps?


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RE: Bearing Current Problems on non-VFD Induction Motors

If the issue is static electricity the cause is not magnetic induction. It is most like most likely to do with the of type of varnish or adhesives used on the rotor.

RE: Bearing Current Problems on non-VFD Induction Motors

(OP)
thanks Compositepro,

Could be static build up, but this is a very isolated case with simimlar conditions as many other units in the field.  The units operate in LNG or LPG typically.  We use a double VPI process with epoxy (100% solids) on all stators, but there is no added adhesive to the rotors.

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