Motor Bearing Insulation 2

[NickParker]

My specification says, The NDE bearings for motors frame 225 and larger (≥30kW approximately) shall be insulated to prevent bearing currents. The bearing insulation shall be performed by insulating the
bearing seat on the motor shaft or through the use of current-insulated rolling-contact bearings.

Vendor is offering the NDE bearing without insulation and the reason he says that since Y-D starters are used, insulated NDE bearing is not required.

My question is, how the Y-D starter prevents the bearing currents?

 

[waross]

Y-D starter as opposed to VFD?
Possibly send back his quote with a note:
"Rejected. Does not comply with specifications."
He may try harder, or he may be unable to bid on the job.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Tmoose]

"My specification" -
In case it becomes de$ireable to re-evaluate the "specification" are the folks still around at your company that created that specification ?

Much information is available on bearing currents and damage mitigation.
A search of Eng-Tips and Internet wide will hit info from ABB and many others.

My recollection is that there are significantly different requirements about which bearing(s) need insulation, and which should and even should not be insulated. Stuff like motor type, drive type and more make the difference. A few differences of opinion and singular experiences might have snuck in as well.

 

[che12345]

Dear Mr. NickParker (Electrical)
"... My specification says, The NDE bearings for motors frame 225 and larger (≥30kW approximately) shall be insulated to prevent bearing currents. .....The bearing insulation shall be performed by insulating.... Vendor is offering the NDE bearing without insulation and the reason he says that since Y-D starters are used, insulated NDE bearing is not required...My question is, how the Y-D starter prevents the bearing currents?..."
1. The specification is taking an easy way out by a blanket coverage for all motors >30 kW. It is in case (in the near future?) that any of these motors which is [presently with Y-D starting may? change to VFD drive]; without taken cost implication into consideration.
2. For (mechanical contactor) [Y-D starting], insulated bearing on NDE is NOT required. However, with (VFD drive), [insulated bearing on NDE is a MUST] to break the hf circulating current between the bearings; created by the drive.
3. Y-D starter does NOT [prevents] the bearing currents. Modern motors with Y-D starting, D-running; any lf bearing current is eliminated. The motor does NOT produce any hf circulating current. Y-D starter does nothing to the prevention of the hf circulating current, as it does NOT exist. Note: The hf circulating current is created by the VFD drive, not by the motor.
Che Kuan Yau (Singapore)

 

[Gr8blu]

NickParker: All motors - regardless of type - have built-in eccentricities in both the electrical and magnetic circuits. These irregularities lead to a difference in shaft potential between one end and the other. Usually, the higher potential will occur at the end where the outside connections are made to the stator (or, in the case of DC, rotor) windings. To break up the circuit that now exists through the shaft and bearings to ground, we insert an insulating material somewhere. Different manufacturers approach it differently: if we break it up at the NDE, we (most likely) interrupt current flow where it's being generated - but we don't necessarily protect the machine from currents created by the driven equipment (think static electricity from a drive shaft rubbing on something, or a plastic material being processed in an extruder). To fix that issue, it makes more sense to place the insulation (and grounding method) at the end closest to the driven load.

All sources that are power electronics (compared to "line" sources) will generate irregularities in the waveform. It's the nature of the beast. These irregularities will exacerbate any irregularities inherent in the machine design, making circulating currents more likely. The soft-starter's argument is that the motor isn't actually operating CONTINUOUSLY on a drive, so it shouldn't matter. I'd be tempted to tell them fine - we'll sell you a car without doors, windows, or roof because it almost never rains or snows.

If you're worried about circulating currents damaging your bearings, force the vendor to include both an insulated bearing arrangement (at one end or the other, or both) AND a suitable grounding connection to the shaft - which is going to be different for high-frequency switching mechanisms like drives than it would be for the "grid" style power source.

Whether the Y-D starter requires an insulated bearing depends on how the switch is made from one configuration to the other. If it's basically a mechanical switch action that just "reconnects" the winding then no ... strictly speaking, an insulated bearing is not required (for the start method). It's still a good idea from other perspectives, though. If the reconfiguration involves some sort of IGBT or thyristor (or even diode bridge) method, then the insulated bearing is a MUST.

Converting energy to motion for more than half a century

 

[petronila]

NickParker,

If your specification is requesting a motor with NDE bearing for your motors, your vendor SHOULD provide exactly what you need no mater the starting method. It is a preventive step because current circulation by the bearings is not only caused for a VFD action or Y-D switching. There are other causes as magnetic dissymetry associated in larger motors with segmented laminations or motors with uneven stator-rotor gap. Also, bearing current can circulate by electrostatic discharges in blowers and fan aplications (mainly belt driven).

Other causes includes external electromagnetic sources nearby the motors, like transformers producing strong electromagnetic fields.

My suggestion is that any vendor SHOULD fulfill your requeriments and thats include a NDE insulated bearing, also they SHOULD include that information in the NAMEPLATE or install an aditional bearing nameplate, that will help the motor repair shops to recognize that they MOST install the insulated bearing when repairing.

As EASA AR 100 Standard recommends (4.2.7-Page 15), once you have received it the motors, you can test the bearing insulation of each motor using a Megometer (500 v). It should be at least 1 MOhm.

Petronila

 

[electricpete]

We have several large DOL motors with only NDE bearing insulated. The disadvantage is, you can't readily test that NDE bearing insulation (you'd have to uncouple, support the DE and remove the DE bearing). With both bearings insulated, you can test bearing insulation whenever uncoupled.

=====================================
(2B)+(2B)' ?

 

[petronila]

Hi Pete,

Just to clarify, that I am saying the bearing insulation should be tested after motor reception rather than after commisioning.

I am not aware if some industry is testing the bearing insulation after commisioning, but these practice don´t make sense to me because if the bearings are operating as intended and vibration analysis is not indicating any problem why I have to stop and uncouple the motor for testing the bearing insulation?

Unusual damage of the berings (not mechanical or lubrication damage)could be caused by bearing currents. To evidence this kind of problem, the bearings should be washed and disected looking for frosted or dull balls/rollers, flutting, pitting of rolling elements or a "picket-fence" pattern in the bearing races.

Petronila

 

[electricpete]

Hi Petronila. No I wasn't making any comment about what you said. But when you talked about testing, it reminded me there is no easy way to test on our motors that have only one bearing insulated. On our motors with both bearings insulated we do test from time to time, either for troubleshooting or sometimes when the machine is uncoupled for other reasons.

It's nice to have that testing option available even though I can say in my 20 years I've never seen current related damage on a non-vfd motor at our plant. BUT, we have a set of sleeve bearing motors with both bearings insulated and oil supplied from a a sump common to the driven pump... it seems to be vulnerable to degraded insulation on those oil inlet or outlet piping connections. So we test periodically and fix when necessary (which beats time-based replacement). AND... on other motors it has happened twice that people inspecting the driven equipment accused the motor of creating circulating current that caused damage to their equipment(on a vertical pump bearing and a vertical pump seal).... in both cases luckily they occured on motors that had both motor bearings insulated and I was able to dispute what they were claiming because I was able to test the bearing insulation. So if I had my druthers I'd rather have both bearings insulated even on our non-VFD large motors.

=====================================
(2B)+(2B)' ?

 

[che12345]

Dear Mr. NickParker
[COLOR=]" 1. ...My specification says, The NDE bearings for motors ... (≥30kW approximately) shall be insulated to prevent bearing currents. The bearing insulation shall be performed by .... Vendor is offering the NDE bearing without insulation and the reason he says that since Y-D starters are used, insulated NDE bearing is not required. 2. My question is, how the Y-D starter prevents the bearing currents?..."[/color]
1. In principle, the spec can state whatever (essential or superfluous) without taking the practicality and cost into consideration. Therefor, the vendor shall comply in FULL, in order to be accepted. If the vendor is unable to comply FULLY or has an alternative solution, may propose "deviation offer" stating the reasons and perhaps the cost difference, for consideration. The buyer, who pays has the final say.
2. ALL motors do produce bearing circulating current, but of very low non-damaging level to the bearings. This is evident that DOL, Y-D , AT starting have been around for more than 100+ years without any serious bearing damages; until the recent introduction of the VFD drive. It is evident that the damaging hf circulating current between the bearings is caused by the VFD drive.
3. Insulated bearing on the DE or NDE is a MUST with VFD drive. Insulating bearing on both ends which needs addition earthing device to earth the shaft is superfluous.
4. Technically, the vendor's proposal is valid, that with Y-D "mechanical contactor" starting, insulated bearing in one end is not required. Acceptable or rejected is dependent on the competency of the specifier; taking all other factors into consideration , which the vendor may not be aware of.
Che Kuan Yau (Singapore)

 

[edison123]

What che said.

Muthu

http://www.edison.co.in