Insulated bearing 1

[Guardiano]

Hello forum members,

We are in the process of ordering one 1,200 kW 5,5 kV wound electric motor to drive a rock crusher.Is it essential that the non drive bearing should be an insulated one?Though I know that for VFD application insulated bearing on NDE side is compulsory.The crusher would not be driven by a VFD.

Thanks

Guardiano

 

[petronila]

Hello Guardiano

From my point of view if you don´t will drive the motor with a VFD is not necessary,but you most check if the motor are going to be installed near of statics energy sources The risk for damage to bearings can occur when transient voltages exceed 0.5V or the electric current flow is more than 0.1A/mm2 (related to the contact area of rolling elements).

Regards

Carlos

 

[electricpete]

Most motor OEM's provide insulated bearing for NDE regardless of VFD application of very large motors. As I undsrtand it, the reason is that stator lamination material comes in standard size sheets... if motor stator lamination is too large it cannot be constructed from a single piece and instead several segmented stator laminations are used in a given plane. This introduces magnetic asymmetries due to variations in the gaps where lamination segments butt against each other, which can cause power frequency bearing circulating current... only necessary to insulate one bearing to avoid it since the power frequency is not capacitively coupled like vfd's.


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[electricpete]

I believe your motor is probably large enough that segmented laminations would be used and insulated ODE bearing is traditionally required.

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[Skogsgurra]

I second pete.

Gunnar Englund

http://www.gke.org

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

 

[petronila]

I am not agree,this phenomena could happened too due to a wrong winding design (magnetic field asimmetry).
If the customer are going to buy a new motor the manufacturer most provide the right one and if it depends of the own manufacturer stator lamination or winding design they knows very well about own problem and they will avoid this one with insulated bearings or other means.

I think in this case Guardiano is asking due to he is going to specify the motor.

Regards

Carlos

 

[electricpete]

Hi Carlos - out of curiosity, have you ever seen a motor this large without insulated ode bearing?

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[electricpete]

Carlos may be right... we can probably trust the OEM's to do the right thing.

I will mention one more thing, at our plant none of the large motors have vfd.... and all motors from around 800hp up have insulated ode bearing at least and some as small as 500hp have it. I don't recall whether it was in original purchase spec (before my time) or whether the OEM just provided it.... I will poke around and check some specs when I get a chance.

There is another option that appears on two of our large non-vfd vertical motors which is: BOTH bearings insulated. It costs more, it is not required for bearing protection, but it has the advantage that you can actually check the bearing insulation resistance with the machine uncoupled....

Otherwise (with only ODE bearing) you cannot check insulation resistance of ODE bearing without removing NDE bearing. For horizontal sleeve bearing motor, that is fairly easy to do with minimal disassembly (support the rotor on jacks and roll one bearing out). But for vertical motor with rolling element lower bearing, it is pretty much impossible to check upper bearing insulation resistance without very substantial disassembly. So you end up with the disconcerting fact that you have insulated upper bearing but no way to check that it remains insulated. Insulating lower bearing would solve this.

So, insulating both bearings will cost more but give you a little extra capability to test your motor... if you as a customer think that's worthwhile, then you should specify it (or at least require it to be priced as a separate option so you can decide if it's worthwhile based on the quoted cost).


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[electricpete]

I should have mentioned that of course the machine must be uncoupled to check bearing insulation resistance no matter what.

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[electricpete]

In the interest of discussing all pro's/con's... an insulated bearing may be slightly less mechanically reliable than a non-insulated bearing. We had an insulating anti-rotation pin on sleeve bearing break, which allowed the bearing to move axially, which created problems when the oil ring hit the oil ring groove and almost stopped moving (we almost wiped bearing as result... but noticed it just in time). If it was a steel anti-rotation pin for non-insulated bearing, I'm sure that wouldn't have happened.

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[electricpete]

The following is an excerpt from the EPRI Large Motor Purchase Specification (EPRI document 1011892):

To prevent the flow of bearing-to-shaft circulating currents, bearings shall be insulated when either of the following applies:
(a) The motor rating is 1005 hp (750 kW) or larger
(b) It is deemed necessary by the Vendor
(c) The shaft voltage (shaft end to shaft end) as measured on test, exceeds 100 mV for rolling element bearings or 200 mV for sleeve bearings.
When insulation is provided, either both bearings shall be insulated with a removable ground connection provided at the drive end or a double-insulated insert shall be provided at the non-drive end. In either case, the intention is that the bearing insulation can be checked without dismantling the motor.

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[electricpete]

The double-insulated design they refer to would make it even easier to check the bearing insulation resistance without even uncoupling ... we have that on our large turbine generator... I have never heard of it on a motor.

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[edison123]

If it is a white metal sleeve bearings motor, one end (usually NDE) is insulated to prevent white metal pitting due to shaft currents (VFD or not). In motors with anti-friction bearings, insulated bearings is not a standard, as far as I know. I think the direct metal-to-metal contact in the rolling bearings is the reason the bearing currents in non-VFD machines is not such a big issue.

Muthu

http://www.edison.co.in

 

[ScottyUK]

I have seen many vertical motors on pump duty with an insulated NDE bearing. They are usually insulated with a ceramic material, and are very expensive relative to a standard bearing of the same dimensions. My understanding of the reason for fitting the insulated bearing was more-or-less in line with what ePete initially suggested.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Yes, our plant has at least 4 families of vertical rolling-bearing motors that have insulated top bearing.

The EPRI spec still requires it based on motor size and based on shaft voltage readings for smaller motors. (I think rated torque would be a better predictor of mootor physical size than horsepower, but I guess they used horsepower for simplicity). I think the voltage readings were taken from NEMA MG-1.

The reasons why electric current can be damaging for rolling bearings even though there is apparent metal-to-metal contact is not clear to me. Maybe it is the tiny tiny film (1 micron) that exists for elastohydrodynamic lubrication? Beats me. Maybe Gunnar or others will chime in on that.

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[rmw]

It is because that the bearings don't run with metal to metal contact (sleeve bearings). The shaft runs on a "wedge" of oil lifted off of the shaft to prevent metal to metal contact which would result in rapid wear of soft bearing materials.

With the shaft lifted, if there are stray or circulating currents in the shaft that want to find their way to ground and if the devices provided for that purpose don't do their job (what - brushes fail?) then when they find their way through the bearing the current arcs across the oil film and the damage to bearings is due to the arcing.

One insulated bearing prevents the currents from circulating by breaking the circuit. Then the grounding device(s) can take any currents generated on the rotor to ground without it having damage to the bearing(s).

Pete, would the shift in the motors in your plant that have insulated bearings and those that don't be due to the shift from roller bearings to sleeve bearings?

rmw

 

[electricpete]

We have insulated bearings on both roller and sleeve bearings (all that are 800hp and above, some between 500-800hp). The determining factor based on our plant would seem to be size of the motor. It seems remarkably consistent with the EPRI spec, although our plant was designed early 1980's... EPRI spec came out in the last few years.

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[Skogsgurra]

I usually get involved with variable speed drives and there the insulated bearings are a mixed blessing. I think that I have shown in some thread that a flash-over in the oil film leaves the outer ring at a new potential that is either positive or negative. The next PWM edge takes the rotor and inner ring to a new potential (opposite the earlier) and the potential difference between outer ring and inner ring gets higher than it would/could be if the bearing weren't insulated. The result is an EDM event with higher voltage involved, but lower capacitance (rotor's capacitance in series with bearing insulation capacitance.

In many machines, you find that both rotor-frame and bearing insulation capacitance is in the 5 - 10 nF order of magnitude. If both are equal, the resulting capacitance is half the uninsulated capacitance. And if outer ring takes on a potential equal to, say, 10 V while the rotor goes to -10 V when the flash-over occurs, you will then have twice the energy (voltage squared and capacitance halved) discharged. And, since energy dissipates in the steel, you get more damage in a bearing with insulated outer ring.

If the bearing doesn't have an insulated outer ring (type 'Insocoat') but if the bearing is insulated using other methods, the capacitance can be as low as 500 pF - 1 nF and that reduces damage so life will usually not be affected.

But, this thread is about DOL machines. I do not have much experience there and think that all what you said are valid points. Just one comment; the 100 mV/200 mV mentioned - is that an RMS value or a peak value?

If I haven't shown what happens in an insulated bearing, I will search for the measurements and put it here.

Gunnar Englund

http://www.gke.org

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