Insulated bearing
Insulated bearing
(OP)
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
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





RE: Insulated bearing
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
RE: Insulated bearing
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RE: Insulated bearing
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Insulated bearing
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
RE: Insulated bearing
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RE: Insulated bearing
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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Muthu
www.edison.co.in
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If we learn from our mistakes I'm getting a great education!
RE: Insulated 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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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
RE: Insulated bearing
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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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...