Medium voltage motor specification

[nullsig]

My company has a very old medium voltage (4160V) motor specification that I am reviewing/updating. None of the people who created it are still with the company.

There are two requirements that I have been looking into:

• Windings shall withstand a minimum of 5000 full voltage starts.

• Outboard end bearing shall be electrically insulated from the motor frame to eliminate shaft currents. Shaft current potential not to exceed 200mv during full load operation.

I'm having difficulty finding a source/justification for these numbers. Does anyone know of any design standard which would recommend those values? Is there an existing standard or recommendation for how many full voltage starts a medium voltage motor should withstand? I agree that motor ODE bearings should be insulated, but what about the 200mV shaft current potential? I'm thinking about eliminating these requirements but would like to see what others think.

 

[ron]

I've been seeing specs requiring the grounding rings or bearings to be grounded to the shaft via brushes since the motor is likely to be powered from a VFD, and shaft voltages occur

http://ecmweb.com/motors/diagnosing-understanding-motor-bearing-currents

 

[Jraef]

I've never seen a minimum number of starts specified before, interesting approach. Most likely this spec is from some specific mfr who put what we call "spec hooks" into the wording in order to drive users into their specific motor. Unfortunately without knowing who's spec that is, it might be something that gets you no bids, or exceptions. If not, and I had to guess, I'd say it might have been an indirect way to make sure they use phase paper in the slots, and that they dip the windings. Most will do that on an MV motor anyway, but maybe they just intended to eliminate the bottom feeders.

As to the 200mV, that seems arbitrary too, in fact that would be considered high if this were an inverter duty motor. Are you going to run this from an MV VFD? If so, consider grounding the shaft.

 

[nullsig]

I've never seen a minimum number of starts specified before, interesting approach. Most likely this spec is from some specific mfr who put what we call "spec hooks" into the wording in order to drive users into their specific motor. Unfortunately without knowing who's spec that is, it might be something that gets you no bids, or exceptions. If not, and I had to guess, I'd say it might have been an indirect way to make sure they use phase paper in the slots, and that they dip the windings. Most will do that on an MV motor anyway, but maybe they just intended to eliminate the bottom feeders.

As to the 200mV, that seems arbitrary too, in fact that would be considered high if this were an inverter duty motor. Are you going to run this from an MV VFD? If so, consider grounding the shaft.

No, this is for across-the-line motors. From the manufacturers and repair shops I have spoke with an insulated ODE bearing is common practice on medium voltage across-the-line motors. Most of ours have it. I guess motors at that voltage level are more susceptible to induced shaft voltages? I'm not sure.

I've continued to dig and actually just found this EASA white paper on bearing fluting:

If the shaft to frame voltage exceeds 100 millivolts AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough to degrade the bearings.

Could be where they grabbed the 200 mV from... still, this doesn't seem to be a heavily documented practice for manufacturing specs (that I can find).

 

[Besoeker]

My company has a very old medium voltage (4160V) motor specification that I am reviewing/updating. None of the people who created it are still with the company.

There are two requirements that I have been looking into:

I'm having difficulty finding a source/justification for these numbers. Does anyone know of any design standard which would recommend those values? Is there an existing standard or recommendation for how many full voltage starts a medium voltage motor should withstand? I agree that motor ODE bearings should be insulated, but what about the 200mV shaft current potential? I'm thinking about eliminating these requirements but would like to see what others think.

Do you what the power rating is and what application it's being used on?

 

[Besoeker]

No, this is for across-the-line motors. From the manufacturers and repair shops I have spoke with an insulated ODE bearing is common practice on medium voltage across-the-line motors. Most of ours have it. I guess motors at that voltage level are more susceptible to induced shaft voltages? I'm not sure.

I've continued to dig and actually just found this EASA white paper on bearing fluting:


Could be where they grabbed the 200 mV from... still, this doesn't seem to be a heavily documented practice for manufacturing specs (that I can find).

I've read mentions of bearing fluting in relation to PWM drives in a Gambica report. The paper you cited also mentions variable speed drives. I've dealt with drives most of my working life and never come across this phenomenon.
Shaft voltage may damage the bearings and/or the bearing lubricant resulting in failure.

 

[nullsig]

I've read mentions of bearing fluting in relation to PWM drives in a Gambica report. The paper you cited also mentions variable speed drives. I've dealt with drives most of my working life and never come across this phenomenon.
Shaft voltage may damage the bearings and/or the bearing lubricant resulting in failure.

Did a quick Google, there's some good explanations here on why manufacturers might supply an insulated ODE bearing for non-VFD applications.

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.

VFD-induced bearing wear is just a fact of life here, as it is in most industrial facilities. Older VFDs don't seem to be as problematic due to the lower switching frequencies. Short cable lengths, appropriately rated cable, and load filters/reactors can all mitigate the effects.

Do you what the power rating is and what application it's being used on?

This is a generic specification for 4160V motors.

 

[Besoeker]

Did a quick Google, there's some good explanations here on why manufacturers might supply an insulated ODE bearing for non-VFD applications.

Appreciated, thank you.

VFD-induced bearing wear is just a fact of life here, as it is in most industrial facilities. Older VFDs don't seem to be as problematic due to the lower switching frequencies. Short cable lengths, appropriately rated cable, and load filters/reactors can all mitigate the effects.

Also, older PWM inverters had somewhat slower IGBTs so the switching dv/dt was lower.

Our UK cable installation practices sometimes differ from that of USA. Power cabling is almost always steel wire armoured - SWAPVC or XLPE with the armour earthed (grounded). EMT (conduit) is rarely used for power. PVC conduit is sometimes used for lighting circuits.

 

[Bugman1400]

Does the spec mention that a grounding brush shall be included? It should be. Back in the old days before the idea of shaft currents were accepted, 4160 motors did not typically have shaft brushes. We would make our own homemade shaft brush and ground it to the case. Perhaps the spec requirement for shaft voltage to be below 200mV is a roundabout way of requiring the mfr to furnish a shaft brush.

 

[Tony S]

Insulated bearings for higher power ratings is quite normal.

I fell foul of this when the bearings on two 2500HP mill motors fail in a month. The motors had pedestal bearings mounted on mica insulators at both ends. The NDE bearing would have a deliberate earth/ground strap, the DE bearing remaining insulated.
After the first failure we tested the bearing insulation weekly @500V DC everything seemed fine. With the 2^nd bearing failure we were under the spotlight.

The cause was laughably simple. A new plant operator was given the motor platforms as his cleaning area. A handy place to leave his pry-bar was leaning against the motor DE bearing. The platform vibration wore the paint away and there was our short circuit.

 

[AZElectrical]

I agree with Tony S. regarding the use of insulated bearings. My company is currently looking to install one on a 5500hp synchronous motor due to some modifications to the exciter and the discovery of bearing wear due to circulating current in the shaft.

Regarding the number of starts for the winding to withstand -- IEEE 1665 is a standard for re-winding synchronous generators rated 1MVA and above. Appendix G lists a typical value for baseload units as 3,000 lifetime total starts and 10,000 for frequently cycled units. Note these numbers are not explicitly directed at the stator winding. NEMA MG1 may have some language on number of lifetime starts, but my recollection is that the standard deals with number of consecutive starts.

 

[nullsig]

Does the spec mention that a grounding brush shall be included? It should be. Back in the old days before the idea of shaft currents were accepted, 4160 motors did not typically have shaft brushes. We would make our own homemade shaft brush and ground it to the case. Perhaps the spec requirement for shaft voltage to be below 200mV is a roundabout way of requiring the mfr to furnish a shaft brush.

Unfortunately grounding brushes are not rated for hazardous areas due to the sparking potential. However, Inpro/Seal recently released a combination bearing seal and grounding ring which carries an explosion proof rating... I'm hoping that will solve all my bearing current problems.