Insulated Bearings

Dear all,

Need your insight on a matter related to Non-Driven End and Driven End insulation of bearings. In most cases, various motor manufacturers recommend that NDE side should be insulated whereas DE side should not.

Logically, I see that whether DE or NDE insulation is there on the bearings, it serves the purpose. Moreover, we do not need insulated bearings in non-VFD driven motors. What do you say?

My specific inquiry is that why focus is given on using insulated bearings on NDE side only. Why not DE? Moreover, If I do have grounding brush on my shaft anywhere, does that play a part in this?

Regards,
Mutahir
 
MRasool...
At least in the USA, there is no NEMA standard, yet. It does however, recommend the phenomenon be investigated! And, you will find that motor mfgrs vary so there is no general agreement as to which end is to be isolated !
Regards, Phil Corso
 
Thanks Phil. This remains a mystery still.
One expert told me that since DE is near the pump or gearbox, due to this close proximity DE bearings are insulated to prevent flow of circulating currents towards the driven equipment.
One expert told me that since NDE is on the other side and easier to replace the bearing there that is why NDE is usually Insulated.
Opposite opinions.
 
Shaft current can be caused by dissymmetry in the magnetic circuit or by an uneven air gap.

Usually, the PWM of the inverter switches in a pattern ..6 times per rotation of the stator magnetic field, ..(6 switching states)...usually.... you have other systems and this will be higher exp on a power stage that has more than 6 power devices.

This switching is one of the sources of common-mode noise/voltage....square waves have loads of odd harmonics

In a nutshell, inverters are as noisy as hell loads of harmonics.

There is loads of material on bearing currents ...I think you judge the size of the problem.....

I thought about your question and thought I should know this..so instead of a long drawn out reply, see the image.

But.....

Harmonic voltage is generated between the AC motor side and DC neutral points of the drive's DC bus.
Motor and inverter housings should be grounded, for obvious reasons, this creates paths to the ground.
In inverters, a capacitive link is found between the IGBT wafer and heat sink plate.
The frame grounding makes a closed loop with the capacitive couplings.
This capacitive link to earth is the source of noise and current ....that potentially flows through the bearing.

If earthing on the load side( motor to pump ...to whatever ) is different then current will flow there as well...but it should be made the same.........but this is not the case from motor NDE to inverter...there is another path....IGBT wafer and heat sink plate.

.... fundamentally the harmonics on the inverter side create an easy path to earth through a capacitor. The common-mode frequency is somewhere in the region of 1∼30 Mhz, ...basically high, so it will go through the capacitance with ease.......Xc = 1/2Pi f C...sorry I had to throw it in.

So on the drive end, something is being dealt with...same earth( does not necessarily mean no current !! )...but on the NDE potentially nothing is being dealt with( if no isolating bearing )

If the whole system was floating then you would get no current flow to earth...but that is not allowed...unless you like getting riveted...or you work in the Philipines where there are some strange ideas on earthing and neutral points as I found when I was commissioning a production line...no neutral..just random earth points all over the show acting as localized neutrals!
 

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MRasool...
At least in the USA, there is no NEMA standard, yet. It does however, recommend the phenomenon be investigated! And, you will find that motor mfgrs vary so there is no general agreement as to which end is to be isolated !
Regards, Phil Corso
I think there is a Federal Communications Commission in the U.S that regulates common-mode noise levels...I'm not familiar with it....but there is always someone regulating something.
 
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