For large electrical motors designed for VFD operation we often install insulated bearings in order to avoid bearings being destructed by currents circulating through the shaft and body of the motor.

We often assume that these currents are caused by harmonics in the supply current, but what can be the mechanism behind creating these currents?

Is it simply some noise, or do the harmonics cause some parasitic Homo-polar or Uni-polar generation effects due to geometric differences in the magnetic circuit at the two ends of the machine, or are there other explanations?

What you are experiencing is Torsional Oscillation of the Motor Shafts. this this due is due to an harmonic rich system. You need to install devices to reduce system harmonics. another method it to alter your motor connections at the peckerhead from Star to Delta. this will significantly reduce Torsional Oscillation of Motor Shafts. Installing insulated bearing will not solve this unique problem.I hope this helps

The variable frequency drive of the motor is the main source of harmonics. Harmonics are generated when the VFD rectify the AC power of the feeder to the motor and then invert the DC power again to variable AC. There are some mitigating procedures to reduce the harmonic effects and also mitigating procedures to reduce the problem of harmonic currents getting into circulation to the motor bearings.

Magnetic dis-symmetry in the air gap results in a voltage being developed along the shaft. This in turn leads to a shaft current that tends to flow through the bearing and on to ground. Insulated bearings prevent the damage to the ball bearing to an extent. If one tends to measure the voltage across the shaft by connecting a voltmeter across the ends of the shaft, what one gets to measure is the stator flux induced voltage!

Practically, if you change the tires of your motor vehicle and the rims and alignment are defective and poor respectively, then the life expectancy the new tires are about 10%. So, it is important to address the circulating currents via neutral and grounding conductors.

Remember that your drive is generating harmonics and also the motor. together they add in the neutral and return to the machine via the ground or neutral conductor creating heat and vibration.

Using Delta configuration will trap and circulate the harmonics in the device which created it, hence Delta supply Delta connected load. after all of this then we move on to coupling components or look everything as a whole for corrective action.

Now on the matter of grounding the machine, resistance grounding may be considered or a zig-zag trans could be used.

Sorry I do not understand those comments related to delta or wye connection or even those related to ground or not to ground and....torsional problems???

Bearing current and shaft voltages can be present not only under pwm operation...they can be present under 60 hz sinewave operation and this effect has been known since 1924.

During 60 hz sinewave operation bearing current problem is related to electromagnetically induced problems.

Under pwm operation the problem is predominantly an electrostatic phenomenon

For a detailed explanation u can check at:

Effect of pwm inverters on AC motor bearing currents and shaft voltages by Jay Erdman, IEEE transactions APEC conference, Dallas Texas March 1995.

System electrical parameters and their effects on bearing currents by Doyle BusseL IEEE transactions APEC conference, san jose california, march 1996.

Regards