Motors

Why Insulated bearings ?

Insulated bearings

To reply this question first we’ve to understand Shaft Current/Voltage which occurs in electric motors and generators due to leakage, induction, or capacitive coupling with the windings of the motor. It can occur in motors powered by variable-frequency drives, as often used in heating, ventilation, air conditioning and refrigeration systems. DC machines may have leakage current from the armature windings that energizes the shaft. Currents due to shaft voltage causes deterioration of motor bearings, but can be prevented with a grounding brush on the shaft, grounding of the motor frame, insulation of the bearing supports, or shielding.

Shaft voltage can be induced by non-symmetrical magnetic fields of the motor (or generator) itself. External sources of shaft voltage include other coupled machines, and electrostatic charging due to rubber belts rubbing on drive pulleys.

Every rotor has some degree of capacitive coupling to the motor’s electrical windings, but the effective inline capacitor acts as a high-pass filter, so the coupling is often weak at 50–60 Hz line frequency. But many Variable Frequency Drives (VFD) induce significant voltage onto the shaft of the driven motor, because of the kilohertz switching of the insulated gate bipolar transistors (IGBTs), which produce the pulse width modulation used to control the motor. The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.

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When an electric current passes through the bearing it can cause micro-cratering in the raceways of inner and outer rings and on the rolling element surfaces (fig.1). The heat, which is generated by the discharges, causes local melting that creates small craters and changes in the structure of the metal. As a result of this initial damage, a “washboard pattern” may be found on the raceways and rolling elements (for roller bearings) (fig.2). This secondary damage is wear caused by the dynamic effect of the rolling elements when they roll over the smaller craters. Current discharges also cause the lubricant in the bearing to change its composition, degrade rapidly and fail prematurely (fig.3). Once bearing damage from electric erosion has begun, increased noise levels, reduced effectiveness of the lubricant, increased heat and finally excessive vibration, all contribute to drastically decrease bearing service life.

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Insulated Bearings eliminate the path to ground through the bearing for current to flow. However, installing insulated bearings does not eliminate the shaft voltage, which will still find the lowest impedance path to ground. This can potentially cause a problem if the path happens to be through the driven load or through some other component.

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