187 -189 Handcroft Road Croydon, England CR0 3LF Tel: 020 8684 8262

Insulated Bearings

When a stray current in an electric motor uses a bearing as its path to ground, bearing damage can occur. The most common causes of stray electric currents are: asymmetry in the motor’s magnetic circuit, unshielded power cables, and fast switching pulse width modulated (PWM) frequency converters used in modern variable frequency drives (VFDs). The increasing popularity of VFDs is directly linked to the increase in electric current related bearing damage.

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. 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). 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.

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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