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What Is The Main Purpose Of A Shaft Grounding Ring?
In modern industry, most motors are driven by variable frequency drives (VFDs). VFDs regulate voltage and frequency through high-speed switching. However, this has a side effect: the PWM waveforms output by the VFDs contain high-frequency components, which generate an asymmetric electric field within the motor. This induction generates a voltage between the motor shaft and ground—this is called shaft voltage.
As shaft voltage accumulates over time, its value increases. Once this voltage exceeds the insulation breakdown voltage of the bearing oil film, it instantly breaks down the oil film. Once the oil film breaks down, the high voltage seeks a path to escape, generating a momentary pulsed current, known as shaft current.
This high-frequency, high-energy shaft current, when passing through the bearing, instantly generates tiny spark discharges at the contact point between the ball and raceway. Frequent occurrence of these discharges can produce an effect similar to electro-erosion, leading to the following common problems:
Pits and grooves: These "washboard"-like pits and grooves form on the surfaces of the bearing raceways and balls.
Grease degradation: High-temperature electrical sparks can cause premature grease aging and deterioration, leading to loss of lubricating properties.
Noise and vibration: Damage to the bearing surface can produce unusual noise and vibration during operation.
Bearing failure: This ultimately leads to premature bearing failure well before the end of the design life, causing motor downtime.
How do shaft grounding rings prevent bearing corrosion? First, we must understand that the protection principle of shaft grounding rings is to provide a "low-impedance preferential path." For example, Volsun shaft grounding rings and conductive brushes are primarily composed of conductive fibers and a metal body, offering lightweight, low-resistance, and physically stable current transmission. Volsun utilizes a cantilever design, with the flexible fiber brush tip fitting tightly against the shaft surface. This path, formed by the conductive fiber bundle and wire, has a resistance far lower than the insulation resistance of the bearing oil film. When shaft voltage begins to accumulate, the shaft grounding ring path, due to its extremely low impedance, preferentially dissipates the majority of the shaft current through it.
Simply put, this provides an easier, lower-impedance path for the shaft current to escape to ground before it breaks through the bearing oil film. This prevents shaft voltage from accumulating to a level sufficient to penetrate the bearing oil film, effectively dissipating the shaft current's path through the bearings and ultimately protecting the bearings from electrical corrosion.
In summary, the shaft grounding ring provides a low-impedance, preferential discharge path, directing harmful shaft voltage and current to ground, preventing them from penetrating the bearing oil film. This prevents sparks from corroding the bearing raceways and balls, effectively addressing electrical corrosion in motor bearings and significantly extending the life of both the motor and bearings.
Are you facing electrical corrosion issues with motor bearings? Contact us for a customized solution!
