What is Electrical Discharge through bearings?
Shaft and bearing currents have been described as shaft voltages, circulating voltages, circulating currents and bearing currents.
Shaft voltage only becomes a problem when it leads to bearing current and consequential damage to the motor bearings. If this voltage, referred to as “common mode voltage” or “shaft voltage”, builds up to a sufficient level, it can discharge to ground through the lubricant film on the bearings. Current that finds its way to ground through the motor bearings in this manner is called “bearing current”.
How does it damage our motors?
When an electric current passes through a rolling bearing, electrical discharges take place through the lubricant between the raceways of inner and outer ring and the rolling elements. The heat, which is generated by the discharge, causes local melting of the bearing metal surface. Craters are formed and particles of molten material are transferred and partly break loose.
This type of damage reduces the life of bearings that should last from 6 to 10 years to as little as 4 months.
How does it look?
Why does it happen?
The basic causes and sources for bearing currents are:
- Magnetic dissymmetry in the motor.
- Asymmetrical, non-shielded cabling (poor cabling).
- Electrostatic discharges.
- Capacitive coupling between stator and rotor when motor is supplied from a VFD.
- Generating a common mode voltage which raises the three phase winding neutral potential significantly above ground potential resulting in discharges from shaft to ground with the current path being through either or both bearings to ground.
Could it be identified with vibration analysis?
Yes it can!
A high resolution spectrum of the 2 - 4 kHz range can indicate an abnormally high band of energy that is a hill-like shape as the amplitude gradually increases to a rounded peak and then decreases. As the fault condition progresses over time , vibration levels at the bearing fault frequencies will become apparent.
In the Augury spectrum you will see the peaks clearly in the acceleration and envelope and then after at the velocity domain.
What could be done to reduce the effects?
- Shaft grounding brushes can reduce, but not eliminated, bearing currents due to magnetic dissymmetry and electrostatic discharges
- Insulating both bearings is a method to reduce effects of capacitive coupling (best to insulate inner race.
- Reducing the switching frequency of the VFD can result in reduced bearing currents.
- Insulating the housing is a method providing bearing insulation. The thicker the housing insulation the lower the capacitance.
Download a PDF of this article below
