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AEGIS® Bearing Protection - Why Shaft Grounding?

 

Examples of Shaft Voltage Readings

         

 

Shaft Voltage Reading High Peak to Peak

High Peak to Peak common mode voltage –
Typically 20 to 120 volts peak to peak. The waveform image shows the capacitive coupled common mode voltage on the shaft of the motor. The “six-step” wave form is the result of the 3 phases of pulses from the VFD. The timing of the pulse width modulation (PWM) pulses to the motor from the drive determines what the wave form looks like. Sometimes it will look like a square wave.

This six-step or square wave is what is seen when there is no bearing discharge and the peak to peak shaft voltage is at its maximum level. The voltage level may eventually overcome the dielectric in most non-isolated bearings and begin discharging.

High amplitude EDM discharge pattern

High amplitude EDM discharge pattern –
Typically EDM discharges can occur from 6 volts peak to 80 volts peak depending on the motor, the type of bearing, the age of the bearing, and other factors. The waveform image shows an increase in voltage on the shaft and then a sharp vertical line indicating a voltage discharge. This can occur thousands of times in a second, based on the carrier frequency of the drive. The sharp vertical discharge at the trailing edge of the voltage is an ultra-high frequency dv/dt with a typical "discharge frequency" of 1 to 125 MHz (based on testing results in many applications).

Low amplitude voltage discharge pattern

Low amplitude voltage discharge pattern –
Typically the peak to peak voltages are 4 to 15 volts peak to peak. The waveform image shows a more continuous discharge pattern with lower dv/dt frequencies between 30 KHz to 1 MHz. The lower voltage is due to greater current flow in the bearings which is the result of the bearing lubrication becoming conductive. As discharges occur in the bearings, the lubrication is contaminated with carbon and metal particles. The lower impedance to the shaft voltages results in lower peak to peak voltages. This condition is usually found in motors that have been in operation for many months or years.

Peak to Peak voltage with AEGIS ring installed

Peak to Peak voltage with AEGIS® ring installed –
With the AEGIS® ring installed, you will typically see discharge voltage peaks around 2 to 3 volts on a bare steel shaft surface. The voltage readings may be decreased with the application of AEGIS® Colloidal Silver Shaft Coating which allows for a more efficient electron transfer to the conductive micro fiber tips. The waveform image shows the low peak to peak waveform of a motor with the AEGIS® SGR ring installed and discharging the shaft voltages.


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