AEGIS® Shaft Voltage Tester™ Digital Oscilloscope


Digital Oscilloscope and Multimeter with Conductive Microfiber Probe Tips

Is your motor safe from destructive shaft voltages? For the first time you can get all of the tools you need to find out in one place. The AEGIS® Shaft Voltage Tester™ Digital Oscilloscope bundle includes the AEGIS-OSC-9100 100 MHz digital osciloscope, shaft voltage probe tips, and a shaft grounding simulator. Armed with these tools, you can quickly and easily find whether your motors are at risk of premature bearing failure due to VFD-produced bearing currents.


Electrical Bearing Damage: Capacitive coupling between windings and rotor can create voltage on a motor's shaft — voltage that can discharge through bearings, damaging them and shortening motor life. For motors controlled by variable frequency drives, however, these voltages can be as high as 10-40 volt peak. At these higher levels, voltages can easily cause electric electrical discharge in the motor bearings, causing pitting, fusion craters, and fluting, which eventually lead to premature bearing and motor failure.

Until now, the only way to tell whether a motor was at risk for such bearing damage was to buy, rent, or borrow an oscilloscope and then buy some type of shaft voltage probe. Now, for the first time, there is a total shaft voltage detection system available in one package.

With the AEGIS® Shaft Voltage Tester™ Digital Oscilloscope, plant maintenance personnel can take shaft voltage readings from motor shafts quickly and easily – readings that confirm or deny the presence of shaft voltage discharges that can damage motor bearings.  Users who already have a 10X oscilloscope with bandwidth at least 100 MHz may be able to measure shaft voltage with it and an appropriate set of our Shaft Voltage Probe™ Tips

Note: For motors powered off the line by sine wave AC power, these voltages are typically 100-200 millivolts at 60/50 Hz and will not cause problems with electrical bearing discharges.

The following equipment is required to test for damaging shaft currents:

Recommended Testing Equipment

AEGIS SVP kit with Fluke Meter
  • Oscilloscope with a 10:1 probe. We recommend a minimum bandwidth of 100 MHz to accurately measure the shaft voltage waveform.
  • *Recommended: AEGIS SVP Shaft Voltage Tester with Shaft Voltage Probe Tips, Magnetic Base, and Shaft Grounding Simulator PN: AEGIS-OSC-9100MB-W2

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 pulses to the motor determines how the shaft voltage waveform appears. 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 microfiber 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.