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Verticle Pump Motor Shaft Grounding

Vertical Pump Motor Shaft Grounding:
Hollow and Solid Shaft Motors

Bearing Currents from VFDs: The extremely high on/off switching speeds of the pulse width modulation (PWM), generated by the insulated gate bipolar transistors (IGBT), induce damaging voltages onto the motor shaft through parasitic capacitive coupling between the stator and rotor.


Electrical Discharge Machining (EDM) occurs when currents arc through the dielectric oil film between the rolling elements and the bearing race. This causes fusion craters, severe pitting, and eventually bearing fluting (a washboard-like pattern in the bearing race) which results in premature bearing failure.

High Frequency Circulating Currents in Large AC and DC Motors: In addition to potential bearing failures in motors from VFD induced EDM currents, AC and DC motors above 100 hp (75 kW) may also experience bearing failures caused by high frequency circulating currents. VFD induced high frequency circulating currents are in the kilohertz or even megahertz range and circulate through the motor’s bearings because of magnetic flux imbalances in the stator. This type of VFD induced current becomes the more dominant destructive current in higher hp/kW motors.


Improving the conductivity of the steel shaft surface enhances the shaft voltage discharge capability in AEGIS® shaft grounding applications. Maintaining a highly conductive shaft surface is especially important in critical applications or in applications where the conductive shaft surface of steel could become compromised. Environmental elements could create a potential for decreased conductivity on the shaft of the motor.

Apply AEGIS® CS015 Colloidal Silver Shaft Coating to any VFD driven motor shaft prior to installing AEGIS® Bearing Protection Ring.

VFD Installed on 25 HP motor
25 hp motor -
AEGIS® installed on drive end

Bearing Current from VFDs
Vertical Hollow Motor

Motors up to 100 HP (75 kW)

Motors up to 100HP

Any motor controlled by a variable frequency drive (VFD) requires bearing protection. Motors of 100 hp down to fractional hp motors may experience bearing failures when operated on a PWM drive.

  • Motor frame must be well grounded.
  • Install one AEGIS® SGR Bearing Protection Ring on either the drive end or the non-drive end of the motor. The standard SGR is simply installed with 2 or 3 mounting clips fastened to the motor end bell. Other mounting options include the bolt through and split ring designs. A NEMA/IEC kit may be needed if there is a shaft shoulder at the end bell.
  • * Coat the shaft with Colloidal Silver Shaft Coating to improve the conductivity of the shaft. Part number CS015.

Motors 100 HP to 1000 HP (75 kW to 750 kW)

Motors over 100 HP

Large motors above 100 hp may have VFD induced EDM currents as well as high frequency circulating currents when they are controlled by VFDs. To protect the bearings, insulate the bearing on one end and install an AEGIS® SGR on the other end.

  • Insulation on one end (usually NDE) and AEGIS® SGR on opposite end.
    Motor frame must be well grounded
  • Non-Drive End: Bearing journal should be insulated or Insulated/Ceramic Bearing installed to disrupt circulating currents.
  • Install AEGIS® SGR Bearing Protection Ring on opposite end of insulation
    and Insulated/Ceramic Bearing (usually DE).
  • * Coat the shaft with Colloidal Silver Shaft Coating to improve the conductivity of the shaft. Part number CS015.

© 2010 Electro Static Technology, an ITW Company


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