Product Manual

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Shaft Grounding Devices
Shaft grounding is recommended (NEMA MG1 31.4.4.3) as an effective
means of bearing protection for motors operated from inverter power.
Shaft voltage occurs in motors powered by variable frequency inverters
(VFD) These VFDs induce shaft voltages onto the shaft of the driven motor
because of the extremely high speed switching of the insulated gate
bipolar transistors (IGBTs) which produce the pulse width modulation
used to control AC motors. The presence of high frequency ground
currents can cause sparks, arcing and electrical shocks and can damage
bearings. One grounding device is adequate to bleed down inverter-
sourced shaft voltages, thereby protecting both bearings for motors as
large as 6085 frame.
There are four common techniques that can minimize or eliminate this
bearing damage caused by these ground currents” Faraday shield,
insulated bearings or ceramic bearings, a ground brush or a grounding
ring. Shielding the cable or wire between the motor and the VFD can
also significantly improve these spikes as well.
Faraday Shield
An electrostatic shielded induction motor (ESIM) is one approach to the
shaft-voltage problem, as the insulation reduces voltage levels below
the dielectric breakdown. This effectively stops bearing degradation
and offers one solution to accelerated bearing wear caused by fluting,
induced by VFDs.
Grounding Brush
Grounding the shaft by installing a grounding device provides an
alternate low-impedance path from the motor shaft to the motor case.
This channels the current away from the bearings. It significantly reduces
shaft voltage, and therefore bearing current, by no allowing voltage to
build up on the rotor.
Shaft Grounding Ring
A shaft grounding ring (SGR) is similar to a grounding brush, except
that this brush makes use of conductive micro fibers, creating a low
impedance path from the motor.