Mimco Equipment LLC

Shaft grounding devices

Shaft grounding devices minimize shaft voltage and prevent current damage. Research has shown shaft
voltage can happen due to leakage, induction, or frictional buildup in other parts of an
electrical system. Shaft groundings stop the energy from causing harm to vulnerable
motor parts.

Motors that are operated with variable frequency drive (VFD) are more susceptible to
bearing failure. In standard generated sine waves, the voltage is balanced; this means
that the amplitudes of the wave form on both the positive and negative sides are equal.
In PWM (pulse wave modulated) generated sine waves, produced by VFD’s, the
voltages are not balanced. Because VFD output power is not balanced (i.e., the voltage
of the three phases does not sum to zero), VFDs create a capacitive voltage on the
shafts of the motors they control.

These shaft voltages will try to discharge to ground by
the lowest-resistance path available to them. Usually, the path of least resistance lies
through the bearings, so that’s where discharge will occur, in the form of an electrical
arc. These arcs create pits in the bearing surface. Although these pits are individually
tiny, arcing can occur thousands of times per second, and each arc creates a new pit.
The damage due to pitting is cumulative and can cause bearing failure in as little as a
few months.

Shaft grounding ring

The shaft grounding ring is placed near either end of the shaft. The carbon fiber brushes
attached to the ring contact the shaft. The brushes now become the easiest path to
ground for current running through the shaft. The current no longer passes through the
bearings, which eliminates bearing failure caused by the unbalanced PWM wave.
Our engineers recommend that motors being operated with a VFD be tested for shaft
voltage amplitude and frequency. AEGIS Shaft Grounding Rings are highly resistant to
contamination and other detrimental environmental factors. The  AEGIS mounts the rings
interiorly which reduces these detrimental influences and is an especially important
factor when used in severe environments.

The motor should be installed in its running condition, and should be tested through the
entire RPM/frequency range and load. This will help determine the best corrective
action to implement if excessive shaft current is present. Larger motors (generally 400
frame size and up: 404/405T) with excessive shaft current may need the additional
protection from an insulated bearing, or installing a shaft grounding ring at each bearing.
Shaft grounding rings are a small cost in comparison to the initial purchase price of the
motor and VFD. And this cost is drastically less when downtime and replacement costs
are considered. Anytime a motor is being operated with a VFD, consideration in
purchasing a shaft grounding ring and other protection is highly recommended.
If you have any questions about shaft grounding rings contact Joseph Melloni at jam@mimco.com