standing voltage wave phenomenon at
the motor terminals, which can lead to a
doubling effect. The drive manufacturer
will normally specify the maximum cable
distance that is allowed before other corrections are needed.
Similar to having a filter between the
brain and the mouth – thereby keeping
your foot out of your mouth and you out
of embarrassing situations – installing line
filters prior to the VFD and load reactors
after the VFD can successfully reduce the
voltage transients emitted from VFDs, and
thus also reduce the deterioration effects.
However, as in the case with insulation,
these filters will only reduce and cannot
eliminate the issue.
Therefore, the industry has come up
with several additional solutions located
at the motor itself, which have varying
degrees of success. Table 1 summarizes
six different techniques that have varying
costs and viability.
As the table illustrates, shaft ground-
ing rings (SGRs) seem to offer the most
reliable solution to the issue. Some
manufacturers’ SGRs, like those of AEGIS,
are engineered with special conductive
microfibers that safely discharge VFD-
induced shaft voltages by providing a very
low-impedance path from shaft to frame,
bypassing the motor’s bearings entirely.
AEGIS’s SGR bearing protection ring
uses the principles of ionization to boost
the electron transfer rate and promote
extremely efficient discharge of the high-
frequency shaft voltages induced by VFDs.
It is also common to combine SGRs with
other methods such as insulated bearings
for the belt-and-suspenders approach.
Author’s Note: The author would like to thank
Allan Stevens, PE, associate partner with Syska
Hennessy Group, for keeping him honest and
providing a ‘sparky’s’ viewpoint on wandering
electrons and VFDs.
Based in Madison, Wis.,
Steve Tredinnick, PE,
is vice president of energy
services for Syska Hennessy
Group, which has more than
16 locations across the U.S.
He has more than 27 years’
experience related to building
heating, ventilation and air-conditioning systems.
The past 16 years of his work have been focused
on district energy systems. Tredinnick is a graduate of Pennsylvania State University with a degree
in architectural engineering. He is a member of
IDEA and ASHRAE and is currently immediate past
chair and current handbook chair of ASHRAE TC
6. 2 District Energy and current chair of the Chilled
Water Plant Subcommittee of ASHRAE TC 6. 1
Steam and Hydronic Systems. He may be reached
Table 1. Voltage Transient Reduction Techniques.
Faraday shield between stator and rotor
Difficult to implement and very expensive
Shaft currents will now seek other paths
(pump or fan bearing); also expensive to
implement and must be replaced eventually
Nonconductive ceramic ball bearings are used and protected Shaft currents will seek other paths (pump
or fan bearing); must be special-ordered,
have long lead times and require maintenance
The conductive particles will increase mechanical
wear of bearing, requiring more maintenance
Will wear out and require regular maintenance
for cleaning, can corrode and lose effectiveness
over time, and need to be monitored and replaced
Effectively blocks induced current with a capacitive barrier
Motor bearings are protected with added layer or coating
of nonconductive material
Shaft grounding brushes
Shaft grounding rings (SGRs)
Conductive particles within the grease provide a
lower-impedance path through the bearings
Carbon or metal brushes contact the motor shaft and
the motor’s grounded frame, providing an alternate
current pathway to ground – resulting in a more practical
and economical solution
Similar to grounding brushes, the carbon microfiber
brushes redirect current from shaft to motor frame
and bypass bearings; SGRs are maintenance-free
Add moderate costs to the pump or fan but
are extremely effective
Source: Compiled by Steve Tredinnick, Syska Hennessy Group, from various sources including Shaft Grounding – A Solution to Motor Bearing Currents, H. William Oh and Adam
Willwerth, ASHRAE (SL-08-025).