Efficient Plant May 2018 - 32

motors

Pinpoint Funny
Motor-Ground
Currents
Howard W. Penrose, Ph.D., CMRP
aka 'Motor Doc'

Damage from harmful voltage impulses can manifest as fluting or frosting in
motor bearings due to shaft-current potential between a unit's frame and rotor
shaft. This type of damage was the cause of ground voltage and current in the
split bearings shown here.

UNUSUAL BEARING OR WINDING FAILURES in across-theline motors, improper operation of variable-frequency-drives (VFDs),
control issues, and other unusual electrical, electronic, and mechanical
issues can be the result of modern technology. Add in special grounding
cases such as HRG or LRG, or ungrounded or floating systems, and
things really get interesting.
With increased application of electronics, parasitic voltage and current in the ground system can be an issue. Yet, when investigating faults,
how often do we check ground current? And what value are we seeking?

Howard Penrose, Ph.D., CMRP, is founder and president of MotorDoc
LLC, Lombard, IL (motordoc.com). Among other things, he also is current chair
of the Society for Reliability and Maintenance Professionals, Atlanta (smrp.org).
Contact him directly at howard@motordoc.com.

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Unfortunately, there are no standards in place for ground current-
just general guidelines based upon expected leakage current
through motor-system insulation. The guidelines are used by some
ground-fault-circuit-interruption-solution vendors to set trip values.
What you should expect to see are values below kVA/1,000 A of the
circuit. Example: In a 480-V system with a peak operating current of
500 A, the leakage current should remain below 0.24 A (240 mA).
Some causes for having higher ground current include:
unshielded cables with unbalanced voltage
other electronic 'noisy' ground systems
leakage due to contamination
VFDs with unshielded cables
electro-magnetic induction
unbalanced voltage
ground faults
ground loops
soft starts.
Winding faults can occur in these instances because, along with
ground currents, there will be a corresponding voltage potential on the
motor's frame. When voltage also contains impulses, the voltage-toground reference across the insulation system, regardless of how clean
the sine wave is to the windings, will also see those impulses. Such
impulses act as if they are part of the motor voltage and can harm the
insulation system. You may also find fluting or frosting on the inner
and outer races of across-the-line-motor bearings due to shaft-current
potential. This unexpected finding was determined to be the cause of
common-mode ground voltage and current in the split bearings shown
in the photo.
You can check for such conditions by measuring the motor ground
using an ammeter. Always investigate conditions in which the current
exceeds 1 A. An oscilloscope can be used to evaluate voltage across
the motor, including identification of any high-frequency noise or
fast-rise-time impulses. Once the condition is identified, investigate to
ensure that:
shielded cables are grounded at both ends
grounds are part of a solid cable or located away from:
+ individual phases
+ ground loops
+ or other conditions, as identified above.
Overall, ground conditions definitely can cause unusual reliability problems in motors and electronic systems. Although not often
included in failure investigations, identification of such conditions
is relatively straightforward. Chasing down the cause can be more
challenging. EP

MAY 2018

http://www.motordoc.com http://www.smrp.org http://www.EFFICIENTPLANTMAG.COM

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