Case Studies

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Lastly, to cover his bases, he
selected Harmonics from the
menu to make sure frequencies
weren’t contributing to the
problem (Fig. 7 & 8).
Analysis
From his unbalance measure-
ments, he could see that an
unbalance was causing an
excessively large phase current
value. He checked the motor
specifications and confirmed
that phase current exceeded
the motor’s FLA (Full Load
Amperage) rating. Looking at
the data for all three phases,
he traced the current unbalance
to excessive loading on one
voltage phase.
The consultant traced the
voltage unbalance to a set of
equipment installed three years
ago. It turned out that all of the
internal single-phase loads
were connected to the same
phase. That last new equipment
installation caused such a
significant power system volt-
age unbalance that it created a
current unbalance at the motor,
increasing the operating
temperature of the conductors
and motor windings to beyond
the limits.
Conclusion
To resolve the situation, the
consultant balanced the inter-
nal single-phase loads between
the three phases, reducing
the overall voltage unbalance
and consequently, the current
unbalance at the motor. This
also reduced the elevated
phase current value and operat-
ing temperature at the motor.
He took new benchmark
measurements for future moni-
toring, transferred all of his
saved screens to his computer
and printed a before-and-after
report for the facility mainte-
nance manager.
While not a power quality
expert, the manager could see
the difference between the
screens. Now he understood
why power quality measure-
ments needed to be taken
before and after new equip-
ment installation. All of the
motor repair and downtime
costs had been un-necessary.
When the consultant suggested
setting up a regular preventive
maintenance schedule, the
manager agreed.
Switching to the Unbalance
screen, he checked the voltage
and current values as well as
the phase diagram, to see if the
motor was operating within
acceptable limits (Fig. 4-6).
2 Fluke Corporation Power Quality Case Study: Three phase motor failure
Fig. 4 Unbalanced phaser amp Fig. 7 Harmonics
Fig. 6 Unbalanced table
Fig. 5 Unbalanced phaser volt
Fig. 8 Harmonics table