Case Studies

Application Note
For three years in a row, a
particularly large three-phase
motor would fail twice a year.
The facility maintenance
manager called in both the
electrical contractor and the
motor manufacturer, who
pointed fingers at each other
but failed to resolve anything
on site. The facility was left in
the middle, with no corrective
action, cyclical motor repair
costs, and lost production from
the repeated downtimes.
Tired of what was clearly a
pattern failure, the facility
manager hired an independent
consultant. The consultant told
the facility manager he would
perform a complete power
quality survey of the electrical
distribution system feeding the
motor, determine its operating
characteristics and work from
there to solve the problem.
Measurements
The consultant connected his
Fluke 434 three-phase hand-
held power quality analyzer to
the circuit supplying energy to
the motor and pushed the View
Config button. The diagram
confirmed that his connections
were proper and that the power
type was three-phase Delta
(Fig. 1).
Three phase
motor failure
From the Fluke Digital Library @ www.fluke.com/library
From there, he pushed the
Scope button and looked at
waveforms and numeric values
for all three phases. Judging by
the differences between the
magnitudes at the top of the
screen, there appeared to be a
balance problem between the
phases (Fig. 2).
To gather more information,
he switched to the numeric
readout on the Voltz/Amps/
Hertz screen. The current read-
ing there was even higher and
still unbalanced (Fig. 3).
Measuring tools: Fluke 434 Power
Quality Analyzer
Features: Scope, Unbalance,
Harmonics
Power
Quality
Case
Study
Fig. 1 Three phase Delta
Fig. 2 Unbalance voltage wave
Fig. 3 Unbalance voltage wave