Case Studies

Application Note
Problem description
Motor failures can be mysteri-
ous. Often, the mechanical
loads on the motors have not
changed and other loads
connected to the same service
appear to work normally — yet,
the motors just fail. The cause
of these mysterious failures is
frequently a power quality
deficiency. Consequently, the
motor rewind industry has
become involved in power
quality troubleshooting. In this
case history, the supervisor of
a motor rewind shop investi-
gated a rash of failures at a
small machine shop.
His investigation process
began with a series of ques-
tions to search for any pattern
that might be common to the
failed units, and to see what
had changed since the time
before the problem surfaced.
Machine shop
motor failures
From the Fluke Digital Library @ www.fluke.com/library
Often, the pattern is one of
age. In this case, there was no
age pattern — some of the failed
motors were nearly new. Others
were much older, and some of
these had been rewound at the
motor shop. But, there was a
pattern: all the failed units had
been carrying full mechanical
loads and the windings showed
evidence of overheating.
The machine shop itself
hadn’t undergone any changes.
But an insurance company had
moved into a new building next
door. And they had done so at
about the time when the motor
failures began. The same utility
transformer supplied both
buildings (see Fig. 1).
Measurements
The motor shop supervisor
suspected that non-linear loads
within the insurance building
were distorting the voltage
being supplied to the machine
shop. He connected his Fluke
43B phase-to-phase at the
machine shop’s main service
panel. The Fluke 43B showed
Machine
Shop
Insurance
Company
Utility
277/480
120/208
Fig. 1 Utility connections for the machine shop and insurance company
Operator: Motor rewind shop
supervisor
Measuring tools: Fluke 43B Power
Quality Analyzer
Features used: Voltage, current,
harmonic spectrum and THD
Power
Quality
Case
Study