Case Studies

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Fluke Corporation
PO Box 9090, Everett, WA USA 98206
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PO Box 1186, 5602 BD
Eindhoven, The Netherlands
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©2003 Fluke Corporation. All rights reserved.
Printed in U.S.A. 8/2003 2103564 A-ENG-N Rev A
produced by the 5th harmonic
subtracts from the torque
generated by the fundamental.
This causes the motor to draw
more fundamental current to
offset the negative torque. The
additional current produces
additional internal heating.
In the case of the machine
shop, non-linear loads in the
neighboring insurance building
caused 5th harmonic distortion
on the supply voltage. Computer
and office machine loads
connected to the 120 V recepta-
cles were generating 3rd, 5th,
7th, 9th, and 11th harmonic
currents. When these currents
reached the 120/208 V trans-
former, the balanced portion
of the zero sequence harmonics
(e.g., 3rd, 9th) were trapped in
the delta primary but the 5th,
7th, and 11th passed through.
These reached the secondary
of the utility transformer, where
they caused the voltage distor-
tion measured at the machine
shop service.
the voltage waveform was flat-
topped, with a total harmonic
distortion (THD) of 7.8 %. It’s
significant to note that the 5th
harmonic was the dominant
bar on the harmonics display —
a fact key to the analysis of the
problem.
The supervisor made some
additional measurements on
motors operating in the
machine shop. He compared
the current drawn by fully
loaded motors to the full load
amps (FLA) value shown on
the nameplates. The measured
values were, by a small
amount, consistently higher
than the nameplate values.
Theory and analysis
The 5th harmonic is “negative
sequence” when compared to
the fundamental. That is,
5th harmonic current flowing
in a 3-phase induction motor
will produce a magnetic field
that tries to run the motor back-
wards. In effect, the torque
Most standard induction
motors require supply voltage
distortion (THD) to be 5 % or
less, if the motor carries a full
mechanical load. This means
harmonic distortion on the
utility voltage must be no more
than 5 %, if the end-user facil-
ity has fully loaded induction
motors.
Solution
The utility, when notified about
the voltage distortion problem,
agreed to install a separate
transformer serving only the
machine shop. The Fluke 43B
showed voltage distortion on
the new transformer was only
3.3 % — less than half of the
previous value.
2 Fluke Corporation Power Quality Case Study: Machine shop motor failures