Efficient Plant October 2017 - 59
feature | solution focus
mechanical phenomena will be modulated
onto the current waveform where they can
be detected and analyzed.
Dunn turned to the ALL-TEST Pro
On-Line II (ATPOL II) energized motor-testing instrument from ALL-TEST Pro
LLC (alltestpro.com, Old Saybrook, CT) to
perform the electrical-signature analysis. He
connected the device to the system, which
was set to a flow rate of 5,600 gpm for the
test. The motor's terminal box remained
open, and the data collector was placed
nearby. Because the ATPOL II collects and
transmits data with Bluetooth, Dunn and
the maintenance team were able to position
themselves at a safe distance.
The data was collected in two segments.
The first was a high-resolution, low fmax
(100 Hz) current acquisition of 50 sec.,
which primarily shows issues at running
speed (misalignment, unbalance), and
below the synchronous speed (rotor bar or
load related issues).
The second segment was a high-frequency capture and FFT of voltage and current
electrical issues including power harmonics, power-factor issues, voltage, and
current, versus nameplate and balance
high-frequency mechanical faults,.
Results of the electrical-signature analysis
The motor was electrically perfect with
balanced current and voltage, and 90+
The motor and pump were mechanically
excellent with no indications of misalignment or unbalance, bearing issues,
rotor, or stator issues.
Tests showed a significant fluctuation in the
current draw, pulsing at about 10 Hz. Dunn
and the team collectively determined that
this was caused by non-laminar flow, or
turbulence in the system.
The indication of turbulence on the
power plant's two under-performing vertical
service water pumps was a revelation. More
surprising to the maintenance team was
the fact that the turbulence had not been
detected with any of the other tests.
One of the key benefits of electrical-signature analysis is that it can show mechanical issues in the driven load-even in the
case of a vertical pump with an impeller
some 30 ft. below the surface. This is in
addition to detailed information on other
parts (mechanical and electrical) of a system. Once the maintenance team understood the root cause of its pump problems,
they were able to take the necessary steps to
remedy the situation and get the units back
to operating in a way that meets the power
plant's performance requirements. EP
The indication of
turbulence on the
pumps was a
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