Case Studies

3 Fluke Corporation Performing power quality studies and troubleshooting loads
“That proved right then and
there that this machine was
our culprit,” McArthur explains.
“We needed to have its drive
system brought up to date using
today’s technology. We had
to get rid of those second and
fourth harmonics. We knew
that once those were gone the
transformer would perform the
way that it should. And that’s
what happened.”
Finding the cause of
voltage drops
In another case, McArthur
used his recently acquired
1735 Power Logger to help a
homeowner place the cause of
lights dimming and computer
screen fluctuations squarely on
the shoulders of her electric
utility. Because of local zoning
laws, McArthur’s client was
not permitted to tear down her
existing home to build a new
one. As a result, she added
3,000 square feet to her home.
She uses the addition as her
primary residence while the
older dwelling has become a
“mother-in-law apartment” or,
in this case, a rental apartment.
The consultant determined
that the total calculated load
for the two dwellings is 305
amps. The meter base is rated
at 400 amps. However, the
utility is supplying the housing
units with only a single-phase
25 kVA transformer having a
full load rating of 104 amps.
“The problem,” McArthur
reveals, “is that she has a four-
ton heat pump. Every time the
heat-pump motor starts, the
locked rotor amps (LRA or motor
inrush) is 140 amps. When we
considered the fundamental
load and did the math, it was
pretty clear what we were
going to see when the motor
started: The lights dim because
the instantaneous voltage drop
was five or six volts (during
each episode).”
The client also reports that
depending on the load on the
transformer, not only do her
lights dim but the computer
screen in her home office some-
times shrinks slightly when the
heat pump comes on. McArthur
explains that utilities often
load a 25 kVA transformer up
to 200 percent of capacity. He
says that the loading on the
transformer supplying his client
is more than that—205 or 209
percent at peak times.
“I kept telling the utility that
for a nice steady-state rms load
they might get away with this
way of operating,” McArthur
reveals, “but any time you inject
a motor starter or something
that takes an instantaneous
high current into the system,
you are going to see a voltage
sag. The sag will manifest itself
in the lights. The utility kept
telling my client, ‘The problem’s
in your home.’”
The consultant made an
agreement with the homeowner
that he would monitor the elec-
trical system at the meter base
to see “what’s being delivered
and what the effects are.” He
attached the power logger for
a week’s worth of monitored
data.
McArthur found that the volt-
age sags were averaging three
percent and happening at a
rate of two to three per hour.
McArthur believes that his find-
ings will require his client’s
utility to address the problem
and upgrade the transformer,
but he also proposes additional
tests to augment his findings.
“The 1735 did a beautiful job
during the 7-day test,” McAr-
thur says. “Using the instru-
ment and its accompanying
software, I was able to average
events over the two-minute,
20-second sample period. But I
want to go back and do at least
one more test.”
McArthur plans either to do
one-second sampling for a
couple of hours or to sample in
the “event” mode. He explains
that in his initial sampling,
the interval of two minutes
and 20 seconds represented a
considerable number of cycles.
He believes that by using one-
second sampling, he will be
able to get down to 60 cycles
and “see” the sags in real
time. Then, the sags will, in
McArthur’s own words, “show
up deeper.” Regarding using
the events mode, McArthur says
that he should be able to set
up to detect each sag when it
occurs, with similar results—
sags represented as deeper.
At this writing, the 1735 is so
new to McArthur that he is still
exploring its capabilities. Still,
he anticipates many uses for
the 1735 Power Logger in his
day-to-day work. He is espe-
cially interested in the instru-
ment’s ability to log power
every 15 minutes for 30 days,
on the same schedule that utili-
ties use and the National Elec-
tric Code specifies.
1
1For details about using the Fluke 1735
Power Logger in this way, see the Fluke
Application Note, “30 Day Load Studies with
the 1735 Power Logger”
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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Web access: http://www.fluke.com
©2007 Fluke Corporation. All rights reserved.
Specifications subject to change without notice.
Printed in U.S.A. 5/2007 3034602 A-EN-N Rev A
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