Application Note

2 Fluke Corporation If you knew how much it cost to run that equipment
Compressor number two was a newer, more
energy-efficient design than its counterpart.
The facility was being charged $0.07 per
kilowatt hour (kWh). The run times and kilowatt
energy usage were extracted from the logged
data and the actual operating costs determined.
Such calculations are relatively simple and can
easily be built into a spreadsheet for plant-wide
usage. The bottom line: Operating compressor
number two as the primary plant air compres-
sor resulted in an annual estimated savings of
$29,510. Obviously, logged data proved to be
much more accurate than “best guess” based
solely on nameplate data. Installing power loggers
and later downloading data into a PC for analysis
takes only a few minutes. A small investment in
this case for a large return.
What does it cost to run fifty grinders
during lunch break?
At another manufacturing plant the question was
asked, “What is it costing to allow those fifty
grinders to run during lunch break?” The logic
for allowing grinders to run while not in use was
that the grinders were relatively small loads as
compared to much larger equipment, and the cut-
ting oil would remain circulating throughout the
grinder. And, since the grinder was effectively
“idling” with no load, it just simply was not worth
the effort to shut the grinder down for such short
periods.
To verify the decision, a recording digital mul-
timeter with an ac clamp was used to determine
the actual operating cost of one grinder during
the lunch break period. Interestingly enough, the
savings came to only $0.55 for the one grinder.
However, multiplying the fifty-five cents times
fifty grinders yielded a lunch time savings of
$27.50. Given the variables of shift work and
holidays, the annual estimated savings of shutting
off the grinders during the lunch periods came
to just under $8,000. Once again, a thirty-minute
check revealed a significant savings that could
be achieved by the push of only two buttons per
grinder: one for “stop” before lunch and one for
“start” after lunch.
Operational costs for larger motors
Larger motors for various applications should
always have their operational cost known. In one
facility a 100-horsepower (HP) motor was used to
pump water from a holding pond several hundred
feet to where the water was used for process
cooling. The motor ran continuously during plant
operations. Other options were being explored
for cooling water. The question that needed to be
answered to determine payback before a decision
could be made was, “How much is it costing us
to run the current pumping system we have?” By
recording the kilowatts consumed by the motor
and the number of hours it was operating during
a plant cycle, it was determined the 100 HP motor
was almost always operating at its full capacity
of 100 HP. It was costing the company $33,241
annually. Business decisions were then made to
replace the existing system with a more efficient
motor and pump design.
But, why not just rely on nameplate data to
determine the cost to operate a motor? After all,
NEMA motors are marked with required ratings
including their horsepower and efficiency ratings.
The answer is that motors rarely operate at their
nameplate specifications. Expect actual operating
costs to vary.
Time
Active power total minimum
(Watts)
Active power total average
(Watts)
Active power total maximum
(Watts)
08:10:07 0ms 32110.238 32097.152 32031.729
08:10:17 0ms 32064.441 32090.611 32142.949
08:10:27 0ms 32097.152 32103.695 32129.865
08:10:37 0ms 32097.152 32103.695 32149.492
08:10:47 0ms 32090.611 32123.322 32123.322
08:10:57 0ms 32084.068 32110.238 32136.408
Data Logger information imported into a spreadsheet for analysis. This 100-horsepower centrifugal pump motor, if fully loaded, should be consuming nearly 80,000 watts
(80 kW). The 32,000 watts (32 kW) indicates the motor is only partially loaded, operating inefficiently, and that it may be a candidate for a VFD—which could result in
significant energy savings.
Readings averaged
every 10 seconds over a
one-minute period
Average watts consumed by the
motor over the previous 10 seconds