Fluke 360 AC Application Note
Application Note
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
“Let’s just oversize the motor and we can run it lightly loaded—that
will save us some money and be easier on the motor.” This is a false
belief among some who select and install motors. Properly sizing
motors for a given load results in driving loads more efficiently, saving
energy, and saving dollars. Motors typically are most efficient when
they are 90 % to 95 % loaded. Just because a motor says “25-hp”
on the nameplate does not mean the motor is producing 25 horse-
power as it operates. The motor may be producing quite a bit less
depending on the load requirements. If the motor constantly runs at
these reduced horsepower requirements money is being wasted and
you should consider replacing it with a correctly sized motor.
Additionally, you need to base
the size of conductors and the
fuses or circuit breaker supply-
ing that motor upon the full load
current rating of the motor, how
often it is expected to oper-
ate, and other factors. Installing
larger conductors and break-
ers than needed is a wasted
expense. It is also important to
realize that even at low horse-
power requirements a motor still
draws a relatively large amount
of current. For example, a motor
operating under no load at all
still draws about 50 % of its
rated current.
When you replace a
motor, match the motor
to the job
When replacing motors it is
important to match the motor
to the job. In addition to select-
ing the proper voltage, phase
(three phase or single phase),
design letter, and code letter, be
sure to select the proper horse-
power rating. If the motor has
been replaced previously, or is
running a pump, fan, or other
equipment that was not sized as
part of an entire system by the
original equipment manufacturer
(OEM), you may not be select-
ing the right size motor. Taking
some basic voltage and current
readings to estimate your own
horsepower requirements will
provide you with a more efficient
system.
Such information is valuable
when conducting an energy
study. If the motor load varies
90 % or less from full load for
extensive amounts of time, the
application may be right for a
variable speed drive and, thus,
significant savings. For instance,
if motor horsepower require-
ments can be reduced with a
variable speed drive to decrease
motor speed to 90 % of the
motor’s full rated speed, then
energy consumption is reduced
to 73 % of what is required
for full-speed operation. Just
another reason to know the load
requirements of your equipment.
In some cases the motor may
be overloaded, drawing more
than its rated current. Whether
it is bad bearings, a misaligned
shaft or other maintenance
issues, or just excessive load on
By Randy Barnett
Determining load horse-
power, wiring, and
breaker size for safe and
efficient installations
Run a Fluke iFlex™ Fluke Current Probe around a single conductor. Or,
you can center the jaws of a clamp meter around a single conductor.
the motor, one detrimental effect
is occurring for sure: excessive
heat is being produced in the
windings. Heat deteriorates insu-
lation and is the leading cause
of motor failure. While properly
sized and installed overloads will
trip the motor at typically 115 %
to 125 % of the full load current
value on the nameplate, the heat
developed during this time is
sure to shorten motor life.
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