Application Note

3 Fluke Corporation Proper use of a clamp meter in commercial and residential settings
Variable speed drives are more
and more common, since they
save considerable energy.
The Fluke 376 is the ideal
clamp to make these motor and
drive measurements:
• Loading: The current draw
of the motor, measured as
an average of the three
phases, should not exceed
the full load amperes rating
of the motor (times the
service factor). On the other
hand, a motor that is loaded
below 60 percent of full load
amperes—and many of them
are—is less and less effi-
cient, and the power factor
decreases as well.
• Current balance: Current
imbalance can be an indica-
tion of problems with the
motor windings (for example,
different resistances on field
windings due to internal
shorts). Generally speaking,
imbalance should be under
10 percent. (To calculate
imbalance, first calculate
the average of the three
phase readings; then find
the highest deviation from
the average and divide by
the average.) The extreme
of current imbalance is
single phasing, when there
is no current on one of the
three phases. This is usually
caused by an open fuse.
• Inrush current: Motors
started across the line (by
mechanical starters) will
have an inrush current (vari-
able speed drives do not
have inrush current). The
inrush current is approxi-
mately 500 percent on older
motors to as high as 1,200
percent on energy-efficient
motors. This inrush current,
if it is too high, is a common
cause of voltage sags as well
as nuisance tripping. The
“inrush” function of the Fluke
376 clamp meter is a unique
capability, designed to trig-
ger on the inrush current and
capture its true value.
• Peak loading (shock loads):
Some motors are subject
to shock loads, which can
cause enough of a current
surge to trip the overload cir-
cuit in the motor controller.
Think of a saw running into
a knot. The min/max function
can be used to record the
worst-case current drawn by
shock loads.
Whether in residential or com-
mercial settings, the clamp
meter is the electrician’s indis-
pensable test tool.
Work safely
The high voltage and currents
present in electrical power
systems can cause serious
injury or death by electrocution
and burns. Consequently, only
trained, experienced electricians
who have knowledge of electri-
cal systems in general and the
equipment under test should
perform testing and modifica-
tion of electrical systems.
Fluke cannot anticipate all
possible precautions that you
must take when performing the
measurements described here.
At a minimum, however, you
should:
• Use appropriate safety equip-
ment such as safety glasses,
insulated gloves, insulating
mats, etc.
• Be sure that all power has
been turned off, locked out,
and tagged in any situation
where you will be in direct
contact with circuit compo-
nents. Be certain that the
power can’t be turned on by
anyone but you.
• Read and understand all
of the applicable manuals
before applying the infor-
mation in this application
note. Take special note of
all safety precautions and
warnings in the instruction
manuals.
• Do not use instruments on
applications for which they
are not intended, and always
be aware that if the equip-
ment is used in a manner not
specified by the manufac-
turer, the protection provided
by the equipment may be
impaired.
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. [phone] or
Fax [phone]
In Europe/M-East/Africa [phone] or
Fax [phone]
In Canada (800)-36-FLUKE or
Fax [phone]
From other countries [phone] or
Fax [phone]
Web access: http://www.fluke.com
©2009-2012 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 7/2012 1989065C_EN
Modification of this document is not permitted
without written permission from Fluke Corporation.
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up and running.
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