Application Note

Inrush current
Introduction
Energy shortages around the
country, especially in California,
have increased the urgency for
plants to install, or replace with,
high efficiency motors. While
they consume less electricity than
their older, less efficient coun-
terparts, they draw considerably
more at startup than the older
motors. Startup or inrush current
in these high efficiency motors
can run as high as 11 times the
running current as compared to a
multiplier of seven to eight times
for earlier motors. While this is
usually not a serious problem,
the installer must be aware of
changes to over current protec-
tion devices (OCPDs) to support
these inrush current increases.
Since motors make up 40 %
or more of the electrical load in
most plants, it is not surprising
that plant electrical personnel
spend 20 % of their time either
installing or troubleshooting
them. Regardless of the job at
hand, it is useful and often nec-
essary, to know what the value of
the starting or inrush current is.
This can help to identify where a
starting problem is located, either
in the motor or somewhere in
the starting circuit. This measure-
ment may also be recorded in a
preventative maintenance log.
Previously, digital clamp
meters have had various features
that were supposed to capture
and display the starting current of
motors. These features have been
called peak, peak hold, min/
max, min/max hold, etc. While
all of these features provide a
reading that is higher than the
running current, they tend not to
be repeatable over several start
cycles. Part of the reason they
do not give an accurate picture
is that they don’t always start
taking measurements synchro-
nously with the start of the motor
inrush current.
Inrush
To provide repeatable motor
inrush measurements, the new
Fluke 370 series of clamp meters
utilizes a “triggered” mode that
allows synchronizing of the
measurement with the actual
starting current. The technician
first “arms” the inrush function
of the clamp meter. The meter
is then triggered by the inrush
current. Once triggered, it takes a
large number of samples during a
Running Current
In-Rush Current
Current
Time
Figure 1. Motor starting current.
100 millisecond period and then
digitally filters and processes the
samples to calculate the actual
starting current. The result is
a more accurate, synchronous
indication of the start current not
previously available in a clamp
meter.
Peak, min/max, and
inrush
It’s important to understand that
different brands of clamp meters
will use different terms to de-
scribe the same measurement. In
addition, the actual operation of
the feature may be significantly
different from what the name
would imply. While there is too
wide a variation across brands to
detail in this space, the following
are the terms Fluke uses:
Application Note
From the Fluke Digital Library @ www.fluke.com/library