Fluke 373, 375, 376, 381, 374 Service Guide

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 coun­terparts, 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 protec­tion 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 nec­essary, 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 measure­ment 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

Figure 1. Motor starting current.

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 synchro­nously 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

In-Rush Current

Current

Application Note

Running Current

Time

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 de­scribe 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:

From the Fluke Digital Library @ www.fluke.com/library

Analog peak

The earliest Fluke handheld
instrument to have a highest
value capture feature was the
8024A digital multimeter (DMM).
It had an analog peak hold circuit
(labeled “Peak Hold”), which
captured the highest peak value
that lasted for ten milliseconds
or longer, regardless of when it
happened in time. From a strict
definition point of view, this was
a correctly labeled feature since it
did capture the actual peak value,
but it didn’t necessarily measure
the inrush current.

Digital min/max

Newer generations of Fluke
DMMs, like the Fluke 27, had a
min/max function (more cor­rectly labeled digital min/max).
It looks at a portion of the a to d
cycle to obtain its value, which
happened at a fixed rate based
on the system clock. However,
it made its measurements when
it wanted to, which in many
cases did not coincide with the
startup event, so it could miss the
event partially or completely. The
Fluke 87 was the first handheld
instrument to have both a one

millisecond analog peak and
100 millisecond digital min/
max, allowing for measuring of
relatively short or medium long
events. Again, the 100 millisec­ond min/max suffered the same
limitations as the earlier model
27; the measurements were not
synchronous with the occurrence
of an event. The first Fluke clamp
meter to have a max hold feature
was the Fluke 36. Although this
was implemented as an analog
track and hold, in ac the Fluke
36 looked at the output of the
ac analog rms converter. That
significantly slowed the response
rate so the 36 was only useful
for events that lasted several
hundred milliseconds or longer.
Unfortunately this was too slow
for short-term events, like inrush
current. Min/max on the Fluke
376 Clamp Meter is a digital min/
max—similar to the one on the
Fluke 27—and it is updated every
time the display updates. Its sam­pling window opens for about
100 milliseconds every 400 mil­liseconds and takes a number of
readings, and updates the min or
max registers as appropriate. This

type of min/max is most useful for
longer term events, such as those
occurring on heavily loaded or
long wire run circuits, to record
more regular voltage drops or
load increases, but is not ideal for
inrush measurements.

Inrush on the Fluke 43B vs.
the Fluke 370 Series

Fluke has inrush functions on
other products, such as the Fluke
43B Power Quality Analyzer,
but while they share the same
name, they are different. The 370
Series takes a large number of
samples precisely at the begin­ning of the starting current for a
100 millisecond period and then
digitally filters and processes the
samples to calculate the actual
starting current. In contrast,
when “inrush” is selected from
the Fluke 43B menu, it digitally
captures the current (amps)
waveform. A cursor can then be
used to pick out the instanta­neous amps value at any point of
the sampled amps waveform and
the duration of that value. This
capability is ideal for examining
reduced voltage (a two stage)
starter waveform.

Why does the display show a value higher

than the rating of the circuit breaker?

1 Second

200 mS

Time

100 mS

20 40 60 80 100 300 1000 2000

Figure 2. Breaker clearing time for 20 amp breaker.

For 20 amp breaker

Breaker Open in
Shaded Areas

Amps

Inrush current can be four-to-eleven
times the normal running current
depending on the type of motor. For
example, if the running current of a
motor is eight amperes and the start­ing current multiplier is five times the
running current, the Fluke 370 Series
clamp meter displays a reading of
about 40 amperes, even though the
circuit breaker is rated for 20 amperes.
The reason the breaker or overload
unit does not trip is because both
of these devices operate on a time
versus current curve. This indicates
how much current passes through
the breaker and for how long a time
period without opening the circuit.

2 Fluke Corporation Inrush current