Fluke 289 Service Guide

Multimeter measurements

on variable frequency

drives using the new

Fluke 289 DMM

Editor’s note: For similar instructions using the

Fluke 87V DMM, reference Fluke article 12345.

In the past, motor repair meant dealing
with traditional three-phase motor failures
that were largely the result of water, dust,
grease, failed bearings, misaligned motor
shafts, or just plain old age. But, motor
repair has changed in a big way with the
introduction of electronically controlled
motors, more commonly referred to as
variable frequency drives (VFD). Drives
present technicians with a unique set of
measurement problems that can vex the
most seasoned pro.

This application note describes the elec­trical measurements you can make dur­ing the installation and commissioning of
a drive, as well as other measurements
to aid in diagnosing bad components, and
other conditions that may lead to prema­ture motor failure in VFDs.

Application Note

Troubleshooting
philosophy

There are many different ways
to go about troubleshooting an
electrical circuit, and a good
troubleshooter will always find
the problem—eventually. The
trick is to track down the problem
as quickly as possible, keeping
downtime to a minimum.

The most efficient procedure
for troubleshooting is to begin at
the motor, and then systemati­cally work back toward the elec­trical source, looking for the most
obvious problems first. A lot of
time and money can be wasted
replacing perfectly good parts
when the problem is nothing
more than a simple loose con­nection.

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

Next, take care to make
accurate measurements. Nobody
makes inaccu rate measurements
on purpose of course, but it’s
easier to do than you may think,
especially when working in a
high energy, noisy environment
like that of an VFD. Choosing the
right test tools for troubleshoot­ing, the drive, the motor, and
the connections is of the utmost
importance. This is especially true
when making voltage, frequency
and current measurements on the
output side of the motor drive.
Until now the only meter on
the market able to make these
measurements accurately was
the Fluke 87V Digital Multimeter.
Fluke has just introduced the next
generation of high performance
Multimeter, the new Fluke 289,

incorporating the same selectable
low pass filter found in the
successful 87V DMM. The filter
allows the meter to make
accurate drive
output
measure­ments that
will now
agree with
the motor
drive controller
display indica-
tions. No longer
does the technician have to
guess whether the drive is
operating correctly and deliv-
ering the correct voltage, current,
or frequency for a given control
setting.

Drive measurements

Input side measurements

You can use any good quality
true rms multimeter to verify
proper input power to the drive.
The input voltage readings
should be within 2 % of one
another when measured phase
to phase with no load. A sig­nificant unbalance may lead
to erratic drive operation and
should be corrected when dis­covered.

Output side measurements

Measurements on the output
side of a pulse width modulated
(pwm) motor drive have been
difficult or impossible to make,
especially accurate measure­ments, that would agree with
the drive controller displayed
values. The traditional true rms
multimeter will not provide accu­rate answers. This is because
the voltage applied to the motor
terminals by the VFD is a pulse
width modulated non-sinusoidal
voltage.

A true-rms DMM will give
an accurate reading of the heat­ing effect of the non-sinusoidal
voltage applied to the motor, but
will not agree with the motor
controller’s output voltage read­ing which is displaying the
rms value of the fundamental
component only (typically from
30 Hz to 60 Hz). The issues are
bandwidth and shielding. Many
of today’s true rms digital multi­meters have a wide bandwidth,
sometimes out as far as 20 kHz
or more, which allows the meter
to respond not only to the fun­damental component that the
motor really responds to, but also
to all of the high frequency com­ponents generated by the pwm
drive.

In addition, if the dmm is
not shielded for high frequency
noise, this can also lead to poor
measurement performance due to
the high noise levels generated
by the drive controller. Because
of their higher bandwidth and
shielding issues, many true rms
meters will display readings as
much as 20 to 30 % higher than
the drive controller is indicating.

289 dual display showing frequency
and voltage

Fluke’s new 289 multimeter,
with it’s ¼ VGA dot matrix dis­play, has the ability to display
multiple parameters at the same
time and utilize the selectable
low pass filter, giving the trou­bleshooter or engineer the ability
make accurate voltage, current
and frequency measurements
on the output side of the drive
either at the drive itself or at the
motor terminals and also display
the filtered output voltage and
frequency (motor speed) simulta­neously. With the filter selected,
the 289 readings for both volt­age and frequency (motor speed)
should now agree with the
associated drive control display
indications if they are available.
When measuring output current
with a clamp accessory, the low
pass filter allows for accurate
current measurements and fre­quency measurements simultane­ously when using the Hall-effect
type clamps. All of these mea­surements are especially helpful
when making measurements
at the motor location when the
drives displays are not in view.

Making safe measurements

Before making any electri­cal measure ments, be sure you
understand how to make them
safely. No test instrument is com­pletely safe if used improperly,
and you should be aware that
many test instruments on the
market are not appropriate for
testing variable frequency drives.
Also insure that the appropriate
personal protective equipment
(PPE) is used in accordance with
the specific working environ­ment and the measurements to
be made. If at all possible, never
work alone.

Safety ratings for
electrical test equipment

ANSI and the International
Electro technical Commission (IEC)
are the primary independent
organizations that define safety
standards for test equipment
manufacturers. The IEC 61010
second edition standard for test
equipment safety states two
basic parameters, a voltage rat­ing and a measurement category
rating. The voltage rating is the
maximum continuous working
voltage the instrument is capable
of measuring. The category rat­ings depict the measurement
environment expected for a given
category. Most three-phase VFD
installations would be considered
a CAT III measurement environ­ment with power supplied from
either 480V or 600V distribution
systems. When using a digital
multimeter for measurements
on these high energy systems,
insure that the
multimeter is
rated at a mini­mum for CAT III
600V and pref­erably for CAT
IV 600V/CAT
III 1000V. The
category rating and voltage limit
are typically found on the front
panel of the multimeter right at
the input terminals.

The new Fluke 289 DMM
is dual rated CAT IV 600V
and CAT III 1000V.

Refer to the ABC’s of DMM Safety
from Fluke for additional information
on category ratings and making safe
measurements.

2 Fluke Corporation Multimeter measurements on variable frequency drives using the new Fluke 289 DMM