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Measurement of Adjustable Speed Drives with Fluke Meters Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 1
Measurement of
Adjustable Speed Drives
with Fluke Meters 2 Fluke Corporation Measurement of Adjustable Speed Drives with Fluke Meters
Table of Contents
Introduction ..................................................................................................................................... 3
Troubleshooting Philosophy ...................................................................................................... 3
Making Safe Measurements (Sidebar) ..................................................................................... 4
Safety Ratings for Electrical Test Equipment .............................................................................. 4
Adjustable Speed Drive Theory ................................................................................................. 6
Simple Things First ...................................................................................................................... 8
Resistance Measurements ............................................................................................................. 8
Voltage Drops .................................................................................................................................. 8
Temperature Measurements .......................................................................................................... 8
Motor Measurements .................................................................................................................... 9
Temperature Measurements .......................................................................................................... 9
Overloading ..................................................................................................................................... 9
Voltage Measurements ................................................................................................................... 9
Current Imbalance Measurements .............................................................................................. 10
Single Phasing .............................................................................................................................. 11
Overvoltage Reflections - Theory (Sidebar) .............................................................................. 12
Overvoltage Reflections - Troubleshooting ............................................................................... 14
Bearing Currents ........................................................................................................................... 17
Leakage Currents .......................................................................................................................... 17
Measurements at the Inverter .................................................................................................. 18
Control Circuit Noise ..................................................................................................................... 18
Volts/Hertz Ratio ........................................................................................................................... 18
Inverter Drive Circuits .................................................................................................................. 18
Voltage Source Inverters .......................................................................................................... 18
PWM Inverters ........................................................................................................................... 19
The DC Bus .................................................................................................................................... 20
DC Voltage Too High ................................................................................................................. 20
DC Voltage Too Low ................................................................................................................. 21
AC Line Input ................................................................................................................................. 22
Diode Bridge .............................................................................................................................. 22
Voltage Notching ....................................................................................................................... 23
Voltage Unbalance .................................................................................................................... 23
Harmonics and IEEE-519 Compliance ................................................................................... 24
ASD Measurement Guide for Fluke Test Instruments
......................................................... 26 Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 3
Introduction
Most experienced motor
technicians are well prepared
to deal with traditional three
phase motor failures caused
by water, dust, grease, failed
bearings, misaligned motor shafts,
or even just old age. However,
modern electronically controlled
motors, more commonly referred
to as adjustable speed drives,
present a unique set of problems.
This application note will focus
on electrical measurements that
can be used to diagnose bad
components and other conditions
that may lead to premature
motor failure in adjustable speed
drives (ASDs).
There are many different ways
to go about troubleshooting an
electrical circuit, and a good
troubleshooter will always find
the problem - eventually. The
procedure set forth in this
application note will begin
looking at the motor and work
back towards the electrical
source. Additionally, it will
emphasize checking the simple
and easy things first. A lot of
time and money can be wasted
replacing perfectly good parts
when the only thing wrong is
a loose connection.
Make accurate measurements.
Of course nobody makes inaccu-
rate measurements on purpose,
but this is easier to do than you
think when working in a high
energy, noisy environment like
ASDs. Dont use grounded test
instruments if it can be avoided.
They can introduce noise into
a measurement where none
existed before. Avoid touching
instruments and probes if
possible while taking the
reading, as electrical noise
can get coupled through your
hands which may also affect
the reading. Use current clamps
which are well shielded and
terminated with BNC connectors.
Troubleshooting Philosophy
Current clamps that put out
10 mV/amp or 100 mV/amp will
have better signal to noise ratio
than 1 mV/amp clamps when
making current measurements
less than 20 amps.
Finally, document electrical
measurements at key test points
in the circuit when the system
is functioning properly. If a good
drawing doesnt exist - make
one. A simple one-line, or even
block diagram will do. Write
down voltage and temperature
measurements at key test
points. This will save lots of
time and head-scratching later. 4 Fluke Corporation Measurement of Adjustable Speed Drives with Fluke Meters
Before any electrical measure-
ments are made, be sure you
understand completely how
to make them safely. No test
instrument is completely safe
if used improperly, but you
should also be aware that many
test instruments on the market
are not appropriate for testing
adjustable speed drives. The
following information is
explained in more detail in the
Fluke application note, ABCs of
Multimeter Safety and the video
with the same name.
1
The main
points are summarized below.
If you install or maintain
modern adjustable speed drives
(ASDs), then please dont skip
this section as there could be
hazardous voltages that exist in
your measurement application
that you are not aware of.
Safety ratings for electrical
test equipment
The International Electro-
technical Commission (IEC)
is the primary independent
organization that defines safety
standards for test equipment
manufacturers. There is much
confusion about what these
standards mean and how they
should be used to determine
the right instrument for the
right application. The following
section will help clarify this
selection process.
The IEC 1010 standard for
test equipment safety states two
basic parameters; a voltage rat-
ing and an overvoltage category
rating. The voltage rating is the
maximum continuous working
voltage that can be measured.
This is fairly straightforward
and simple, although Im sure
many of you have noticed that
your DMM or scope will often
give a reading higher than its
maximum voltage rating. Your
test instrument should never
be operated above