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TitleMeasuring Adjustable Speed Drives
Tags Electric Current Alternating Current Rectifier Power Inverter
File Size11.4 MB
Total Pages28
Document Text Contents
Page 1

Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 1

Measurement of
Adjustable Speed Drives

with Fluke Meters

Page 2

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

Page 14

14 Fluke Corporation Measurement of Adjustable Speed Drives with Fluke Meters

Figure 12. Leading edge of PWM pulse with
no reflected voltage

Figure 13. Leading edge of PWM pulse with
reflected voltage (ringing)

The signals in Figures 12 and
13 were captured by triggering
on a single pulse using single
shot mode with cursors enabled
to make the peak voltage
measurement along with rise
time. While this measurement
requires more button pressing
and scope “know how,”
the automated rise time
measurement may be worth the
trouble. Manually resetting the
single shot trigger periodically
will give you a sampling of
various peak voltages for the
different pulses. Also, slowly
raising the trigger voltage
will give you an idea of the
maximum peak when the
scope stops triggering.

Overvoltage reflections —
troubleshooting
As mentioned earlier, fast rise
times on the ASD output pulses
and long cable runs between
the ASD and the motor will
cause overvoltage reflections
approaching double the DC
bus voltage and even higher.
An oscilloscope is required to
discover the full extent of this
problem, as seen in Figures 10
and 11 below.

Figure 10. Normal PWM signal

Figure 11. Overvoltages due to reflected waves

Figure 10 shows the ASD
L-L voltage measurement at the
motor terminals with six feet
of cable, while Figure 11 shows
the ASD L-L voltage with
100 feet of cable. Notice the
difference in peak voltage
measurements — about 210
volts. Also notice that there is
only 5 Vrms difference between
the two waveforms (small digits
on the display). This means
your voltmeter will not find
this problem.

Very few scopes will trigger
as nicely and easily as the Fluke
ScopeMeter® 123 Test Tool did
for the measurements in Figures
10 and 11. For other scopes
use the procedure outlined
below to measure the extent
of the overvoltages.

Page 15

Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 15

Assuming you have identified
a true overvoltage, or ringing
problem, then something must
be done about it. The simplest
solution is to shorten the cable.
Table 3 shows the maximum
length of the cable before the
peak voltage goes beyond 1.15
times the dc bus, (highest “safe”
motor voltage) for various rise
times and reflection coefficients
( L ) of 0.9 and 0.8 (see previous
section for more details).

The table only shows where
the peak overvoltages start
for a given rise time and length
of cable. The peak overvoltages
will continue to increase to
almost double the dc bus
voltage as the cable lengthens
or rise time gets faster. The
peak voltages can even exceed
voltage doubling if the reflected
voltage occurs on top of existing
ringing due to the distributed
leakage inductance and
coupling capacitance.

Length where Vpeak Length where Vpeak
>1.15 x DC bus voltage >1.15 x DC bus voltage

PWM Pulse Rise Time and L = 0.9 and L = 0.8

0.1 µs 2.8 feet 3.1 feet

0.2 µs 5.5 feet 6.3 feet
0.5 µs 14 feet 16 feet
0.7 µs 19 feet 22 feet

1 µs 28 feet 31 feet
1.5 µs 42 feet 47 feet
2 µs 55 feet 63 feet

3 µs 83 feet 94 feet
4 µs 111 feet 125 feet
5 µs 138 feet 156 feet

6 µs 166 feet 188 feet
7 µs 194 feet 219 feet
8 µs 221 feet 250 feet

9 µs 249 feet 281 feet
10 µs 277 feet 313 feet

Table 3. Maximum cable lengths for various rise times and reflection coefficients before peak
voltages begin to exceed 1.15 times the DC bus voltage

The real danger of this
overvoltage condition is the
damage it can do to the motor
windings over a period of time,
which may not show up as a
problem when the PWM drive is
first installed. Many PWM drives
are installed without taking into
consideration the overvoltage
effects of long cabling between
the PWM output and the motor.
And while improved efficiency
of the newest and latest PWM
drives are achieved by making
the rise times faster on the
output pulses, this can make
the overvoltage problem even
worse, and the need for shorter
cabling even greater.

If your motor has already
failed and has to be rebuilt,
better insulated wire such as
Thermaleze Qs, or TZ Qs (by
Phelps-Dodge), should be used
to rewind the motor. The main
advantage is that it provides
significantly more protection
against overvoltages without
adding insulation thickness
and the same stator can be
used without modification.
If the motor has been damaged
beyond repair then a motor
designed to meet NEMA MG-31
specifications (sustained Vpeak
≤1600 volts and rise time
0.1 µs) should be used as a
replacement motor for PWM
applications where sustained
overvoltages may be occurring.

Page 27

Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 27

AUTOHOLD
RUN

SCOPE
MENU

123 SCOPEMETER 20 MHz

A A COMCOM B B A COM B

SAVE

PRINT

F1 F2 F3 F4

mV

V

mV



V

V Hz A

s TIME ns

A BA B

USER
OPTIONS

V Hz A

80i-400
• 1-400A
• ±3% of Rdg
• 30 mm (1.18 in)
• 1 mA/Amp

Fluke 89-IV
• 50,000-count resolution

with instant readings

• 0.025% basic dc
accuracy

• 100 kHc ac bandwidth
• Internal memory allows

for stand-alone logging
and storage of up to
1000 measurements

• Time-stamped MIN/MAX/
AVG

• Real-time clock
• Temerature readings in

ºC or ºF

• Lifetime warranty
• CAT III 1000V safety
• Optional FlukeView

software

Fluke 43
The Fluke 42 Power Quality
Analyzer performs the
measurements you need
to maintain power systems,
troubleshoot power problems
and diagnose equipment
failures. All in a rugged
handheld package.

• Combines the most useful
capabilities of a power
quality analyzer,
multimeter and scope

• Measure power, harmonics,
and catch intermittents

• Monitoring functions help
track intermittent problems
and power system
performance

• Menus use familiar
electrical terminology

• Complete package with
voltage and current probes,
PC software and cables

• Applications manual, power
quality troubleshooting
video and users manual
explain measurements
step-by-step

ScopeMeter®190 Series
• Bench-level performance

in a rugged, reliable
handheld oscilloscope

• 200 MHz bandwidth
• 2.5 GS/s real time

sampling

• 27,500 points record
length

• Advanced trigger modes
• Up to 1000V isolated

inputs

• 4 hour battery life
• Built-in meter and

recorder

80i-1000s
• Optional accessory. AC

current 100 mA to
1000A rms continuous,
1400A rms peak. BNC
connection.

• Use with 39/41B or
ScopeMeter

ON

OF
F

80i-500s AC
• Included with both the

Fluke 39 and 41B. AC
current up to 500A rms,
700A rms peak, with
best accuracy of 2% of
reading from 45 Hz to
65 Hz. BNC connection.

80i-110s
• Optional accessory.

Measures ac and dc
current in two ranges
from 50 mA to 100A.
Best accuracy is 3% and
frequency response is
dc to 100 kHz. BNC
connection.

ON

OL

OU
TP

UT
100

mV
/A

10m
V/A O

FF
INP

UT 100
Ap

k MA
X

ZE
RO

ZE
RO

+
-

+
-

II

80T-IR
For fast non-contact
temperature measurements.
Range: -18°C to 260°C (0°F to
500°F). Accuracy: ±3% of
reading or ±3°C (5°F) which-
ever is greater. Internal switch
selection for °C or °F. For use
with DMMs or ScopeMeter.

FlukeView® Software
• Capture screen images or waveforms to

document and archive measurements

• Use data in spreadsheet programs for
detailed analysis

• Save and retrieve setups for fast
preparation of measurement routines

• Supports popular PC file formats (BMP
and PCX) for image storage

A mA COM V

TEMPERATURE

A

AutoHOLD

CANCEL

FAST MN MX

SETUP

MIN MAXHOLD REL

% msHz RANGE

dB

dB

ac+dc

ac+dc

ac+dc

ac+dc

F

nS

mA

mA
A

mV

V

mV

V

OFF

C

A

A

A

TRUE RMS MULTIMETER89

A mA COM V

TEMPERATURE

A

CLEAR MEM
VIEW MEM

LOGGING

SAVE

YES

NO

400mA
FUSED

10A MAX
FUSED

CAT
1000V

43 POWER QUALITY ANALYZER

HOLD
RUNPRINT

ENTERF1 F2 F3

RECORDMENU

SAVE

1 2

Fluke 87
• 0.1% basic dc accuracy
• 100 mV to 1000V ac &

dc

• 0.1 mA to 10A, all fused
• 33/4 digit, 4000 count

digital display

• Analog bar graph pointer
• Min/Max/Avg recording
• Frequency, duty cycle,

capacitance

• Input AlertTM
• True-rms, backlit display,

1 ms peak hold, 41/2 digit
mode

• Touch Hold® and
Relative modes

• Lifetime warranty

100ms AVG H

k

40
0 1 2 3 54 6 7 8 9 0

87 TRUE RMS MULTIMETER

MIN MAX RANGE HOLD H

REL Hz

mA
A

A

mV

V

V

OFF

!

A COM VmA A

1000V MAX

400mA MAX
FUSED

10A MAX
FUSED

PEAK MIN MAX

ScopeMeter® 123
• 0.5% basic dc accuracy
• 5 mV to 600V ac & dc
• 20 MHz bandwidth
• 32/3 digit, >5000 count

display

• 2 channel Min/Max/
Trend Plot recording

• Frequency, duty cycle,
pulse width

• Direct readout in amps
and °C/°F

• True-rms, backlit display,
40 nS glitch capture

• Touch Hold and Relative
modes

• Print Screen capability
• Three-year warranty

Page 28

28 Fluke Corporation Measurement of Adjustable Speed Drives with Fluke Meters

Acknowledgments
[1] A. von Jouanne, P. Enjeti, V. Stefanovic,

“Adjustable Speed AC Motor Drives: Applica-
tion Problems and Solutions”, Seminar notes,
pp. 3.1-6.20, PowerSystems World Conf. 1996

[2] IEEE Recommended Practices and Require-
ments for Harmonic Control in Electrical
Power Systems - Std 519, New York, NY: IEEE
Inc., 1992

[3] EC&M, “Practical Guide to Motors and Motor
Controllers”, pp. 9-16, Intertec Electrical
Group, 1991

Special thanks to Dr. Annette von Jouanne for
her assistance preparing this application note
and to Oregon State University for the use of
their Motor Systems Resource Facility.

Fluke Corporation
PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands

For more information call:
U.S.A. (800) 443-5853 or
Fax (425) 356-5116
Europe/M-East/Africa (31 40) 2 678 200 or
Fax (31 40) 2 678 222
Canada (800) 36-FLUKE or
Fax (905) 890-6866
Other countries (425) 356-5500 or
Fax (425) 356-5116
Web access: http://www.fluke.com

©1999 Fluke Corporation. All rights reserved.
Printed in U.S.A. 11/99 1264691 A-ENG-N Rev C

Printed on recycled paper.

Fluke. Keeping your world
up and running.

Application Notes
In Tune with Power Harmonics
Literature #B0221UEN
Electrical Power Circuit Mea-
surements for ScopeMeter
Literature #B0241UEN
ABCs of Multimeter Safety
Literature #B0317UEN
Electrical Troubleshooting with
Fluke Multimeters
Literature #B0171UEN

Educational Videos

ABCs of Digital Multimeter Safety
P/N 609104
Understanding and Managing
Harmonics
P/N 609096

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