CT256 Going to Ground: AC Drives-Wiring,
Bonding and Grounding Best Practices
Copyright ©2012 Rockwell Automation, Inc. All rights reserved.
VFD Wiring, Bonding and Grounding Best Practices
• Generation of Common Mode Current or Line
to Ground Noise
• Interference with electronic equipment
•
•
•
•
•
PLC’s, DCS’s
PLC communication networks
Sensors, Encoders
AM radio interference
0-10 volt or 4 -20 ma signal interference
• VFD Faults and Failures
8-2
What a VFD input/output voltage and current looks
like
M
3.75MVA
460V
5.75%
T


1) Ref A:
2) Ref B:
T
100 A 2 ms T
10 V 2 ms
8-3
What a VFD common mode output voltage or Vng
(neutral to ground) look like
3.75MVA
460V
5.75%
4 kHz Period between Pulses
With Frequency Spectrum from DC to >6MHz
8-4
Electrical Characteristics of VFD output cable and motor
showing parasitic capacitance to ground
and high frequency common mode current
Conduit
U
Drive
PE
IMOTOR
Ill
R01
L01
R02
L02
R03
L03
Ilg
Cll
Clg
Clg-c
Motor
Cll-m
Clg-m
Frequency Spectrum from DC to >6MHz
8-5
Common Mode Current Path and Effect on Electronic
Equipment
AC Drive
Input Transformer
A
R
B
XO
Ilg
C
(+)
Vdc
bus
(-)
S
Ilg
PE
U
T
Ilg
EARTH
GROUND
V
Motor Frame
Motor Tach
W
C lg-m
Logic
Ilg
Common Mode
Current Path
Ilg
C lg-c I
lg
PE
Potential #1
Potential #2
Potential #3
Interface Electronics
0-10V, communication,
4-20 ma,sensor interface,etc
Potential # 4
True Earth Ground (TE)
Common Mode Noise (Frequency Spectrum from DC to > 6MHz)
Current Path causing ground EMI Interference
8-6
Grounding Practices to Minimize Interference
• The word “Grounding” has many meanings
• What do you think of when you hear or use the word grounding or
ground?
(Confidential – For Internal Use Only) Copyright © 2008 Rockwell Automation, Inc. All rights reserved.
7
Ground
• Different meanings for different individuals and profession
– Farmer
– Electrical tradesman: Safety, NEC codes, Lightning protection
– RF engineer; Ground plane and ground plane dimensions for radio transmission and
reception
– Industrial electronic engineer; Safety, bonding of grounds between electronic systems for
EMI control
– Circuit board design engineer: Power Supply common, power supply connections to ground
for reliability, Circuit signal returns
(Confidential – For Internal Use Only) Copyright © 2008 Rockwell Automation, Inc. All rights reserved.
8
Grounding & Bonding to Solve a Problem
Lightning
Protection
Grounding & Bonding to Solve Problem
Safety
Grounding & Bonding to Solve Problem
Overload Protection
Grounding & Bonding to Solve Problem
AC Drive
Input Transformer
A
R
B
XO
Ilg
C
(+)
Vdc
bus
(-)
S
Ilg
PE
U
T
Ilg
EARTH
GROUND
V
Motor Frame
Motor Tach
W
C lg-m
Logic
Ilg
Common Mode
Current Path
Ilg
C lg-c I
lg
PE
Potential #1
Potential #2
Potential #3
Interface Electronics
0-10V, communication,
4-20 ma,sensor interface,etc
Potential # 4
True Earth Ground (TE)
Common Mode Noise Current Path causing ground EMI
Interference
Control Electro-Magnetic Interference
8 - 12
Grounding & Bonding - Solutions
Grounding & Bonding Inside the panel
Grounding & Bonding Inside the panel
Scrape Paint
Scrape Paint
Panel Layout– The Problem
Panel Layout– The Problem
Panel Layout– The Problem
Panel Layout is Important
UVW
ASD Output
Conduit / Armor
bond to cabinet
Cabinet Back Plane
RST
A
PE
ASD - 1
PE
ASD - 2
A
B
PLC or
Electronics
ASD Input
Conduit / Armor
Bond to cabinet
Transient Common
Mode Current on Armor
PE
PE
ASD - 3
ASD - 4
PE
PE
PE Copper Bus
B Transient Common
Mode Current on PE Wire
Optional PE to
Building Structure
B
Better panel layout allows Ilg noise on output armor to flow back to input
armor and out of PLC ground Better panel layout allows Ilg noise on output
armor to flow back to input armor and out of PLC ground
8 - 19
Panel Layout- Solutions
Panel Layout- Solutions
Panel Layout- Solutions
Containment of Common Mode Current Paths –
Typical Installation
AC Drive
Input Transformer
A
B
XO
Ilg
Ilg
PE
C
Ilg
S
Ilg
V
Vdc
bus
T
(-)
Ilg
W
s
t
r
a
p
PE
Potential #1
Motor
C lg-m
Motor PE
GND wire
PE
Ilg
Motor Frame
U
(+)
Common Mode Current Path
EARTH GROUND
Potential 4
Conduit
R
Accidental
Contact of
conduit
Potential #2
Ilg
Potential #3
CONDUIT PROVIDES GOOD WIRING NOISE CONTROL BUT ACCIDENTAL
CONDUIT CONTACT TO GROUND MAY ALLOW Ilg NOISE PATH BACK TO
GROUND
8 - 23
Additional Containment of Common Mode Current
Paths - Better
Input Transformer
AC Drive
A
R
B
XO
Ilg
Ilg
PE
Shielded Cable / Armor
with PVC Jacket Motor Frame
C
Ilg
S
V
Motor
Vdc
bus
T
(-)
Ilg
W
PVC
C lg-m
PE
Ilg
PE
Common Mode
Current Path
EARTH GROUND
Potential 4
U
(+)
Ilg
Potential #1
Additional
Motor PE
GND wire
Potential #2
Potential #3
Shielded output cable/armor with PVC jacket PROVIDES BETTER WIRING & NOISE CONTROL WITH NO ACCIDENTAL
ARMOR CONTACT TO GROUND BUT IF NOT USED ON INPUT MAY ALLOW Ilg NOISE PATH BACK TO GROUND
8 - 24
Common Mode Choke as a Solution
A
AC Drive
R
B
XO
S
Vdc bus
(+)
Ilg
W
PE
PE
Common Mode
Bus Capacitors
EARTH GROUND
Potential 4
Ilg
IlgCommon Mode
Current Path
Potential #1
Motor
C lg-m
(-)
C
Motor Frame
U
V
T
PE
Common Mode
Choke
Potential #2
Motor PE
GND wire
C lg-c Ilg
Potential #3
Implementation of CM core in drive output, CM cores are smaller than output
line reactors.
8 - 25
Common Mode Choke (Ferrite Core) Properties
Inverter
output
Voltage
Vu-v
70 nS
Ipeak
Common
Mode
Ilg Current
Ilg Current
With
Common
Mode
Chokes
6 MHz Oscillation
Slower 1.5 us to 5 us risetime
Lower 200 kHz to 63 kHz Oscillation
Lower 1/3 Ipeak
CM ferrite cores attenuate the high frequency noise peak amplitude and more
importantly the ILG noise current rise time that develops CM noise ground
voltage Critical Distances become Longer.
8 - 26
Inductive Loads Switched On and Off
Very Strong Electro Magnetic Emissions From Non Supressed Inductive Loads Switching on and off
Relays, Contactors, Solenoids Electro Magnetic Brake coils
8 - 27
Drive Input
• Power Distribution - Chapter 2 of DRIVES-IN001
– Grounding
• Symptoms: Equipment Damage, Overvoltage Faults, Ground Faults
– Solid Ground
– High Resistance Ground
– Ungrounded
– Rockwell can support all three schemes
• Some competitors only support solidly grounded
28
Drive Input
• Power Distribution - Chapter 2 of DRIVES-IN001
– Solid Ground Wye– Recommended
• Controlled path for common mode noise current
• Consistent line-to-ground voltage reference that minimizes
insulation stress
• Accommodation for system surge protection schemes
• Fewer Technical Support Calls when this
scheme is used
• The only system allowed for CE compliance
29
Drive Input
• Power Distribution - Chapter 2 of DRIVESIN001
– High Resistance Ground Wye
• Potential nuisance ground fault trips causes drive
overvoltage
• Potential equipment damage
30
Drive Input
• Power Distribution - Chapter 2 of DRIVES-IN001
– Ungrounded
• System level ground fault detection or suppression may be required
(use of zig-zag transformer for ground fault detection)
• High voltages can be present between the
chassis of the drive and the internal power
structure components causing undue stress on insulation
• Dangerous – ground faults on the primary
will show primary voltage potential to ground
on the secondary side
31
Drive Input
• Drive Configuration – 750-IN001 or 520-UM001
– Power Jumper Configuration
• Symptoms: Equipment Damage, Ground Faults
32
Drive Input
• Drive Configuration – 750-IN001 or 520-UM001
– Power Jumper Configuration
33
Drive Panel
• Drive Configuration – DRIVES-IN001
– Grounding Practices
• Symptoms: VFD Faults Overvoltage/Over currents, Ground
Faults, Communication Faults
• Proper grounding practices for short circuit/safe touch
• Improved bonding using unpainted surfaces, flat braided ground
straps
• Motor Ground directly back to drive, then from drive to panel
ground
• Motor Cable shield directly back to drive (either PE or drive
chassis), then from drive to panel ground, 360 degree
terminations.
34
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Copyright © 2011 Rockwell Automation, Inc. All rights reserved.
Copyright ©2012 Rockwell Automation, Inc. All rights reserved.
36