Differential and common
mode passive motor filters
Dennis Kampen, EWG
dennis.kampen@block-trafo.de
Tel. +49 (0)4231 678 178
Agenda
• Differential mode and Common Mode
• Differential mode filters
– Topologies
– effects
• Common mode filter
– Topologies
– Effects
• Materials
• Summary
Differential and Common Mode
definition
Differential mode voltage
inverter
uab  t   ua0  t   ub0  t 
 
 
 
load
grid
Common mode voltage
uM0  t 
 
The inverter generates no pure symmetrical voltage system.
A common mode voltage remains.

ua0  t   ub0  t   uc0  t 
 
 
3
 
Distribution of disturbances
Differential mode and common mode
HF-disturbances are not damped by line
reactors or harmonic filters.
Differential mode path
Grid impedance
Line inductor
Differential mode path
Common mode path
parasitic
motor/
cable
capacitance
The motor side inverter generates differential mode disturbances between the phases on
motor and line side. Common mode currents are driven over the motor and cable
capacitances to Earth and back over the line side to the input rectifier and the DC link.
Distribution of disturbances
conducted emissions>150kHz
Differential mode
Common mode
The grid can be safely protected against disturbances of all frequencies with a line inductor
or a hamonic filter and in addition an EMI-Filter.
Motor filter
Motor terminal voltage
Inverter output voltage
wave forms without filter
Motor current
Motor starpoint – Earth voltage
cable
grid
motor
Motor leakage current
Line leakage current
Motor phase voltage
Differential mode motor filters
motor choke,
dv/dt-filter
Inverter output current
Motor current
Motor
terminal
Leiter
Leiter
Spannungvoltage
am Motor
Spannung / V
800
400
200
0
-200
-400
-800
40.00m
50.00m
Zeit /s
60.00m
Motor starpoint – Earth voltage
grid
Cable
motor
Motor leakage current
Reduction of differential mode dv/dt <500V/µs
 prevention of motor terminal overvoltages
 reduction inverter output current peaks
Differential mode motor filters
motor choke or dv/dt filter
WR
filter
cable
motor
Phase-phase motor voltage
Phase-Phase-output voltage
Undamped du/dt-filter
Strongly damped
du/dt Filter
no filter
No filter  high overvoltage peaks because of long cable reflections
Undamped du/dt filter  defined overvoltage peaks , low dv/dt
Ohmic damped dv/dt filter  low voltage overshoots, but high losses
Differential mode motor filters
why dv/dt filter?  motor insulation
Motor insulation stress depends on amplitude and rise time of the motor voltage.
Phase-phase motor voltage
Literature: PHD Thesis Norbert Hanigovski, 2005
Differential mode motor filters
SFB – sine filter
Motor terminal voltage
Motor current
Motor starpoint – Earth voltage
grid
Cable
motor
Motor leakage current
Generates nearly pure sinusoidal differential mode motor voltages and motor
currents, because fundamental frequency<filter resonance frequency<switching
frequency  long motor cables are possible, no HF losses in motor
Desing of dv/dt filter and sine
filter
insertion loss through AC simulation
inverter
Uin(f)
Uout(f)
Design of dv/dt filter
• switching frequency<filter resonance frequency
• ohmic damping often required
Gain 
Uout (f )
Uin (f )
design of sine filter
• fundamental frequency<filter resonance frequency
<switching frequency
Effect of motor filters on DM
motor terminal voltage
No filter
Motor choke, dv/dt-filter
Differential mode
motor terminal
voltage
SFB sine filter
dv/dt
Peak voltage
No filter
No filter
Motor choke, dv/dt-filter
Motor choke, dv/dt-filter
SFB sine filter
SFB sine filter
Ground
Cable length /m
Cable length /m
Effect of motor filters on inverter
output current
Inverter output current (ideally)
No filter
No filter: high output current peaks
With any inductive filter: elinination of output current peaks.
 Reduction of inverter stress
27.09.2012
Passive Filter
With motor choke or any
other inductive filter
13
Common mode motor filters
Motor terminal voltage
Motor current
SFB
Motor starpoint – Earth voltage
grid
Cable
motor
Motor leakage current
Differential mode motor inductors, dv/dt-filters or sine filters have nearly no
impact on the common mode disturbances.
So, how to damp common mode disturbances?
Common mode motor filters
common mode choke
grid
Cable
motor
SFB
Simple damping of common mode currents.
Damping depends on cable capacitance.
Resonances possible. Switching frequency is not
filtered. With a further connection to DC link,
a LC-filter is formed with high common mode
filter performance.
Literature:
Common mode motor filters
allpole sine filter - SFA
Motor current
Motor terminal voltage
Motor starpoint – Earth voltage
grid
Cable
motor
Motor leakage current
Line leakage current
The DC-link connection short cicuits the
common mode disturbances optimal to the
DC-link.
Effect of motor filters on motor
star point voltage
No filter
Motor star point
to ground voltage
SFA allpole sine filter
dv/dt
No filter
Peak voltage
No filter
Ground
SFA allpole sine filter
Cable length /m
SFA allpole sine filter
Cable length /m
Effect of motor filters on motor
bearing currents
No motor filter: maximum bearing currents
SFA allpole sine filter: no bearing currents
Only differential mode motor filter:
small reduction of bearing currents by 5%-10%
Effect of motor filters on
conducted EMI on line side
No filter
Motor choke or differential mode sine
filter
No filter
SFA allpole sine filter
Small effects of differential mode filters, high impact of common mode filter
Core materials in passive
filters
Electrical steel
High efficiency steel 1
High efficiency steel 2
Medium efficiency steel 1
Medium efficiency steel 2
Standard efficiency
Foto: metal-traders.de
Product examples for electrical steel:
50Hz transformers
Motor chokes, sine filter<400Hz
Three phase filters
High power up to 3000A
Core materials in passive
filters
Powder compounds, amorphous materials
Sendust /KoolMu /X-Flux /Iron Powder /HighFlux /Megaflux
Ring core
UI-core
EI-cores
METGLAS,
amorphous C-cores
Core materials in passive
filters
Ferrite
Ferrit-transformers
<1W until > 100kVA
Ferrit inductors
<1A until 1000A
Winding: Copper/Alu: round, rectangle, litz wire, HF-litz wire, foil
Complete filters
product examples
Sine filters > 400Hz
Sine filters higher power
Potted filters
EMI-Filters
AC-Side
DC-side
Summary
Differential mode filters reduce…
• dv/dt and motor peak voltages (less motor insulation stress)
• inverter output current peaks (less inverter stress)
• Low cost: Motor choke, High quality: sine filter SFB
Common Mode filters reduce…
• dv/dt and peak shaft voltages (less motor insulation stress,
elimination of bearing currents)
• conducted and radiated EMI, maybe shielded cabels are not
necessary
• High quality: allpole sine filter SFA
Summary
Thank you very much!
Question?