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High Frequency M odeling
of
Induction Motor Drives for
EM1 and Overvoltage Mitigation Studies
L.
Arnedo and
K.
Venkatesan
Center for Power Electronic Systems
Electrical and Com pute Engineering Department
Unive rsity of Puerto Rico,Mayaguez C ampus
The present work deals with a comparative study of different
overvoltage mitigation techniques and
their
effect upon
the
COD
ducted EMI emissions in
an
nduction
motor
dlive system.
A
de-
tailed
PSplce model
of
the system has been developed
considering lumped parameter model
with 64
seclious for the
cable and high frequeucy models for the
IGBT
PWM nverter
and Induction motor.
Simulanon
results are compared
with
er-
perimental results.
Four
overvoltage mitigation techniques such
as
RLC
nverter
output
Nter, modified RLC inverter output
fX-
ter, RC Blter at motor te d n a l s and dv/dt control
are
consid-
e r e d
1.
IR0DUCTION
Advances
in power electronic switching devices such
as
Power MOSFETs and IGBTs have enabled high tequency
switching
operatious
and hence improved the performance of
PWM inverters for feeding induction motors. However these
new technologies have created new problems related to Elec-
tromagnetic interference (Emand over-voltages at the ter-
m i na l s
of electric ma chi es [I].
Presently EMC regulations are more stringent, imposing
additional design objectives for power electronic systems.
Some
forms
of filtering are required
for
the input and out-
put(s)
lines of
equipmen t However, the optimum design a p
poach is to minimize E M at the source of emission. This
reduces the size and volume of the filter and reduces the
p a -
sibility EMI being radiated internally to other sen sitive com-
ponents m the equipment.
When an induction motor is con nected to a PWM IGBT
inverter through a cable, over-voltage is caused at its termi-
nals w i g lectric s t r e ss
on
nter-- insulation
of
motor
windings.
There are a lso parasitic currents referred as cam -
mon mode (CM) and differential mode OM) currents flow-
ing through the parasitic capacitances of the inverter, cable
and m ot a. These high frequency currents create EMI prob-
lems in
the
system.
The frequency range o f interest for con-
ducted E M
n
power electronics
is
usually from IO KHz to
IO
MHZ
[Z].
The over-voltage phenomenon ha s destructive effectsupon
both cable and machine insulation system due the energy
. contained in the transient overshoot caused by voltage wave
reflection
at
the electric machine
terminals.
This phenomenon
is also directly related with the conducted EM . There exists a
close relationship hetween the over-voltage phenomenon and
the E M problem through the rise and
fall
times of the volt-
age pulses generated by the PWM Inverter. For voltage
.
.
pulses with short rise times the voltage at motor terminal and
the magnitude of the CM nd DM currents will increase and
for voltage pulses with large rise time the voltage at motor
terminal and the magnitude of the CM and DM urrents will
decrease [3]. The overvoltage mitigations techniques change
the rise and fall times of the incident pulses to cable or motor.
It is important that the EMI and over voltage characteris-
tics of the system must be analyzed and predicted in the d e
sign stage. The simulation model of the system taking into
account the noise cw en t paths would
he
useful
for imple
menting E M mitigation circuits in system design.
In this work, a PSpice model of an electrical drive system
that allows prediction of over voltage at the terminal of the
motor and conducted EMI in presence of long feeders is d e
vel& The m odel is used to study the effect
of
over-
voltage mitigation techniques upon conducted emissions.
11.
DEVELOPMENTOF SYSTEM
MODELXNP-SPICE
A. HFPWMInvener Model
For an accurate EMC model of the inverter it is necessary
to take into a w u n t the
HF
parasitic paths. Fig. 1shows he
HF quivalent circuit for one leg o fth e inverter [4].
~~~
( . c .
-
Fig 1
Fig. 1
Inverter
model for
EM1
studies
The most important parasitic paths of this circuit are: the
parasitic inductance
of
the emitter
Le
and the internal para-
sitic capacitances of the IGBT. The value of L s taken from
d a ic e datasheet and parasitic capacitances of IGBT are in-
cluded in the IGBT P-spice model. Stray inductances of the
connecting wires (I.,) have very small values and affect prin-
cipally the differential conducted emissions. For this study
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this inductance has been neglected because the P W M nverter
is enclosed in a package and the length of the conn ecting wire
is very small. The Stray capacitanceC, between the collector
and grounded heatsink is measured mth an impedance ana-
lyzer.
In
the
HF
range the equivalent circuit of the
DC
ink
ca-
pacitor consists of the series combina tion of capacitance , re-
sistance and inductance, as sh o w in fig.2
Ls
Rs
C s
- : -
Ca- bd-
FO
.
F .
2 . G
Fig.
2
High
kquency model ofDC
link capacitor
When the frequency ncreases the impedance of the capaci-
tor
decreases linearly at rate of -2OdEVdecade. Tb e im pedance
of the indu ctor increase until it
equals
that of the capacitor at
the point of resonance. At
t h i s
point the impedance
is %.
For
higher frequencies the impedance of the inductor increases at
rate
of
+20dB/decade. The im pedance of the DC link capaci-
tor bas a strong effect upon the d ifferential conducted
emis-
sions [4].
The parameters
of
the inverter used are given in append ix
B.
H
Cable m o d 1
The
transient phenomena of cables have been explained
in
detail using transmission
lie
theory
[ 5 ]
and different cable
model configurations used in studies of vo ltage reflection are
presented in
[a] In
P-spice program
although
there are circuit
simulation elements for transmission
lines,
the options
are
limited for multi-conductor cables. For o b t a i i g an ads
quate model of the cable for high fiq ue nc y studies software
Maxwell 2D xtractor is used.
The Maxwell 2D extractor
uses
finite element method to
compute the circuit parameter matrices such as inductance
and capacitance for any arbitrary multi-conductor transmis-
sion line. These circuit parameters depend upon the geometry
of the structure and the characteristics of the materials that
make up the s fmchue. Once computed, tbese circu it parame-
ters c nbe transformed into a P-spice s sub circuit forming the
lumned
A
reoresentation of the cable. In order
to
model aw
experimentally determined values closely agree with the cal-
culated values as
shown
in the same figure.
Fig. 3
Cable
on ode
impedance
C. Induclion
Motor model
A
motor model
as
s h o w in fig.4 suitable
for
low and bigh
frequencies is used
[SI.
The model is based
on
the expeainten-
tal observation of frequency
response
and an approximation
of the distributedHFmotor model presented in
[9]
wfiere
it
is
possible to identify three dominant capacitancesC,
Ci,
C, t
high teqnencies.
l l
Limp
m
odel
Fig.
4 Induaimmotor
model for
wide frequency
range
In above C
q i
and %representing phase
or
neutral
to
ground capacitance, phase
to
phase capacitance, phase to neu-
tral capacitance and eddy
loss resistor
are effective
at high
frequencies. Series impedance elements c onsisting of %, L
and C, are associated with phase to ground and neutral to
ground
currentpaths
at medium frequencies.
propriately the cable in a wide frequency range
64
umped
sections have been used [7]. Fig 3shows the variation of
common mode impedance with frequ ency as calcu lated for a
six meter SJ 4-14
AWG
cable using 64 lumped sections. The
The advantages of this model are that the parameters can
be determined by frequencyresponse tests and
the
model can
be used
for
over voltage and condu cted
EMI
studies. This can
be implemented in P-spice or Saber for analysis of inverter
fed induction motor drive systems.
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I ll. S W AT IO N F OVR-VOLTAGE AND
E M
ble twe
SJ-4
14
AWG.
a 1/3 HP
three
Dhase 208
V
induc-
tion -motor and LISN' was modeled
;or low
and high
points ofmeasurements
are
indicated in
figure
,
effectivenessOf
the
n predicting over-voltages f i q e n c Y
studies,
Typical results =e given below, The
at motor terminals and conducted EMI in the system has been
verified
through
experimental results. An induction motor
Drive
system consisting
of
a
three
phase
PWM
inverter,
Ca-
Fig. 5 Complete Dri
Figure 6shows the simulated and experimental voltage
pulses at the inverter and motor erminals when the lengtb
of he cable connecting he inverter and motor s sixmeters
and the
rise
time
of
the inverter voltage pulse is 110 ns.
I*' _ ..,
BXP*ri .(
. ,
I
..
'*
-.&
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VI
OVERVOLTAGEITIGATION ECHNIQUES
Conventional
Ourput
Filter
A
low-pass filter to reduce the d dd t
of
the inverter volt-
form
s passed
viaually
unchanged except
for
the delayed
rise and fall limes [IO]. Fig.
9
shows the inverter
output
il-
The developed models are
used
to analyze the effect of
over-vo ltage mitigation techniqu es such a s Inverter output
filter, motor terminal filter and dv/dt control upon con-
ducted emissions,
To
compare the effectiveness
of
these
age pulses is used at the inverter output The p m ave
...
a
strategies and their effect upon conducted emissions the
same induction motor drive system is studied with the in-
corporation of different types of filters
Fig.
9
Drive
System
Wim
Inverter
Output Filter
Figure.10 shows the line to line voltage at the inverter
and at motor terminals without and with filta. The rise
time of the inc ident pulse is 11Ons.
A
filter
has
been de-
signed to reduce the over-voltage at th e terminals of the
motor to
10%
of overshoot
and
increasing the rise tim e of
the incident pulses ffom 1 Ons to 32Ons. Although the fil-
ter reduces the overvoltage at motor terminals, there is no
significant reduction
in
the conducted
EMI
emissions up to
4 M H z as shown
in
Fig. 11;this is
because
the magnilude
of the ulmmon mode voltage is still the same.
Sf
m
u
a
Fig
-
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Ill
Fig. 12 .r)rve system
with
modified invertet output
filter
...... ...... _i..... . in-&..................
.........
i
.....
. . . .
. . .
. . . .
. . .
. . . . .
\ . . . . .
....
...
.... ....
;.
....
..
.... i
....
i , : : I
1 ; : i $ i j
Fig.
13
Voltage at motm terminsls aud
EMI
conducted emis-
I
Y
*
a
*
2 1 1
a
e
W.
sions
using modified inverter output filter
RC
tennination
Filter
The motor terminal over-voltage
can he
educed with a
first order resistor-capacitor filter that is connected in par-
allel with the motor taminals [IO]. With
a
designed filter
the ova sho d pacentage is 14%. The total EMI emission
does
not change appreciably compared to a
system
without
filter. Fig. 14
shows
the voltage at the inverter, motor ter-
minal and the spectrum obtained througb simulation up to
5.2 MHz
.................
......... .....,..
...................
D
.- .
. . . .
_ __
.....
_
.....
......
....
2
Fig. 14.Voltage
at motor
terminals and
EMI
onducted emir
.2S6
P_
......
si using
RC
en&ation
DV/DT Control
In d d d t control the output voltage rate
can be
con-
trolled by adding pby%ically a small capacitor between the
IGBT gate to collector to increase the Miller capacitance
[12]. The inverter voltage rise tim e
cbanges d e n
capaci-
tor of
0.15
nF is placedbetween he gate and collector. The
inverter voltage rise time without dvldt control is 1Ions
whereas with dv/dt
control
it increases o
524ns.
The over-
shoot at motor tamina ls with a rise time of 52411s s
6%.
Fig.
15 shows
the voltage a t inverter terminals, at motor
terminals and
EMI
conducted emissions.
A
decrease
of
10
dBuV
in the total EMI emission s is
obtained.
412
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V.
ONCLUSION
The use of RLC low pass inverter output filtes is al-
though effective to mitigate the over voltage at motor ter-
minals,
it does not have an appreciable effect in the
reduction of the total E M emissions. More reduction of
the total E M emission s is possible by inc reasing the value
of th e filter capacitor but thiswould increase the losses in
the filter.
Modified low pass RLC output filter represen ts an alter-
native to contro l effectively both the ov er voltage and
EMI
emission s without increasing significantly filter
losses.
A
reduction
of
both ovenoltage and EMI conducted muis-
sions is obtained.
The RC filter at motor term inals is a sim ple solution for
the ov er voltage control but it is an expensive solution due
to the high power losses in the filter and this technique
does not reduce the E M emissions.
Control with a capacitor placed between the collector
and gate of the IGBT educes the over voltage at motor
termina ls due to the decreased dv/dt of the inverter output
pulses. It is also effective in reducing conducted EMI
emissions.
The over voltage reduction techniques based upon low
pass filters have disadvantages such as the power
losses
in
the damping resistors of the filters, the lags introduced by
the filters and the sue of the filters in increasing the vol-
ume of the final product
Phase
Volts
Hcrtz
Input
ACKNOWLEDGEMENT
This work was supported primarily by the ERC
Program
of the National Science Foundation under Award EEC-
9131617.
APPENDIX
TABLE I
HIGH AND
MEDIUMFEQX
PNUMEIERS
FOR
%w OTOR
1
3
208i230
5otf.o
I
TABLE U
LOW FFSQuurcYP RI\METuIsFOn% Hp
MOTOR
I = 13.5 R L 4 . 4 2 0 6 H
L
.61R
Frenucncy-
60Hz
Lq-=L1-=20.95E-3H
J ~
= 0 . 0 0 1 5 1 5 k ~ z
TABLE m
PWM GBT INVERTERPUUMRERS
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