Download EMI in Modern AC Motor Drive Systems

in that year, where he was engaged in the research and development
of communication network architectures and communication switching software. He joined the Division of Medical Informatics, Shimane University Hospital, in 2004. He moved to the Department of
Medical Informatics, Shimane University School of Medicine, in
2005. His research interests include information processing systems
in hospitals as well as computational learning theory and its application. He is a member of the Institute of Electronics, Information
and Communication Engineers of Japan, the Japanese Society for
Artificial Intelligence, the Japan Society for Health Care Management, and the Japanese Society for Clinical Pathway.
Takato Kudou was born in Oita, Japan, in
1963. He received his B.Eng., M.Eng. and D.
Eng. degrees in communication engineering, all
from Kyushu University, Fukuoka, Japan, in
1985, 1987 and 1990, respectively. From 1990
to 1994, he was a Research Associate of the
Department of Computer Science and Communication Engineering, Kyushu University. In 1994,
he joined the Department of Electrical and Electronic Engineering, Oita
University, and is currently an Associate Professor of the same University. His research interests have been on electromagnetic direct/inverse
scattering and FDTD analysis of the electromagnetic environment. Dr.
Kudou is a member of the IEICE of Japan, the IEE of Japan and the
Japanese Society of Medical and Biomedical Engineering.
Takashi Kano was born in Tokyo, Japan, in
1949. He received his B.Eng. degrees from
Sophia University, Tokyo, Japan, in 1973. He
received his Ph.D. degree from Toa University,
Yamaguchi, Japan, in 2004. Since 1973, he
has worked at the Department of Medical Engineering Service in Mitsui Memorial Hospital for
32 years. Since 2006, he has been working at
the Department of Biomedical Engineering, Faculty of Health and
Medical Care, Saitama Medical University. Prof. Kano is a member
of the Japanese Society of Medical and Biomedical Engineering, the
Healthcare Engineering Association of Japan and the Japanese Society
of Medical Instrumentation.
EMC
EMI in Modern AC Motor Drive Systems
Firuz Zare, Queensland University of Technology Brisbane, QLD,
Australia Email: [email protected]
Abstract—In this paper, several aspects of high frequency related
issues of modern AC motor drive systems, such as common mode
voltage, shaft voltage and resultant bearing current and leakage
currents, have been discussed. Conducted emission is a major problem in modern motor drives that produce undesirable effects on
electronic devices. In modern power electronic systems, increasing
power density and decreasing cost and size of system are market
requirements. Switching losses, harmonics and EMI are the key
factors which should be considered at the beginning stage of a
design to optimise a drive system.
Introduction
Nowadays, more than 60 percent of the world’s energy is used
to drive electric motors. Due to growing requirements of speed
control, pulse width modulated inverters are used in adjustable
speed drives. Rapid developments in semiconductor technology have increased the switching speed and frequency of power
switches dramatically. In a motor drive system, a voltage source
converter with hard switches generates high dv/dt, which
causes leakage currents due to stray capacitances in an electric
motor. As shown in Fig.1, a modern power electronic drive
consists of a filter, a rectifier, a DC link capacitor, an inverter
and an AC motor. Many small capacitive couplings exist in the
motor drive systems which may be neglected at low frequency
analysis but the conditions are completely different at high
frequencies.
Electromagnetic Interference (EMI) is a major problem in
recent motor drives that produces undesirable effects on electronic devices. In modern power electronic systems, increasing
power density and decreasing cost and size of a system are market requirements. Switching losses, harmonics and EMI are the
key factors which should be considered at the beginning stage
of a design to optimise a drive system.
Common mode voltage creates shaft voltage through electrostatic couplings between a rotor and stator windings and the rotor and a frame which can cause bearing currents when the shaft
voltage exceeds a breakdown voltage level of the bearing grease.
An increase in the carrier frequency of voltage-source Pulse
Width Modulated (PWM) inverters based on high-speed
switching devices has improved operating characteristics of the
inverters. High speed switching can generate the following serious problems due to high dv/dt:
• Ground current escaping to earth through stray capacitors
inside a motor
• Conducted and radiated noises
• Shaft voltage and bearing current
Models of parasitic couplings and high frequency components for an inverter fed induction motor drive system
are investigated to determine suitable models to predict
©2009 IEEE
53
Voltage
Supply
Diode
Rectifier
Filter
dc-Link
Capacitor
Cables
Inverter
Motor
Filter
PE
Fig. 1. A power electronic motor drive system with capacitive couplings.
Stator (s)
End
Motor
Winding Frame
Stator
Cww
Crs
Cwr
Winding (w)
Shaft
Rotor
Ballbearing
Cball
Ballbearing
Cwr
Cww
End
Winding
Cws
Stator
Rotor (r)
Cball
Fig. 2. A view of stator slot and capacitive coupling in an induction motor.
Outer Race
Inner Race
Ball
Outer Race
dBO
CBO
Ball
Shaft
C
dBI
S
CBI
Inner Race
(a)
(b)
(c)
Fig. 3. (a) general structure of a ball bearing (b) a view of ball, outer and inner
races and capacitive couplings (c) simple model of a ball bearing.
b­ earing currents and shaft voltage over a wide frequency
range. A high frequency model of an electric motor is an
important issue for power electronic engineers, which helps
them to analyse leakage and bearing currents and to design
EMI filters. At low frequency, an equivalent circuit of an
electric motor consists of inductances and resistances without considering stray capacitances and skin effect. These
issues become more important at high speed switching applications due to high dv/dt.
54
Fig.2 shows the capacitive couplings of an induction motor and a
view of stator slot, where:
Cwr is the capacitive coupling between the stator winding and rotor
Cws is the capacitive coupling between the stator winding and stator
frame
Crs is the capacitive coupling between the rotor and stator frame
Cww is the capacitive coupling
between turns of stator winding
Cball is the capacitive coupling of
ball bearing
Fig.3 shows a general structure of
a ball bearing and shaft in an AC machine. As shown in this figure there
are some balls between outer and inner races with lubricated grease between the balls and the races. There
are capacitive couplings between the
outer and inner races. During operation, the distances between the balls
and races may be changed and will
vary the capacitance values and resultant electric field between the races
and balls. Due to this fact, this capacitance has a nonlinear value. Lubricated grease in the ball bearing cannot
withstand a high voltage and a short
circuit through the lubricated grease
may happen, thus this phenomenon
can be modelled as a switch.
PWM inverters have been found
to be a major cause of motor bearing
failures in inverter motor drive systems. All inverters generate common
mode voltages relative to the ground,
which make bearing current through
motor parasitic capacitances.
According to Fig.4.a, phase voltages and a common mode voltage (Vn)
can be derived based on the power
converter voltages (Va, Vb,Vc). Each
leg voltage of a three phase inverter
is given by:
Va 5 Van 1 Vn
Vb 5 Vbn 1 Vn
Vc 5 Vcn 1 Vn
(1)
And then:
Va 1 Vb 1 Vc 5 1 Van 1 Vbn 1 Vcn 2 1 3Vn
(2)
Van 1 Vbn 1 Vcn 5 0
(3)
It is clear that in a three-phase system:
So, the common mode voltage can be calculated as:
©2009 IEEE
Vn 5 1 Va 1 Vb 1 Vc 2 /3
(4)
In a three phase power inverter, a
DC voltage is converted to three
phase voltages with 120° phase
shift. Fig.4.b shows three phase leg
voltages of an inverter with common
mode voltage.
The trend in increasing switching frequency improves the quality
of current waveforms in motor drive
systems but due to short switching
time, a high dv/dt is produced across
the motor terminal. The leakage
current is created by a high voltage
stress during switching time and capacitive coupling in an AC motor.
Fig.5 shows a simple equivalent model of an induction motor
which contains main capacitances
between the windings, rotor and
the stator frame.
Va
Vb
Vc
Van
Vbn
Vcn
Vn
(a)
60
40
20
Analysis of an Electric
Motor at High Frequency
0
The stray capacitance between the
–20
windings and the stator frame is the
most significant parasitic compo–40
nent compared to the other stray
capacitances which generate signifi–60
cant conducted emission noise. At
high frequency, an electric motor
(b)
can be modeled as distributed capacitors, inductors and resistors as
shown in Fig.6 and the maximum Fig. 4. (a) three phase voltage source inverter (b) PWM voltage.
frequency can be determined using
the standing wave’s equation. We consider one v section to
model the motor at high frequency as shown in Fig.6.b and
Rotor
Winding
only the stray capacitance between the windings and the stator
has been considered. Each motor has different high frequency
Cwr
parameters due to its structure, size and materials.
Cws
Crs
Cball
Calculation of Cws
The first step is to measure the magnitude and the phase values
of the impedance in terms of frequency based on two different
connections of the windings.
The first measurement is based on Fig.7.a where three phases are connected to each other at the terminal sides. Thus, the
impedance between the phases and the stator (phase and magnitude values) can be measured in terms of the frequency using
an impedance analyzer.
Based on this configuration, the impedance of the stray capacitance, Cws, is much higher than the impedance of Ls and
Rloss at low frequencies and the model can be simplified as two
parallel stray capacitances.
Fig.8 shows measurement results of the impedance between
the phases and the stator frame of a 5.5 KW motor. According
to Fig.8, the phase value is around –90 at ,40 KHz which
addresses the fact that the two capacitors are connected to each
other in parallel. The stray capacitance of the electric motor has
been calculated using the magnitude and the phase values from
Stator Frame
Fig. 5. A high frequency model of an induction motor.
10 kHz up to 40 kHz and the result shows that Cws is almost
around 0.9 nF. The capacitance value is calculated based on the
measurement results and 0 Zcws 0 5 1/vCws.
Calculation of Ls and Rloss
The second measurement is based on Fig.7.b where three
phases are connected to each other and the impedance between
the phases and star point is measured. Based on this configuration, the impedance of the stray capacitance is much higher
than the impedance of Ls and Rloss (Fig.9.a) at low frequencies
©2009 IEEE
55
C
C
C
R-Lstator
R-Lstator
R-Lstator
1
C
2
Rloss
n
C
C
C
Ls
Phase
C
n
Cws
(a)
Cws
(b)
Fig. 6. (a) a distributed model of an electric motor at high frequency (b) a simple
model of an electric motor.
and it can be simplified as an inductive and a resistive load as shown in
Fig.9.b.
Fig.10 shows measurement results of the impedance between the
phases and the star point. The inductance and resistance values can
be calculated using the measurement results and based on the circuit diagram shown in Fig.9.b. The
inductance value is decreased from
5.77 mH down to 5.56 mH when
Ls
Phase
n
Ls
RLoss
Phase
n
RLoss
cws
cws
(a)
Stator
(a)
(b)
Fig. 7. Two different connections to measure the impedance between
(a) the phases and the stator (b) the phases and the star point.
Fig. 9. Circuit diagram for measurement between phases
and star point (a) a model of motor (b) simplified model for
low frequency range.
718
1 · 104
(b)
1 · 103
Z1〈1〉
Z2〈1〉
1 · 103
100
100
10
10
1
1 · 104
4.73
1 · 105
Z2〈0〉
(a)
1 · 106
1
1 · 104
72.2
P2〈1〉
1 · 106
Z1〈0〉
(a)
1 × 104
1 · 107
100
1 · 105
1 · 107
1 × 107
100
P1〈1〉
50
50
0
0
–50
–50
–100
1 · 104
–85.5
1·
105
P2〈0〉
1·
106
1·
107
–100
1 · 104
1×
104
1 · 105
1 · 106
P1〈0〉
(b)
1 · 107
1 × 107
(b)
Fig. 8. Measurement results (a) magnitude and (b) phase
values between the phases and the stator of a 5.5 KW motor.
56
Fig. 10. Measurement results (a) magnitude and
(b) phase values between the phases-star point
of a 5.5 KW motor.
©2009 IEEE
the frequency is increased from 10 kHz up to 40 kHz. At low
frequency (below 100 kHz), the input impedance of the windings (according to Fig.9.b) is ZRL 5 Rloss3jvLs /jvLs 1Rloss.
The magnitude and phase values of this configuration, which
are used to extract the resistance and inductance values, are
given in Fig.10.
The simulations have been carried out based on the following equations driven from the above figures:
Zstator_winding 5
Zwinding_winding 5
Rloss
Cws
Cws
Fig. 11. A high frequency model of a motor with a short cable.
ZCws
Ls 5 5.56 mH; Cws 5 0.9 nF; Rloss 5 2215 V; Lcable 5 378 nH;
Rcable 5 3 V
2
ZCws
1 ZRL 2
1 ZRL
1 · 104
3.1 × 103
The 5.5 kW motor is modeled based on this analysis and the
simulation and experimental results are shown in Fig.12. The
cable impedance is negligible at frequencies below 4-5 MHz
but it becomes significant above that frequency range.
Active EMI Filters
The use of Active EMI filters based on current injection is a
proper solution to cancel common mode high frequency currents. Fig.13 shows a block diagram of an active EMI filter
with a common mode transducer.
The filter composes of an emitter follower using complementary transistors and a common mode transformer to measure the
78.3
100
Phase_exp〈1〉
1 · 103
50
100
0
1·
arg(Zsimul(f )) · 180
π
–50
Z_exp〈1〉
Zsimul(f)
1
1 · 104
1 × 104
n
2 3 ZCws 1 ZRL
Where ZRL 5 Rloss 3 jvLs /jvLs 1Rloss and 0 Zcws 0 5 1/vCws
There is a short cable from the motor terminals to
the impedance analyzer, which can be modeled as a R_L
in series with the motor as shown in Fig.11. The second
resonance frequency shown in Fig.10 is associated with
this inductance and the first stray capacitance, Cws. Thus
the inductance value of the cable can be calculated using
Lcable 5 1/ 1 2pf 2 2Cws 5 378 nH. The resistance value of the
cable can be calculated at the second resonance frequency.
Based on this analysis, the parameters of the 5.5 kW motor
have been extracted as follow:
10
Ls
Lcable
Stator
ZCws 1 ZRL 1 ZCws 2
2
2.79
Rcable
–91.1
105
1·
106
1·
107
7
Z_exp〈0〉, f
1 × 10
–100
1 · 104
1 × 104
1 · 106
1 · 105
Phase_exp〈0〉,
(a)
f
1 · 107
1 × 107
(b)
1 · 103
79.087
100
Phase_exp〈1〉
741.102
50
arg(Zsimul(f )) · 180
π
100
0
10
–50
Z_exp〈1〉
Zsimul(f )
3.378
1
1 · 104
1 × 104
1·
–85.5
105
1·
Z_exp〈0〉, f
106
1·
107
1 × 107
(c)
–100
1 · 104
1 × 104
1 · 106
1 · 105
Phase_exp〈0〉,
f
1 · 107
1 × 107
(d)
Fig. 12. Experimental and simulation results of a 5.5 kW motor; (a) magnitude (b) phase values between phases-stator
(c) magnitude (d) phase values between phases-star point.
©2009 IEEE
57
Vs
duce undesirable effects on electronic devices. In modern power electronic systems, increasing power density and decreasing
cost and size of system are market requirements. Switching
losses, harmonics and EMI are the key factors which should
be considered at the beginning stage of a design to optimise
a drive system. In most power electronic designs, EMI issues
have not been taken into account as one of the main factors;
and mitigation techniques for EMI are considered at the last
stage of design!
Motor
Ileakage
Active
EMI Filter
References
Fig. 13. An active EMI filter.
leakage current. A high frequency amplifier generates the same
current and injects it into the motor drive circuit in order to bypass the current as shown in Fig.13. In this case, the leakage current generated by dv/dt is circulated through the active filter.
There are few issues to design an active EMI filter which
should be considered:
• A high band width power amplifier to generate the leakage
current
• A three-phase or a single-phase current transducer with symmetrical configuration to measure current accurately
• A separate power supply for the active EMI filter.
Conclusions
A high frequency model of an electric motor has been analyzed
in this paper. One of the advantages of the high frequency
model of the electric motor is to estimate and analyze the leakage and bearing currents to design EMI filters using simulations. This paper addresses a simple method to extract high
frequency parameters of an electric motor based on the measurement results. It is recommended to estimate the behavior
of the motor in a frequency range, when the motor acts as a
capacitive or an inductive load according to the phase values of
the impedance. Then, the parameters can be calculated directly
from the magnitude and the phase values of the impedance.
In this paper several aspects of high frequency related issues
of modern AC motor drive systems, such as common mode
voltage, shaft voltage and resultant bearing current and leakage currents, have been discussed. Conducted and radiated
emissions are major problems in modern motor drives that pro-
58
[1] Firuz Zare, “Power Electronics E-Book”, www.peeeb.com; from this website, you can virtually attend lectures and tutorials and also access to lecture
notes and computer Labs.
[2] Firuz Zare, “High frequency model of an electric motor based on measurement results”, Australian Journal of Electrical & Electronics Engineering
(AJEEE), Vol 4, No 1, 2008, page 17-24.
Biography
Dr. Firuz Zare received his B.Eng degree in
Electronic Engineering from Gilan University,
his MSc degree in Power Engineering from K.N
Toosi University and his Ph.D. degree in Power
Electronics from Queensland University of Technology in 1989, 1995 and 2001, respectively.
He spent several years in industry as a team
leader and development engineer where he was
involved in electronics and power electronics projects. Dr. Zare won a
student paper prize at the Australian Universities Power Engineering
Conference (AUPEC) conference in 2001 and was awarded a Symposium Fellowship by the Australian Academy of Technological Science and Engineering in 2001. He received the Vice Chancellor’s
Award in research in 2009 and faculty excellence award in research
as an early career academic from Queensland University of Technology in 2007. Dr. Zare has published over 60 journal and conference
papers and technical reports in the area of Power Electronics. He has
been invited as a reviewer and technical chair of national and international conferences and presented several seminars and workshops. He
presented a half-day tutorial at the 2007 IEEE International Symposium on EMC in Hawaii, at the EMC Asia Pacific Workshop in
Singapore in May 2008 and at the 2008 IEEE International Symposium on EMC in Michigan. Dr. Zare is a senior lecturer at the
Queensland University of Technology, Australia and a senior member
of the IEEE. EMC
©2009 IEEE