STUDY OF ELECTROMAGNETIC
ELECTROMAGNETIC INTERFERENCE IN INVERTER DRIVEN ASYNCHRONOUS
ASYNCHRONOUS MOTORS
MOTORS
ELECTROMAGNETIC
ETIC INTERFERENCE IN INVERTER
STUDY OF ELECTROMAGN
DRIVEN ASYNCHRONOUS MOTORS
MOTORS
Eng. Ioan ŢILEA PhD-student1, Prof. Eng. Călin MUNTEANU PhD1
1
Technical University of Cluj-Napoca, Electrical Engineering Faculty
REZUMAT. Interferenţele electromagnetice
electromagnetice conduse din motoarele asincrone alimentate de invertoare PWM sunt o
problemă actuală în sistemele de acţionare cu turaţie variabilă. Interferenţele electromagnetice conduse din aceste
sisteme se datorează motorului electric şi a cablului de alimentare. Analiza
Analiza caracteristicii impedanţei motorului electric
şi a cablului de alimentare sunt fundamentale pentru interpretarea emisiilor de mod comun şi diferenţial din sistemele
de acţionare cu turaţie variabilă.
variabilă.
Cuvinte cheie: supratensiuni la motor, invertor motor, interferenţe electromagnetice.
ABSTRACT. Conducted electromagnetic interference (EMI) in induction motor fed from pulse width modulation (PWM)
inverters is one of the most difficult current technical problems in variable speed system. The conducted EMI emissions
in inverter motor system is related to the inverter load which is the induction motor and cable. The analysis of the
impedance characteristic of the motor and cable is fundamental for understanding the common and differential mode
of emissions.
emissions.
Keywords: motor overvoltage, inverter-fed motor, electromagnetic interference.
1. INTRODUCTION
More than 60% of the world’s energy is used to
drive electric motors. Due to growing requirements of
speed control, pulse width modulation inverters are
used in adjustable speed drives. Rapid developments in
semiconductor technology have increased the switching
frequency of power electronics dramatically. Increase in
the carrier frequency of pulse width modulation and the
faster switching rates of the power electronics can
induce serious problems in inverter fed induction motor
drive system.
One of the most difficult technical problems is the
generation of high frequency currents flowing in all
parts of the drive system due to capacitive couplings in
the motor drive system. As shown in Figure 1, a modern
motor drive system consists of a filter, a rectifier, a DC
link capacitor, an inverter and an AC motor [2]. The
EMI problems are very high in the cable, motor area
because of the high frequency of the inverters output
voltage.
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 were the influence of the parasitic
capacitance is noticeably higher.
When the inverter is connected to the motor through a
long cable because of the cable inductance, stray
capacitance distributed between the cable wires and high
rise times of the PWM voltage from the inverter,
overvoltages appear at the motor terminals [1], [9], [12].
Overvoltages at the motor terminal stress the motor
winding insulation reducing its life and causes partial
discharges that damage the insulation.
Common mode voltage creates shaft voltage through
electrostatic couplings between 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 [3], [5].
The overvoltage and conducted EMI problems in
variable speed system with high power motors and long
cables can be clarified by motor impedance analysis.
This paper analyzes conducted EMI generated by a
PWM inverter fed induction motor drive system
analyzing the overvoltages occurring at the motor
terminals when the motor is connected to a PWM
inverter through different cable lengths.
Using a finite element analyses (Maxwell 3D
software) for the extraction of the induction motor high
frequency equivalent circuit and a circuit analysis
(Simplorer) for the inverter fed induction motor drive
system.
_____________________________________________________________________________________
Buletinul
AGIR nr. 3/2012 ● iunie-august
1
Buletinul AGIR nr. 3/2012 ● iunie-august
695
WORLD
ENERGY
SYSTEM
CONFERENCE
– WESC
2012
_____________________________________________________________________________________
WORLD
ENERGY
SYSTEM
CONFERENCE
- WESC
Fig.1. Motor drive system with capacitive couplings.
2. ASYNCHRONOUS MOTOR HIGH
FREQUENCY MODEL
At low frequencies, the equivalent circuit of an
electric motor consists of inductances and resistances
without considering the motor capacitances. At high
frequencies, the electric motor can be modeled as
distributed capacitors, inductors and resistors. The stray
capacitance of electric motors is very important in
predicting EMI problems [6-7].
Asynchronous motor winding physical construction
is very complicated and detailed determination of its
capacitance is difficult.
Using Maxwell software a finite element analysis of
the induction motor is carried out to obtain the
inductance and capacitance of the motor. The numerical
3D model of the induction motor simulated in Maxwell
is presented in Figure 2. For an accurate determination
of the induction motor capacitance and inductance a 3D
model of the motor must be analyze for the reason that
it incorporates the end winding of the induction motor.
To acquire the equivalent inductance of the
induction motor a magnetostatic simulation must be
done and for the equivalent capacitance of the motor an
electrostatic simulation. Using the results from the
magnetostatic and electrostatic simulations an
asynchronous motor equivalent circuit can be obtained
consisting of inductances and capacitances.
The induction motor inductances and capacitances
obtained from the Matrix solution in Maxwell are
exported in Simplorer for circuit analyses. The
investigated motor is three phase induction motor with
an output power of 11 kW, rated voltage 400 V and a
speed of 900 rpm.
Fig.2. Numerical model of the induction motor.
3. OVERVOLTAGE ANALYSIS OF
INVERTER FED INDUCTION MOTOR
For analyzing the overvoltages at the motor terminals
when the motor is connected to a PWM inverter the
simulation model presented in Figure 3, is used.
The model consists of an inverter, equivalent high
frequency circuit of the cable and the induction motor
capacitance and inductance imported from the finite
element analysis.
_____________________________________________________________________________________
Buletinul AGIR nr. 3/2012 ● iunie-august
2
696
Buletinul AGIR nr. 3/2012 ● iunie-august
STUDY OF ELECTROMAGNETIC
ELECTROMAGNETIC INTERFERENCE IN INVERTER DRIVEN ASYNCHRONOUS
ASYNCHRONOUS MOTORS
MOTORS
Fig.3. Inverter fed induction motor simulated model
Overvoltage in inverter fed induction motor is due to
parasitic capacitance in the cable and the motor [4], [10],
[11]. Based on the results from a frequency sweep of the
motor it is possible to determine the resonance
frequency, which is the result of the winding parasitic
capacitance.
Current measurements from a frequency sweep of
the induction motor are presented in Figure 4. The
resonance frequency of the induction motor, when the
motor is connected to the inverter through a 1m long
cable (Fig.4.a), is 3 kHz, this is the frequency range
were there is a minimum load and this voltage
component is not attenuated by the motor. The
resonance frequency of the inverters load is
significantly affected by the motor cable (Fig.4.b) the
resonance frequency with a 10 m long cable is 1 kHz
and with a 50 m long cable is 0.5 kHz (Fig.4.c).
The parasitic capacitance of the motor cable moves
the resonance frequency towards the lower ranges. It is
a key reason for appearance of serious overvoltage
problems in inverter fed induction motor drive system
with long motor cable. Identifying the resonance
frequency of the inverter load (motor and cable) is
helpful for avoiding serious overvoltage problems in
inverter driven motor system.
b) Motor connected to the inverter through a 10m cable;
c) Motor connected to the inverter through a 50m cable;
Fig.4. Current measurements from a frequency sweep when the
inverter is connected to the motor through different cable lengths.
When a PWM voltage is applied to the motor
through a long cable, the parasitic capacitance and
inductance of the cable generates overvoltages at the
motor terminals.
The moment that the PWM voltage is “off” because
of stored energy in the cable inductance the parasitic
capacitance of the cable is charging.
a) Motor connected to the inverter through a 1m cable;
_____________________________________________________________________________________
Buletinul
AGIR nr. 3/2012 ● iunie-august
3
Buletinul AGIR nr. 3/2012 ● iunie-august
697
WORLD
ENERGY
SYSTEM
CONFERENCE
– WESC
2012
_____________________________________________________________________________________
WORLD
ENERGY
SYSTEM
CONFERENCE
- WESC
When the PWM voltage is “on” the motor is subjected
to a theoretical surge of two times the amplitude of the
inverter output voltage. With each cycle “on” and “off”
of the PWM voltage the parasitic capacitance of the
cable is being charge and discharge.
a) Inverter output voltage;
b) Motor terminal voltage;
Fig.5. Inverter and motor terminal voltage waveforms.
In Figure 5, the inverters output voltage and motor
terminal voltage is presented, the overvoltage at the
motor terminal is clearly visible (Fig.5.b) due to the
parasitic capacitance of the motor cable, were the
inverters output voltage presents no voltage spikes
(Fig.5.a). The inverter output voltage has an amplitude
of 500 V and a fundamental frequency of 50 Hz with a
carrier frequency of 3 kHz, the induction motor is
connected to the inverter through a 1m cable (Fig.5).
In Figure 6 the voltage at the motor terminal is
presented when the motor is connected to the inverter
through different cable lengths and the carrier
frequency of the inverter output voltage is 1 kHz.
When the motor cable has a 1 m length the
overvoltage at the motor terminal (Fig.6, a) is clearly
visible, but the number of oscillations are fewer because
the resonance frequency of the cable and motor is
higher than the 1 kHz carrier frequency of the inverter
output voltage.
If the inverter is connected to the motor through a 10
m cable, the overvoltage at the motor terminal (Fig.6, b)
becomes a serious problem because the 1 kHz carrier
frequency of the inverter output voltage is much closer
to the resonant frequency of the inverters load (cable
and motor), thus the number of overvoltage oscillations
are much higher. When a 50 m cable is used to supply
the motor from the PWM inverter, the motor terminal
voltage is severely affected by the high parasitic
capacitance of the cable (Fig.6, c).
The parasitic capacitance of the motor cable moves
the resonance frequency towards lower frequencies,
amplitude and oscillation number of the overvoltage are
getting higher with cable length.
a) Motor connected to the inverter through a 1m cable;
_____________________________________________________________________________________
Buletinul AGIR nr. 3/2012 ● iunie-august
4
698
Buletinul AGIR nr. 3/2012 ● iunie-august
STUDY OF ELECTROMAGNETIC
ELECTROMAGNETIC INTERFERENCE IN INVERTER DRIVEN ASYNCHRONOUS
ASYNCHRONOUS MOTORS
MOTORS
b) Motor connected to the inverter through a 10m cable;
c) Motor connected to the inverter through a 50m cable;
Fig.6. Voltage waveforms measured on the motor terminal when the inverter is connected to the motor through different cable
lengths.
4. CONCLUSIONS
Determining the resonant frequency of the motor
and cable is very helpful in predicting serious EMI
problems in inverter driven motor system.
Overvoltages in inverter fed induction motor system,
with long motor cable can be predicted using the
method presented in this paper.
Finite element analysis of the motor for extracting
the motors high frequency equivalent circuit is useful
when the motor is still a project, before entering
production.
ACKNOWLEDGMENT: This paper was supported by the
project "Improvement of the doctoral studies quality in
engineering science for development of the knowledge based
society-QDOC” contract no. POSDRU/107/1.5/S/78534,
project co-funded by the European Social Fund through the
Sectorial Operational Program Human Resources 2007-2013.
BIBLIOGRAPHY
[1] Luszcz J., Iwan K., Conducted EMI propagation in inverter
fed AC motor, Electrical Power Quality and Utilisation,vol. 2,
2006.
[2] Zare, F. EMI in modern AC motor drive systems, IEEE
International Symposium on EMC, 2007.
[3] WEG Equipamentos Eletricos S.A. Induction motors fed by
PWM frequency inverters. Technical Guide, 2006.
_____________________________________________________________________________________
Buletinul
AGIR nr. 3/2012 ● iunie-august
5
Buletinul AGIR nr. 3/2012 ● iunie-august
699
WORLD
ENERGY
SYSTEM
CONFERENCE
– WESC
2012
_____________________________________________________________________________________
WORLD
ENERGY
SYSTEM
CONFERENCE
- WESC
[4] Kosei T. , Kotaro W. Propagation of inverter surge and
voltage distribution in motor winding, Electrical Engineering
in Japan, vol. 161, 2007, pp. 771–777.
[5] GAMBICA/REMA Working Group, Motor shaft voltages
and bearing currents under PWM inverter operation, Technical
Guide, 2002.
[6] Arnedo L., Venkatesan K., High frequency modeling of
induction motor drives for EMI and overvoltage mitigation
studies, Electric Machines and Drives Conference, IEMDC'03,
IEEE International.
[7] Moreau M., Idir N., Modeling of Conducted EMI in Adjustable
Speed
Drives,
Electromagnetic
Compatibility,
IEEE
Transactions, 2009, pp. 665 – 672.
[8] Bartos S., Dolezel I., Theoretical and experimental
investigation of parasitic effects in induction motor drives
supplied from semiconductors inverters, Acta Electrotechnica et
Informatica ,vol. 8, 2008, pp. 3-9.
[9] Ogasawara S., Akagi H., Analysis and reduction of EMI
conducted by a PWM inverter-fed AC motor drive system having
long power cables, Okayama University Digital Information
Repository, 2000.
[10] Mutoh, N. Konuma T., A motor-terminal surge and induced
EMI noise controlling method effective in EVs with the high
density packaging structure, Vehicle Power and Propulsion
Conference, VPPC '08, IEEE, pp. 1-7.
[11] Włodek R. , Roehrich J., Analysis of overvoltage on motor
winding insulation fed by PWM pulses, High Voltage
Engineering and Application (ICHVE), International
Conference 2010, pp. 584 – 587.
[12] Wang L., Ngai-Man Ho C., High-Frequency Modeling of the
Long-Cable-Fed Induction Motor Drive System Using TLM
Approach for Predicting Overvoltage Transients, IEEE
Transaction on Power Electronics, vol. 25, no. 10, 2010.
About the authors
Eng. Ioan ŢILEA PhD-student.
Technical University of Cluj-Napoca, Electrical Engineering Faculty, Department of Electrotechnics and
Measurements, Baritiu Street 26-28, 400027 Cluj-Napoca, Romania.
email: Ioan.TILEA@ethm.utcluj.ro
Graduated at the North University of Baia Mare, Engineering Faculty, study program – Electromechanic.
From 2011 he is a PhD student at the Technical University of Cluj-Napoca, Department of Electrotechnics and
Measurements. His research topic is electromagnetic phenomenons in electric motors powered in a harmonic
polluted regime.
Prof. Eng. Călin MUNTEANU PhD.
Technical University of Cluj-Napoca, Electrical Engineering Faculty, Department of Electrotechnics and
Measurements, Baritiu Street 26-28, 400027 Cluj-Napoca, Romania.
email: Calin.Munteanu@ethm.utcluj.ro
He received the MSc. degree in electrical engineering from Technical University of Cluj-Napoca, in 1989, and the
PhD degree in electrical engineering in 1999. He joined the Electrotechnics Department from Technical
University of Cluj-Napoca in 1991. Since 2003 he is Professor and the Head of the EMC Laboratory. His scientific
work is related to EMC, electromagnetic fields, numerical computation, optimal design techniques.
_____________________________________________________________________________________
Buletinul AGIR nr. 3/2012 ● iunie-august
6
700
Buletinul AGIR nr. 3/2012 ● iunie-august