measurement of conducted electromagnetic emissions in

8/11/2019 Measurement of Conducted Electromagnetic Emissions In

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50 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 41, NO. 1, FEBRUARY 1999

Measurement of Conducted Electromagnetic Emissions in

PWM Motor Drive Systems Without the Need for an LISN

L. Ran, J. C. Clare, K. J. Bradley, C. Christopoulos

Abstract This paper presents a technique for measuring the conductedelectromagnetic emissions produced by pulse-width modulated (PWM)inverter induction motor drive systems. The method does not require an

artificial line-impedance stabilizing network (LISN) but does, however,allow the emission levels to be calculated as if an LISN were present.Testing can be performed when an LISN is either unavailable, prohibi-tively expensive, or impractical to include in the supply. This is often thecase for large drive systems or for systems already installed in the field.A normal RF voltage probe and a spectrum analyzer are used to measurethe spectra of the common-mode and differential-mode excitation sourcesdue to the inverter switching. Line inductors for high-frequency (HF)isolation are required for some of the tests, but the cost and complexity ofthese compared to an LISN is low. Common-mode and differential-modeThevenin equivalent circuits are then derived from measured impedances.

The emissions for any defined supply impedance (including an LISN) canthen be determined. A laboratory test on a 15 kW PWM drive systemis carried out to verify the accuracy and effectiveness of the proposedmethod.

Index Terms Conducted emissions, electromagnetic compatibility,

power converters, variable speed drives.

I. INTRODUCTION

Conducted electromagnetic emissions produced by variable-speed

ac drive systems are of increasing concern in industry. Filtering is

normally required to achieve compliance with regulations and this can

add considerable cost to the product and to product development.

Extensive tests in the frequency range up to 30 MHz are required

to identify the noise signature of the system so that compliance

with international standards can be demonstrated and/or suitable

filtering and suppression techniques can be determined. Standard

measurements usually require that the drive is supplied from a

line impedance stabilizing network (LISN) [1]. The function of the

LISN is to isolate the drive (at high frequencies) from the utilitypower supply and to provide a means of measuring the conducted

emission by diverting the RF currents through a defined impedance

(for example, 50

resistive). However, as discussed below, it is very

desirable in some circumstances to be able to measure the conducted

emissions of the drive system without using an LISN.

For large drive systems, say above 100 kW, an LISN of comparable

rating can be prohibitively costly. Depending on the emission mecha-

nism being considered it may be necessary to operate the drive system

at full load and this excludes the possibility of using an LISN of

lower rating. Drive manufacturers may not wish to invest in specialist

electromagnetic compatibility (EMC) testing facilities like the LISN

and it is, therefore, particularly useful to be able to obtain information

about the system performance using available instruments [2]. LISN-

free measurements are also useful during drive commissioning wherethe inclusion of an LISN in the supply may be impractical. In some

situations, the use of an LISN is undesirable because of its effect on

the system performance. For example, the LISN can influence rectifier

Manuscript received May 27, 1997, revised April 30, 1998. This work wassupported by the industrial partners and EPSRC in the LINK PEDDS ProjectEMC in Power Electronic Converters and Drives under Grant GR/K40932.

The authors are with the Department of Electrical and Electronic Engineer-ing, The University of Nottingham, Nottingham, NG7 2RD, U.K.

Publisher Item Identifier S 0018-9375(99)01535-5.

commutation [3] and significantly change the emission signature of

the drive (this is particularly true for dc drives). In such cases, it is

useful to be able to test the drive without the LISN. Some of these

difficulties can be overcome by the use of a voltage probe to make

measurements [1] and this is the method normally applied at high

powers. However, the approach described here allows the emissions

to be predicted for any defined supply impedance. It is, therefore,

useful for assessing the effectiveness of proposed filtering strategies

and can also be used for precompliance testing. No previous workon this subject has been reported.

Although it is possible to use numerical models to predict the

emissions of drive systems, the accuracy that can be achieved is

always limited by the need to simplify the representation of very

complex phenomena [4], [5]. For example, it is difficult to model

the full details of the switching processes within the inverter, which

are significant in determining the emission level. The stray parameters

distributed in the system are seldom accurately known and neither are

the high-frequency characteristics of various components including

the induction motor, cables, and the auxiliary circuits connected to

the power electronic stage. Modeling techniques exist to determine

some of these parameters but very sophisticated tools are required and

they very quickly become unwieldly for a system of this complexity.

As a result, it is useful to predict the conducted emission level of thedrive system based on measurements and, thus, to validate numerical

model predictions.

The study of this paper assumes that the LISN characteristics

are well defined and, thus, can be modeled analytically. Based on

Thevenins theorem, a voltage probe is used to measure the common

and differential mode sources of the drive, which is supplied directly

from the utility supply. The Thevenin impedances of the drive are

measured off line taking into account the conduction pattern of the

inverter. Unwanted effects due to the supply system are minimized

by setting up the test system according to the mode concerned.

From these measurements, the conducted emission level is calculated

as if an LISN were present. The calculated results are, therefore,

always referenced to a standard system configuration using an LISN

and the experiment is, thus, repeatable. The Thevenin representationallows the effectiveness of filtering strategies to be predicted and also

provides information about the different modes which are difficult to

separate with conventional tests. Comparative studies are used to

confirm the accuracy of the proposed method.

II. DOMINANT MODES OF C ONDUCTED EMISSIONS

Fig. 1 illustrates the configuration of a typical PWM inverter

induction motor drive system with a voltage-type dc link assuming an

LISN is present. The dashed lines indicate the dominant oscillation

modes, which are concerned with the conducted emissions. Viewed

from the LISN, both common and differential modes are present if

the drive is not fitted with an internal filter. Common-mode emissionsare driven by a voltage created by the inverter switching, which is

developed between the heatsink/earth and the dc link [4], [5]. This

voltage is characterized by a series of sharp edges associated with

each switching event in the inverter. Differential mode oscillations

between the output phases are also created by each switching. These

lead to differential mode currents on the LISN side due to the parasitic

inductance and resistance of the dc link capacitor bank [5]. In this

paper, we consider only differential mode emissions created by the

high-frequency switching of the inverter and by the control electronics

00189375/99$10.00 1999 IEEE


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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 41, NO. 1, FEBRUARY 1999 51

Fig. 1. System configuration and dominant modes.

Fig. 2. Measured spectrum of LISN earth current.

supplied via the inverter dc link. Differential mode emissions due to

the input rectifier bridge have been considered elsewhere [3].

Based on the principle of superposition, each mode can be analyzed

individually. The common-mode emissions can be identified from the

LISN earth current whose spectrum is shown in Fig. 2 for the 15-

kW test-drive system running at 40-Hz output frequency. The signallevel is generally above 40 dB

A up to about 10 MHz. The spike

at 24 MHz in the current spectrum is due to clock pulses on the

control board. The corresponding common mode components in the

LISN output voltage will be 34 dB greater due to the 50-

LISN

impedance.

Differential mode emissions can be measured at the LISN by

opening the LISN earth conductor or by connecting a choke of large

inductance in series with it (for personnel safety). The common-mode

components are, thus, suppressed in the LISN voltage. In doing this,

it is assumed that system is balanced and the common-mode current

does not, therefore, give rise to any differential mode emissions.

The measured spectrum of the differential mode LISN voltage when

the LISN earth conductor is disconnected is shown in Fig. 3. The

significant emission level illustrates that the dc link-capacitor bankis not able to completely restrict the differential mode current to the

output side.

The spectra in Figs. 2 and 3 were measured with an LISN included

in the test setup. The purpose of this study is to develop a method

whereby these emission characteristics can be reconstructed from

measurements made without an LISN when the drive system is

connected directly to the utility power system. Assuming that the

characteristics of the LISN are well defined, this can be achieved

provided that the drive system can be reduced to a Thevenin equiva-

lent for each mode as shown in Fig. 4. The common and differential

mode emissions are calculated assuming that each phase of the LISN

can be represented by its 50-

impedance although the true LISN

Fig. 3. Measured spectrum of differential mode emissions at LISN.

Fig. 4. Equivalent circuits for common and differential modes.

impedance could be used for more accuracy. For the common mode,two phases of the LISN appear in parallel since it is assumed that

two of the input rectifier bridge diodes conduct at any time. For the

differential mode, two LISN phases appear in series for the same

reason. It is also necessary to account for the fact that each input

phase is only active for at most 240 of each supply cycle due to the

diode bridge. During discontinuous conduction the diode conduction

times will be shorter, 240 , represents the worst case and is, therefore,

used in the calculations. The common mode current and differential

mode LISN voltage are then given by

(1)


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Fig. 5. RF voltage probe with isolation.

0

(2)

In order to compute the total RF voltage measured at the LISN it

is necessary to combine the common mode and differential mode

effects. Since the relative phase is not known it is only possibleto add the magnitudes at each frequency. This gives the worst-case

condition and will be most accurate at frequencies where one of the

modes dominates. Potential errors will be greater at frequencies where

the modes are of comparable amplitudes.

Where impedance is inserted between the drive and the LISN

(for example, filters or cables) then (1) and (2) can be modified

accordingly to take this into account. The remaining problem is to

determine measurement techniques so that the Thevenin equivalent

circuits of the drive system can be found. Measurement of the sources

and

and the impedances

and

are

considered separately below.

III. MEASUREMENT OF THEVENIN EQUIVALENT SOURCES

By definition, the EMF in a Thevenin equivalent is the terminalvoltage measured when the corresponding external part of the system

is open circuited. For the common mode, this requires that the supply

earth conductor is disconnected. The voltage is then measured using

a standard voltage probe [1] connected between the dc link and

the heatsink of the semiconductor devices, which is mounted on

the drive case. In order to ensure that the earth path is indeed

open circuited, the voltage probe should be augmented with a 1:1

RF transformer with small insertion loss as shown in Fig. 5. The

transformer used is a mini-circuits-type FTB-1-6, which has a flat-

frequency response (within 1 dB) over a frequency range 10 kHz-250

MHz; the transformer stray capacitances are, therefore, negligible at

the frequencies considered here. A voltage probe of this type can be

easily made or obtained commercially. The measured voltage then

needs to be scaled by the attenuation ratio of the probe (50/1550 or29.8 dB) assuming the response is constant over the whole frequency

range. Alternatively, the probe can be calibrated more accurately

using an impedance analyzer. We use the latter method.

It is important for the validity of the method that the measured

Thevenin equivalent voltage is dependent only on the drive and its

load and is independent of the supply impedance. This is investigated

by comparing a measurement of the common-mode source made

with and without the LISN, as shown in Fig. 6. Although the supply

impedance changes significantly when the LISN is included, the effect

on the spectrum of the common-mode source is almost negligible.

This confirms that the measured source depends only on the drive

and the load and is, therefore, a true Thevenin equivalent.

Fig. 6. Measured spectrum of common mode excitation source.

Fig. 7. Measured spectrum of differential mode excitation source.

It has been observed experimentally that the spectrum of the

common-mode voltage source is almost identical when measured

from either of the dc link buses. This is because virtually no common-mode current flows through the dc link capacitor. For common

modes, the potentials of the two dc buses are merely different by

a constant voltage

, which can be neglected when analyzng the

high-frequency EMC phenomena. The large differential-mode current

flowing through the dc link-capacitor bank will not significantly

influence the common-mode emissions.

The differential mode Thevenin source

is theoretically

the voltage measured across the dc link-capacitor bank when the

supply is open circuit. This introduces a difficulty since it is obviously

impossible to operate the drive in this way. To overcome this problem,

line inductors are inserted for the test, which effectively open circuit

the supply at the frequencies of interest, but allow the drive to operate

normally. This inductance also swamps the supply impedance and

again ensures that the measurement is independent of the supplycharacteristics. Fig. 7 shows the spectrum of

measured using

the voltage probe described earlier. Despite the compact construction

and high-quality decoupling components used in the inverter, the

considerable level of this signal illustrates the difficulty (particularly

from the EMC viewpoint) caused by the high levels of

generated by modern power devices.

IV. DETERMINATION OF THEVENIN IMPEDANCES

For the common-mode emissions, the three output phases of the

pulse-width modulated (PWM) inverter are effectively in parallel [4],

[5]. At any time, each phase is connected either to the positive

or negative dc bus through a conducting device in the inverter


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Fig. 8. Circuit to measure common mode impedance.

Fig. 9. Measured common mode impedance.

and the two dc buses can be considered to be an equipotential in

the analysis of common mode emissions [4], [5]. Therefore, the

drive system is reduced to a constant topology circuit based on

which the Thevenin impedance of the system can be defined. With

the supply disconnected, the arrangement to measure the common

mode Thevenin impedance

is illustrated in Fig. 8. Note that

the Thevenin impedance includes the output cable and load. The

measurement is carried out off-line and temporary links between the

dc buses and the output phases are used to mimic conducting devices

in the inverter. These links should be as short as possible and, if

practicable, can be made directly on the IGBT modules. According

to the preceding argument, the combination of connection between

the ac and dc terminals does not significantly affect the measurement.

An impedance analyzer is used to perform the measurement. The

measured

for the test system (which includes a 10 m motor

cable) is shown in Fig. 9. This clearly shows transmission line effects

due to the relatively long motor cable.

When an impedance analyzer is not available,

can be

estimated using numerical models based on simplified representations

of the drive system components [4], [5], but this is less satisfactory.

The common mode impedance of the induction motor

can

be calculated from an equivalent circuit which models its high-

frequency behavior [4][6]. The total common-mode admittance

can then be expressed as [7]

cosh

sinh

sinh

cosh

(3)

where

and

are the characteristic impedance and propagation

parameters of the motor cable for the common mode,

is the cable

length, and

is the total stray capacitance of the IGBT switches

to the heatsink. Equation (3) allows

to be calculated from

Fig. 10. Computed common mode impedance.

relatively straightforward measurements made on the motor, cable,

and power-device modules [4], [5].

Fig. 10 shows

for the test setup calculated using (3).

Comparison with Fig. 9 shows that the general shape of the measured

impedance is reproduced. The transmission-line effects due to the

long cable are accounted for although the measured impedanceis more complicated than that predicted by the numerical model.

The difference between the measured and calculated impedance is

generally less than 25

implying that when the computed impedance

is used to calculate the emissions at the LISN side, the resultant error

will be within 6 dB. Clearly, it is desirable to use the measured

impedance if possible, but the numerical model will give useful

results when an expensive impedance analyzer is not available.

It is relatively straightforward to determine the Thevenin

impedance for the differential mode

. This is the impedance

of the dc link capacitor in parallel with the impedance of the

output circuit reflected through the inverter to the dc link. Strictly,

this varies with the inverter switching pattern. However, for any

sensible practical inverter with a moderate length output cable the

external output circuit impedance is much greater than the shuntdc link impedance at high frequencies and its effect on

can be neglected.

is, therefore, taken simply to be the

shunt impedance of the dc link-capacitor bank, which, again, can

be measured with an impedance analyzer. Alternatively, when an

analyzer is not present, it can be assumed that this impedance

simplifies to a constant inductance, which may be measured with an

LCR bridge. A constant measured inductance

is assumed for

the results presented here.

V. EXPERIMENTAL VERIFICATION

The method proposed in this paper is used to predict the emission

levels by measuring impedances and voltage sources when a LISN

is not used. The predicted emission levels are, however, calculated

based on the assumption that a LISN is present so that the results arecomparable with standards which specify the EMC performance with

well-defined test conditions. Therefore, the accuracy of the proposed

method can be verified by comparing the predicted emission level

with that measured when a LISN is used.

Fig. 11 shows the predicted spectrum of the LISN earth cur-

rent (representing the common-mode emissions) together with the

corresponding measurement shown previously in Fig. 2. In the com-

putation, (1) has been extended to include the short (1 m) supply

cable linking the LISN and the drive. Good agreement is demon-

strated. Fig. 12 shows the predicted spectrum of the differential

mode emissions together with the measurement shown previously

in Fig. 3. Generally, there is reasonable agreement although there


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Fig. 11. Comparison of measured and predicted spectra of LISN earthcurrent.

Fig. 12. Comparison of measured and predicted differential mode LISNvoltage spectra.

is a loss of accuracy at the low-frequency end. The agreement has

been considerably improved compared with what was achieved using

numerical simulation models [4], [5]. Higher order effects such as

the complex nature of the switching edges and the stray components

that are impractical to include exactly in a numerical simulation are

inherently taken into account in the measurement method proposed.

If the Thevenin equivalent circuits for the drive/load system are

known, it is also possible to predict the emissions when filters

are introduced between the drive and LISN. To investigate this a

simple filter supplied by the drive manufacture was included in the

experimental setup. This filter consists of line chokes and lineline

capacitors placed on the supply side of the chokes as shown in

Fig. 13. Differential mode currents are virtually eliminated at the

LISN since they are diverted through the capacitors. The series

impedance of the chokes also serves to attenuate the common mode

current. With this filter in place, the common mode current is

shared by all three LISN phases since the line to line capacitors are

effectively a short circuit at high frequencies. Two of the line chokes

are involved in the common mode path assuming that two diodes in

the input rectifier are conducting so that the equivalent circuit is as

shown in Fig. 14. If the chokes are represented using a single series

reactance function

then (1) can be modified to compute

the common mode LISN voltage as follows:

0

(4)

Fig. 15 shows the measured and computed spectra of the LISN

output voltage with the filter in place. Good agreement is again

Fig. 13. Filter arrangement.

Fig. 14. Common mode equivalent circuit with filter in place.

Fig. 15. Comparison of measured and predicted common mode emissionsat LISN with filter included.

obtained. In order to further improve the accuracy of the calculation,

the impedance characteristics of the differential mode filter can be

measured. These results show that once the Thevenin equivalent

circuits are determined, the proposed method can be extended to look

at the effectiveness of various filtering techniques.

VI. CONCLUSIONS

A new method is proposed to measure the conducted emissions of a

PWM inverter induction motor-driven system without using a LISN.

The drive system is represented as a Thevenin equivalent, which can

be used to predict the emissions at the input side. The common and

differential mode excitation sources are determined on-line and the

uncertainty of the utility power supply system impedance does not

affect the measurement. The Thevenin impedances can be measured

off line. Using a spectrum analyzer and an RF voltage probe, the

proposed method is easy to implement and can include many complex

effects that are difficult to model numerically. The accuracy of the

method is verified using measurements on a 15-kW test-drive system.

The predicted conducted emission levels can be calculated as if an


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LISN were used and can be used for precompliance testing. The

method is expected to be useful and cost effective when a LISN is

not available for large PWM drive systems or when the inclusion of a

LISN is impractical. Although a voltage probe is conventionally used

at high powers to make measurements without a LISN, the proposed

method yields extra information about the individual modes and the

emission level can be calculated for any defined supply impedance.

This makes the method suitable for predicting the effectiveness of

filtering strategies as well.

APPENDIX

TEST-DRIVE SYSTEM DATA

Supply system 415 V, 50 Hz;

PWM inverter IGBT type, 5 kHz switching frequency;

Induction motor 415 V, 15 kW, four pole, cage rotor;

Motor cable four core cable, 10 m unscreened;

LISN cable four cable, 1 m unscreened;

LISN Rohde and Schwarz ESH2-Z5;

37 nH;

Input filter line-line capacitor 150 nF, line choke 145

H (measured at 100 kHz).

REFERENCES

[1] International Standards, CISPR 16-1, International ElectrotechnicalCommission, 1993.

[2] T. Williams, EMC for Product Designers. Newnes, Oxford:Butterworth-Heinemann, 1992.

[3] R. Scheich and J. Roudet, EMI conducted emissions in differentialmode emanating from an SCR: Phenomena and noise level prediction,

IEEE Trans. Power Electron., vol. 10, pp. 105110, Mar. 1995.[4] L. Ran, S. Gokani, J. C. Clare, K. J. Bradley, and C. Christopou-

los, Conducted electromagnetic emissions in induction motor drivesystemsPart 1: Time-domain analysis and identification of dominantmodes,IEEE Trans. Power Electron., vol. 13, pp. 757767, July 1998.

[5] L. Ran, S. Gokani, J. C. Clare, K. J. Bradley, and C. Christopoulos,Conducted electromagnetic emissions in induction motor drive sys-

temsPart 2: Frequency domain models,IEEE Trans. Power Electron.,vol. 13, pp. 768776, July 1998.

[6] E. Zhong and T. A. Lipo, Improvements in EMC performance ofinverter-fed motor drives, IEEE Trans. Ind. Applicat., vol. 31, pp.12471256, Nov./Dec. 1995.

[7] C. Christopoulos, Principles and Techniques of Electromagnetic Com-patibility. Boca Raton, FL: CRC, 1995.

Electric and Magnetic Fields

Created by Electrosurgical Units

Robert Martin Nelson and Howard Ji

Abstract One of the places where it is critical to prevent equipmentmalfunction due to electromagnetic interference is in the hospital oper-ating room. Unfortunately, the electromagnetic environment in hospital

operating rooms can be quite harsh. The electromagnetic fields presentin this environment are created by distant sources such as AM andFM broadcast transmitting antennas as well as local sources such as

electrosurgical units. In this paper, measured values are reported ofelectric and magnetic field strengths created by typical electrosurgical

units. Measurements were taken in operating rooms at the Departmentof Veterans Affairs Medical Center in Fargo, ND, and were made for threeelectrosurgical units (ESU) operating modes. All three components of theelectric and magnetic fields were determined as a function of frequency.Field strengths as large as 153 dB V/m and 76 dB A/m were measured1 m from the electrosurgical unit.

Index Terms Antennas, electric field measurement, electromagneticcompatibility, electromagnetic interference, magnetic field measurement.

I. INTRODUCTION

Electrosurgical generators (which are also called electrosurgicalunits) are commonly used in surgical procedures to cut tissue or

stop blood flow. The electrosurgical unit (ESU) uses high-frequency

currents to accomplish these tasks. The electric and magnetic fields

created by these currents can induce electrical noise in nearby elec-

tronic devices. In the work reported here, the electric and magnetic

fields emanating from a typical ESU were measured as a function of

frequency in an operating room environment. Antennas were placed

1 m from both the ESU and the specimen being operated on. The

fields emanating from the ESU were measured while the device was

used in various operating modes and with various power levels.

After a review of the background of this problem, the objectives

and scope of the paper are set forth. Section II describes the measure-

ment methodology and development used in the project, including

a description of the measurement site, equipment being tested andmeasurement equipment. In addition, a typical measurement session

is described and measurement concerns are discussed. Measurement

results are provided in Section III, including both the electric and

magnetic field strengths emanating from the equipment under test

as well as ambient fields. Concluding remarks are provided in

Section IV.

A. Background

The phenomenal changes that have occurred in the electronics

industry in the past several years have had a drastic effect on

the technology and instrumentation used in modern medical care.

Virtually every aspect of modern medical care involves the use of

electronic devicesfrom electronic monitoring systems to magneticresonance imaging units to ESUsthe use of high-speed electronics

has virtually revolutionized the instrumentation used in hospitals and

Manuscript received May 13, 1994; revised September 15, 1998. Thiswork was supported by a grant from NSF/ASEND (National Science Foun-dation/Advancing Science Excellence in ND).

R. M. Nelson is with the Department of Electrical Engineering, NorthDakota State University, Fargo, ND 58105 USA.

H. Ji is with Cisco Systems, San Jose, CA 95134 USA.Publisher Item Identifier S 0018-9375(99)01536-7.

00189375/99$10.00 1999 IEEE

measurement of conducted electromagnetic emissions in

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