Reduction of Audible Noise of a Traction Motor at PWM Operation

Reduction of Audible Noise of a Traction

Motor at PWM Operation

Hanna Amlinger

Licentiate Thesis

Stockholm, Sweden

2018

Academic thesis with permission by KTH Royal Institute of Techno-logy, Stockholm, to be submitted for public examination for the degree of Licentiate in Vehicle and Maritime Engineering, Tuesday the 6th of February, 2018 at 13.00, in E2, Lindstedsvägen 3 (floor 3), KTH - Royal Institute of Technology, Stockholm, Sweden.

ISBN 978-91-7729-649-2 TRITA-AVE 2017:93 ISSN 1651-7660

c

Hanna Amlinger, 2018

Postal address: Visiting address: Contact:

KTH, AVE Teknikringen 8 amlinger@kth.se

SE-100 44 Stockholm Stockholm

Abstract

A dominating source for the radiated acoustic noise from a train at low speeds is the traction motor. This noise originates from electromagnetic forces acting on the structure resulting in vibrations on the surface and thus radiated noise. It is often perceived as annoying due to its tonal nature. To achieve a desirable acoustic behavior, and also to meet legal requirements, it is of great importance to thoroughly understand the generation of noise of electromagnetic origin in the motor and also to be able to control it to a low level.

In this work, experimental tests have been performed on a traction mo-tor operated from pulse width modulated (PWM) converter. A PWM converter outputs a quasi-sinusoidal voltage created from switched volt-age pulses of different widths. The resulting main vibrations at PWM operation and their causes have been analyzed. It is concluded that an appropriate selection of the PWM switching frequency, that is the rate at which the voltage is switched, is a powerful tool to influence the noise of electromagnetic origin. Changing the switching frequency shifts the fre-quencies of the exciting electromagnetic forces. Further experimental in-vestigations show that the trend is that the resulting sound power level decreases with increasing switching frequency and eventually the sound power level reaches an almost constant level. The underlying physical phenomena for the reduced sound power level is different for different frequency ranges. It is proposed that the traction motor, similar to a thin walled cylindrical structure, shows a constant vibration over force re-sponse above a certain frequency. This is investigated using numerical simulations of simplified models. Above this certain frequency, where the area of high modal density is dominating, the noise reducing effect of further increasing the switching frequency is limited.

Keywords: electromagnetic noise, pulse width modulation (PWM), switching frequency, traction motor

Sammanfattning

Traktionsmotorn är en starkt bidragande källa till ljudet från ett tåg vid låga hastigheter. Elektromagnetisk krafter i motorn verkar på dess yta så att denna vibrerar vilket resulterar i utstrålat ljud. Detta ljud uppfat-tas ofta som irriterande eftersom att det är väldigt tonalt. För att minska det utstrålade ljudet från motorn samt för att uppfylla lagkrav, är det väldigt viktigt att förstå de bakomliggandet orsakerna till hur ljud från elektromagnetiska krafter uppkommer och även att kunna kontrollera detta ljud till en låg nivå.

I detta arbete har experiment utförts på en traktionsmotor matad från en pulsbreddsmodulerad (PWM) frekvensomriktare. En PWM omrikta-re geneomrikta-rerar en sinusliknanade spänning utifrån switchade spännings-pulser av olika bredd. De dominerande vibrationerna vid PWM mat-ning och dess bakomliggande orsaker har analyserats. Ett lämpligt val av PWM switchfrekvens, dvs. hur snabbt spänningen switchas, är ett ef-fektivt sätt att påverka ljudet som uppkommer från elektromagnetiska krafter. När switchfrekevensen ändras, ändras de exciterande elektro-magentiska krafternas frekvens. Ytterligare experimentella mätningar visar att trenden är att ljudtrycket minskar med ökad switchfrekvens och till slut når ljudtrycket en nästan konstant nivå. De bakomliggan-de fysikaliska orsakerna till minskningen av ljudtrycket är olika för oli-ka frekvensområden. En hypotes är att traktionsmotorn, likt en tunn-väggig cylinder, har ett konstant förhållande mellan resulterande vibra-tion och exciterande kraft över en viss frekvens. Detta studeras närma-re genom numeriska simuleringar av fönärma-renklade modeller. Detta skulle betyda att över denna frekvens, i området som domineras av hög mo-daltäthet, är effekten av att öka switchfrekvensen ytterligare med avsikt att reducera ljudet begränsad.

Nyckelord: elektromagnetiskt ljud, pulsbreddsmodulering, switchfrek-vens, traktionsmotor

Acknowledgements

First of all, I would like to that The KTH Railway Group and Bombardier Transportation for the financial support of this research.

To my supervisors Ines Lopes Arteaga and Siv Leth - thank you both for all your support, encouragement, feedback and discussions during this work!

I would also like to thank all present and former colleagues at Bom-bardier: my colleagues at the Control Dynamics department always an-swering all my questions related to converter control and all acousti-cians within the Center of Competence Acoustics helping out with ques-tions related to acoustics and acoustic measurements. A special thanks to Fredrik Botling with whom I shared a large part of this journey for the good collaboration, to Florenece Meier for all the interesting discus-sions and all your valuable input and feedback; and to Daniel Jansson, Mattias Hill and Tommy Sigemo for all the help with measurements. Thank you to all colleagues at KTH, even though I have not been at KTH very often you have always made me feel very welcome!

Last but not least, to all my loving friends and family, without you this would never have been possible. Karin, Fredrik and especially Lina: it has been so valuable to share my thoughts with someone who has already been on a similar journey, thank you for always being there and listening and cheering! Christoffer, thank you for your endless love and support; and Linnea, there is nobody or nothing that reminds me what it is really important in life the way you do - I love you!

Hanna Amlinger Västerås, January 2018

Dissertation

This thesis consists of two parts: The first part gives an overview of the research area and work performed. The second part contains the following research papers (A-B):

Paper A

H. Amlinger, F. Botling, I. L. Arteaga, and S. Leth, “Operational Deflec-tion Shapes of a PWM-fed TracDeflec-tion Motor,” in Rotating Machinery, Hybrid

Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8: Proceedings of the 34th IMAC, A Conference and Exposition on Structural Dynamics 2016,

J. De Clerck and D. S. Epp, Eds. Springer International Publishing, 2017, pp. 209-217.

Paper B

H. Amlinger, I. L. Aretaga, and S. Leth, “Reduction of Radiated Acoustic Noise of a Traction Motor at PWM Converter Operation”, submitted to IEEE Transactions on Industry Applications (January 2018).

Division of work between authors

H. Amlinger initiated the direction of and performed the studies, made the analysis, and produced the papers. I. Lopez Arteaga and S. Leth supervised the work, discussed ideas and reviewed the work.

Publications not included in this thesis

Conference papers:

H. Amlinger, I. L. Aretaga, and S. Leth, “Impact of PWM Switching Frequency on the Radiated Acoustic Noise from a Traction Motor,” in

Proceedings of the 20th International Conference on Electrical Machines and Systems (ICEMS),2017, Sydney, Australia. Available: IEEE Xplore. F. Botling, H. Amlinger, I. L. Arteaga, and S. Leth, “Vibro-Acoustic Modal Model of a Traction Motor for Railway Applications,” in Rotating

Ma-chinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8: Proceedings of the 34th IMAC, A Conference and Exposition on Structural Dynamics 2016,J. De Clerck and D. S. Epp, Eds. Springer International Publishing, 2017, pp. 197-208.

Contents

I

OVERVIEW

1

2 Acoustic noise of electromagnetic origin in traction motors 9

2.1 Pulse width modulation . . . 9 2.2 Electromagnetic forces . . . 12

3 Summary of appended papers 19

3.1 Paper A . . . 20 3.2 Paper B . . . 21

4 Conclusions 23

Bibliography 25

Part I

1

Introduction

1.1

Background

The electrification of the traction systems used for rolling stock emerged at the end of the nineteenth century. However, experiments in elec-tric rail have been traced back to the mid-nineteenth century. The first known electric locomotive was built by Robert Davidson and was pow-ered by galvanic cells, i.e. batteries. The first electric passenger train was presented by Werner von Siemens in Berlin in 1879. It was run-ning on a 300 meter long circular track and the electricity was supplied through a third, insulated rail situated between the tracks. Much of the early development of electric locomotion was driven by the increasing use of tunnels, particularly in urban areas. Smoke from steam locomot-ives was noxious and municipalities tended to prohibit their use within their limits. Much has occurred since the first electric locomotive in the end of the nineteenth century and in 2006, 240,000 km (25% by length) of the world rail network was electrified and 50% of all rail transport was carried by electric traction. [1]

Nowadays, the pollution of smoke from steam locomotive is not of-ten on the agenda but one topic that is instead more commonly dis-cussed is the pollution of noise. According to the World Health Organiz-ation (WHO), noise is the second largest environmental cause of health problems, after the impact of air quality (particulate matter). Road traffic is the most dominant source of environmental noise in Europe. Expos-ure to road traffic is followed by rail traffic noise, aircraft noise and in-dustrial noise. In 2012, it was reported that nearly 7 million people in Europe was exposed to rail traffic noise above 55 dBlen. Complementing

the reported data with estimations, it was estimated that the total num-3

CHAPTER 1. INTRODUCTION

ber of people exposed to railway noise was nearly 14 million. 55 dBlen

is the EU threshold for excess exposure, indicating a weighted average during the day, evening and night [2]. It is reported that exposure to noise in Europe contributes to about 910 thousand additional prevalent cases of hypertension, 43 thousand hospital admissions per year, and at least 10 thousand premature deaths per year related to coronary heart disease and stroke [3].

The attention for noise related to trains and railways has increased during the last years. This is due to the enhanced awareness of noise pol-lution from trains and the railway sector and the resulting consequences on health and wellness. This is also recognized in legalization, in con-tractual requirements from customers on suppliers in the railway in-dustry as well as in the expectations from citizens and travelers. Since 2002 there are technical specifications for interoperability (TSI) [4] in-cluding limits for noise emission of new vehicles operating in Europe. The latest revision is from January 2015 with reduced permitted noise levels compared to the previous versions.

Figure 1.1: Example of typical speed dependency for different noise sources, figure from [5].

There are many types of acoustic noise on a train, e.g. rolling noise, aerodynamic noise and noise related to the traction and auxiliary sys-tems (compressors, transformers, traction motor etc.). The train speed has a large impact on the radiated noise, and the importance of the different sources adding up to the total noise level differs for different 4

1.1. BACKGROUND

Figure 1.2: Electromagnetic noise characteristics during acceleration.

speeds of the train as is illustrated in Figure 1.1 [5].

As can be seen in Figure 1.1, the noise related to the traction system is dominating at low speeds when the rolling noise is very low. This applies especially to electromagnetic noise from the drive system caused by magnetic forces acting on structures. An important contributor to the electromagnetic noise on trains is the traction motor. This noise is passengers on platforms as well as in the compartments exposed to. The noise of the traction motor is often perceived as quite annoying due to its tonal nature. A typical frequency spectra during acceleration for a motor is shown in Figure 1.2 and as can be seen it is characterized by narrow-band harmonics in a wide frequency range.

To be able to reduce the noise of a train and also to meet the TSI requirements it is of great importance to understand and predict the acoustic noise of electromagnetic origin, for example from the traction motor. It is desirable to control the acoustic noise of the traction motor to a low level, preferably already on the design stage. The generation of acoustic noise of electromagnetic origin from the traction motor is a process involving several different physical domains. Looking at it from a modeling point of view this process could be divided into five differ-ent sub-domains as shown in Figure 1.3. The converter control controls the power converter using so called pulse width modulation (PWM) to output a pulse width modulated 3-phase voltage to the traction motor. The voltage results in electrical currents in the 3-phase windings of the motor which is described in the electrical domain. These motor cur-rents induce a magnetic field that generates magnetic forces in the air-5

CHAPTER 1. INTRODUCTION

Figure 1.3: Generation of acoustic noise of electromagnetic origin.

gap of the motor. The magnetic field is rotating and has both tangential and radial components. The tangential components result in the desired torque of the motor. The radial components are unwanted and act radi-ally on the stator and cause vibrations. The resulting deflections of the stator are described in the structural domain. The response is depend-ent of a match in both frequency and spatial order between the exciting forces and the structural natural modes. The vibrations on the surface of the motor generate pressure variations in the air, thus radiated acoustic noise of the motor.

The resulting radiated noise of a traction motor can be controlled in multiple ways. One possibility is to change the physical structure of the motor and thereby affect the structural response of the motor. Another possibility is to change the excitation of the structure by changing the forces resulting from the converter control and the used PWM method. This thesis focuses on the latter.

1.2

Objectives

The overall objective of this thesis is to gain a thorough understanding on the generation of acoustic noise of electromagnetic origin in a trac-tion motor to be able to reduce the resulting noise to a low level by controlling the excitation forces. More specific, the following research questions are studied:

• What are the causes of the main vibrations (and hence also radi-ated noise) of a traction motor at PWM operation?

• How is the radiated noise of a traction motor affected by the PWM switching frequency?

• Is it possible, from simplified modeling of the motor, to determ-ine a engdeterm-ineering optimum value of the switching frequency that result in a good acoustic behavior?

1.3. THESIS OUTLINE

1.3

Thesis outline

This thesis is organized in two parts. Part I gives some background to the problem studied and a motivation for the research area. Then the problem studied and the objectives of this thesis is presented. In chapter 2, an introduction to pulse width modulation and the generation of acoustic noise of electromagnetic origin in traction motors is given. The contributions of this thesis are briefly presented in Chapter 3 where each of the appended papers is summarized. The main conclusions of the work are listed in Chapter 4. Part II contains the full contribution of this thesis in the form of appended papers.

2

Acoustic noise of

electromagnetic

origin in traction

motors

The acoustic characteristics of induction motors and the corresponding sources have been subject to research for a very long time. One of the earliest works on the subject was presented already in 1921, investigat-ing different sources for acoustic noise of electrical machines and how this noise could be reduced by an improved mechanical design [6]. The characterization of acoustic noise generated by Maxwell forces, i.e. noise of electromagnetic origin, was later addressed in 1954 [7]. Since these early works, the research has continued and also included new areas and aspects as the development of the related electrical machines and technology has evolved [8–10].

2.1

Pulse width modulation

The traction motor 3-phase voltage is generated from a voltage source converter having a constant DC-link voltage Udcas input. The output

from this converter cannot be varied constantly, it can only be switched between discrete levels. The aim of the converter control is to appropri-ately choose the switching instants. This is done using a pulse width modulation (PWM) method with the purpose to select the switching instants so that the voltage-time area over time of the switched pulse pattern coincides with that of the desired reference waveform.

CHAPTER 2. ACOUSTIC NOISE OF ELECTROMAGNETIC ORIGIN IN TRACTION MOTORS

A very basic PWM method is the carrier-based method where the switching instances are determined from intersections between the de-sired reference and a high-frequency carrier. Typically the carrier is a triangular or a sawtooth wave. When the reference is higher than the carrier, the output is in its high state. Otherwise it is in the low state. This is illustrated in Figure 2.1 where the red line is the reference, the blue line the carrier and the green line the switched output voltage.

0 0.2 0.4 0.6 0.8 1 time [s] -1 0 1 voltage [V] 0 0.2 0.4 0.6 0.8 1 time [s] -1 0 1 voltage [V]

Figure 2.1: Principle of pulse width modulation.

The ratio between the switching frequency fcand the fundamental

frequency of the reference f0is defined as the pulse number n:

n= fc

f0. (2.1)

The effect of an increased switching frequency, i.e. increased pulse num-ber, is illustrated evaluating the frequency spectra of the pulse width modulated signal for two different pulse numbers. The result is plotted in Figure 2.2 and as can be seen, an increased switching frequency shifts the harmonic content to higher frequencies. A change of the harmonic content of the PWM output voltage also affects the motor currents. As an example, the motor current for a certain application is plotted for the different pulse numbers in Figure 2.3. It is evident that the desired current wave form is less disturbed for the higher pulse number.

An important measure for evaluating the performance of a specific PWM method is the harmonic distortion. This factor quantifies to what 10

2.1. PULSE WIDTH MODULATION 0 20 40 60 80 100 Harmonic order 10-2 10-1 100

Harmonic magnitude (p.u.)

(a) 0 20 40 60 80 100 Harmonic order 10-2 10-1 100

Harmonic magnitude (p.u.)

(b)

Figure 2.2: Harmonic components for pulse number (a) 9 and (b) 24.

degree undesirable harmonics are created by the used PWM method. The consequences of the harmonics are for example: additional losses in equipment connected to the converter, torque pulsations in electrical machines fed by the converter and also acoustic noise. A commonly used measure for the harmonic distortion is the weighted total harmonic distortion (WTHD) which is calculated as

WTHD = 1 Urms,1 v u u t ∞

∑

where Urms,kis the RMS voltage of the kth harmonic and k = 1 is the

fun-damental frequency. If the two examples above are again considered, the WTHD is reduced from 10.6% to 3.9% as the pulse number is increased from 9 to 24.

CHAPTER 2. ACOUSTIC NOISE OF ELECTROMAGNETIC ORIGIN IN TRACTION MOTORS 0 0.005 0.01 0.015 0.02 time [s] -1 -0.5 0 0.5 1

norm. motor phase current [-]

(a) 0 0.005 0.01 0.015 0.02 time [s] -1 -0.5 0 0.5 1

norm. motor phase current [-]

(b)

Figure 2.3: Example of one motor phase current for pulse number (a) 9 and (b) 24.

There are numerous different PWM methods, except different carrier-based methods there are for example also various programmed methods and direct methods. For programmed methods, the switching instances are computed off-line with the objective to achieve a certain perform-ance. For direct methods, the switching instances are determined dir-ectly, typically with the aim to keep a control variable within a specific tolerance band. Despite the used PWM method, the main objective is still to determine the switching instances to output the desired funda-mental waveform and at the same time preferably minimize unwanted harmonic distortion [11].

2.2

Electromagnetic forces

The electromagnetically generated acoustic spectrum can be determined from the radial component of the air gap Maxwell forces. The electro-magnetic forces result from the interaction of two different flux density waves, i.e. a combination of two permeance harmonics and two mag-netomotive (mmf) harmonics. The electromagnetic forces can be classi-fied into three different groups [12]:

1. forces resulting of the interaction of the stator and rotor magneto-motive force (mmf) and permeance slot field harmonics,

2. forces resulting of the interaction of the fundamental stator mmf and a harmonic mmf caused by a PWM time harmonic of the stator current,

2.2. ELECTROMAGNETIC FORCES

mode 0 mode 1 mode 2

Figure 2.4: Examples of spatial modes of order 0, 1 and 2.

3. forces resulting of the interaction of the permeance slot field har-monics and and a harmonic mmf caused by a PWM time harmonic of the stator current.

The three different groups of electromagnetic forces have been fur-ther studied, and analytical expressions for the the force harmonics with highest magnitude have also been derived [13–16].

All electromagnetic forces can be characterized by their frequency and their mode order, that is the circumferential spatial shape. Examples of mode order 0, 1 and 2 are shown in Fig. 2.4. The resulting vibrations are dependent of a match in both frequency and spatial order between the exciting forces and the structure.

The main electromagnetic forces in the first group, the so called slot-ting vibrations, are listed in Table 2.1. Variables kr and ks are positive

integers from the Fourier series of the permeance distribution, Zr and

Zs are the number of rotor and stator slots respectively, s is the slip,

and p the number of pole pairs. The higher kr and ksare, the lower are

the permeance harmonics, and the lower is the corresponding force har-monic. Therefore, the force harmonics with highest magnitude are given by kr= ks= 1.

Table 2.1: Frequency and spatial order of the main slotting vibrations

Name Frequency f Spatial order m

F_slot− fs(krZr(1−s)/p−2) krZr−ksZs−2p

F_slot0 fs(krZr(1−s)/p) krZr−ksZs

F_slot+ fs(krZr(1−s)/p+ 2) krZr−ksZs+ 2p

The characteristics of the forces in the second group, the so called PWM vibrations are listed in Table 2.2. The fundamental stator flux density is of order p and frequency fs where p is the number of

pole-pairs and fsthe fundamental stator frequency. The harmonic flux

CHAPTER 2. ACOUSTIC NOISE OF ELECTROMAGNETIC ORIGIN IN TRACTION MOTORS

Table 2.2: Frequency and spatial order of pure PWM force lines

Name Frequency f Spatial order m

Fpwm− fs−ηsfns p−p= 0

Fpwm+ fs+ ηsfns p+ p = 2p

ity caused by a PWM time harmonic of the stator current is of order p and frequency fs

nwhere fns is the frequency of the PWM harmonic.

De-pending on the propagation direction of the harmonic mmf, the interac-tion of these flux density waves results in two groups of force harmon-ics, F−

pwmand Fpwm+ . As can be seen in Table 2.2, all pure PWM vibrations

have a spatial order of 0 or 2p

Figure 2.5: Relationship between amplitude of harmonics and modulation index, fIis the

fundamental stator frequency (figure from [17]).

For PWM with a triangular carrier, the harmonics fs

ncan be written

as

fns= n1fs±n2fc, (2.3)

where fcis the PWM switching frequency and n1and n2have an

oppos-ite parity. The amplitude of the harmonics of the motor current depends on both the carrier type of the PWM as well as the modulation index. The relationship between the amplitude of the current harmonics and the modulation index for PWM with a triangular carrier is shown in Fig. 2.5. As can be seen, the amplitude of 2 fc± fs is dominating for lower

modulation indexes but at high modulation indexes the amplitude of

fc±2 fsis instead largest [17].

Substituting these frequencies in Table 2.2 gives the frequencies cor-responding to the main peaks in the PWM related vibration response for a triangular carrier signal and their spatial orders as listed in Table 2.3. 14

2.2. ELECTROMAGNETIC FORCES

Table 2.3: Frequencies and spatial orders of main PWM-related vibration peaks for a tri-angular carrier

( fs

n = 2 fc±fs) ( fns = 4 fc±fs) ( fns = fc±2 fs) ( fns = 3 fc±2 fs) Spatial

Frequency f Frequency f Frequency f Frequency f order m 2 fc−2 fs 4 fc−2 fs fc+ fs 3 fc+ fs −2p

2 fc 4 fc fc+ 3 fs 3 fc+ 3 fs 0

2 fc 4 fc fc−3 fs 3 fc−3 fs 0

2 fc+ 2 fs 4 fc+ 2 fs fc−fs 3 fc−fs 2p

The main electromagnetic forces in the third and last group, the so called slotting vibrations, are listed in Table 2.4. They have the same spa-tial order as the slotting vibrations but they occur at a higher frequency.

Table 2.4: Frequency and spatial order of slotting PWM vibrations

Frequency f Spatial order m

F_slotpwm− fs(krZr(1−s)/p−1)−ηsfns krZr−ksZs−2p

Fslotpwm fs(krZr(1−s)/p−1)−ηsfns krZr−ksZs

Fslotpwm fs(krZr(1−s)/p+ 1) + ηsfns krZr−ksZs

F_slotpwm+ fs(krZr(1−s)/p+ 1) + ηsfns krZr−ksZs+ 2p

The first group of forces is present also at sinusoidal supply but the other two groups are added at PWM operation and therefore a motor that is quiet at sinusoidal supply can still be noisy at PWM supply.

The number of force harmonics is infinite, but the amplitude of the corresponding harmonics in the vibration response is inversely propor-tional to m4 where m is the spatial order. Therefore, only the lowest spatial order forces result in vibration levels (and thereby noise) of sig-nificance. For traction motors, the spatial orders of interest are those of order zero to four.

The power spectral density of an accelerometer measurement dur-ing an acceleration is plotted in Figure 2.6. The acceleration is constant, hence time is corresponding to motor speed. In the obtained spectrum, vibrations resulting from all thre groups of electromagnetic forces can be observed. The slotting vibrations are depending on motor speed with the corresponding force lines starting in the origin. Both PWM vibra-tions and slotting PWM vibravibra-tions are located around multiples of the switching frequency (1050 Hz in this case) but with different depend-ence on the motor speed.

CHAPTER 2. ACOUSTIC NOISE OF ELECTROMAGNETIC ORIGIN IN TRACTION MOTORS

Figure 2.6: Spectrogram of one accelerometer during constant acceleration

With an appropriate selection of number of stator and rotor slots [18], the influence of the forces related to permeance slot field harmonics can be reduced and the main vibration forces will be related to PWM vibra-tions, i.e. the second group of electromagnetic forces. The character-istics of these vibrations are related to the PWM switching frequency, the stator frequency and the pole pair number. Therefore, the PWM switching frequency can be used as a design parameter to to influence the acoustic performance of the traction motor.

The impact of PWM switching frequency on the radiated acoustic noise is addressed in several publications investigating different aspects of the problem. It has been concluded that it is of great importance to avoid coincidence with the natural frequency of the stator. If the PWM forces acting as excitation matches the natural modes and frequencies of the structure, higher levels of vibration and acoustic noise are cre-ated [19, 20]. The recommendation is to select a switching frequency be-low or above the frequency region of the stator natural modes of order 0 and 2p since these are the spatial orders of the pure PWM forces [16]. Another aspect of increasing the switching frequency is that the electro-magnetic force excitation is reduced due to an reduced harmonic content 16

2.2. ELECTROMAGNETIC FORCES

in the motor currents. However, since the structural resonances are of great importance for the resulting noise level, there is not necessarily a proportional relationship between the reduced harmonic current amp-litude and the resulting acoustic noise [21].

When designing a system for high power applications, as a traction converter is, there is always a tradeoff between performance, size, cost and operating switching frequency of the switching devices used. The progress in silicon carbide (SiC) material opens for replacing the conven-tionally used silicon power semiconductors with silicon carbide power semiconductors. This enables the possibility for much higher switching frequencies for high power applications with the efficiency and perform-ance still retained [22,23]. Except improved acoustic performperform-ance, other advantages are reduced losses, reduced temperature and reduced size. Since these new devices enables a new range of feasible PWM switching frequencies for traction converters and train applications, it is of great interest to fully understand the behavior in this frequency range and the implications for the system in total. This also include the effect on the resulting radiated acoustic noise of the traction motor.

The evaluation of different PWM methods with respect to acous-tic performance has also been subject of research. There are numerous commonly used PWM methods [24] and on-line methods have in com-mon that strong current harcom-monic components are present in specific frequency bands. These frequency bands in combination with the mech-anical design, i.e. structural behavior, determine the resulting radiated noise [25, 26]. There are also possibilities to implement off-line optim-ized PWM methods. With these, there are openings to for example min-imizing specific problematic harmonic components [25, 27]. However, off-line methods might be difficult to implement and not always pos-sible to use in practice.

A common approach to reduce the acoustic noise at PWM opera-tion is various randomizaopera-tion techniques [28,29]. However, it should be noted that at randomization the total energy is not reduced but spread over a wider range of frequencies. Thereby, tonality is reduced but of-ten the sound pressure level does not decrease and at times it even in-creases [25, 30].

3

Summary of

appended papers

The contribution of the appended papers includes:

• Experimental measurements and evaluation of operational deflec-tion shapes of a tracdeflec-tion motor at PWM operadeflec-tion (Paper A) • Experimental investigation and evaluation of the impact of an

in-creased PWM switching frequency on the radiated acoustic noise of a traction motor. Initial validation of the proposal that the trac-tion motor shows a constant mobility behavior at higher frequen-cies (Paper B)

CHAPTER 3. SUMMARY OF APPENDED PAPERS

3.1

Paper A

Operational Deflection Shapes of a PWM-fed Traction Motor

The operational deflection shapes of a three-phase induction motor fed by a PWM frequency converter are studied. At operation, the radial de-flections of the stator are measured with a mesh of accelerometers in a lab environment. The tests are performed for different motor speeds and different PWM switching frequencies. The frequency and spatial modes of the deflections with largest amplitude are determined. The resulting vibrations are dependent of match both in frequency and space between the exciting forces and responding structure.

The dominating deflections all have spatial mode 0 or 4. They can all be related to electromagnetic forces resulting from the PWM that are of spatial mode 0 and 4. These PWM forces are a result of the combina-tion of the fundamental flux density and its time harmonics. The fun-damental flux density is given by the stator frequency, and its harmon-ics by the stator frequency and the switching frequency of the PWM. The frequency of the PWM forces can therefore be determined from the stator frequency and the switching frequency.

There is a significant difference in amplitude of the largest deflections, and radiated noise, for the two different tested switching frequencies. Changing the switching frequency shifts the frequencies of the excit-ation forces and thereby influences the match between the structural mode natural frequency of the stator and the PWM excitation. An ap-propriate selection of the PWM switching frequency is therefore a power-ful tool to influence the acoustic radiation of the motor originating from the electromagnetic excitation.

Figure 3.1: The operational deflection shape at 2036 Hz. The switching frequency is 1050 Hz and motor speed 940 rpm.

3.2. PAPER B

3.2

Paper B

Reduction of radiated acoustic noise of a traction motor at PWM con-verter operation

The impact of the PWM switching frequency on the radiated acoustic noise from a traction motor is studied. A four pole 150 kW traction mo-tor is tested at PWM converter operation in a lab environment. The test is repeated for four different motor speeds and the PWM switching fre-quency is stepwise increased. The resulting sound pressure level as an average of three different microphone positions is quantified and ana-lyzed. It is found that the sound pressure level decreases as the switch-ing frequency increases. The trend for all four tested motor speeds is similar and for higher switching frequencies, the sound pressure level seems to asymptotically reach a constant level.

It is proposed that the traction motor shows similar behavior as a cyl-indrical structure approaching a plate with a constant mobility above a certain frequency. If the excitation from the PWM forces is in the constant mobility region there are no structural advantages to increase the switching frequency even higher. For lower frequencies, the modal density is lower and coincidence between the exciting forces resulting from the PWM supply with structural natural modes of the motor res-ult in high noise levels. The frequency where the constant mobility re-gion is entered could be used as an engineering optimum with respect to acoustic noise for the switching frequency. Noise and motor losses are decreasing with increasing switching frequency, converter losses on the other hand increase with switching frequency. As a trade-off on system level it is desirable to select a high switching frequency resulting in low noise, but not too high to have acceptable converter losses.

0 5 10 15

Norm. switching frequency (-)

-35 -30 -25 -20 -15 -10 -5 0

Norm. sound pressure level (-)

140 rpm 552 rpm 970 rpm 1380 rpm

Figure 3.2: Normalized average sound pressure level for increasing switching frequency for the four motor speeds tested.

4

Conclusions

The main conclusions of this work are:

• An appropriate selection of the PWM switching frequency is a powerful tool to influence the acoustic radiation of the motor. A changed switching frequency shifts the frequencies of the excita-tion forces and thereby influences the match between structural mode natural frequencies and the PWM excitation.

• The sound pressure level decreases as the switching frequency in-creases and for higher switching frequencies, the sound pressure level seems to asymptotically reach a constant level.

• It is proposed that the traction motor shows constant mobility above a certain frequency and there are no structural advantages to in-crease the switching frequency even higher than moving the PWM excitation forces into this constant mobility region. This could be used as an engineering optimum for the switching frequency with respect to acoustic noise.

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Part II