Manufacturing Effects on Iron Losses in Electrical Machines

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DEGREE PROJECT, IN ELECTRIC POWER ENGINEERING , SECOND LEVEL STOCKHOLM, SWEDEN 2015

Manufacturing Effects on Iron Losses in Electrical Machines

KONSTANTINOS BOURCHAS

KTH ROYAL INSTITUTE OF TECHNOLOGY ELECTRICAL ENGINEERING

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Manufacturing Effects on Iron Losses in Electrical Machines

Konstantinos Bourchas

Master of Science Thesis in Electrical Machines and Drives at the School of Electrical Engineering

Royal Institute of Technology Stockholm, Sweden, June 2015

Supervisors: Dr. Alexander Stening(ABB LV Motors) Dr. Freddy Gyllensten (ABB LV Motors) Examiner: Docent Juliette Soulard (KTH)

XR-EE-E2C 2015:006

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Manufacturing Effects on Iron Losses in Electrical Machines.

KONSTANTINOS BOURCHAS

Copyright c 2015 by Konstantinos Bourchas.

School of Electrical Engineering Department of Energy Conversion Royal Institute of Technology SE-100 44 Stockholm

Sweden

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List of Symbols

β exponential coefficient [-]

µ₀ vacuum permeability [(T· m)/A]

µr relative permeability [-]

A cross sectional area [m²]

B flux density [T]

B_r remanent flux density [T]

f frequency [Hz]

H magnetizing field [A/m]

H_c coercive field [A/m]

I₁ current in the primary winding [A]

k_ec eddy current loss coefficient _W

kg·(T·Hz)²

kexc excess loss coefficient _W

kg·(T ·Hz)^1.5

k_hyst hysteresis loss coefficient _W

kg·T²·Hz

l length of single Epstein strip [m]

l_m effective path length of flux [m]

m total mass of test specimen [kg]

ma active mass of test specimen [kg]

M_s saturation magnetization [A/m]

N₁ number of turns of primary winding of Epstein frame [-]

N2 number of turns of secondary winding of Epstein frame [-]

p_ec eddy current loss density [W/kg]

p_exc excess loss density [W/kg]

pF e iron loss density [W/kg]

p_hyst hysteresis loss density [W/kg]

P_c total losses of test sample [W]

Pm measured power [W]

P_s specific iron losses [W]

R_i total resistance of the instruments that are connected to the secondary winding [Ohms]

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List of Abbreviations

ELE Exponential Law Extrapolation FEM Finite Element Method

HAZ Heat Affected Zone

LASER Light Amplification by Stimulated Emission of Radiation MMF Magnetomotive Force

RD Parallel to the rolling direction SST Single Sheet Test

TD Perpendicular to the rolling direction

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Abstract

In this master thesis, the magnetic properties of SiFe laminations after cutting and welding are studied. The permeability and the iron loss density are investigated since they are critical characteristics for the performance of electrical machines. The magnetic measurements are conducted on an Epstein frame for sinusoidal variations of the magnetic flux density at frequencies of 50, 100 and 200 Hz, according to IEC 404-2. Mechanical cutting with guillotine and cutting by means of fiber and CO₂ laser are performed. The influence of the fiber laser settings is also investigated. Especially the assisting gas pressure and the power, speed and frequency of the laser beam are considered.

In order to increase the cutting effect, the specimens include Epstein strips with 1, 2 and 3 additional cutting edges along their length. It is found that mechanical cutting degrades the magnetic properties of the material less than laser cutting. For 1.8% Si laminations, mechanical cutting causes up to 35% higher iron loss density and 63% lower permeability, compared to standard Epstein strips (30 mm wide). The corresponding degradation for laser cut laminations is 65% iron loss density increase and 65% permeability drop. Material of lower thickness but with the same Si-content shows lower magnetic deterioration. Additionally, laser cutting with high-power/high-speed characteristics leads to the best magnetic characteristics among 15 laser settings. High speed settings have positive impact on productivity, since the cutting time decreases.

The influence of welding is investigated by means of Epstein measurements. The test specimens include strips with 1, 3, 5 and 10 welding points. Experiments show an iron loss increase up to 50% with a corresponding 62% reduction in the permeability.

A model that incorporates the cutting effect is developed and implemented in a FEM- based motor design software. Simulations are made for a reference induction motor.

The results indicate a 30% increase in the iron losses compared to a model that does not consider the cutting effect. In case of laser cut core laminations, this increase reaches 50%. The degradation profile considers also the deteriorated magnetizing properties. This leads to increased nominal current up to 1.7% for mechanically cut laminations and 3.4%

for laser cut laminations. This corresponds to a 1.4% and 2.6% reduced power factor, respectively.

Keywords – electrical machine, induction motor, iron losses, relative permeability, guillotine, fiber laser, CO2 laser, laser settings, cutting effect, welding, electrical steel.

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Sammanfattning

I detta examensarbete studeras hur de magnetiska egenskaperna hos SiFe-pl˚at p˚averkas av skärning och svetsning. Permeabilitet och järnförlustdensitet undersöks eftersom de

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ar kritiska variabler för elektriska maskiners prestanda. De magnetiska mätningarna genomfördes p˚a en Epstein ram med en flödesfrekvens p˚a 50, 100 och 200 Hz, enligt IEC 404-2. Effekterna av mekanisk skärning med giljotin samt skärning med fiber- och CO2-laser studerades. Inverkan av olika fiberlaserinställningar undersöktes ocks˚a genom att variera gastrycket, skärhastigheten samt frekvensen och effekten av laserstr˚alen.

För att öka skäreffekten inkluderades Epsteinremsor med ytterligare 1, 2 och 3 längsg˚aende skärsnitt. Det visas att mekanisk skärning har en mindre p˚averkan p˚a de magnetiska egenskaperna hos materialet än vad laserskärning har. Mätningar p˚a pl˚at med 1.8% Si visar att d˚a prov med tre extra längsg˚aende giljotinklipp används kan permeabiliteten reduceras med upp till 63% och järnförlusterna kan öka med upp till 35%. Motsvarande resultat för laserskurna pl˚atar visar en permeabilitetsreduktion p˚a upp till 65% och en järnförlustökning p˚a upp till 65%. Ur studien av de tv˚a studerade skärprocesserna framkommer även att tunnare pl˚at p˚averkas mindre negativt än tjockare pl˚at. Ett antal olika inställningar har provats för att utreda hur olika parametrar p˚averkar effekterna av laserskärning. Studien indikerar att skärning med hög effekt och hög hastighet ger den minsta p˚averkan p˚a materialets magnetiska egenskaper. Vilket även har en positiv inverkan p˚a produktiviteten vid laserskärning.

Epsteinprover har även utförts för att undersöka vilka effekter som introduceras d˚a SiFe-pl˚at svetsas. Provstyckena bestod av remsor med en, tre, fem och 10 svetspunkter.

Experimenten visar en järnförlustökning med upp till 50% samt en permeabilitetsreduktion upp till 62% d˚a pl˚atarna svetsats samman tv˚a och tv˚a.

En modell för att studera effekterna av de förändrade materialegenskaperna vid skärning p˚a en induktionsmotor utvecklas och implementeras i en FEM-baserad mjukvara. Resul- taten tyder p˚a en järnförlustökning med 30% d˚a skäreffekten orsakad av giljotin beaktas.

Vid simulering av laserskuren pl˚at kan denna ökning vara s˚a stor som 50%. Det framkommer även att laserskärningen kan reducera effektfaktorn s˚a mycket som 2.6%.

Nyckelord - elektrisk maskin, induktion motor, järnförluster, relativ permeabilitet, giljotin, fiberlaser, CO2-laser, laser inställningar, skäreffekt, svetsning, elektromagnetisk pl˚at

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Acknowledgments

This thesis concludes my MSc in Electric Power Engineering at KTH, Royal Institute of Technology in Stockholm. The thesis was conducted at ABB LV Motors at V¨aster˚as, Sweden.

First of all, I am grateful to my main supervisor at ABB, Dr. Alexander Stening, for his support during my master thesis project. His regular feedback helped me to improve the content and the text of the thesis. I truly appreciate the discussions we had that helped me to develop myself as motor designer. His understanding and feedback made this master thesis an experience of a lifetime.

Secondly, I would like to thank my manager at ABB, Dr. Freddy Gyllensten. It has been my honor working with him. He gave me very valuable feedback and many ideas which improved the scientific significance of the current master thesis. He was always eager to share with me his experience on motor design, something that helped me a lot to deeply understand many aspects of this field.

Moreover, many thanks go to my examiner at KTH, Docent Juliette Soulard for her feedback on the text of my report. Additionally, in her course, I gained the initial high level knowledge on motor design. She also helped me to finish the latter stages of the master thesis (presentation at KTH) as soon as possible and I am really thankful to her for that.

Furthermore, I would like to thank Dr. Arvid Broddefalk and Magnus Lindenmo from Surahammars Bruks AB. Their help and support throughout the project was very valuable. Many thanks go also to Mats Dahlen from Gerdins AB for the productive cooperation we had during my project.

Next, I would like to thank my colleagues at the group of Technology Development of ABB LV Motors, Lic.Tech. Rathna Chitroju, Lic.Tech. Kashif Khan and Dr. Dan Fors.

The working environment was great and they helped me a lot with the discussions that we had.

I would also like to thank Dr. Andreas Krings from ABB Corporate Research. His suggestions in the early stages of the project helped me a lot and gave me good scientific directions.

I am also thankful to Lic. Tech. Mats Leksell for the cooperation that we had during my time in the Eco Marathon team of KTH and during my time as research assistant at E2C lab. He gave me the opportunity to develop my skills as engineer and to gain valuable hands-on experience. I will always be grateful for that.

I am also very thankful to my close friend and KTH classmate Alexandra Kapidou for her support and all the great moments that we spent at V¨aster˚as, during our master thesis elaboration. I would also like to thank my dear friend and KTH classmate Tin Rabuzin. We spent a lot of quality time discussing about our future and our dreams. I

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am also thankful to my friend Nikolaos Apostolopoulos who has also supported me as brother during the last two years and he gave me real help to take important decisions.

Additionally, I would like to thank all of my friends in Stockholm, who helped me to have a wonderful time in Sweden during the last two years. I will not say all of the names because I am afraid that I will forget someone. I hope that we will continue be in touch for the rest of our lives.

I am also grateful to my family. First to my parents Georgios Bourchas and Elli Geralidou for their endless support throughout my life. Without them, I would not be able to fulfill any of my dreams. I will always be thankful to them because they made me what I am today. Secondly, my two sisters Lina and Kally have offered me endless support through their love and I am grateful for that. I would also like to apologize to them for the time that I have not spent with them because of my studies abroad. Finally, I wish to express my deepest gratitude to Konstantina Nikolaou for her love and understanding not only during my thesis but also during those two years of my master studies.

Konstantinos Bourchas Stockholm, Sweden June 2015

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Chapter 1 Introduction

This chapter describes the background and the scope of this master thesis. Moreover, an outline of the thesis is presented.

1.1 Background

The last 20 years, the climate change has raised concerns worldwide [1]. This is the reason why many regulatory authorities have established legislations regarding the decrease of the energy consumption, aiming at the reduction of the CO₂ emissions. The European Union has established a policy to combat the environmental pollution and the climate change. One of the goals of this policy is a 20% increase in the energy efficiency by 2020 and 27% by 2030 [1].

Electric motors account approximately for 65% of the energy use in industry [2]. That means that any increase of the efficiency of these motors can potentially lead to major energy savings. IEC 60034-30-1:2014 is a standard that regulates the efficiency levels of the industrial induction motors around the world [3]. Figure 1.1 illustrates the range of the efficiency classes as defined by IEC 60034-30-1:2014.

Figure 1.1: Efficiency classes of low voltage, 4-pole induction motors according to IEC 60034-30-1:2014 [3].

The design of high-efficient motors requires accurate motor models. The iron loss 1

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models are often considered as the main source of error for the prediction of the motor efficiency. The estimation of the iron losses in the stator and the rotor of the motor depends on the analytical description of the physical phenomena that cause these losses [4]. Furthermore, the processes during the production of an electrical motor lead to deterioration of the magnetic properties of the core materials. Therefore, they should be taken into consideration [5]. According to [6], the major source of steel degradation is cutting. This master thesis focuses on the effect of different cutting techniques on the stator and rotor material’s magnetic properties. The effect of welding is also investigated.

1.2 Thesis Scope

The main objective of this thesis is to investigate the change of the magnetizing and iron loss characteristics of electrical steel due to cutting by means of guillotine and laser.

The project is divided in different stages as follows:

• Measurement and analysis of the effect of mechanical and laser cutting.

• Comparison between the two cutting techniques.

• Measurement and analysis of welding effect.

• Development of finite element method (FEM) model that incorporates the experimental results.

1.3 Outline

This master thesis consists of three parts. The first part is a literature review on the processes that affect the iron losses of the electrical steel. The second part includes experimental results which concern the cutting and the welding effect. Finally, the third part of the thesis presents a FEM model that incorporates the experimental results. The thesis report is separated in six chapters with the following content.

Chapter 1 presents the background and the scope of the project.

Chapter 2 gives an overview of the physics, the models and the evaluation of the magnetic properties of the ferromagnetic materials.

Chapter 3 discusses the main manufacturing steps that influence the properties of the magnetic materials.

Chapter 4 presents the experiments which are conducted on the Epstein frame regarding the magnetic properties of SiFe laminations after cutting and welding.

Chapter 5 introduces a FEM model that incorporates the cutting effect.

In Chapter 6, conclusions are drawn and suggestions for future investigations are made.

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Chapter 2 Ferromagnetic Materials

Ferromagnetic materials consist of ferromagnetic domains which are small areas, where the magnetic dipoles are parallel to each other [7]. A basic characteristic of ferromagnetic materials is the hysteresis. In this chapter, the loss mechanisms are presented. Addition- ally, the main iron loss models are given and the recommended methods for the evaluation of the magnetic properties of the ferromagnetic materials are discussed.

2.1 Iron Losses

The iron losses are also referred as core losses. They are created by the varying magnetic field in the iron parts of the machine. The two basic components of the iron losses are the hysteresis and the eddy current losses. Both of these components result in the same physical phenomenon which is Joule heating.

2.1.1 Hysteresis Losses

The hysteresis losses are mostly dependent on the microstructure of the magnetic material [8]. The electrical steel consists of uniformly magnetized regions, called domains. When no external field is applied, the statistical sum of the magnetization of all the domains is zero [9]. The neighboring domains are magnetized in an opposite direction and the border that separates two such domains is called domain wall. The domain wall is actually an energy zone through which the magnetization gradually changes direction [4].

When an external field is applied, the domain wall moves in the direction of the field.

As a result, the area of the domain whose magnetization is aligned to the field grows at the expense of the area of the neighboring domain which has opposite magnetization [10].

However, non-magnetic impurities (like carbon and sulfur) can be found in the electrical steel. These impurities act as pinning sites and they hinder the domain wall motion [4, 9]. In this case, the domain wall overcomes the pinning sites by being subjected to increased external field. After a certain field, the domain wall rapidly overcomes (jumps) the pinning site. This phenomenon is called Barkhausen jump and through this rapid movement, eddy currents are induced. These eddy currents cause Joule losses which in this case also called hysteresis losses [4]. The Barkhausen effect is one of the main reasons of the hysteresis loop behavior of the magnetic material.

For low values of external field H, the domain walls do not overcome the pinning sites. Therefore there are no Barkhausen jumps. At this region, the magnetization is

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reversible, which means that if the magnetizing field is removed, then the magnetization of the material returns to zero. The slope of the BH curve in this region is expressed by the initial susceptibility [11]. For higher magnetizing fields, the magnetization of the material is no longer reversible [11]. If the flux density in the material reaches saturation and the external field is removed, the material sustains a remanent magnetization which is expressed by B_r. In order to demagnetize the material, an opposing coercive magnetic field H_c should be applied. This behavior of the ferromagnetic material is illustrated by the hysteresis loop as depicted in Figure 2.1.

Figure 2.1: Initial BH curve and hysteresis loop of ferromagnetic materials.

2.1.2 Eddy Current Losses

The variation of the magnetic flux over time induces an electrical field in the magnetic core, which causes the flow of eddy currents. According to Lenz law, these currents tend to oppose the field that produced them. This current flow results in Joule losses, also called eddy current losses [12, 13]. The most effective method to reduce the eddy current losses is to divide the core into thin sheet laminations as depicted in Figure 2.2.

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2.2. IRON LOSS MODELS 5

Figure 2.2: Solid and laminated iron core to reduce the flow of the eddy currents (red lines). The vector of flux density B is perpendicular to the surface of the core.

2.2 Iron Loss Models

The estimation of the iron losses is one of the most challenging aspects in the design and analysis of an electrical machine. There are many different analytical approaches which estimate the iron losses for different induction levels and frequencies. In this section, the most common iron loss models are presented.

2.2.1 Models based on the Steinmetz Equation

Steinmetz was the first who developed an analytical approach to predict the iron losses in 1892 [14, 15] . Equation 2.1 is called Steinmetz Equation and expresses the iron losses of the material. This equation is valid only for sinusoidal flux density waveforms.

p_{F e} = k_SEf^αBˆ^β (2.1)

where p_{F e} are the specific iron losses (W/kg), f is the frequency and ˆB is the peak value of the flux density. The coefficients k_SE, α and β are obtained through fitting in the experimental results.

Based on the Steinmetz’s initial empirical equation, several iron loss models have been developed. The Modified Steinmetz Equation [16] is such model and can be used for arbitrary flux density waveforms. The Modified Steinmetz Equation is given in formula 2.2.

p_{F e}= k_SEf_eq^α−1Bˆ^βf (2.2) where f_eq is an equivalent frequency which depends on the rate of change of the flux density and is expressed as:

f_eq = 2

∆B²π² Z T

0

dB dt

!2

dt (2.3)

Other approaches based on the Steinmetz Equation can be found in [17].

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2.2.2 Separation Models

Another approach is splitting the iron losses in two or three terms. These terms correspond to the hysteresis, the eddy current and the excess losses. Table 2.1 summarizes the models that are based on the separation approach.

Jordan[18]

p_{F e}= p_hyst+ p_ec = k_hystf ˆB²+ k_ecf²Bˆ² (2.4)

Pry and Bean[19]

p_{F e} = p_hyst+ η_ap_ec =

khystf ˆB²+ ηexckecf²Bˆ² (2.5) Bertotti[4]

p_{F e}= p_hyst+ p_ec+ p_exc

= k_hystf ˆB²+ k_ecf²Bˆ²+ k_excf^1.5Bˆ^1.5 (2.6) Jacobs[20]

p_{F e} = k_hyst⁰ f ˆB²+ (k_ec+ a₁Bˆ^a²) ˆB²f² (2.7) where k_hyst⁰ = khyst(1 + _B^B^min

max(r − 1))

Table 2.1: Separation models for iron loss estimation.

Jordan in [18] separates the iron losses in hysteresis and eddy current losses. Both terms depend on the amplitude of the flux density. However, the hysteresis term depends on f (static losses), while the eddy current losses depend on f² (dynamic losses) [9].

Even though Equation 2.4 holds for NiFe laminations, it is not accurate for SiFe [9].

This is the reason why, Pry and Bean in [19] introduced a correction factor η_excto minimize the discrepancy between the measured and the predicted eddy current losses.

Bertotti in [4] gave a physical meaning to this discrepancy by introducing a third term, which is the excess (or anomalous) losses. This term corresponds to the mesoscopic scale in the magnetization process and depends on the eddy currents due to the domain wall motion, assuming that the hysteresis losses and the Barkhausen effect are disregarded [4].

Jacobs in [20], evolved Bertotti’s model in order to take into account the rotational losses (through the constant r) and the high order losses (through the constant α₂), which is caused by the magnetic saturation. The separation models are valid for a frequency range, where the skin effect is negligible [8].

2.2.3 Hysteresis Models

Apart from the iron loss models that are based on the Steinmezt Equation and the separation concept, there are also models considering the hysteresis behavior of the magnetic

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2.3. CHARACTERIZATION OF MAGNETIC PROPERTIES OF ELECTRICAL STEELS7 material. Such models are more complicated, but they have higher accuracy. The Preisach model is one of these models [21]. More details about the various iron loss models can be found in [17].

2.3 Characterization of Magnetic Properties of Elec- trical Steels

In this section, the basic methods of characterization of the magnetic properties of the electrical steels are presented. Figure 2.3 gives an overview of these methods.

Figure 2.3: Overview of magnetic material characterization methods.

The characterization of magnetic materials is obtained through magnetic measurements. The magnetic measurements of electrical steels concern the determination of the magnetizing and iron loss characteristics of the material. The Epstein frame measurements and the Single Sheet Test (SST) are two methods for the characterization of strip shaped laminations. The geometry of the samples is simple and their dimensions are determined by standards. The catalogue data of the electrical steel manufacturers are obtained using Epstein frame measurements. The major drawback of these methods is that the geometry of the test specimens is not representative of the actual motor application.

Another method for the magnetic characterization is the measurements on a ring core topology [22]. The main advantage of this method, compared to the Epstein and SST, is that the geometry is representative of the stator yoke of electrical machines. Further to mention, this topology offers a closed flux path without any airgaps [9]. An alternative approach based on the ring core topology is conducting measurements on an actual stator core. The principles of operation are the same as in the ring core topology. However, the stator teeth cause fringing effect, which means that the flux is not uniformly distributed.

These effects can be corrected through models as presented in [23, 24]. More methods and detailed description of magnetic measurements can be found in [25].

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2.4 Magnetic Measurements by means of the Epstein Frame

Measurements with the 25 cm Epstein frame is the standardized method to characterize the magnetic properties of electrical steel and follows the IEC 404-2 standard [26]. The samples consist of rectangular strips. Therefore they are easy to cut and measure. Ad- ditionally, the effect of the cutting direction is canceled, since strips that are cut in the rolling direction (RD) and strips are cut transversally to the rolling direction (TD) are measured simultaneously in the Epstein frame. The main characteristics of the Epstein measurements, following the IEC 404-2 standards, are summarized below:

• The Epstein frame consists of 4 coils, as depicted in Figure 2.4. The strips under test are inserted in these coils. Each of these coils includes a primary (excitation) and a secondary (measurement) winding. The primary windings are connected in series, as illustrated in Figure 2.5. The same applies for the secondary windings.

• The Epstein strips form a square which has double-lapped joints. This way, each of the four branches has the same length and cross sectional area.

• The strip width shall be 30 mm while the strip length shall be in the range of 280 mm-320 mm.

• The number of strips shall be a multiple of 4.

• lm is the effective magnetic path of the Epstein frame and equal to 0.94 m (Figure 2.4).

Figure 2.4: The 25 cm Epstein frame [26].

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2.4. MAGNETIC MEASUREMENTS BY MEANS OF THE EPSTEIN FRAME 9

Figure 2.5: Connection of windings in the Epstein frame.

The waveforms of the magnetizing current and the output voltage are illustrated in Figures 2.6-2.7. The magnetizing current is regulated so that the voltage in the secondary is sinusoidal. This way, the condition of sinusoidal flux density is satisfied, as IEC 404-2 defines.

Figure 2.6: Example of the current in the primary winding of the Epstein frame.

Figure 2.7: Voltage in the secondary winding of the Epstein frame.

The total losses of the test specimen (i.e all the strips) are determined by Equation 2.8.

P_c = N₁ N2

P_m− (1.111| ¯U₂|)² Ri

(2.8)

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Where,

• P_c are the total losses of the test sample.

• N1 is the total number of turns of the primary winding.

• N₂ is the total number of turns of the secondary winding.

• P_m is the measured power.

• | ¯U₂| is the average value of the rectified voltage that is induced in the secondary winding.

• R_i is the total resistance of the instruments that are connected to the secondary winding.

The specific iron losses are then determined by Equation 2.9.

P_s = P_c ma

(2.9) Where m_a is the active mass of the test specimen and is defined as shown in Equation 2.10.

m_a = m · l_m

4l (2.10)

Where, l is the length of one Epstein strip and m is the total mass of the test specimen (includes all strips of the Epstein frame).

The calculation of the magnetizing characteristics of the tested material lies on the determination of the values of the magnetizing field H and the corresponding induced flux density B. The magnetizing field H is obtained through the current in the primary winding I₁ and is given by Equation 2.11.

H(t) = N₁

l_mI1(t) (2.11)

The induced flux can be obtained directly by means of a fluxmeter or by digitally integrating the voltage of the secondary winding, as given in Equation 2.12 [9].

B(t) = − 1 N₂A

Z

u₂(t)dt (2.12)

Where A is the cross sectional area of the test specimen and is given by Equation 2.13.

A = m

4 · l · ρ_m (2.13)

with ρ_m being the conventional density as determined by IEC 404-13.

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2.5. SUMMARY 11

2.5 Summary

In this chapter the physical phenomena behind the losses of the ferromagnetic materials were shortly presented. Furthermore, analytical models to estimate the iron losses were discussed. Historically, Steinmetz was the first to develop a mathematical model that describes the iron losses as a function of the induction and the frequency. Nowadays, there are many approaches towards the iron loss estimation. The selection of an iron loss model is a trade-off between complexity of implementation and accuracy of estimation.

Additionally, the major methods of magnetic measurements were presented. SST and Epstein frame require samples with simple geometry. These are also the two methods used by the electrical steel manufacturers. On the other hand, the ring core measurements use specimens that are more representative of the motor geometry. The method by means of the Epstein frame was thoroughly discussed, since it is used in the current thesis project.

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Chapter 3 Manufacturing Effects on Iron Losses in Electrical Machines

The accurate estimation of the iron losses depends not only on the use of a sophisticated iron loss model, but also on the incorporation of the manufacturing effects that deteriorate the magnetic properties of the electrical steel. In this chapter, the major processes that cause degradation of the magnetic material, according to the literature, are presented.

3.1 Introduction

The production of an electrical machine consists of different manufacturing processes.

Each of these production steps induce mechanical and thermal stresses to the magnetic material used in the active parts of the motor. These stresses change the magnetic and the electrical properties of the material. In this chapter, the major manufacturing effects are presented.

Figure 3.1: Overview of manufacturing effects on iron losses in electrical machines.

3.2 Forming the Core Laminations

The electrical steel laminations used in the stator and rotor cores of electrical machines are typically obtained by cutting through punching or laser. In the case of Epstein or SST strips, guillotine cutting is mainly used. The use of this tool emerges from the fact that the guillotine cutting is closer to the punching process which is used in mass production of stator and rotor cores. The standards covering the Epstein measurements

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suggest guillotine as the cutting technique for the test specimens [26]. Laser cutting is mainly used for the production of stator and rotor cores of prototype motors or small scale production, since the adjustment of the punching tool into a new design has relatively high cost.

3.2.1 Mechanical Cutting

The major factor for the deterioration of the magnetic properties of the ferromagnetic materials is the cutting process [27, 28, 29]. This degradation is caused by the induced mechanical stresses [30] during cutting. These stresses lead to an increase in the materials’

specific iron losses and a drop of the relative permeability [31].

3.2.1.1 Affected Area due to Mechanical Cutting

In [32], SST measurements indicate that there is a degradation of the magnetic properties of SiFe laminations in an area which can be greater than 10 mm from the cut edge.

According to [33], the magnetically deteriorated area of high Si-content laminations can be found up to 15 mm from the cut edge, while the respective distance for low Si-content is 10 mm. Similar results are obtained in [34], where experiments on concentric ring cores are analyzed. According to these measurements, punching can create a degradation zone up to 10-20 mm from the punched edge, while the results in [35], where same configuration is used, confirm that the degradation zone can extend up to 10 mm from the edge. In [30, 36, 37, 27, 38, 39, 40, 41, 42, 43] the lamination strips are cut in thinner pieces so that the cutting length is increased. In [34, 35], similar experiments were conducted using concentric toroidal cores instead of strips. These results indicate that the degradation of the material depends on the punched width. In [33, 44] search coils along the lamination strips are used in order to obtain the flux density at several distances from the cut edge. These measurements result in the determination of the material degradation as a function of the absolute distance from the cut edge. Additionally, microhardness tests in [45] indicate a strain deformation up to 0.5 mm from the cutting edge, which results in degradation of the magnetic properties.

3.2.1.2 Effect of Mechanical Cutting on Hysteresis and Eddy Current Losses The magnetic degradation due to mechanical cutting by means of guillotine or punching, causes increase of the iron losses and decrease of the relative permeability. According to [41, 46], the cutting process mainly affects the hysteresis component of the iron losses.

The increase in the total iron losses can thereby be modeled by increasing the value of the hysteresis loss coefficient.

However, the plastic deformation after cutting also affects the eddy current losses.

The reason for this is the degradation of the insulation which leads to lower apparent resistivity. Since the mechanical deformation due to punching is very limited (up to tens of micrometers) [30], the deterioration of the insulation is insignificant and the increase in the eddy current losses is very low.

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3.2. FORMING THE CORE LAMINATIONS 15 3.2.1.3 Effect of Mechanical Cutting on the Magnetizing Current

The deterioration of the magnetizing properties practically means that the core needs more magnetizing field in order to develop a certain induction level. In [47], magnetic measurements on grid geometry are presented. This geometry consists of stacked stator laminations, where only slots are punched. According to the results, almost 10%

additional magnetizing current is needed to maintain the same flux under a pole.

In [34], measurements on concentric ring core specimens indicate an increase in the magnetizing current with an increased cutting length.

3.2.1.4 Si-Content

The process followed in order to produce a high silicon electrical steel is more expensive and meticulous than with low silicon content. The manufacturing procedures in the case of high Si-content lead to low impurities in the electrical steel and larger grain size.

The silicon in the electrical steel increases the resistivity and therefore decreases the eddy current losses. The larger grains improve the hysteresis characteristics of the steel, leading to lower hysteresis losses.

In [27], SST measurements are conducted in high, medium and low Si-alloyed grades.

It is shown that for the same induction levels, the exciting field increases with increasing Si-content. The cutting effect is more significant for higher Si-content steels. According to [6, 27], the content of Si in the steel laminations plays a major role in the degradation of the material. More specifically, an increased Si content in steel laminations, leads to a higher increase of the exciting field and iron losses for a specific cutting length [27].

In [33], SST measurements on SiFe alloys indicate that the magnetic deterioration in the case of high Si-content material expands up to 15 mm from the cut edge, while the corresponding area for low Si-content expands less than 10 mm from the cut edge.

However, the authors mention that the most influential factor concerning the extent of deterioration is the grain size and not the Si-content.

In [39], three SiFe grades with different Si-content are tested by means of SST measurements. Through the experiments, the stress tensor for different directions inside the material is determined. The authors conclude that the stresses after mechanical cutting are higher for high silicon laminations.

3.2.1.5 Cutting Perpendicular or Parallel to the Rolling Direction

Cutting of electrical steel laminations can be performed in parallel or transversally to the rolling direction of the mother coil. The cutting direction has a large impact on the magnetic characteristics of the electrical steels.

In [41], the authors conduct Epstein measurements on strips that are cut parallel (RD) and perpendicular (TD) to the rolling direction. The results show that cutting has lower impact for strips that are parallel cut. Similar experimental results are presented in [27, 37]. Machine manufacturers usually assemble the stator and rotor cores by stacking core laminations that are twisted 90^o or with lower angle. This way, the anisotropy of magnetic properties is canceled out.

In case of segmented stator cores, there is flexibility in the orientation of the cutting edges of the teeth and yoke. In [41], the authors suggest that the stator segments should

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be cut so that the teeth, which present the highest induction, are oriented RD while the yoke which normally has lower level of induction, can be cut TD.

3.2.2 Laser Cutting

Laser stands for Light Amplification by Stimulated Emission of Radiation. Laser cutting is a non-contact method of cutting and it is mainly used during the manufacturing of prototypes or small-scale production motors. Laser cutting causes irreversible damage to the magnetic characteristics of the electrical steel due to the high temperatures that are developed during cutting [31].

3.2.2.1 CO₂ and Fiber Laser

The laser machines are classified depending on the source of the optical gain. Two major types are the CO₂ and the fiber laser. The CO₂ laser has been commercially available since the 1970s and it belongs to the category of gas lasers. That means that the source of optical gain is a gas, usually carbon dioxide. The fiber laser is an improved version of the Nd:YAG (Neodymium-doped Yttrium Aluminium Garnet) laser that has existed since the 1980s. The optical gain medium of a fiber laser is an optic fiber which is doped with rare earth elements. The advantage of the fiber laser is the considerably higher cutting speed (as high as three times) than a corresponding CO₂ laser, in the case of laminations that are less than 4 mm thick. Moreover, the running cost of a fiber laser is up to 50%

lower than that of a respective CO₂ laser. Its maintenance is less expensive as well [48].

3.2.2.2 Laser Settings

The performance of the laser cutting technique is regulated by parameters like the type of laser, the power, the cutting speed, the beam spot size, the type of assisting gas and the gas pressure. Regulating these settings leads to different magnetic properties of the magnetic material. More information based on literature can be found in section 3.5.2.

3.2.2.3 Spatial Distribution of Degradation

SST measurements with different power, speed and gas pressure settings in [49] indicate no significant loss variations when specimens with large length are cut. This revealed that the magnetic degradation due to laser cutting is dependent on the geometrical shape of the cut sample. Similar results are obtained in [40], where the degradation is measured in the whole width of the strips and a relation to the geometry of the samples is recognized.

Another characteristic of the laser cut laminations that reveals the nature of the spatial distribution of the magnetic degradation is that the heat affected zone (HAZ) is dependent on the thermal history of the cutting process which means that the largest degradation is evident in the region that is cut first [40, 49].

3.2.3 Comparison between Mechanical and Laser Cutting

Punching induces shearing forces at the cut edges causing plastic deformation while laser causes thermal stresses at the edges [40, 49].

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3.3. CORE ASSEMBLY 17 In [31], Epstein measurements on 2% SiFe steel indicate 6% higher losses for laser cut laminations compared to punched ones. In [39], SST measurements on 0.31% and 2.98% Si laminations indicate that laser cutting gives better results than punching. That happens when small samples are concerned, while X-ray analysis reveals that laser causes higher internal stresses than mechanical cutting.

SST measurements in [49], reveal that the losses of 3% Si laminations after laser cutting are up to 15% higher than the corresponding losses after mechanical cutting. Additionally, the same experiments show that in the case of laser cut samples, higher field strength is required to reach a certain level of induction. According to the same paper, the laser cutting technique provides limited possibilities of improvement of the material’s magnetic properties due to the induced thermal stresses.

The spatial distribution of the magnetic deterioration is also different for the two cutting techniques. Experiments in [40] highlight that the degradation of the mechanical cut strips appears close to the cut edges, while the corresponding degradation for laser cut strips is evident in the total width of the strip.

Finally, when mechanical and laser cutting methods are compared, it should always be taken into consideration that the laser cutting performance is not dependent on time, while the quality of the mechanical cutting degrades with time. This is the reason why the punching and guillotine tools require maintenance when the sharpening of the cutting blade is discussed.

3.2.4 Abrasive Water Jet

Another method of lamination cutting which could be considered as an alternative to the laser cutting is the abrasive waterjet cutting. SST measurements in [50] show that this cutting technique causes very low deterioration in the magnetic properties of electrical steels and compared to mechanical and laser cutting gives the best results. Although this is the best cutting method, regarding the magnetic results, this technique is not widely used due to the low speed (800 mm/min for 0.5 mm thick laminations) [50].

3.3 Core Assembly

In this section, the methods used for stacking the core laminations are presented. After pressing, the three main techniques for holding the stack together are welding, cleating and gluing.

3.3.1 Pressing during Stacking

The next manufacturing step after cutting the core laminations is the pressing. This process affects both eddy current and hysteresis losses. The damage of the insulation coating affects the eddy current losses, while the forces applied may deform the material and therefore the magnetic properties degrade and the hysteresis losses increase [41].

Measurements in [41] show that an unpressed lamination stator core has 185% lower losses than a pressed one. Additionally, in [6] and [51], ring core measurements indicate an increase of 400% in the change of the specific iron losses (∆pF e) between two cores that are pressed with 1 MPa and 8 MPa respectively.

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3.3.2 Welding

During the welding process, the lamination stack of the core is assembled through welding seams in the direction of the active length of the machine. During this process, mechanical and thermal stresses are induced and degrade the magnetic properties of the material [51].

Additionally, welding causes short circuits between the laminations which decrease the effective resistivity of the core and therefore increases eddy current losses [35].

In [51], magnetic measurements on ring core topology are performed and the result indicate that as the number of welding seams increases there is an increase in the iron losses and a drop in the permeability.

In [52], the welding effect is investigated on a toroidal core topology with 8 welding seams and NO20 laminations. The results are compared to a non welded, taped core and the outcome is that the magnetic properties of the material are significantly degraded and the specific iron losses increased.

Finally, in [41] the authors investigate the welding effect on a stator core topology. As reference a taped stator core is used and the studied core has 12 welding seams. It is shown that the additional losses are decreasing with the increase of frequency and induction level, which means that the loss increase in this case is caused by the degradation of the magnetic properties of the material. It is also worth mentioning that the same study concludes that there is an increase in iron losses of 0.5-1% per welding seam, when stator yoke carries maximum flux density.

3.3.3 Cleating

Cleating is a method used for holding the lamination stack together. In this technique, metal strips are placed into slots in the periphery of the stator core. These strips are called cleats. Once the laminations are pressed together, the two ends of the cleats are bent over so that they create a holding tab [53]. It is believed that cleating causes lower degradation than welding. This is due to the fact that cleating does not induce any thermal stresses and it does not cause short circuits between the laminations.

3.3.4 Gluing

An alternative method of holding the lamination stack pressed together is gluing, also called sticking. Gluing is mostly used in applications where light weighted stator cores are required and there is no extra material available for welding or cleating [53].

The use of this technique is described in [51], where a toroidal core is assembled. An adhesive varnish is applied to the core laminations. Afterwards, the stack is assembled through a heating process. The gluing has very low or negligible effect on the magnetic properties of the material, since the varnish has non magnetic content. Therefore, any possible degradation due to gluing is because of the thermal treatment. The results of the experiments in [51] indicate that welding with 2 seams increases the iron losses by nearly 80% compared to a glued core, while the corresponding difference of a welding with 6 seams is 400%.

However, this technique has the drawback of being expensive, limiting its usage mostly to special applications with low-weight requirements [53].

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3.4. MOTOR ASSEMBLY 19

3.4 Motor Assembly

After manufacturing the stator and rotor cores, the assembly of the motor takes place.

In this step of the process, the motor takes its final form.

3.4.1 Shaft Insertion

In inner rotor designs, the shaft of the electrical motors is the part that transmits the torque from the rotor to the load.

The shaft should not move relatively to the rotor core. By heating the rotor core, it expands and the shaft can be then inserted. As a next step, the rotor is rapidly cooled down and the shaft is then embedded into the rotor core. This process degrades the magnetic properties of the rotor core laminations due to the thermal stresses as well as due to the mechanical stresses when the core shrinks and applies a compressive force to the shaft. These mechanical stresses as shown in [45], affect the magnetic properties of the material. Therefore the hysteresis losses are affected.

3.4.2 Pressing into Frame

Another manufacturing process is the insertion of the stator core into the frame. These two parts should be in good contact, since the frame assists in the motor cooling. The process followed for this manufacturing step starts with the heating of the frame. Once the frame has expanded, the stator core is inserted. Afterwards, the assembled parts are cooled and the stator core is then embedded in the frame. During this process, compressive stresses from the frame to the stator core are induced. These mechanical stresses deteriorate the magnetic properties of the ferromagnetic material. In [54], the iron losses of a surface mounted PM motor are measured before and after the insertion in a cast aluminum frame. The results indicate an increase of 10% in the iron losses between the two cases. Additionally, according to [5], the degradation of the steel after inserting the core in the frame is more significant for laminations of higher Si content.

3.4.3 Rotor Machining

The process of machining takes place mainly when induction motors are manufactured.

After casting the aluminum into the rotor bars, there may be imperfections, like aluminum leftovers on the rotor’s surface. Machining removes these remnants of aluminum and it also ensures that the rotor has the correct dimensions for the air gap requirement. Machining is also used in other types of machines so that the air gap width is obtained with the expected accuracy.

Machining damages the insulation of the outer part of the rotor laminations and creates short-circuits among them. In [9, 23], magnetic measurements are performed on two identical CoFe stator cores. The one core is just stacked and compressed, while the other one is also glued and machined. The results indicate an increase in the eddy current losses for the machined core.

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3.5 Manufacturing Mitigations

In this section, the processes that mitigate the material degradation due to the different manufacturing steps are presented. These processes either recover the magnetic properties of the material or they regulate the manufacturing process so that it degrades less the magnetic properties of the material. Stress relief annealing belongs to the first category, while the fine tuning of the laser settings and the maintenance of the punching machine belong to the second one.

3.5.1 Annealing

Stress relief annealing is a technique, used to recover the magnetic properties of the ferromagnetic materials after cutting.

In [35], two identical induction motors are tested, while only one of the stator cores is annealed. At rated voltage, an iron loss reduction of 15% is found. In [45], the authors use the annealing process to verify that the degradation of the magnetic properties due to cutting arise from the plastic strain in the cut edges. This strain is removed through annealing of 720^oC for 2 hours and a decrease in the maximum permeability is found.

Annealing can also be performed after laser cutting. Particularly, in [31], laser cut laminations are tested. The annealing is performed for four cases:

• Laminations just after cut

• Cut and annealed

• First annealed and then cut

• Annealed, cut and then annealed again

The hysteresis characteristic of the fourth case is the superior one with the lowest value of coercive field and the highest value of magnetization knee. In [23], magnetic measurements on a CoFe stator core before and after annealing are performed. The annealing temperature is 720^oC and the duration is 2 hours. The un-annealed core has 17 times higher coercive field Hc while the maximum value of induction B is 3.5 times lower. This result highlights the necessity of annealing, when CoFe laminations are used.

In [37], annealing is applied in 1% SiFe Epstein strips for 1 hour at temperatures from 450ôC to 700ôC. It is shown that annealing at 700ôC decreases the iron losses 15 times more than annealing at 450ôC.

The effectiveness of annealing depends on the temperature and the time. In [55], the annealing is performed on a SiFe stator core at approximately 800^oC for 8 minutes. It is shown that for a flux density of 1.5 T, the iron losses of the sample decrease by 4.9%. In this case, annealing does not have large impact on the iron losses. One reason for this could be the short duration of annealing (8 minutes).

3.5.2 Tuning of Laser Settings

A method to decrease the manufacturing effect due to laser cutting is to fine tune the settings of the laser in order to achieve lowest degradation of the electrical steel lamination.

The type of the laser tool and the tuning of settings like the power, the cutting speed,

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3.6. SUMMARY 21 the beam spot size, the type of assisting gas and the gas pressure have a large influence on the deterioration of the cut magnetic material.

Measurements in [39], indicate that the pulsed mode laser with low speed provides better results than the continuous mode. The internal stresses after cutting with pulsed mode are higher. This can be explained by the fact that the internal stresses are translated as effective pinning sites for the domain walls of the magnetic material. Therefore, the speed of the domain walls is drastically decreased and the eddy current losses drop as well [39].

Finally, SST measurements in [56], between a CO₂ and a fiber laser show that the specific iron losses are almost the same when strips are cut both in RD and in TD. An increase of the energy input at constant power changes the relative permeability of the material. However, the relation of change is not linear. Best permeability characteristics can be seen for 4kJ/m while the worst magnetic properties are evident at 24kJ/m, which is the highest tested energy value.

3.5.3 Maintenance of Punching Machine

Similarly to the tuning of the laser settings, the maintenance of the punching machine has a large impact on the deterioration that the cutting causes to the steel laminations.

This maintenance concerns the regrinding (sharpening) of the cutting blade.

Schmidt in [37] compares the specific losses of Epstein strips for a sharp and a blunt cutting tool. When the strips are cut in the rolling direction, the blunt blade causes approximately 7% higher iron losses than the corresponding sharpened tool. Similar study in [41] highlights that a newly sharpened punching tool causes up to 4% lower losses than the catalogue values, while a worn-out tool causes up to 6% higher losses than the respective catalogue values.

3.6 Summary

The manufacturing process introduces deterioration in the magnetic properties of the electrical steel that is used in the stator and rotor core laminations. The material degradation consists of a reduced permeability and increased specific iron losses. Taking into consideration the material deterioration after the major manufacturing steps can lead to more accurate estimation of the characteristics of the produced motor.

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Chapter 4 Measurements

In this chapter, the conducted experiments regarding the cutting and welding effects are presented. In Section 4.1, an overview of the experiments is presented. The test setup and the repeatability of the measurements are also described. In Section 4.2, the results from the Epstein measurements with mechanically cut laminations of M400-50A, M270-50A and NO20 are presented. Sections 4.2.1, 4.2.2 and 4.2.3, concern the results of the Epstein measurements on M400-50A, M270-50A and NO20, respectively, while in Section 4.2.4, a comparison of the results for the three grades is presented. Section 4.3 concerns the measurements on laser cut laminations. Different laser settings are tested and the cutting effect on laser cut laminations of M270-50A is presented. In Section 4.5, experiments regarding the influence of welding are shown.

4.1 Introduction to the Experiments

The purpose of the experiments is the investigation of the cutting effect due to the two major cutting techniques, namely punching and laser cutting. Since the development of a new punching tool is an expensive process, a guillotine cutting is used instead. Apart from the cutting effect, the influence of welding is also investigated.

This degree project was held at ABB LV Motors. The laminated materials, the guillotine and the measurement equipment were provided by Surahammars Bruks AB, a part of TATA Steel group, which is the second largest electrical steel manufacturer in Europe.

The laser cutting was conducted at Gerdins AB, a company that specializes in components, cable systems and cutting technology. The welding was made at the factory of ABB LV Motors.

Two challenges in this project were the planning of the experiments and the time scheduling. Before starting the experimental procedure, a preliminary time plan was made. However, many changes to the initial plan were made, due to the consideration of new investigations.

4.1.1 Motivation

Studies on the cutting and welding effect have been done before, as presented in the literature review (see Chapter 3). However, in most cases, a qualitative analysis is presented.

The purpose of this project is to gain the absolute values of the material characteristics after cutting and welding. Thus, the results can be implemented in the analytical and finite

23

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element models of electrical machines. Only few references investigate laser cutting for different lamination widths and testing of a broad range of laser settings. Moreover, the suggested method for the investigation of welding effect requires the standard equipment for magnetic material characterization (Epstein frame). The laminated materials that are under investigation are selected because they are typical for electrical machines. To have a more complete investigation, laminations with the same Silicon content and different thickness, as well as laminations with different Silicon content and the same thickness are selected.

4.1.2 Test Setup

The experiments were conducted with guillotine and laser cut laminations. For mechanical cutting, a guillotine at Surahammars Bruks AB was used. Figure 4.1 illustrates this machine. The guillotine is adjusted to cut standard Epstein strips (30 mm wide). Cutting thinner strips was challenging and time consuming, because a non standardized method of cutting should be adopted.

The laser cutting was performed at Gerdins AB by means of fiber and CO₂ lasers.

The laminated materials were sent from Surahammars Bruks AB to Gerdins AB. Figure 4.2 depicts the fiber laser that is used at Gerdins AB.

After cutting the test specimens, their magnetic properties were measured in the Ep- stein frame provided by Surahammars Bruks AB. The frequencies used in the experiments are 50, 100 and 200 Hz. The reason for this selection is the limitations of the measurement equipment that is used. The selected range of frequencies are representative for industrial line fed motors, where the electrical frequency is 50 or 60 Hz. A training was taken at the factory of Surahammars Bruks AB, to learn cutting with guillotine and conducting measurements on the Epstein frame.

4.1.3 Repeatability

To ensure the validity and reliability of the experimental results, each measurement was repeated for three samples. The magnetic characteristics presented are the mean values

Figure 4.1: The guillotine at Surahammars Bruks AB.

Figure 4.2: The fiber laser machine at Gerdins AB.

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4.1. INTRODUCTION TO THE EXPERIMENTS 25

Figure 4.3: The Epstein frame used for the measurements of the magnetic properties at Surahammars Bruks AB.

Figure 4.4: Zoom of the overlapping strips at the one edge of the Epstein frame (red region of Figure 4.3). Epstein strips with 2 additional cutting edges are used in this configuration.

of these three measurements, unless otherwise stated. Thus the calculated values are more representative of the mother coil properties. Figures 4.5-4.6 illustrate the relative standard deviation of the permeability and iron loss density of the three samples in the case of mechanically cut M400-50A 15 mm wide strips as a function of the flux density.

The relative standard deviation expresses the variation of the measurements from the mean value and is given by Equation 4.1.

RSD = s

¯

x × 100 (4.1)

Where s and ¯x are the standard deviation and the mean value, respectively, of these measurements. The standard deviation is expressed by Equation 4.2.

s =r P(x − ¯x)²

n − 1 (4.2)

Where x is the measured value and n is the number of values.

The maximum values of RSD in the conducted measurements are shown in Table 4.1.

max. RSD of µr at 1 T max. RSD of pF e at 1 T

Sample 7.5 mm M400-50A 7.5 mm M400-50A

Value 1.1 % 1.6 %

Table 4.1: Maximum max values of RSD.

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0

0.5 1 1.5 2 2.5

B (T)

RSD of relative permeabilty (%)

Figure 4.5: RSD of the relative permeability of mechanically cut 7.5 mm wide M00-50A strips.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0 0.5 1 1.5 2 2.5 3

B (T)

RSD of iron loss density (%)

Figure 4.6: RSD of the iron loss density of mechanically cut 7.5 mm wide M00-50A strips.

4.1.4 Tested Grades

The laminated materials that are under investigation are M400-50A, M270-50A and NO20.

M270-50A and NO20 contain more Silicon than M400-50A. M400-50A and M270-50A have a thickness of 0.5 mm, while NO20 is a 0.2 mm thick lamination. Table 4.2 summarizes the basic characteristics of the tested materials.

Table 4.3 summarizes the experiments that were carried out during this project. The magnetic measurements of non standard Epstein strips (less than 30 mm wide) required extra time and effort. The reason for this is that the sub-strips, which constitute a standard Epstein strip (see Figure 4.7), should be attached to each other with tape.

Otherwise, the strips could not be inserted in the Epstein frame.

Si-content Resistivity Thickness

M400-50A 1.8% 42 µΩcm 0.5 mm

M270-50A 3.2% 55 µΩcm 0.5 mm

NO20 3.2% 52 µΩcm 0.2 mm

Table 4.2: Overview of the tested material.

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4.2. MECHANICAL CUTTING 27 M400-50A M270-50A NO20 Epstein mea-

surements

Strips Mechanical cutting for

4 strip widths.

X X X 36 1680

Laser cutting for 4 strip widths.

- X - 4 100

Laser cutting with 15 different settings (9 of them measured).

X - - 16 768

Laser cutting with 3 different settings.

- - X 9 648

Welding X - - 5 100

Total 70 3296

Table 4.3: Overview of experiments and number of strips.

4.2 Mechanical Cutting

According to IEC 404-2 which is the standard regarding the Epstein measurements [26], the total width of the strips under test should be 30 mm. In order to increase the cutting effect, the samples are cut along their length in 1/2, 1/3 and 1/4 widths. Therefore, the Epstein tests are conducted on strips whose width is 30, 15, 10 and 7.5 mm. Figure 4.7 illustrates the four type of samples, as they were cut at Surahammars Bruks AB.

Figure 4.7: Schematic diagram of the 4 different configurations of the Epstein strips.From top to the bottom: A standard Epstein strip (30 mm wide) and strips with one, two and three additional cutting edges, respectively.

Manufacturing Effects on Iron Losses in Electrical Machines

Manufacturing Effects on Iron Losses in Electrical Machines

Manufacturing Effects on Iron Losses in Electrical Machines

Konstantinos Bourchas

Contents

List of Symbols

List of Abbreviations

Abstract

Sammanfattning

Acknowledgments

Chapter 1 Introduction

1.1 Background

1.2 Thesis Scope

1.3 Outline

Chapter 2

Ferromagnetic Materials

2.1 Iron Losses

2.1.1 Hysteresis Losses

2.1.2 Eddy Current Losses

2.2 Iron Loss Models

2.2.1 Models based on the Steinmetz Equation

2.2.2 Separation Models

2.2.3 Hysteresis Models

2.3 Characterization of Magnetic Properties of Elec- trical Steels

2.4 Magnetic Measurements by means of the Epstein Frame

2.5 Summary

Chapter 3

Manufacturing Effects on Iron Losses in Electrical Machines

3.1 Introduction

3.2 Forming the Core Laminations

3.2.1 Mechanical Cutting

3.2.2 Laser Cutting

3.2.3 Comparison between Mechanical and Laser Cutting

3.2.4 Abrasive Water Jet

3.3 Core Assembly

3.3.1 Pressing during Stacking

3.3.2 Welding

3.3.3 Cleating

3.3.4 Gluing

3.4 Motor Assembly

3.4.1 Shaft Insertion

3.4.2 Pressing into Frame

3.4.3 Rotor Machining

3.5 Manufacturing Mitigations

3.5.1 Annealing

3.5.2 Tuning of Laser Settings

3.5.3 Maintenance of Punching Machine

3.6 Summary

Chapter 4

Measurements

4.1 Introduction to the Experiments

4.1.1 Motivation

4.1.2 Test Setup

4.1.3 Repeatability

4.1.4 Tested Grades

4.2 Mechanical Cutting