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Degree project in

Magnetic and Electric Characterization

of Materials for Electrical Machines

M.S. Muhit

Stockholm, Sweden 2011

XR-EE-EME 2011:016

Electrical Engineering

Master of Science

Magnetic and Electric Characterization of Materials for Electrical Machines by

M.S.Muhit

Master Thesis

Royal Institute of Technology

School of Electrical Engineering

Electrical Machines and Power Electronics

Stockholm, August 2011

XR-EE-EME 2011:016

ii Magnetic and Electric Characterization of Materials for

Electrical Machines

This thesis aims to characterize materials for electrical machines. Electromagnetic properties (b-h

curves) and electrical resistivity were the main properties investigated in the project work. Two types of

samples are considered: stator lamination sheet and rings made of steel used for structural pieces in

large AC machines.

To facilitate magnetic characterization experiments, an existing test setup was upgraded. Sensors and

amplifiers have been developed. The control system has been upgraded and developed SIMULINK

obtain the results.

The lamination sheet has been tested for frequencies in the range [0.1-150 Hz] and flux density levels up

to 1.5 T. The obtained results depict characteristics hysteresis curves and measured loss figures.

The ring samples (structural steel) are characterized to explore the B-H curves at frequencies in the

range [0.1-250 Hz] and field intensity up to 900 A/m.

For resistivity measurements of the lamination sheets and ring samples, standard resistivity

measurement techniques have been implemented. Magnetic properties, Epstein frame, hysteresisgraph, four point probe, resistivity measurement, soft magnetic material, electrical steel. iii

Abstract (Swedish)

Sammanfattning

magnetiska material, elektroplåt. iv

Acknowledgement

The master thesis was carried out at the laboratory of Electrical Machines and Power Electronics, School of Electrical Engineering, Royal Institute of Technology - KTH. Firstly, I would like to thank my supervisor Associate Professor Juliette Soulard. Her endless support and valuable suggestions helped me carry out the project work. I would also like to thank my co-supervisors, Tek. Lic. Henrik Grop for providing me the sample specimens and have valuable discussions on obtained results; Andreas Krings for his help during the electrical characterization experiments and plenty of worthy suggestions; And, lastly, Seyedali Mousavi for his continuous and consistent support all throughout the project work period. his tech support with computers. I would like to say thanks to the staff of the whole department for supporting and helping me in many ways. Thanks to my friends and well wishers around. Without their inspirations and witty chats, nonetheless with their unconditioned love helped me in every moment. Finally, I would like to dedicate this work to my parents who have never let me flown off the track. Their blessings and inspirations together with endless love have always been by my side.

M.S.Muhit

August 2011

v

Contents

1 Introduction 1

1.1 Purpose 1

1.2 Scope 1

1.3 Structure of the report 1

2 Literature review on magnetic characterization 3

2.1 Basics of magnetic characterization 3

2.1.1 Hysteresisgraph measurements 3

2.1.2 Hysteresis curve 5

2.2 Different types of test setups based on hysteresisgraph principle 5

2.2.1 The Epstein frame 6

2.2.2 Single sheet tester 7

2.2.3 Other test fixtures 7

2.3 Digital feedback control loop 8

2.4 Iron loss in electric machines 9

2.4.1 Steel sample dimensioning, orientation and production 10

2.4.2 Excitation systems 10

3 Development of test bench 11

3.1 Existing setup 11

3.2 Magnetic field strength measurement 12

3.2.1 AC/DC current probe 12

3.2.2 Shunt resistor 12

3.2.3 Current transducer based on LEM module 13

3.3 Magnetic flux density measurement 13

3.3.1 Voltage amplifiers (3 and 4 stage) 13

3.4 Demagnetization block 15

3.5 Post processing of signals and loss density calculation from

hysteresis loop 16

4 Magnetic characterization 18

4.1 Stator lamination sheet 18

4.1.1 Lamination sheet: M600-50A 18

4.1.2 Test fixture: Epstein frame 18

4.1.3 Specimen preparation and loading 19

4.1.4 Test bench for magnetic characterization of lamination sheet 20

4.1.5 Test procedure 20

vi

4.1.6 Obtained results 21

4.1.7 Analysis of results and conclusion 26

4.2 Ring sample 27

4.2.1 Test fixture and sample preparation 28

4.2.2 Test bench setup arrangement 29

4.2.3 Test procedure 29

4.2.4 Obtained results and analysis 30

4.3 System drawbacks and common measurement errors 37

4.4 Conclusion 38

5 Electrical Characterization 39

5.1 Basic concepts 39

5.2 Measuring resistivity of lamination sheet 39

5.2.1 Four point technique 39

5.2.2 Four point probe technique 40

5.2.3 Discussion of results 41

5.3 Measuring resistivity of ring samples 42

5.3.1 Resistivity measurement using current injection 42

5.3.2 Results 43

5.3.3 Discussion of results 43

5.4 Comments on measurement methods 44

5.5 Conclusion 44

6 Conclusions and future work suggestions 45

6.1 General conclusion 45

6.2 Future work 45

Appendix 46

References 51

List of symbols and abbreviations 53

1

1 Introduction

1.1Purpose

The suggested work in this thesis deals with electrical and magnetic characterization of materials used in rotating electrical machines.

Rotating electrical machines are commonly built with different grades of iron, steel and other

metallic alloys. The stator and rotor are commonly built with specific grades of laminated electrical

steel. Proper dimensioning of the machine requires precise estimation of the B-H curves of the laminations and also the iron loss it undergoes during various states of operation. Lamination sheets are usually ordered with specific grades from the manufacturer. The grades

describe material characteristics and losses at two operating points at least (f=50, 60 Hz and B=1, 1.5

T). These loss characteristics may not be sufficient for machines with variable speed drives or with different types of excitation supply. Hence, it requires characterization over wide frequency ranges (0.1-1000 Hz) and varying magnetic field levels (50-500 A/m) high enough to reach saturation flux density. Beside lamination sheets, structural metallic parts are used in the machine for several purposes. Some of these parts are also subjected to flux density variations if they happen to be placed in leakage flux paths. Hence they may undergo eddy current and hysteresis losses too. Due to lack of material characterization, these losses are usually not accounted in the simulation models. Hence,

efficiency is not accurately calculated. To include the structural materials in loss calculation model,

they require characterization.

1.2 Scope

This report aims to describe the activities conducted to measure the magnetic characteristics and

loss figures under sinusoidal excitation of two specific parts of an electric machine: the stator

lamination sheet and especially manufactured rings made of steel used for structural parts. Electrical

characterization has been done to measure the resistivity values of the samples. The results

described in this report are based upon experiments performed in two laboratories of KTH:

electromagnetic engineering lab and electrical machines lab.

1.3 Structure of the report

The report is organized in 6 chapters, the first chapter being the introduction. presents the literature review on magnetic characterization. It discusses on magnetic

characterization techniques, test setups and feedback systems. It also discusses on iron loss in

electrical machines and the different factors affecting it.

3 describes the existing setup for magnetic characterization. It also presents the

development of measuring devices (sensors and amplifiers), SIMULINK blocks for demagnetization of samples and post processing program for facilitating the magnetic characterization analysis. 2 ter 4 addresses the magnetic characterization of the samples. It particularly deals with the arrangement of test setups, sample preparation, test procedure and obtained results. The chapter also contains a reflection around common experimental errors during the experiment. deals with electrical characterization of the samples. A brief description of the adopted resistivity measurement techniques is followed by the presentation of the test procedure, arrangement of setups and analysis of results. Experimental errors are discussed then. contains the final conclusion of the characterization tests. It discusses over the drawbacks associated with the setups and deviation of test results. Future work is also suggested. 3

2 Literature review on magnetic characterization

This chapter focuses on a literature review on the following subjects: magnetic characterization

techniques, test setups and necessity of controlled feedback. Different factors affecting magnetic loss in

the sample are also discussed.

2.1 Basics of magnetic characterization

Characterization of soft magnetic materials aims at defining a given material's suitability on being

selected for a particular application [1]. In simple terms, agnetic characterization can be explained as

an experimental procedure in which the specimens made of the investigated material are subjected to

wide range of polarization levels at different frequencies. Among other parameters, magnetic losses at

different frequency and induction leǀels, relatiǀe permeability (ʅr) and coercivity (Hci) are of significant

interest.

2.1.1 Hysteresisgraph measurements

The response of magnetic materials can vary greatly with applied magnetic field. To obtain desired

characteristics, controlled magnetic field must be applied. A commonly used instrumental setup

facilitating the measurements is a hysteresisgraph or BH meter [4]. Test specimens defining closed

magnetic loop are equipped with an excitation winding and a measurement winding. The response is measured at varying magnetic field strength and frequency. Result of the measurement is a hysteresis curve. Figure 2.1 displays a typical setup. Fig 2.1 Typical hysteresisgraph setup for characterization of soft magnetic materials

Computer

Fluxmeter Bipolar power

supply

Voltmeter

B H

Sample

specimen

Excitation

winding

Measurement

winding 4

An AC sinusoidal current is injected into the excitation winding which induces a magnetic field H in the

sample. The value of current is obtained from the measured voltage drop across the shunt resistor. The

value of applied magnetic field is directly proportional to the current according to line integral form of

Ampere's law.

Where,

݈௠ is the effective magnetic path length of the test specimen

The applied magnetic field in the excitation coil gives rise to voltage across the measurement winding.

The induced voltage is integrated over time to obtain the induced flux density (B) using a fluxmeter.

Where

݂ is the frequency in .

The computer interface allows conducting controlled magnetic measurements. B and H can be simultaneously measured using a data acquisition card.

To obtain initial b-h characteristics curve, the sample needs to be demagnetized in a decreasing

alternating field to begin with. The obtained curves are often used as input properties of magnetic

materials in modeling of electro-mechanical devices, finite element simulations being one of the

possibilities. 5

2.1.2 Hysteresis curve

The resulting plot of a hysteresisgraph measurement is the B-H characteristics curve as shown in figure

2.2

Fig 2.2 Hysteresis loop [23]

Important properties of the soft magnetic material are obtained in points be, cf , a and d shown in figure

2.2. The total area enclosed by hysteresis loop abcdef is a measure of hysteresis loss in one cycle per unit

volume of the material.

2.2 Different types of test setups based on hysteresisgraph principle

Based on hysteresisgraph principle, different types of test fixtures have been developed. The diversity

eases up characterization of different sample geometries such as squares, belt buckles (E-I transformer

lamination), rectangles and other closed geometries. Still though, ring geometry is mostly preferred as it

has a continuous magnetic path length. Air gap causes lowering of relatiǀe permeability (ʅr) and

therefore introduces distortion in the test result. For precise measurements, blocks can be converted to

ring specimen.

Two widely used test setups, Epstein frame and single sheet tester, are discussed in the next section.

They are standardized according to [2, 3].

6

2.2.1 The Epstein frame

It is the most commonly used setup to characterize laminated steel sheets used in electrical machines.

International standards IEC 60404-2 [2] is followed during the test procedures.

Fig. 2.3 Double-lapped joints [2]

Fig 2.4 The 25 cm Epstein frame [2]

The laminated sheets are placed to form a continuous square shaped magnetic circuit with double

lapped joints as shown in figure 2.3 and 2.4. In interlocking fashion, these strips are placed as multiples

of four inside the four coils of the Epstein frame. The orientation of lamination sheets can be in rolling

direction, transverse direction or can be combination of both. In practice, beside the primary H coil (700

turns) and secondary B coil (700 turns), the Epstein frame consists of a H compensation coil (also known

as air flux compensation coil). This coil compensates the H field to avoid distortion of the test results.

More details about this can be found in [2].

7

For characterization of steel sheets using Epstein frame, sample preparation is a tedious process. After

cutting, the samples may sometimes require to undergo stress relief annealing. Loading of samples in the

frame is also a time consuming procedure. In some cases, Epstein frame can be replaced with single sheet tester.

2.2.2 Single sheet tester

The test fixture can perform characterization of steel sheet using a single strip of the test material.

International standard IEC 60404-3 [3] is followed during the test.

Fig 2.5 Diagram of single sheet tester [3]

The test specimen is placed in between two identical yokes which complete the path of flux closure. The

excitation coil can be designed with single continuous winding or several coils connected in parallel

layers. Depending on measuring instruments characteristics, the number of turns in measurement

winding can vary. Also, air flux compensation coil is found inside the primary coils. The basic operation

principle is similar to Epstein frame. The setup is advantageous from sample preparation and loading point of view. Hence it can reduce testing time. The reproducibility of results lies within standard deviation of 2%.

A comparative study of Epstein frame test results and single sheet tester (SST) method can be found in

[5]. It also discusses about the economical importance of SST.

2.2.3 Other test fixtures

Varying types of 2D yoke (round, double sided and planar) test fixtures, toroid testers and three phase

transformers can also be used in magnetic characterization [6]. 8

2.3 Digital feedback control loop

According to international standard, power loss measurement in magnetic materials must be recorded

only under sinusoidal flux density [2]. But magnetic materials are non linear due to inherent hysteresis

characteristics of the material. Hence controlled feedback must be used to retain sinusoidal flux density

[6]. For traditional measurements analog feedback is sufficient enough. Problem arises when measurements

are taken over wide range of polarization levels at high frequencies. In these cases, the feedback loop

needs high performance. According to [6], digital feedback is then to be preferred. Sinusoidal flux density

is obtained by iterative modification of the magnetizing current. Such a solution is discussed in [6]. The

block diagram of the setup is shown in figure 2.5.

Computers equipped with LabVIEW® or dSPACE software incorporated with a high speed data

acquisition system consisting of DAQ (data acquisition and generation card) are used during

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