axial force reduction of a homopolar machine

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COMPEL - The international journal for computation and mathematics

in electrical and electronic engineering

Axial force reduction of a homopolar machineSlawomir Wiak Dieter Gerling Marcin Pyc

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To cite this document:Slawomir Wiak Dieter Gerling Marcin Pyc, (2012),"Axial force reduction of a homopolar machine", COMPEL- The international journal for computation and mathematics in electrical and electronic engineering, Vol. 31Iss 5 pp. 1513 - 1520Permanent link to this document:http://dx.doi.org/10.1108/03321641211248264

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Axial force reductionof a homopolar machine

Slawomir WiakInstitute of Mechatronics and Information Systems,

Technical University of Lodz, Lodz, Poland

Dieter GerlingElectrical Drives and Actuators,

University of the German Federal Armed Forces in Munich,Munich, Germany, and

Marcin PycInstitute of Mechatronics and Information Systems,

Technical University of Lodz, Lodz, Poland

Abstract

Purpose The purpose of this paper is to describe the optimization process of a homopolar machine.The work has been focused on a reduction of axial forces acting on the rotor.

Design/methodology/approach A novel machine design has been proposed and a detailed FEManalysis has been performed.

Findings The source of the axial force in the homopolar machine has been indicated and a newmethod of axial force reduction has been presented.

Originality/value A detailed comparison shows that the applied changes lead to a significantaxial force reduction and allow the usage of cheaper and less prone to axial forces bearings.

KeywordsForce measurement, Stress (materials), Optimization techniques, Homopolar machine,

Axial forces, Finite element methodPaper typeResearch paper

IntroductionIn Gerling and Pyc (2008) a development process of a homopolar machine has beenpresented.

A homopolar machine is an AC excited synchronous machine, equipped with anarmature and excitation winding on the stator side of the air-gap. The machine isequipped with a rotor divided axially and each of the rotors salient poles in each rotorpart has the same polarity. The homopolar flux produced by a ring-formed excitationwinding flows axially through the rotor shaft and closes through the stator teeth andstator yoke. In Figures 1 and 2, the principle of operation of a homopolar machine ispresented (Kummel, 1986).

Because of the fact that both excitation and armature windings are on the stator sideand do not rotate with the rotor, no sliding contact is required and the total machinecan be reduced. It makes the construction very robust and this kind of machine isespecially intended for high speed applications. The stator winding carries sine wavecurrents although there are examples in the literature of homopolar machines withsquare wave currents (Goodier and Pollock, 2002). The windings, as in the case of thepreviously mentioned PM machine, can be realized as a concentrated or lap winding.

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When using a concentrated winding, the winding can be produced in two differentways. It can go along the whole machine whereas the stator poles have to be

mechanically 908 rotated against each other; another possibility is to divide thearmature windings axially into two parts, rotate them 908 electrically and keep the rotorpoles non-rotated against each other. The second solution would require more copper,increase the reliability but its cost and weight as well. The homopolar machine is rarelydiscussed in the literature although it combines many advantages of reluctance andPM machines. It has neither an excitation winding nor magnets rotating with the rotor,and compared to a reluctance drive, it has additional excitation. Due to this fact, notonly reluctant torque, but also much higher electromagnetic torque can be produced

Figure 1.Principle of operation ofa homopolar machine

Excitation

Winding

Rotor Shaft

Flux Path

Rotor Poles

Armature

Winding

Stator Yoke

Figure 2.Flux distribution in ahomopolar machine

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and additionally, the excitation can be freely adjusted. Due to the absence of magnets,there is no risk of demagnetization of magnets.

Current stage of the project

The calculated model was fulfilling project requirements and it has been decided, tobuild a real machine based on the existing computer model. The first impression was arelatively loud operation and vibrations of the machine, increasing with the rotationalspeed. After few hours of operation, an additional noise appeared. It has been statedthat the reason for the noise was a destroyed bearing (Stack et al., 2003). Furtherinvestigations of the bearing proved that there are signs of axial forces acting on theshaft of the machine (ISO, 1995). The next step of the research was to find the origin ofthe source of the axial forces.

Axial forces investigationThe existing FEM model of the machine described in Gerling and Pyc (2008) has been

again deeply investigated. A simulation, with a special focus on the axial forces hasbeen performed. Already the first simulation showed high peak values of forces actingin the axial direction (Figure 3).

The maximum value of 183 N means a peak to peak value of above 360 N and it wasfar beyond the maximum allowed axial force tolerance of the bearing.

The aim of the further work was to reduce the axial force and at the same time, tokeep all significant parameters like torque and torque ripple at least on the same level.

In order to find the source of the axial force, the air-gap region of the machine hasbeen analyzed. A thin air cylinder has been placed in the air-gap and the z-directionforces have been displayed on the surface of the cylinder. It has been found out that thepeak values of the axial forces are concentrated in the air-gap regions where the rotorteeth end (Figures 4 and 5).

In order to understand the background of this phenomenon, the principle ofoperation of a homopolar machine should be considered. In an ideal case, the fluxproduced by the excitation flows axially in the shaft, distributes in the rotor poles andcrosses the air-gap perpendicular to the shaft. This case is shown in Figure 6 by meansof a black dotted line. In the prototype machine, the flux chooses the shortest path andtherefore, the flux crosses the air-gap at an angle different from 908. This produces an

Figure 3.Axial forces acting on the

rotor of the homopolarmachine100

50

0

50

100

150

200

0 5 10 15 20 25 30

Rotor position [mech deg]

Force[N

]


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additional axial force component. It is marked with a red dotted line in Figure 6. Thisphenomenon is not typical for any other machine types and seems to be a problemassociated with the homopolar machine.

In the next step, a new model with changes in rotor teeth has been prepared.All other machine parts remained unchanged. It has been done in order to prove that

Figure 4.Axial forces distributionin the air-gap of thehomopolar machine

Figure 5.Direction of axial forces

acting on the rotor poles

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the previously found phenomenon is really the reason for high axial forces. The ideawas to cut out a part of the stator teeth in their middle part. This should allow the fluxdirection to be perpendicular to the axis. This solution is shown in Figures 7 and 8.

A simulation of the new machine model has been performed. The calculated modelhas been analyzed analogously to the previous version. In the first step, an air cylinderin the air-gap has been created and the z force component has been displayed on it.A direct comparison between two resulting figures showed a significant reduction of

the peak values of the axial force. Comparing to the previous machine version (Figure 4)the peak axial force value on the cylinder has been reduced to 1/3 of the initial value(Figure 9).

It can be seen that compared to the previous machine version, the peak axial forcevalue on the cylinder has been reduced to 1/3 of the initial value.

As the previously mentioned cuts made in the stator teeth did not completelyeliminate the z-component of the force, new considerations regarding the next version ofthe machine were made. The forces acting on the rotor were considered separately foreach of the rotor axial parts. This is shown in Figure 10 and this figure explains how thetotal axial force is created. When the machine rotates, both rotor parts generate an axialforce of the same amplitude. It can be seen that if it was possible to introduce a 30 8shiftbetween the curves, the force would amount to 0 for every rotor position. This would

only be possible if there was an additional 308 rotational shift between each rotor pole oreach stator part. However, that would unfortunately lead to a significant decrease in thetorque the machine produces.

SummaryAs a result of the experiment, the source of the axial force in the homopolar machinehas been indicated and a new method of axial force reduction has been presented.A detailed comparison shows that the applied changes lead to a significant axial force

Figure 6.Flux flow in the prototype

machine (red dotted line)and preferred flux flow(black dotted line)


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Figure 8.Homopolar machine withcuts made in the statorteeth

Figure 7.Cuts made in the statortooth of the homopolarmachine

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reduction and allow the usage of cheaper and less prone to axial forces bearings.Also the noise caused by the pulsating forces acting on the rotor and stator should

meaningly decrease.

References

Gerling, D. and Pyc, M. (2008), Optimisation of a homopolar machine, paper presented at 19thInternational Symposium on Power Electronics, Electrical Drives, Automation andMotion, Speedam, Ischia.

Goodier, E. and Pollock, C. (2002), Homopolar variable reluctance machine incorporating anaxial field coil, IEEE Transactions on Industry Applications, Vol. 38 No. 6.

Figure 9.Axial forces distribution

in the air-gap of thehomopolar machine with

cuts made in the statorteeth

Figure 10.Axial forces acting on the

left and right part of therotor of the homopolar

machine with cuts made inthe stator teeth100

50

0

50

100

0 5 10 15 20 25 30 35 40 45

Rotor position [deg mech]

Force[N]

Force on the left rotor part

Force on the right rotor part


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ISO (1995), Mechanical Vibration Evaluation of Machine Vibration by Measurements onNon-rotating Parts Part 1: General Guidelines, ISO 10 816-1:1995(E), InternationalOrganization for Standardization, Geneva.

Kummel, F. (1986), Elektrische Antriebstechnik, Teil 1. Maschinen, Springer, Berlin.

Stack, J.R., Habetler, T.G. and Harley, R.G. (2003), Effects of machine speed on the developmentand detection of rollingelement bearing faults,IEEE Power Electronics Letters,Vol.1No.1.

About the authorsProfessor Sawomir Wiak, DSc, PhD, MEng, MIEEE, MICS is Dean of Faculty of Electrical,Electronic, Computer and Control Engineering, Technical University of Lodz, Poland. Hisspecializations are: computer science and electrical engineering (computer aided design,computer modeling and simulation, data base and expert systems, mechatronics, engineeringknowledge). He has given invited lectures at: University of Southampton, UK, National TechnicalUniversity of Athens, Greece, University of West Bohemia Plzen, University of Prague, CzechAcademy of Sciences, University of dArtois, France, University of Maribor, University of Pavia,Italy and University of Vigo, Spain. He is a member of: IEEE, ICS, ICS IEEE, Polish Society of

Applied Electromagnetics. He is also a member of: International Steering Committees: SMC,COMPUMAG; Chairman of ISEF; and the Editorial Boards ofCEFC,ICEM,EWOIPE,ACEMP,ELECTROMOTION,ICEMS(China),ICREPQ(Spanish, Portuguese),ICEFHE,IJEET,ICAISC.

Dr Dieter Gerling obtained his diploma and PhD degrees in Electrical Engineering from theTechnical University of Aachen, Germany in 1986 and 1992, respectively. From 1986 to 1999 hewas with Philips Research Laboratories in Aachen, Germany as a Research Scientist and later asSenior Scientist. In 1999 he joined Robert Bosch GmbH in Buhl, Germany as Director. Since 2001he has been Full Professor and Head of the Institute of Electrical Drives at the Universitaet derBundeswehr Muenchen, Germany.

Marcin Pyc received his MSc in Computer Engineering from the Technical University of Lodzin 2003 and PhD degree in Electrical Engineering from the University of the Federal ArmedForces Munich, Germany in 2009. He is presently an Assistant Professor at the TechnicalUniversity of Lodz, Poland. His research interests concern computer aided design, computer

modeling and simulation and especially electrical machines. The main topic of his research ishomopolar machine. Marcin Pyc is the corresponding author and can be contacted at: [email protected]

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axial force reduction of a homopolar machine

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