Proceedings of the 2008 International Conference on Electrical Machines Paper ID 942

Some Problems of the High-Speed Permanent Magnet Miniturbogenerators Development

Janush B. Danilevich, Senior Member, IEEE, Irina Yu. Kruchinina, Member, IEEE,

Victor N. Antipov, Member, IEEE, Yuvenaliy Ph. Khozikov, Anna V. Ivanova Russian Academy of Sciences, Institute of Silicate Chemistry;

2, Makarova emb., 199034 Saint Petersburg, Russia Tel: (+10)-812-3288500, fax: (+10)-812-3281691

e-mail: ikruch@isc.nw.ru, jandan@isc.nw.ru, khozikov@isc.nw.ru, aivanova@isc.nw.ru

Abstract - Investigations for developing ultimate rating microturbogenerators based on modern materials were executed. Maximum permissible electromechanical and mechanical loads depending on minimum permissible core dimensions for microturbogenerators with rating of 100 and 200 kW and speed of rotation 30000–100000 rpm are determined. Complex simulation models for the purpose of chosen generator parameters optimization and for 3D contact problems decision based on numerical finite element method are worked out. For principal decision of mechanical problems for microturogenerator with rating of 200 kW and speed of rotation 100000 rpm and more, it’s necessary to use new materials of higher strength. Consequently the most essential physical characteristics of strip cylinder materials which exert influence on the geometric dimension changes limits were find out to raise microturogenerator capacity.

I. INTRODUCTION

The priority trend of power engineering towards distributed generation leads to develop some local energy sources and nontraditional systems. The synchronous permanent magnet machines (SPM) are more used in different applications in recent years due to their high performance and to the relatively low cost with a high quality of modern permanent magnets [1-4]. Using the high speed (up to 100000 rpm) gas turbine coupled directly with permanent magnet generator rating of 100-200 kW is one of the developing directions. The priority task now is the designing of small bulk mobile power station for wide local users. Such unit miniturbogenerators must have the small weight and dimensions but it’s necessary to solve some problems for its manufacturing. This paper deals with the design approach of a PM miniturbogenerators in order to obtain a combination of machine rating and rotating speed as high as possible using mechanical strength criteria. One of the aims is to investigate the possibility of increasing the rotation speed up to 100000 rpm to obtain rating up to 200 kW. The paper discusses this problems in more detail using the numerical data of some permanent magnet miniturbogenerators designs.

The pre-production model of PM generator rating of 50 kW with 15000 rpm has been designed and testing [5].

II. DESIGN AND CALCULATION SOLUTION

The first task must be solved is the rotor strength providing.

Permissible velocity on the rotor surface is founded within 250-300 m/s for usual materials. The rotor diameter limitations according to speed of rotation in such case are shown in Fig. 1.

0

50

100

150

200

250

300

350

400

450

0 20000 40000 60000 80000 100000 120000Speed of rotation ( rpm)

Rot

or d

iam

eter

( mm

)

300m/c

250 m/c

100kW

200kW

Fig. 1. The rotor diameter limitation for high-speed miniturbogenerators

On the other side the required rotor diameter for generator of some rating power and speed of rotation via electromagnetic performance (the air gap magnetic flux density B, electric loading of armature winding A1 and Arnold’s machine constant Ca) can be founded. Some relationships of geometrical sizes and nominal rating power are presented.

It should be noted that because of high value magnetization frequency for high-speed miniturbogenerators the stator-teeth and stator-yoke magnetic flux density must be decreased and the air gap magnetic flux density should be limited up to 0.5 Т.

We can see in Fig. 1 that the some problems of rotor mechanical strength for rating 100-200 kW and speed of rotation above 50000 rpm are exist.

Realization of the target rated apparent nominal rating power

nnn PS ϕcos= needs stator bore diameter to be equal

31 n

SCD naλ= ,

where Pn - rating power, kW; nϕcos - power factor; λ - the ratio of stator core rated length to diameter; n - shaft rotation speed. Arnold’s machine constant Ca is defined by electromagnetic loads (induction maximal value in an air gap

σB and linear load of a stator winding A1):

978-1-4244-1736-0/08/$25.00 ©2008 IEEE 1

Proceedings of the 2008 International Conference on Electrical Machines

)'/(101.6 17

wfa KkBAC ασ⋅= ,

where 'α - calculated ratio of induction in an air gap average value to its maximal value, kf - form factor representing the ratio of operating emf value to average value, Kw - winding coefficient of the emf fundamental harmonic.

Dependence of linear stator winding load (Ampere density) from stator (anchors) diameter based on the experience of modern electromechanical design is presented in Fig. 2. Magnetic induction values in a gap for high-speed machines should be accepted less usually used ones with the purpose of restriction of losses in steel at high frequency of magnetic reversal. Assuming Kf = 1.11, 'α = 2/π and λ = 2 we shall receive dependence of Arnold’s machine constant on stator bore diameter presented in Fig. 3 that allows to receive dependences of the demanded rotor diameter for given rotation speed and rating power in Fig. 1.

As a result of preliminary researches the design of a microturbogenerator with a radial arrangement of magnets on a rotor is chosen (see Fig. 4).

Am

pere

den

sity

(A/c

m)

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160

Diameter (mm)

Fig. 2. Dependence of a linear load of a stator winding from diameter.

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

0 20 40 60 80 100 120 140 160

Diameter (mm)

Са

(mm

3 /min

/MV

/А)

Fig. 3. Dependence of Arnold’s machine constant on stator bore diameter.

Fig. 4. Rotor design of the synchronous generator with permanent magnets (1 – magnets; 2 – not magnetic inserts; 3 – face flanges; 4 – the strip cylinder).

As a result of preliminary analytical calculations generator

parameters and characteristics are following: − efficiency and losses distribution; − diagram of magnets and volume magnetic energy stock factor; − static overload; − inductive leakage resistance on direct and quadrature axes; − parameters at loading and short circuit; − external characteristic and no-load characteristic; − mechanical stress in the bandage cylinder; − mounting dimensions.

Complex simulation models for evaluation choice of constructive decisions are worked out. Additional mathematical models for the purpose of generator chosen parameters optimization were developed. These models were used to receive electromagnetic field characteristics in no-load mode, rated load and short circuit modes. They are also used for 3D contact problems decision based on numerical finite element method analysis and permit to determine generator ultimate dimensions.

The rotor magnetic circuit consists of some elementary magnets. The traditional construction of radial magnetic flux rotor is shown in Fig. 5.

Fig. 5. The construction of ¼ radial magnet flux rotor (1 - shaft of rotor, 2 - substrate, 3 - nonmagnetic segments, 4 - permanent

magnet segments (elementary magnets), 5 - strip cylinder)

5

1

2

3

3

4

2

Proceedings of the 2008 International Conference on Electrical Machines

Elementary magnet dimensions are determined by required electromagnetic performance and the possibilities of technology of elementary magnet manufacturing and their assembly. The elementary magnet assemblies are divided by nonmagnetic fixings to receive the required number of poles. The magnets and fixings are reinforced at the substrate by the strip cylinder and appropriate tightness.

III. MODELING RESULTS

The calculation and design have been made for miniturbogenerators at rating power 100 and 200 kW and for speed of rotation 30000, 50000, 100000 rpm. The magnet and strip cylinder dimensions have been found. The results of investigation are represented (see Table I).

TABLE I

THE CHOISE OF MAGNET AND STRIP CYLINDER DIMENTIONS

Rating power P, kW

Rotation speed n, rpm

Air gap d, mm

Strip cylinder width h, mm

Magnet high H,

mm

Tensile strength

tσ , MPa

100 30000 6 5 10 410 100 50000 6 5 8 690 100 100000 6 5 8.5 1400 200 30000 8 7 9.5 690 200 50000 8 7 8 795 200 100000 8 7 6.5 1990

The strip cylinder is loaded with centrifugal force from

magnet and fixing during rotation. So it’s necessary to analyze the mechanical stress level in the rotor magnetic circuit to solve the contact problem and to determine assurance factor, the value of contact pressure or some elements permissible separation from the substrate.

In this case a series of simulations of various rotor magnetic circuits must be made. The 3D modeling of rotor fragment with several contact regions has been realized. An external action is the inertial loading on account of rotor rotation. The symmetry of some regions has been adopted as boundary condition. For such approach the peculiarities of the material properties and geometrical sizes must be known.

The detail analysis of mechanical rotor state for all concerned variants has been made. Magnetic segments are conjugated with shaft of rotor via shrink fit with specified tightness to provide fixing magnetic elements on theirs radial surface at rating speed of rotation.

Some results are shown below (see Table II and Fig. 6–9). TABLE II

THE STRESS LEVEL IN DIFFERENT PARTS OF ROTOR

Rating P, kW

Rotation speed n,

rpm

Tensile strength tσ , MPa

substrate strip cylinder

magnets

100 30000 710 420 40 100 50000 1060 630 80 100 100000 1600 1300 140 200 30000 1100 680 70 200 50000 1150 800 100 200 100000 2000 1800 220

Fig. 6. Tensile strength (Von Mises stress) distribution in ¼ part of rotor

(kp/cm2)

Fig. 7. Tensile strength (Von Mises stress) distribution in ¼ part of strip

cylinder (kp/cm2)

Fig. 8. Contact pressure distribution on interface surfaces of magnetic system

units (kp/cm2)

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Proceedings of the 2008 International Conference on Electrical Machines

Fig. 9. Friction contact pressure distribution (kp/cm2)

IV. CONCLUSIONS

Some stress level calculations have been carried out to investigate the possibility of increasing rating of high-speed PM machines as much as feasible taken into account mechanical problems of rotor.

Applying numerical simulation based on FEM it has been established that the strip cylinder can be made from high-strength steel alloy for miniturbogenerators rating of 100 and 200 kW with speed of rotation up to 50000 rpm. At the speed of rotation 100000 rpm rating of 100 kW it is necessary to use composite based on synthetic material with permissible tangential strength nearly 1500 MPa, for instance from organic plastic type OPGA/EDT-N with organic plastic wisp Armos 600-A-NK which has tension strength 2500 MPa.

At rating of 200 kW with using such type strip cylinder it’s possible to reach the speed of rotation only 50000 rpm. To achieve speed of rotation 100000 rpm with rating of 200 kW the new material is needed. In order to solve the problems of applying new materials for rotor it’s necessary to use possibilities of nanotechnologies to develop the new composite materials with appropriate behavior.

ACKNOWLEDGMENT

Authors thank RFBR for support by grant № 07-08-345_a.

REFERENCES

[1] A.S. Nagorny, R.H. Jansen, D. M. Kankam. Experimental Performance Evaluation of a High-Speed Permanent Magnet Synchronous Motor and Drive for a Flywheel Application at Different Frequencies. Book of Abstracts ICEM 2006 – XVII International Conference on Electrical Machines, Chania, Crete Island, Greece, Sept. 2006.

[2] A. Binder, M. Klohr, T. Schneider. Losses in High-Speed Permanent Magnet motor with magnetic levitation for 40000/min, 40 kW. Book of Abstracts ICEM-2004, Poland, Krakow, Sept.2004.

[3] J.F. Gieras, U.Jonsson. Lesign of a High-Speed Magnet Brushless Generator for Microturbines. ICEM-2004, Poland, Krakow, Sept.2004.

[4] Johanes J., Paulides H., Geraint W., Howe D. An Evaluation of Alternative Stator Lamination Materials for a High-Speed. 1,5 MW, Permanent Magnet Generator. IEEE Trans. on Magnetic. Vol.40. NO 4, July 2004, pp.2041-2043.

[5] Danilevich Ya.B. Antipov V.N. High–speed (3000-15000 rpm) permanent magnet generator (design and testing) Book of Abstracts ICEM 2006 – XVII International Conference on Electrical Machines, p3, 2-5 September 2006, Crete Island, Greece.

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