IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 2, FEBRUARY 2012 871

Ultra High Speed Motor Supported by Air Foil Bearings for Air BlowerCooling Fuel Cells

Do-Kwan Hong, Byung-Chul Woo, Ji-Young Lee, and Dae-Hyun Koo

Electric Motor Research Center, Korea Electrotechnology Research Institute, Changwon 641-120, Korea

As accomplishing the research of the PM synchronous motor driven by 15 kW at the rated speed of 120,000 rpm, this paper representsan approach to minimize eddy current losses which occur prominently at high speed. Various simulation and testing techniques aretherefore applied to determine the effect of design parameters. Here, the proposed parameters for analysis are winding types, slot openingwidth and sleeve thickness. This paper based on the obtained results, evaluates the optimum value of the most critical design parametersresponsible for eddy current loss. Finally, this paper studies the developed model which represents a solution for minimizing the causeof eddy current loss by implementation of sinusoidal distribution of air-gap magnetic flux density. It comes to the conclusion that sleevethickness should be chosen as considering a function of mechanical stress and critical speed of rotor. Furthermore, the critical speed ofrotor is evaluated by rotor dynamics. The design efficiency is well accorded with the efficiency test, having a maximum error of 2.7%.

Index Terms—Campbell diagram, core loss in stator, critical speed, eddy current loss in rotor, shrink fit, ultra high speed permanentmagnet synchronous motor (PMSM) for air blower, winding type.

I. INTRODUCTION

R ECENTLY ultra high speed electrical machines have at-tracted considerable interest for many industrial applica-

tions such as machine tools, centrifugal compressors, vacuumpumps, and turbine generators [1],[2]. They are highly efficient,compact, and light weight; hence, they can be potentially em-ployed in future applications of high-speed systems. An electro-mechanical system can be made much smaller and lighter at thesame power level by increasing its operating speed. The con-straints on the design of a high-speed permanent rotor can beclassified as mechanical and thermal constraints. These con-straints affect the choice of materials used in rotors, as wellas the appropriate length/diameter ratio. Moreover, an accurateprediction of the critical speed of a rotor at the design stage isnecessary because operation near the critical speed, bears therisk of bearing fatigue failure and rubbing. Thermal behavior athigh speed operation is one of the critical factors in ultra highspeed motors. An Ultra high speed PMSM (permanent magnetsynchronous motor) which is rated at 15 kW, 120,000 rpm foran air blower cooling fuel cells has been developed, using sev-eral design, analysis and manufacturing techniques.

II. ANALYSIS OF ULTRA HIGH SPEED MOTOR

A. Developed Model

Fig. 1 shows prototype of ultra high speed PMSM. This pro-totype is designed for an air blower application for cooling fuelcells. Air foil journal bearings and thrust bearings are separatelydeveloped for the operation at ultra high speed. Detailed di-mensions of the proposed 3 phase PMSM are summarized inTable I. The main target of the design presented in this paperis to achieve a torque of 1.19 Nm at the rated speed of 120,000rpm in Fig. 2.

Manuscript received July 07, 2011; revised October 04, 2011; accepted Oc-tober 20, 2011. Date of current version January 25, 2012. Corresponding author:D.-K. Hong (e-mail: dkhong@keri.re.kr).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2174209

Fig. 1. Prototype of ultra high speed PMSM. (a) Prototype. (b) Stator(winding).(c) Thrust bearing. (d) Air foil bearing. (e) Rotor and load compressor.

In order to achieve this goal, it requires 1.43 Nm at the basespeed of 100,000 rpm. In this paper the design goal takes intoaccount an approximately 17% margin, considering loss relatedto actual manufacturing.

B. Loss Characteristic of Ultra High Speed Motor

Reducing the loss along with a cooling method of high speedmotor is one of the most significant problems. The losses inhigh speed motor are divided into core loss, copper loss andmechanical loss, as generally in other motors.

1) Stator Losses: Core and copper losses are occurring in thestator. The copper losses are proportional to the square of thecurrent flowing through the phase. Therefore, the copper lossescan be calculated by the winding resistance and the current

0018-9464/$31.00 © 2012 IEEE

872 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 2, FEBRUARY 2012

TABLE IPROPOSED3-PHASE PMSM DIMENSIONS

TABLE IITABLE OF ORTHOGONAL ARRAY �� � �, SAMPLING

TABLE IIIANALYSIS RESULT USING TABLE OF ORTHOGONAL ARRAY �� � �

(1)

where : phase current RMS value, : armature windingresistance.

In case of core losses in stator, the total losses are commonlyexpressed by (2) for sinusoidally varying magnetic flux density

with frequency , and they are divided into hysteresis loss

Fig. 2. Design specification and torque-speed characteristic.

and eddy current loss [3]. The each coefficients are obtained ofmeasurement data of core samples.

(2)

where : maximum flux density, : hysteresis loss coeffi-cient, n: Steinmetz coefficient in the range 1.5 to 2.5, f: fre-quency, : eddy current loss coefficient

To calculate flux density, time-dependent magnet field sim-ulation was completed by FEA method. The FEAs were per-formed with a commercial software package, and the relatedgoverning equations are introduced in [4]

(3)

where : the magnetization of the permanent magnet, : mag-netic vector potential, : conductivity, : electric scalar poten-tials, : reluctivity, : armature current density

2) Rotor Losses: In case of ultra high speed PMSM, the rotorlosses form a larger proportion of the total losses than usual inconventional low to medium speed machines. Since the rotoris composed of non-magnetic materials with conductivity, onlyeddy current loss is considered for PM and the retaining sleeve.The loss in the PM is relatively small or nonexistent among rotorlosses [5]. The loss in conductive retaining sleeve is higher andthe major part of rotor losses. In this paper, therefore, the at-tention is focused on rotor loss in the retaining sleeve. The har-monic eddy current losses generated in the permanent magnetand sleeve can be calculated as follows:

(4)

where :eddy current, : eddy current density, :total sleeve and magnet eddy current loss (total current flowingin conductors), : conductivity

3) Mechanical Strength: As the mechanical equivalentstress, the von Mises stress was computed. A physical interpre-tation of von Mises criterion is that yielding begins when theelastic energy of distortion reaches a critical value. The vonMises criterion is also known as the maximum distortion strain

HONG et al.: ULTRA HIGH SPEED MOTOR SUPPORTED BY AIR FOIL BEARINGS 873

Fig. 3. Analysis of means (ANOM) of core loss, eddy current loss and torqueaccording to types (dotted line is selected level).

energy criterion. The von Mises stress, , is computed withthe principal stresses as follows:

(5)

C. Winding Method

Generally, core losses in stator are essentially caused by thePM flux and do not depend on the armature current. However,the variation of the air-gap magnetic flux density by the arma-ture current can affect eddy current. The effect of core lossesin stator is analyzed considering the variation of the magneto-motive force (M.M.F) distribution according to winding types.Fig. 3 shows winding configuration according to winding types.Core loss in stator, eddy current loss in rotor and torque are arecompared and reviewed according to winding types. The de-signs that were analyzed satisfy the same set of specifications(electric and magnetic loading, rotor speed, sleeve thickness,magnet thickness, and axial length). The designs will help high-light the effect of winding configuration on eddy-current losswhile producing the same active output power.

Winding configuration can be applied as a technique for re-ducing the loss. However, the proper winding should be chosenas depending on required torque and loss.

D. Slot Opening

The distribution of sleeve losses shows that the peak of lossdensity occurs in the portions of the sleeve under the centerof the slot. This can be explained by considering the inducedvoltage which occurs axially down the length of the sleeve.Here, eddy current occurring in the edge of the shoe may becaused by saturation and fringing effect. The effect of differ-ence between flux density at the center region and at the edge ofteeth is taken into account.

Fig. 4. von-Mises stress �� � distribution(120 krpm, 210 �).

E. Sleeve Thickness

Most eddy currents occur in the sleeve. Therefore, this paperdeals with the analysis of parameters responsible for eddy cur-rent loss in the rotor (sleeve and magnet). Sleeve thickness ischosen as another parameter to reduce eddy current loss.

F. Analysis of Means Using Orthogonal Array, Sampling

To determine the contribution and effect of winding type, slotopening and sleeve thickness on core loss in stator, eddy currentloss in rotor and torque, the method of ANOM is used. Samplingis used table of orthogonal array in Table II and its results inTable III.

It calculates the mean value of the core loss in stator, eddy cur-rent loss in rotor and torque value corresponding to the level ofeach parameter and it’s deviation. Fig. 3 shows the characteristicby sleeve thickness. As can be seen from Fig. 3, the eddy cur-rent loss reduction by sleeve thickness is much smaller than theeffect by slot opening and winding types. However sleeve thick-ness should be chosen as in regarding of mechanical stress. It de-termines the percentage contribution of each parameter on coreloss, eddy current loss and torque and the optimal set (full-pitchwinding, slot opening: 2.4 mm, sleeve thickness: 4.5 mm) ofparameters considering torque and loss can be obtained fromFig. 3.

G. Structural Analysis of Shrink Fit

Maximum equivalent stress requires a growing sleeve thick-ness. This result is similar to the 2-D analytic model. Fig. 4shows von-Mises stress distribution in interference 0.065 mmfor shrink fit(operating condition: 120 krpm, 210 ). Inconelwhich has yield stress, 1100 MPa is used for sleeve material.The generated maximum von-Mises stress is 963 MPa in caseof sleeve thickness 5 mm. Safety factor is 1.14 ( 1100/963).

H. Electrical Analysis of Developed Model

Fig. 5 shows Wye connecting, distributed winding, flux den-sity contour and flux path of the designed PMSM using 2-DFEA. It has inner rotor type and consists of 2 poles and 12 slots.This paper deals with analyzing parameters to reduce iron lossand eddy current loss which occurs prominently at high speed.Here, the proposed design parameters for development are con-sidered such as winding types, slot opening width and sleevethickness. This paper based on the obtained results analyzes themost critical design parameter in eddy current loss by rotatingat the rated speed of 120,000 rpm and researches to reduce eddycurrent loss. Fig. 6 shows core loss in stator and eddy currentloss in rotor. The maximum eddy current loss is about 183 W.And the maximum core loss in stator is about 148 W. Table IV

874 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 2, FEBRUARY 2012

Fig. 5. Winding pattern, flux density contour and flux path (full-pitch windingtype, slot opening: 2.4 mm, sleeve thickness: 4.5 mm).

Fig. 6. Core loss and eddy current loss.

TABLE IVCOMPARISON DESIGN AND TEST RESULT OF THE DEVELOPED PROTOTYPE

shows the comparison result between simulation and experi-ment. The efficiency result has an error within 3%. It is hardfor a dynamo system to cover the performance test of the pro-totype in the ultra high speed operation. Therefore, the energymethod is applied to measure prototype performance.

I. Rotor Dynamic Analysis of Developed Rotor

The comparison of the critical speed obtained from 3-D rotordynamic simulation considering gyroscopic effect and rotatingeffect is shown in Fig. 7.

The supported bearing stiffness of air foil bearings was setto 1e6 N/m in simulation. The air foil bearings can be usedin high speed. The critical speed of rotor should be above theoperating speed of 120,000 rpm. Generally operating is not af-fected in the lateral and conical whirling critical speed becauseof the damping of air foil bearing, even though damping has nota great effect. The result of 3-D FEM is well accorded with theone of modal testing in maximum 5%. However, the bendingwhirling critical speed must be avoided. Fig. 7 shows a Camp-bell diagram. The crossing points represent the critical speed.

Fig. 7. Campbell diagram (sleeve thickness: 4.5 mm).

The critical speed of 3rd forward whirling of the developedmodel is 192,429 rpm. The separation margin between the oper-ating speed and the bending whirling critical speed needs to belarge. An appropriate separation margin is typically 20 30%.The developed model has 60.36% separation margin. All theperformances of the ultra high speed motor prototype are veri-fied successfully, as shown in Table IV.

III. CONCLUSION

This paper deals with the ultra high speed motor for air blowerthat is designed and manufactured for cooling fuel cells at therating of 15 kW, 120,000 rpm. Several techniques of analysishave been applied to develop the ultra high speed motor. Theelectrical design of the developed motor meets the requirementsconsidering core loss in stator and eddy current loss in rotor.The losses in dependence of varying sleeve thickness are cer-tainly analyzed in ultra high speed operation. Simultaneously,the structural stability and critical speed of rotor is evaluatedaccording to sleeve thickness. The shrink fit effect should beconsidered from a structural point of view with respect to op-erating condition and interference. The rotor dynamic analysisshows that the critical speed of the rotor is higher than operatingspeed with sufficient separation margin, as proven in a Campbelldiagram. All performances of the ultra speed motor prototypeare verified successfully. The design efficiency is well accordedwith the efficiency test, with a maximum error of 2.7%.

REFERENCES

[1] O. Bottauscio, F. Casaro, M. Chiampi, S. Giors, C. Maccarrone, andM. Zucca, “High-speed drag-cup induction motors for turbo-molec-ular pump applications,” IEEE Trans. Magn., vol. 42, no. 10, pp.3449–3451, Oct. 2006.

[2] L. Zhao, C. Ham, L. Zheng, T. Wu, K. Sundaram, J. Kapat, and L.Chow, “A highly efficient 200 000 rpm permanent magnet motorsystem,” IEEE Trans. Magn., vol. 43, no. 6, pp. 2528–2530, Jun. 2007.

[3] S. M. Jang, H. W. Cho, and S. K. Choi, “Design and analysis of a high-speed brushless dc motor for centrifugal compressor,” IEEE Trans.Magn., vol. 43, no. 6, pp. 2573–2575, Jun. 2007.

[4] L. Zheng, T. X. Wu, D. Acharya, K. B. Sundaram, J. Vaidya, L. Zhao,L. Zhou, C. H. Ham, N. Arakere, J. Kapat, and L. Chow, “Design ofa super high-speed cryogenic permanent magnet synchronous motor,”IEEE Trans. Magn., vol. 41, no. 10, pp. 3823–3825, Oct. 2005.

[5] M. Nakano, H. Kometani, and M. Kawamura, “A study on eddy-currentlosses in rotors of surface permanent-magnet synchronous machines,”IEEE Trans. Ind. Appl., vol. 42, no. 2, pp. 429–435, Mar./Apr. 2006.