Electrical and Mechanical Compatible Design of
15 kW, 150,000 r/min Ultra-High-Speed PM Motor
Toshihiko Noguchi
Graduate School of Integrated Science and Technology
Shizuoka University
Shizuoka, Japan
noguchi.toshihiko@shizuoka.ac.jp

Kohei Fujita
Graduate School of Integrated Science and Technology
Shizuoka University
Shizuoka, Japanfujita.kohei.14@shizuoka.ac.jp

Abstract— This paper describes an electrical and mechanicalcompatible design of an ultra-high-speed permanent magnet(PM) motor of which rated output power is 15 kW and rated
speed is 150,000 r/min. In general, the electromagnetic design ofthe motor must be conducted under some mechanical constraints
such as a rotor diameter and a rotor axial length, while themechanical design is restricted by mechanical strength of theelectromagnetic components such as a PM. In the paper, twocandidates of the rotor structure have been investigated from theviewpoint of electrical and mechanical compatibility, i.e., acylindrical hollow PM (standard SPM) rotor and a solid PM
rotor with a titanium alloy sleeve. Both of the motors can achieve
high efficiency over 96 % and high power density over 55 W/cm3,
but the latter has a remarkable anti-demagnetization
characteristic less than 1 %. Because the permeance of the solid
PM rotor is extremely low, a lot of the leakage magnetic flux is
generated in the motor, resulting in the eddy current losses of themetal shafts as well as the titanium alloy sleeve surrounding thesolid PM. In order to investigate dependency of the shaft material
onto the electromagnetic characteristics, three kinds of materials
are employed for the shafts and are examined through 3-Delectromagnetic analysis. As a result, stainless steel is the best
choice for the shaft material and can satisfy the electrical andmechanical compatibility with high torque generation, highefficiency, high power density, high anti-demagnetization
characteristic, and so forth.

some electrical and mechanical design constraints. In general,
however, the higher-torque design tends to increase themachine volume, so higher-speed (higher-frequency operation)
design is desirable and effective for further efficient
downsizing of the machine.

Keywords: surface permanent magnet synchronous motor(SPMSM), high-speed motor, solid permanent magnet,
demagnetization

High-speed motors have not only electrical but alsomechanical design constraints in the design process, e.g., theirrotor axial length is limited by the axial vibration and theresonance frequency, and their rotor diameter is limited by thecircumference speed and the centrifugal force. In addition,
generally accepted design concepts of the electric machines are
not applicable to the high-speed motors because theirdownsized motor proportion makes it difficult to improve theelectrical performance in the limited motor size without
sacrificing the mechanical performance. Therefore, it issignificantly important to discuss the electrical and mechanicalcompatible design of the high-speed motors.
Table I. Design specifications of 15-kW, 150,000-r/min PM motor.
Over 15 kW

Rated speed

150,000 r/min

Rated torqueOver 1 Nm

Rated efficiency

Over 95 %Maximum power density

Over 50 W/cm3

Demagnetization rate

Within 1 %DC bus voltage

Lower than 250 V

Rated current

77 A (over load 150%)

I. INTRODUCTION
Electrification of mechanically operated systems is widelyand intensively spread to various industry sectors, home
appliance product sectors, transportation and logistics sectors,
and so forth. Electric machines play the most important role
for the electrification of variaous kinds of industrial products.
The most important points to develop electric machines arehigher-power-density and fault-tolerant design as well as higher
efficiency design.
High-power motors are intensively
developed to achieve higher-power-density in recent years, andpermanent magnet synchronous motors (PMSMs) using rareearth PM are widely employed and are applied to various
applications because of their excellent power density andefficiency. In order to enhance the motor output power, it isdefinitely necessary to deliver as high electromagnetic torqueas possible and to rotate at as higher speed as possible underRated output power

Table

Maximum current

115 A

Rotor diameter

Smaller than 43.3 mm. Fundamental design outline of ultra-high-speed PM motor.
Motor typeSurface permanent magnet
synchronous motor (SPMSM)

Number of phases

3 phases

Number of poles

2 poles

Stator configuration

Concentrated winding structure

Permanent magnet(Br = 1.34 T, HCB = 963 kA/m,
BHmax = 350 kJ/m3)

N43TS Nd-Fe-B

20HX1300
Electromagnetic steel plates

978-1-7281-3398-0/19/$31.00 ©2019 IEEE

(0.2 mm thick, 0.54 μΩm,
Bmax = 1.5 T)lglm

Windings

Stator core

Shaft

Shaft diameter

Rotor outer diameter

Several ultra-high-speed PM motors have been developedby the authors, which feature the extremely wide air gap to
reduce the synchronous inductance. It is possible to achieve
the required output power specifications and to realize theexcellent efficiency characteristic over 96 % with the wide-airgap design, but the anti-demagnetization design is still difficult
without sacrificing the outstanding electrical and mechanicalperformance of the motors. In the paper, the electrical andmechanical configuration of the rotor is mainly discussed,
focusing on the anti-demagnetization characteristic of the PM
mounted on the rotor. In addition, the rotor shaft material isinvestigated from the viewpoint of electromagnetic
characteristics, and the appropriate material is chosen for theshaft to satisfy the electrical and mechanical requirements atthe same time. The paper describes the electrical andmechanical compatible design of the 15-kW, 150,000-r/min
ultra-high-speed PM motor on the basis of the above
discussion.

Permanent
magnetFig. 1. Cross section of hollow magnet ultra-high-speed PM motor.

B [T]
Br

II. DESIGN TARGET AND BASIC DESIGN CONCEPT OF ULTARAHIGH-SPEED PM MOTOR

B. Basic Design Concept of Ultra-High-Speed PM Motor
TABLE II shows designed parameters of the ultra-highspeed PM motor to be discussed in the paper. The cross sectionof the surface PM type motor is sown in Fig. 1, which is atypical surface permanent magnet synchronous motor(SPMSM). The rotor is actually reinforced by glass fiber or
carbon fiber, which is wound around the rotor PM with epoxy
resin. The SPM motor delivers only the magnet torque, butcannot deliver reluctance torque because of no saliency. The
rotor diameter is too small (less than 43.3 mm) to employ
interior permanent magnet (IPM) structure for the reluctancetorque generation. Two-pole six-slot combination is employed
to reduce an electrical frequency to drive the motor with aninverter. A NdFeB cylindrical shaped PM is mounted on therotor shaft, of which maximum energy density is BHmax = 350
kJ/m3; hence the wide-air-gap structure is possible to employ,
and the synchronous inductance can also dramatically be
reduced with effective suppression of the permeance variationin the circumference direction. The stator is constituted with aconcentrated winding six-tooth six-slot iron core for the
purpose of reducing the leakage magnetic flux and the eddy
current loss in the PM. The stator iron core employs 0.2-mm
electromagnetic steel plates to reduce the iron loss, and 3-mm
thick filets are on the tooth ends to avoid violent permeancevariation in the circumference direction and to increase thewinding coefficient to retrieve more PM magnetic flux.

pu
0
H [A/m] Hc
HmFig. 2. B-H characteristic and operating point of permanent magnet.

Rotor outer diameter

A. Design Target of Ultra-High-Speed PM Motor
TABLE I indicates a design target of the 15-kW, 150,000r/min ultra-high-speed PM motor. The motor also has a 150-%
momentary overload specification for the higher acceleration.
The rotor diameter is limited under 43.3 mm so that thecircumference speed of the rotor does not exceed the sonicvelocity in the air because turbulence between the rotor and theconcentrated winding stator may occur.

Bm

lg
lm

Permanent
magnetProtection slleve
Windings
Stator coreFig. 3. Cross section of solid magnet ultra-high-speed PM motor.

Protection
sleevePermanent
magnetShaftsFig. 4. Cross section of solid permanent magnet rotor.

III. PRINCIPLE AND CONFIGURATION OF TWO-TYPE ULTARAHIGH-SPEED PM MOTORS
A. Demagnetization Characteristic of Conventional SPM
Configuration
An approach to improve the anti-demagnetization
characteristic is to raise a permeance coefficient of the motor.
The permeance coefficient is defined as the following equation,
which determines the operating point of the PM:
pu =

lm

ag

am K c l g=lm

Dm + lm

Dm - lm

K c lg

,

(1)

where
pu : permeance coefficient,lm : PM thickness,
am : average cross section area of PM,
K c : Carter’s coefficientlg : air gap length,

Table. Design parameters and major specifications of motor models.
Motor model# 1 (hollow)# 2 (solid)

Rotor outer diameter

37.2 mm

39.0 mmlm

8.1 mm14 mm

lg

10.7 mm5.55 mm

Permeance coefficient

1.1

3.3

Number of winding turns
Winding space factor

2250.8 %

2250.8 %

Current density

4.78 A(rated operation)

4.78 A(rated operation)

Glass fiber

Titanium alloy(0 Ωm)(2.3 μΩm)

Stator winding resistance

5.5 mΩ

5.3 mΩ

Stator winding inductance
Efficiency at rated
operating point

37 μH

38 μH

97.4 %

96.9 %

57.9 W/cm3

60.4 W/cm3

Protection sleeve

Maximum power density

ag : average cross section area of air gap, andAs described previously, the cylindrical ring NdFeB PM isemployed, and the air gap length is much longer than that of thestandard PM motor, i.e., almost same as the thickness of thePM. On the other hand, it is indispensable to enlarge the rotor
shaft diameter to make the resonant frequency as high as
possible, resulting in prevention of the mechanical shaft
vibration. Therefore, the SPM type ultra-high-speed motor
must have the low permeance coefficient due to the low valueof lm, which is limited by both of the rotor shaft diameter andthe rotor outer diameter. The thinner PM makes the antidemagnetization characteristic worse.
B. Demagnetization Characteristic of Conventional SPM
Configuration
Keeping the wide air gap configuration to make thesynchronous inductance as low as possible, a new rotor
configuration is discussed, where the rotor is fully filled with
the PM and has no shaft. Figure 3 illustrates a cross section of
the ultra-high-speed PM motor, of which rotor is filled with asolid NdFeB PM. As can be seen in the figure, it is possible tomaximize lm and to minimize am; thus the permeancecoefficient can be enlarged without sacrificing the inherent
wide air gap.
Figure 4 illustrates an axial cross section of the solid PM
rotor. This configuration of the rotor does not allow having theshaft in the PM, so it is necessary to reinforce the solid PM
rotor with a stiff and robust metal sleeve in order to prevent therotor from scattering. By taking advantage of the mechanicalreinforcement by the metal sleeve, it is possible to allocate themetal shaft and the solid PM rotor on the identical axis, and to
joint them tightly. A titanium alloy is employed for the metal
sleeve because the mechanical strength is relatively high, andthe titanium alloy sleeve makes it possible to connect the shafts
to the both ends of the solid PM rotor and to reinforce the rotor
at the same time. Since the titanium alloy has as low

12
Demagnetization rate (%)

Dm : PM outer diameter.

10
8
6
4
2
0

#1

#2
Motor models

Fig. 5. Comparison of demagnetization characteristics.
Flux density [T](Contour plot)(b) # 2(a) # 1
Fig. 6. Flux density distribution of permanent magnets on demagnetizedsituation.

permeability as air, it is possible to reduce the synchronous
inductance because the rotor including the protection sleeve
can be regarded as air. This great feature contributes to drive
the motor at a lower terminal voltage.
In the following sections, the SPM type motor #1 with thecylindrical hollow PM shown in Fig. 1, and the motor #2 ofwhich rotor is fully filled by the PM shown in Figs. 3 and 4 areelectrically compared. An electromagnetic analysis software
JMAG-Designer 18.0TM is used to evaluate the designed motor
performances.

IV. CONDITIONS OF ELOCTROMAGNETIC ANALYSIS ANDEVALUATION

The anti-demagnetization characteristic and the power lossof the rotor are the most important performance indexes toevaluate the two motors. It is desirable to reduce thedemagnetization rate less than 1 %, and to control the copper
loss and the iron loss to maximize the efficiency at the rated
operating point. In the following section, the electromagnetic
analysis results are compared between the two motors from theviewpoints of the demagnetization characteristics, the power
losses, and the maximum efficiency operating points. The
demagnetization characteristics are examined under theconditions of 200-°C PM temperature and the maximum
negative d-axis current, and the no-load electromotive forces(e.m.f.) before the demagnetization at room temperature andafter the demagnetization described above are compared.
When the power losses and the efficiency are discussed, themotor temperature is assumed to be 75 °C. The maximum
efficiency can be obtained when the equation below is satisfied:
Ws + Wr = Wc ,500
400
Loss (W)

Design parameters and the major specifications of the twotype motors are listed in TABLE III. Both motors have a flat
proportion, and have stator overhang of 1 mm for each end of
the stator iron core to receive as much PM magnetic flux as
possible. The synchronous inductance of the two motors is
reduced down to approximately 37 uH owing to the wide air
gap configuration. It is considered, however, that the #2 motorwith the solid PM rotor has much more leakage inductancebecause its inductance is almost same as that of the #1 motor
regardless of the no-shaft configuration in the PM. The
magnetic flux density in the stator back yoke and the stator
teeth is approximately 1.4 T for both motors, and the two
motors have achieved high efficiency over 96 % and highpower density over 55 W/cm3.

Eddy-current loss (Protection sleeve)
Eddy-current loss (Shaft)
Eddy-current loss (PM)Stator iron loss
Copper loss

300
200
100
0

#1

#2
Motor models

Fig. 7. Comparison of total losses at rated operation and loss analysis results.
Stator iron loss
Eddy-current loss (PM)Eddy-current loss (Shaft)
Eddy-current loss (Protection sleeve)28.7 %

70.1 %

0.28 %
0.95 %

Fig. 8. Analysis result of iron loss of # 2 motor at rated operation.
Stator flux
Permanent magnet flux

41.0 %

59.0 %(2)

where
Ws : stator iron core loss,
Wr : rotor iron core loss, and
Wc : stator winding copper loss.

It is found from this equation that the maximum efficiency
can be obtained if the iron losses and the copper loss are almost
same. The maximum efficiency operating points are evaluatedfrom the viewpoint of this condition.
Figure 5 shows demagnetization characteristics of the two
motors, and the magnetic flux density distributions after thedemagnetization are shown in Fig. 6. It is confirmed from Fig.
5 that the demagnetization rate of #2 motor is less than 1 % of
#1 motor. This significant difference can be observed in the
demagnetization areas in the PMs as shown in Fig. 6.
Therefore, the solid PM rotor is remarkably effective to preventthe demagnetization, compared with the standard SPM rotor.
The total power losses and their analysis results areindicated in Fig. 7, where the iron loss analysis results areshown in Fig. 8 and the eddy current loss of the titanium alloy
sleeve is shown in Fig. 9, respectively. The all results are
estimated at the rated operating point. It is found from Fig. 7

Fig. 9. Detail of eddy-current loss in protection sleeve.

that the total power loss of #1 motor is less than that of #2
motor because of the eddy current loss caused in the titanium
alloy sleeve on the rotor. The percentage of the eddy current
loss in the titanium alloy sleeve of the #2 motor is 28.7 %(117.8 W), and is a dominant cause of efficiency degradation.
The reason why the eddy current is generated in the titanium
alloy sleeve is a permeance variation caused by theconcentrated-winding-structure based slot ripples. According
to Fig. 9, it can be found that the eddy current loss caused by
the PM magnetic flux is 41 %, and that caused by the stator
magnetic flux is 59 %. The permeance variation of the motor isbasically small because of the wide air gap, but the stator
leakage magnetic flux easily interlinks to the titanium alloy
sleeve, which is the major reason of the eddy current
generation. The temperature rise of the PM caused by the eddy

current loss in the titanium alloy sleeve may be a great concern,but the specific heat and the thermal conductivity of thetitanium alloy are very small. Therefore, the possibility of thethermal demagnetization is considered to be very low. The
efficiencies of the two motors are almost same because theratios between the iron losses and the copper loss are very
similar as shown in Fig. 7.

Stator Flux
Permanent magnet Flux
Shaft

StatorMagnet

Shaft

Stator Stator

StatorMagnetV. DISCUSSION ON SHAFT MATERIAL OF SOLID PM ROTOR
A. Electromagnetic Characteristics Depending on Rotor Shaft
Material
Figure 10 illustrates the stator magnetic flux behavior in theultra-high-speed PM motor with the solid PM rotor. The rotor
shaft material influences the electromagnetic characteristics ofthe motor because the leakage magnetic flux is not negligible
as described in the previous section. If the permeability of the
shaft is higher than that of the PM, the stator magnetic flux
linking to the rotor is reduced because much of the flux goes tothe shaft. In addition, the pole pair makes short circuits in thethree-dimensional magnetic paths through the shaft, which
increases the leakage magnetic flux of the PM. These leakage
magnetic fluxes do not contribute to generate the average
torque, but improve the anti-demagnetization characteristic as aresult. If the permeability of the shaft is as low as air, the stator
magnetic flux linking to the rotor is relatively increased
because there is not significant difference in the permeability of
the rotor and the shaft. The leakage magnetic flux between the
PM pole pair through the shaft is reduced, resulting in
improvement of the average torque generation but in
deterioration of the demagnetization characteristic. In thefollowing section, the electromagnetic characteristics are
discussed from the viewpoint of a trade-off between the
average torque and the demagnetization characteristic, picking
up three kinds of shaft materials.
B. Electromagnetic Analysis Conditions and Methods
Assuming that one of the three-kind metal materials is
employed as the shafts, i.e., (a) S45C, (b) SUS304, and (c)titanium alloy, the average torque, the demagnetization
characteristics, and the efficiency characteristics are examined.
An initial relative permeability of S45C is the highest and1846.2, and that of the titanium alloy is the lowest and almost 1
same as air. The demagnetization characteristics are examinedat the PM temperature of 200-°C and the maximum negative daxis current is given to the motor. The no-load e.m.f. before
the demagnetization at room temperature and after thedemagnetization is compared. When the power losses and theefficiency are examined, it is assumed that the motor
temperature is 75 °C.

Motor models

Shaft material(a) Low permeability(b) High permeability

Fig. 10. Stator flux and Permanent magnet flux depending on shaft materials.
Flux density [T](Contour plot)

Permanent Magnet
Shaft

(a)

(b)
(c)
Fig. 11. Stator flux density distribution depending on shaft materials.
Flux density [T](Contour plot)

Permanent Magnet

Shaft

(a)

(b)
(c)
Fig. 12. Permanent magnet flux density distribution depending on shaft
materials.

Table IV. Electrical characteristics depending on shaft materials.
Demagnetization
Initial relative
Average torqueTotal loss
permeability
rate1846.20.53 %
1.10 Nm
790.3 W

Eddy-current loss
(Shaft)
1.2 W

Efficiency

a

S45C

b

SUS3042.1

0.16 %

1.48 Nm

816.3 W

2.2 W

96.60%

95.62%cTitanium alloy

1.0

1.42 %

1.49 Nm

815.5 W

4.6 W

96.64%

C. Comparison of Analysis Results
TABLE IV lists the analysis results depending on the shaft
materials. Figure 11 shows the stator magnetic flux density
distribution of the three cases. It can be found that the antidemagnetization rate of (b) is almost 1/3 of that of (c) because
the stator magnetic flux linking to the rotor PM is reduced by
the leakage magnetic flux through the shaft. On the other hand,(a) is much better than (b) in terms of the anti-demagnetization
characteristic because the leakage magnetic flux to the S45C
shaft is increased and the counter magnetic flux against the PM
is reduced.
On the other hand, the magnetic flux density distributionsof the rotor are indicated for the three cases. There is about 26% difference in the average torque between (a) and (c), but (b)and (c) can deliver almost the same average torque. As can beobserved in Fig. 12, the magnetic flux density of the shaft in
the case of (a) is much higher than that of (b) and (c), which
implies that the leakage magnetic flux between the PM poles
through the shaft is increased.
The efficiency of (a) is slightly lower than (b) and (c) . The
total power loss and the average torque of (a) is approximately
4 % lower and approximately 26 % lower than those of (b) and(c), respectively. Although the total power loss is improved byintroducing the S45C shaft in the case (a), the average torquedeterioration detrimentally affects to the efficiency, which is
caused by the excessive leakage magnetic flux through theshaft.
VI. CONCLUSION
This paper has described an electrical and mechanicalcompatible design of an ultra-high-speed PM motor, of which
rated output power is 15 kW and rated speed is 150,000 r/min.
In order to satisfy not only the ratings but also high efficiency
over 96 % and high power density over 55 W/cm3, the two
types of the PM motors are investigated, i.e., a motor with acyllindrical hollow PM (standard SPM) rotor and the othermotor with a solid PM rotor. It has been confirmed that both ofthe motors can attain the design target but that the solid PM
rotor configuration makes it possible to improve an antidemagnetization characteristic at the same time.
By
introducing the solid PM rotor structure, the total power loss
increases by approximately 20 %, which is mainly caused by atitanium alloy sleeve, but the demagnetization rate can bedramatically reduced down to less than 1 %.
However, the motor with the solid PM rotor has anotherunavoidable problem, which is the leakage magnetic flux
caused by an extremely low permeance of the solid PM and theprotective titanium alloy sleeve. Particularly, it has been foundthat the stator magnetic flux affects the eddy current in themetal shafts as well as the titanium alloy sleeve surrounding thesolid PM. In the paper, three kinds of metal materials for theshafts have been investigated, i.e., steel S45C, stainless steel
SUS304, and titanium alloy, and dependency of the materials
onto the electromagnetic characteristics has been examinedthrough the 3-D electromagnetic analysis. As a result, stainlesssteel SUS304 is the best choice for the shafts from theviewpoints of the average torque, the efficiency, the antidemagnetization characteristic, and so forth. It has been foundin the investigation that the average torque characteristic andthe anti-demagnetization characteristic have a trade-off
relationship, depending on the permeability of the shaft
material.
For the future work, it is required to reduce the eddy current
loss of the protection sleeve on the solid PM rotor, which
occupies 28.7 % of the total power loss, and to improve theefficiency further without sacrificing the anti-demagnetization
characteristic.
REFERENCES
[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

Hirokatsu Katagiri, Hiroyuki Sano, Kazuki Semba, Noriyo Mimura, andTakashi Yamada"Fast Calculation of AC Copper Loss for High Speed
Machines by Zooming Method,"IEEJ J. Industry Applications, vol. 6,
no. 6, pp. 395-400, 2017.
Kenta Yamano, Shigeo Morimoto, Masayuki Sanada, and Yukinori
Inoue"Design of Surface Permanent Magnet Synchronous Motor Using
Design Assist System for PMSM,"IEEJ J. Industry Applications, vol. 6,
no. 6, pp. 409-415, 2017.
K. Sakai, T. Tokumasu, and K. Itou, “Magnetic Field Analysis of aSuper−High−Speed Permanent Magnet Motor with a New Rotor,”
Journal of the Japan Society of Applied Electromagnetics, Vol. 4, no. 3,
pp. 40-45, 1996.
T. Noguchi, Y. Takata, Y. Yamashita, Y. Komatsu, and S. Ibaraki,“220000 r/min, 2-kW PM Motor Drive for Turbocharger,” IEEJ Trans.
IA, vo. 125, no. 9, pp. 854-851, 2005.
T. Noguchi, T. Komori, “Eddy-Current Loss Analysis of Copper-Bar
Windings of Ultra High-Speed PM Motor,” 3rd International Conference
on Electrical Systems for Aircraft, Railway, Ship Propulsion and Road
Vehicles , Aachen, Germany, (2015-2).
K. Ogata, and T. Noguchi, “Efficiency Improvement of 3-kW, 150,000r/min Ultra High-Speed PM Motor Focusing on Conductor Eddy-Current
Loss,” IEEJ Techinical Meeting on Motor drive Rotating Machinery,
Automotive Machinery,MD－18－074, RM－18－060, VT－18－015,(2018-9).
T. Wada, and T. Noguchi, “Optimum Design Focused on Efficiency andPower Density of 1.5-kW, 150,000-r/min PM Motor Fed by LowVoltage Power Supply,” IEEJ Techinical Meeting on Semiconductor
Power Converter and linear drive, SPC-09-174,LD-09-064, (2009-12).
K. Fujita, and T. Noguchi, “Study on propotion of 15 kW, 150,000 r/min
PM motor,” IEEJ Annual National Conference, 5-031, (2019-3).