Development of 150000 r/min, 1.5 kW PermanentMagnet Motor for Automotive Supercharger
Toshihiko Noguchi *, IEEE Senior Member, and Masaru Kano **Nagaoka University of Technology
Address: 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
Phone: +81-258-47-9510, Fax: +81-258-47-9500
e-mail: tnoguchi@vos.nagaokaut.ac.jp
Abstract—This paper discusses an optimum design of an
ultra high-speed permanent-magnet synchronous motor(PMSM), which is applied to a supercharger of an
automotive engine. Although the motor is driven by aninverter with a 12-V DC bus voltage due to an automotive
power source, it achieves the maximum rotating speed of
150000 r/min and the rated output of 1.5 kW. Since thepower source strictly restricts the motor terminal voltages
and the fundamental operating frequency is as high as 2500
Hz, it is significant to pursue further reduction of thesynchronous impedance in the motor, paying attention to its
permeance coefficient.
In the paper, a FEM-based
electromagnetic field analysis is conducted, followed by atheoretical discussion on the optimum machine design. In
addition, the mechanical structure is discussed to produce areal machine. The developed prototype has a variety of
unique features from electrical and mechanical viewpoints,
and some experimental test results are presented todemonstrate its potential.
Index Terms— ultra high-speed, permanent-magnet
synchronous motor, supercharger, electromagnetic field
analysis, and permeance coefficient.

I. INTRODUCTION
Superchargers are often used on automotive
combustion engines to enhance the engine output power
and to reduce the physical engine size at the same time.
Not only the performance improvement in a powerweight ratio of the engines but also various advantages
over common power plants can be expected, such as aquality improvement of the exhaust gas, a fuel-efficiency
improvement with respect to the output power, and aquick response of the engine torque. A conventional
supercharger has a mechanical linkage with the engine,which uses a timing belt from the crankshaft via
overdrive devices, and it compresses inlet air to theengine cylinders by means of the mechanical power
provided by the engine. Fig. 1 illustrates a mechanicalconfiguration of the traditional supercharging system. As
shown here, many of such conventional superchargers
employ a positive displacement compressor rather than acentrifugal compressor because of limitation of theoperating speed due to the low engine-rotation. Lowefficiency and low boost-pressure are, however, major
drawbacks of the positive displacement compressor,
which prevents further performance improvement of thesupercharged engines.
On the other hand, an electrical drive of thesupercharger is a very promising approach as a next-

generation axulially machine in future automobiles. Fig.
2 depicts an outline of the investigated supercharging
system for the automotive combustion engines. Since theelectrically driven supercharger allows to employ thecentrifugal compressor instead of the positive
displacement compressor, highly efficient operation canbe achieved, e.g., higher operating speed over 100000
r/min, higher boost-pressure, smaller mechanicaldimensions, faster response of the inlet air compression,etc. Furthermore, there is another advantage of theelectric supercharger in a mechanical-linkage-freestructure, which reduces overall mechanical losses andeliminates complicated link mechanism.
This paper discusses an optimum design of an ultra
high-speed permanent-magnet synchronous motor(PMSM), which is specifically applied to the electricsupercharger of the automotive engines. Although the
PMSM is operated by an inverter with a 12-V DC bus
voltage for an adjustable-speed drive, it must attain the
maximum rotating speed of 150000 r/min and the rated
output power of 1.5 kW for electrification of thecentrifugal-compressor-based supercharger. There arevarious difficult technical problems in the machine
Displacement
compressor

EngineBelt & PulleyFig. 1. System configuration of conventional supercharging system.
Inverter

12 V PS

Engine12-V
Battery

Ultra UHS
motor
high-speed
motor

Centrifugal
compressor

Fig. 2. System configuration of electric supercharging system.

TABLE I. Target specifications of ultra high-speed motor.
Assumed engine
Rated output power
Rated speed
Rated torque
Overload capacity and duration

1.5 L class
1.5 kW
150000 r/min
0.0955 Nm
200 %, 1 s

Coil (Winding)
Stator Core

Permanent Magnet

Shaft

lg

lm

TABLE II. Electrical and mechanical design parameters of ultra highspeed motor.
Rated voltage
Rated current
Number of phases
Number of poles
Stator configuration
Winding configuration
Number of stator slots
Stator outer diameter
Stator inner diameter
Stator stack length
Stator tooth widthElectromagnetic steel plates
Rotor shaft diameter
Permanent magnet
Bearings

2.96 V/phase
195 A
3 phase2 poles
Concentrated winding structure
1 turn, 2 parallels per phase
6 slots
92 mm
28 mm
30 mm
10 mm
10JNEX900(0.1-mm thick, 6.5-% silicone,µs=23, Bmax=1.8 T)
12 mm
N-42SH Nd-Fe-B(Br=1.28 T, bHc=955 kA/m,
BHmax=310 kJ/m3)
Angular ceramic-ball bearing
with grease lubrication

design, e.g., drastic reduction of the synchronous
impedance, minimization of the iron and the copper
losses, further improvement of the power-weight ratio,
etc., which should simultaneously be solved withcompromise to some extent. In addition, these electricaldesign requirements must be satisfied all together with acompact and robust mechanical structure design. In thedesign process, a finite-element-method (FEM)-based
electromagnetic field analysis is conducted to make thefine adjustment of the detailed machine shape and to seek
the best design parameter set, focusing on a permeancecoefficient of the PMSM. The mechanical structure isalso discussed to create a real prototype machine, whichis experimentally examined to confirm the basic
performance as a first step of the system development.
II. REQUIREMENTS AND DESIGN SPECIFICATIONS FORULTRA HIGH-SPEED MOTOR
A. Requirements for Ultra High-Speed Motor
Assuming that the centrifugal compressor is employed
for a supercharger mounted on a 1.5-litter class gasoline
engine, the ultra high-speed motor is required to output
the mechanical power from 1.5 to 2 kW at 150000 r/min
or higher. Particularly, when boosting the inlet air
compression, extremely fast response is indispensable toaccelerate the compressor from several thousand r/min to
the rated operating speed of 150000 r/min withinapproximately 0.5 s, which is almost comparable with a
response time of the conventional supercharger. In order
to meet with this requirement, the ultra high-speed motor

Fig. 3. Cross section diagram of ultra high-speed SPMSM.
TABLE III. Design conditions of five different permeance coefficients.
Design typeAir gap length ℓg (mm)Permanent magnet
thickness ℓm (mm)Permeance coefficient

#1
6

#2
5

#3
4

#4
3

#5
2

2

3

0.33

0.6

4

5

6

1

1.67

3

must have an overload capacity double of the rated output
power at least for one second. Taking these requirements
for the motor into account, the target specifications are
determined as listed in TABLE I.
B. Basic Design Concept of Ultra High-Speed Motor
On the basis of the target specifications, detailed
electrical and mechanical design parameters must be
investigated prior to the numerical analysis for the bestsolution. In order to achieve the highest efficiency
among various electric machines, a two-pole three-phasesurface permanent-magnet synchronous motor (SPMSM)
is selected as the best rotating machine for the ultra highspeed motor drive because of no magnetizing current
required unlike an induction motor or a reluctance motor.
A strong Nd-Fe-B permanent magnet of the rotor allows
not only improvement of the motor efficiency but also
drastic size reduction of the rotor to restrict its
circumference speed and to mitigate the centrifugal force
effect, which is of vital importance in the ultra high-speed
motor. Using such powerful Nd-Fe-B permanent magnet
that has 310-kJ/m3 BHmax makes a wide-air-gap design
possible to reduce the synchronous reactance and toobtain a sinusoidal electromotive force (e.m.f.) regardlessof the concentrated stator winding structure.
On the other hand, the stator has a six-teeth and sixslot structure and the concentrated windings because themotor is required to reduce the leakage inductancethoroughly as well as the synchronous reactance. It
should be noted that each phase has a pair of single-turn
windings in parallel. The stator core must be designed tohave as low iron-losses as possible even at 150000 r/min.
Therefore,high-performance
6.5-%
siliconeelectromagnetic steel plates of which thickness is only
0.1 mm are employed to compose the laminated statoriron core. TABLE II is a summary of the basic
conceptual design for the ultra high-speed SPMSM to beinvestigated in the paper.

Air

3000

StatorCore

1000

Losses (W)Left side of equation
Right side of equation

Coil(Winding)
Air
Permanent
Magnet

Shaft

100
0.0

Fig. 4. Generated mesh for FEM analysis only in quarter portion.

0.5

1.0
1.5
2.0
Permeance coefficient

2.5

3.0

Fig. 6. Loss characteristics with respect to permeance coefficient.
Eddy current loss (shaft)
Eddy current loss (permanent magnet)
Copper loss (windings)
Iron loss (teeth)
Iron loss (back yoke)
Mechanical loss (bearings)
77.4%
0.0165%
6.69%
2.78%
2.05%
11.1%

Fig. 5. Example of flux density distribution at rated operation.

Fig. 7. Numerically calculated losses of design type #4.

III. THEORETICAL DISCUSSION AND ELECTROMAGNETIC
FIELD ANALYSIS FOR OPTIMUM MOTOR DESIGN

inverter, the inverter loss must be taken into account.
The maximum efficiency condition of the motor drive
system can be expressed asst

A. Theoretical Discussion for Optimum Motor Design
It is well known that the operation characteristics of
the SPMSM dominantly depend on the permeancecoefficient because it determines an operating point of thepermanent magnet on its B-H curve. Assuming that theinvestigated SPMSM has uniform permeance distribution
along the air gap, the permeance coefficient pu of the
SPMSM is expressed by the following equation:
pu =ag
Dm + l g
lmlm=
am K C l g
Dm − l m K C l g ,(1)where ℓm is a permanent-magnet thickness, am is anaveraged cross sectional area of the permanent magnet, ag
is an averaged cross sectional area of the air gap between
the rotor and the stator, ℓg is an air gap length, Dm is anouter diameter of the permanent magnet, and KC is a
Carter’s coefficient. Since KC normally takes a value ofapproximately 1.2 to 1.5, ag can be regarded as almost
same as amKC; thus, the following approximatedexpression is obtained:
pu ≈

lm
lg .

(2)

This equation indicates that the permeance coefficient isdetermined by the ratio between ℓm and ℓg as illustrated inFig. 3. The permeance coefficient pu is basically
proportional to the e.m.f. if other physical dimensions of
the investigated motor are not changed.
On the other hand, in order to improve the total
efficiency of the motor drive system including themag

Wi + We+ Wm = Wc + W Ron ,(3)
where Wist is an iron loss of the stator core, Wemag is an
eddy-current loss of the permanent magnet, Wm is amechanical loss (bearing friction loss), Wc is a copper
loss of the stator windings, and WRon is a conduction lossof the inverter. The left-hand side terms of (3) are notcontingent upon the line currents of the motor, while theright-hand side terms are almost proportional to square of
the currents. There is a switching loss as well as the
conduction loss in the inverter, but the switching loss
does not affect the maximum efficiency conditionindicated by (3) because it is proportional to the line
currents.

B. FEM-Based Electromagnetic Analysis and Results
TABLE III shows analytical conditions of the ultra
high-speed SPMSM, where five combinations of thepermeance coefficient, i.e., the permanent-magnet
thickness ℓm and the air gap length ℓg, are investigated.
Figs. 4 and 5 are a quarter portion of the generated mesh
(7692 elements and 7909 nodes in the whole mesh) andan example of the resultant magnetic-flux density
distribution, respectively. As shown in this example, themean value of the flux density is approximately 0.45 T in
the air gap, while significant magnetic saturation ishardly observed in the iron core. Fig. 6 shows the loss
analysis result, where the left-hand side terms and theright-hand side terms in (3) are separately drawn withrespect to pu. It can be found from this figure that (3) issatisfied at pu = 1.5, i.e., the total loss is minimized at this(a) Stator core made of 0.1-mm thick, 6.5-% silicone electromagnetic
steel plates and single-turn stator windings.

(a) Front view (load side) of prototype machine.

(b) Nd-Fe-B permanent-magnet rotor before mechanical reinforcement
using carbon fiber.
Fig. 8. Photographs of stator and rotor.

(b) Rear view (opposite side of load) of prototype machine.
Fig. 10. Exterior photographs of prototype machine.

Fig. 9. Three-dimensional computer graphic of motor assembly.

condition. Therefore, the design type of #4 (ℓm = 5 mm
and ℓg = 3 mm), which has the nearest permeancecoefficient to pu = 1.5, is introduced to the prototype
machine production. Fig. 7 represents numericallycalculated losses of the design type #4. As can be seen in
the figure, the mechanical friction loss is dominant,which accounts for 77.4 %, while the stator iron loss andthe eddy-current loss on the permanent magnet are
merely 19.8 % of the total loss. The electrical efficiency
except for the mechanical loss is approximately 95 %.
IV. ELECTRICAL AND MECHANICAL STRUCTURE OF
PROTOTYPE MACHINEAs described in the basic design concept of the ultra
high-speed motor, the prototype has a special electricaland mechanical structure. Fig. 8 shows photographs ofthe stator and the rotor. The laminated stator core
consists of approximately 300 sheets of 6.5-% silicone
electromagnetic steel plates, of which outer diameter is
92 mm, inner diameter is 28 mm, and axially stack lengthis 30 mm. An alphabetically “b”-shaped stator-winding
bar is inserted in each stator tooth with keeping electricalinsulation from the stator core by polyimide taping, and isconnected to a neutral-point end ring together with theother stator windings. Each of the stator windings has
cross sectional area of 31.2 mm2, resulting in current
density of approximately 6.25 A/mm2 at rated load.
Every clearance between the tooth and the stator windingis less than 0.5 mm, which effectively improves themagnetic coupling by reducing the leakage inductance.
On the other hand, the rotor is simply assembled with asteel shaft and the ring-shaped Nd-Fe-B permanent
magnet, and is magnetized so that the flux distribution
becomes sinusoidal. The photograph of the rotor shows
exterior of the permanent magnet before mechanicalreinforcement with carbon fiber. After assembling therotor, 1.5-mm thick layer of the carbon fiber is formed
with special epoxy resin on the permanent magnet surface
against large centrifugal force. Fig. 9 illustrates a threedimensional computer graphic (a bird’s-eye view) of the
prototype motor assembly. All of the metal components
are made with highly precise NC machining tools.
Particularly, bearings are the most important parts torealize the ultra high-speed operation. In the realmachine, a pair of super-precise angular ceramic ballbearings is used because the prototype is just

8

D
SC1

VDC
72 V

LC CT IL

Six-step
inverter

Su S v S w

Ultrahigh-speed
motor

SC2

C +

SPMSM

Sx Sy

Hall sensor singnal (V/div)

Buck chopper

6
4
2
0
-2
60

Sz

ωm* +

-

PI

ωm

+
-

Logic circuitPI

Hall-effectposition
sensor

Hu
Hv
Hw

F/V

Line current (A)40

48-kHz Carrier

experimentally examined and requires neither highreliability nor mechanical endurance in the laboratory
tests. However, the prototype is designed and created tohave extremely high accuracy of µm-order, especially inthe bearing and shaft part. Fig. 10 shows a front view
and a rear view of the assembled prototype motor.
V. EXPERIMENTAL SYSTEM AND TEST RESULTS
A prototype motor was tested to confirm several basic
operation characteristics with an experimental setup
illustrated in Fig. 11. Since the fundamental operating
frequency of the test motor is over 2 kHz, a pseudo
current-source inverter was employed to drive the motor
instead of a common voltage-source PWM inverter. The
pseudo current-source inverter features a currentcontrolled buck chopper part and a six-step inverter part.
The former has a DC bus current feedback loop andcontrols the DC bus current in accordance with a torquecommand coming from a speed control loop. Therefore,
it can be regarded as a regulated current-source, which
achieves PAM of the DC bus current. This approach issuperior to ordinary thyristor-based power converters
because it operates at the switching frequency of 48 kHz,
resulting in drastic reduction of the DC bus inductanceand in millisecond-order fast response of the DC bus
current control. On the other hand, the six-step inverter
commutates the DC bus current, and generates 120-deg
conduction patterns of the line currents. Every time thecurrent is commutated, the synchronous inductance of the
motor and the line inductance generate surge voltages
across the inverter terminals. In the case of the pseudo
current source inverter, however, the DC bus voltage
clamps the surge voltages via body-diodes in the inverter
MOSFETs and a bypass diode of the buck chopper. The

Terminal voltage (V/div)

Measured value
1.89
0.072
0.07

1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
0.0

0.5

1.0

1.5
2.0
2.5
Time (ms/div)

3.0

3.5

4.0

Fig. 12. Operating waveforms at 33000 r/min under no-load condition.
8
Hall sensor signal (V/div)

Ra (mΩ)La (µH)

Designed value
1.96
0.054
0.056

-20

-60
2.0

6
4
2
0
-2
60
40

Line current (A)Motor parameters
E.m.f constant (10-5 V/r/min)

0-40

Fig. 11. Schematic diagram of pseudo current-source inverter drive.
TABLE IV. Measurement result of motor parameters.

20

20
0
-20
-40
-60
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Time (ms/div)

Fig. 13. Operating waveforms at 120000 r/min under no-load condition.

current commutation is controlled by a logic circuit
where three-phase Hall-effect sensor signals generate the120-deg conduction patterns.
TABLE IV indicates comparison between the designed
and the experimentally measured motor parameters. As
listed here, both of the winding resistance and theinductance of the real machine are slightly higher than thedesigned values. It is inferred that the higher resistance isdue to contact resistance between the winding bars and50000 r/min is 0.05 Nm, which is half of the rated value.
Although the output torque waveform is quite choppy as
indicated in Fig. 14, it can be found from the waveform
envelope that the PI regulator in the speed loop linearly
controls the torque.
Fig. 15 shows an experimentally estimated loss
analysis result. There can be seen a good agreement with
the numerically analyzed result shown in Fig. 7 in theoverall loss percentage, where the mechanical friction
loss caused by the bearings is dominant and the statoriron losses and the eddy-current loss of the permanent
magnet account for 22.0 % of the total loss.

Rotating speed (r/min)

60000
50000
40000
30000
20000
10000

Estimated output torque (Nm)

0
0.06

0.04
0.02

VI. CONCLUSION

0.00
-0.02

0.0

0.5

1.0

1.5

2.0

2.5

Time (s)Fig. 14. Speed step response from 15000 to 50000 r/min andexperimentally estimated output torque.
Eddy current loss (shaft)
Eddy current loss (permanent magnet)
Copper loss (windings)
Iron loss (teeth)
Iron loss (back yoke)
Mechanical loss (bearings)
77.7%
0.0163%
5.51%
0.31%
6.32%
10.2%

Fig. 15. Experimentally estimated loss-analysis result.

the neutral-point end ring and that the higher inductanceincludes line inductance and the leakage inductance.
Fig. 12 shows waveforms of the Hall-effect position
sensor signal, a line current and a motor terminal voltage,
which were measured at approximately 33000 r/min in atest run. The current has low amplitude of approximately
10 A because of no-load operation, and its 120-deg
conduction pattern is generated synchronously with theoutput signal of the Hall-effect position sensor. Since themotor terminal voltage is sinusoidal without conspicuous
harmonic distortion, which means the e.m.f. is properly
generated by the permanent magnet on the rotor in spiteof the concentrated winding structure of the stator. Fig.
13 demonstrates operating waveforms of the prototype at
120000 r/min under no-load condition. As can be seen in
the figure, an excellent current waveform with 120-deg
conduction pattern is properly obtained, which is in phasewith the Hall-effect position sensor signal.
In addition, an acceleration test was conducted tocheck the output-torque controllability. In general, it israther difficult to measure the mechanical output at such
an ultra high-speed as 150000 r/min, so the output torqueof the test motor was estimated by acceleration in the
speed step response waveform and a design value of therotor inertia. From Fig. 14, it is inferred that the
maximum output torque in the acceleration from 15000 toThis paper described development of an ultra highspeed SPMSM, of which ratings are 150000 r/min and1.5 kW. The prototype has various unique features fromelectrical and mechanical viewpoints to realize the ultra
high-speed drive fed by a 12-V DC power source likebatteries. A 120000-r/min operation under no-loadcondition and speed step response to 50000 r/min were
examined, and proper operation characteristics wereconfirmed through some experimental tests. Furthermore,experimentally estimated loss analysis result agreed very
well with the FEM-based electromagnetic field analysis
result. Consequently, the dominant loss factor was foundto be a mechanical loss dissipated by the bearings.
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