The International Workshop on Mechatronics 2006
December 12-13, 2006, Supalai Resort, Saraburi, Thailand

Efficiency Improvement and Loss Analysisof Ultra-High Speed Permanent-Magnet Motor
Masaru Kano, and Toshihiko Noguchi
Department of Electrical Engineering, Faculty of Engineering,Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
Tel/Fax: 81-258-47-9510, E-mail: kano@stn.nagaokaut.ac.jp, tnoguchi@vos.nagaokaut.ac.jp
Abstract
This paper describes a design of an ultra-high speed(UHS) permanent-magnet (PM) synchronous motor for asupercharger mounted on an automotive engine. Conventional superchargers are mechanically powered by theengine to compress inlet air to the engine cylinder. Thesupercharger electrically driven by the UHS motor allows
employing a centrifugal compressor. It has higher
efficiency and higher output pressure than a displacement
compressor [1, 2]. Therefore, the electric supercharger can
improve the efficiency of total drive system and output
pressure response without mechanical linkage to the engine.
In this paper, a surface permanent-magnet (SPM) motor(150,000 r/min, 1.5 kW) fed by a low voltage battery (12 V)
is discussed. The simulation results through Finite Element
Method (FEM) analysis are presented for an optimal
permeance coefficient design and efficiency improvement.
Keywords: ultra-high speed PM motor, permeancecoefficient, FEM analysis
1. Introduction
Automotive superchargers are mechanically linked to
an internal combustion engine with a belt from a crankshaft,and compress the air flow to the engine cylinder withmechanical power provided by the engine. Most ofconventional superchargers employ a positive displacement
compressor rather than a centrifugal compressor because of
the operating speed restricted by the engine rotation. The
demerits of the positive displacement compressors arelow-efficiency and low boost pressure.
In this study, therefore, an electric drive of thesupercharger with the centrifugal compressor is investigated. The electric supercharger is assumed to be usedwith a 1.5-2.0-liters gasoline engine. Table 1 shows design
specifications of the UHS PM motor discussed in this paper.
Several problems of the motor should be resolved toachieve the UHS operation. At first, the electricsupercharger is driven by a low voltage battery (12 V),
which is used in most of conventional cars. Next, UHS drive
Table 1. Design specifications of ultra-high speed PM motor.
Supply voltage (V)Rated power (W)Rated torque (Nm)Rated speed (r/min)

1215000.0955150,000

Permanent Magnet

Shaft

Coil (Winding)
Stator Core

lglm

Figure 1. Configuration of ultra-high speed SPM motor.

generates much loss, for example, magnet eddy-current loss
that is proportional to square of the operating frequency,and mechanical loss. Also, it is important to pay attention tomechanical vibration of the UHS drive.
2. Permeance coefficient and motor efficiency
Figure 1 illustrates the configuration of the UHS SPM
synchronous motor. It mainly consists of a rotor shaft, a PM,windings, and a stator core. Several cases of the permeance
Table 2. Analysis conditions of UHS PM motors.
Type of shape
#1
#2
#3
#4
#5
Stator configuration
Concentrated winding
Number of poles
2
Number of stator slots
6
Stator outer diameter (mm)9228
Stator inner diameter (mm)30
Stator stack length (mm)10
Tooth width (mm)Airgap length (mm)65432
Magnet thickness (mm)23456Permeance coefficient
0.33 0.60 1.00 1.67 3.00
Nd-Fe-B, 1.26, 954.9
Magnet, Br (T), iHc(kA/m)
12Rotor shaft diameter (mm)0.1
Thickness of steel sheet (mm)Three-phase sinusoidal waveform
Operating current
In phase with e.m.f.
Phase angle
2
FET ON resistance (mΩ/phase)

3000

6000
5000

left side of (3)right side of (3)1000

Loss (W)Resonance frequency (Hz)

7000

4000
3000
2000
1000
005060708090100110120Shaft length (mm)Figure 2. Relationship between shaft length andmechanical vibration.

100
0.0

0.5

1.5

2.0

2.5

3.0

Figure 4. Comparison between both sides of Eq. (3)at rated operation.

Air
StatorCore

1.0Permeance coefficient pust

WRon 169.2

Wi 40.3

mag

We

15

st

Wimag

We

Coil(Winding)

Wm

Air

Wc

Permanent
Magnet

Shaft

Figure 3. Mesh configuration of motor for FEM analysis.

coefficients shown in Table 2 are examined for design of theUHS motors. In the case of the SPM motor, the permeancecoefficient is expressed by the following equation:aglpu = m(1)am K C l g ,where lm is a magnet thickness, am is an averaged crosssectional area of the magnet, ag is an averaged crosssectional area of the air-gap between the rotor and the stator,
lg is an air-gap length, and KC is a Carter’s coefficient. For
the SPM motor, ag is nearly equal to amKC, so (1) can beapproximated as follows:
lpu = m(2)
lg .
Therefore, the permeance coefficients are determined byonly lm and lg. If other parts of motor shape are the same, thepermeance coefficient is proportional to the motor
electromotive force.
On the other hand, in order to improve the total
efficiency of the motor drive system incorporating theinverter, the inverter loss must be taken into account. The
maximum efficiency condition of the motor drive system
can be described as (3).
Wist + Wemag + Wm = Wc + WRon ,(3)where Wist is an iron loss of the stator core, Wemag is an
eddy-current loss of the rotor magnet, Wm is a mechanicalloss which is bearing friction loss, Wc is a copper loss of thestator coil, and WRon is a conduction loss of the inverter. The
left side terms of (3) don’t depend on the drive current,WRon
Wc 14.4

Wm 197.9

unit: watt

Figure 5. Loss analysis at maximum-efficiency operation.

while the right side terms are proportional to square of thedrive current. Inverters have not only the conduction loss
WRon but also a switching loss. The switching loss is
proportional to the drive current, so it doesn’t affect themaximum efficiency condition shown in (3).
3. Analysis results
3.1 Optimisation of permeance coefficient
Table 2 shows analytical conditions of the UHS motor
to be investigated. Those motors are designed from theviewpoint of the specifications in Table 1, implementationin vehicles, and prevention of the mechanical vibration.
Permeance coefficient is changed from 0.33 (#1) to
3.00 (#5) according to configurations of lm and lg. The stator
stack length shown in Table 2 is one of the most importantparameter, and is fixed at 30 mm. Because the shaft
vibration frequency should be higher than the motor
operating frequency (2500 Hz). Figure 2 shows therelationship between the shaft length and the shaft vibration
frequency. When the stator stack length is 30 mm, the shaft
length is 78 mm, and the shaft vibration frequency is about
5200 Hz, which is more than twice of the operating
frequency. Figure 3 illustrates a mesh configuration of the
motor for 2D FEM analysis. It consists of 7692 elements
and 7909 node points. Besides, it is analyzed at 3-degreeintervals.
Figure 4 shows two kinds of losses when the designed
motor is operated at the rated load. One is the left side of (3),which is summation of the iron loss Wist , the magnet
eddy-current loss Wemag, and the mechanical loss Wm. TheNorth Pole
Shaft
South Pole

Radial Segment(0.5 mm)mm
30
Axial Segment(0.5 mm)

30 mm

Figure 6. Configuration of PM segmentation.

Figure 8. Prototype stator core and six 1-turn windings.

Eddy-current loss in PM (W)12magnet eddy-current loss is reduced by 47.6 %, when themagnet is separated into two pieces in the radial direction inaddition to the axial segmentation. The magnet
segmentation of these six pieces reduces only 1 % withrespect to the total system loss. However, the magnet
segmentation is effective to reduce the temperature rise andto prevent the irreversible demagnetization of the magnet
because the magnet loss can necessarily be decreased.

10864
Axial segmentation
Axial & radial segmentation

200

1

2

3

4

Number of axial segmentation

Figure 7. Relationship between magnet segmentationand eddy-current loss in PM.

other is the right side of (3), which is summation of thecopper loss Wc and the inverter conduction loss WRon.
Figure 4 shows a trade-off between the left side and theright side of (3) as the permeance coefficient is changed. In
the case where the permeance coefficient is 1.4 (cross pointof two curves shown in Figure 4), (3) is fulfilled and its
efficiency becomes the maximum value.
The calculated system loss of #4 in Table 2 is indicatedin Figure 5. For #4, the permeance coefficient value is theclosest to 1.4. The system efficiency is 77.3 % which is a
product of the motor efficiency (84.7 %) and the inverter
efficiency (91.2 %). Figure 5 also shows that themechanical loss and the inverter loss are dominant.
3.2 Loss reduction by means of magnet segmentation
The percentage of the magnet eddy-current loss is only
3.4 % as shown in Figure 5. However, the loss leads tomagnet temperature rise and degradation of magnet
performance, which is called “irreversible demagnetization.” Therefore, the effect of magnet segmentation shownin Figure 6 is investigated. Electrical insulation material(0.5-mm width) is set to radial direction and axial direction
between the magnets shown in Figure 6.
Figure 7 shows 3D FEM analysis results depending on
the magnet segmentation. The magnet eddy-current loss isapparently reduced by 36.9 %, when the magnet isseparated into three pieces in the axial direction. The

4. Conclusion
This paper describes a design of a 150,000 r/min-1.5
kW ultra-high speed permanent-magnet synchronous motor
for a supercharger mounted on an automotive engine. The
permeance coefficient is used for a design parameter, andthe optimal shape of the motor is examined using FEM
analysis. The trade-off between the left side terms and theright side terms of (3) has been calculated through FEManalysis. As a result, the system efficiency of the motor
drive has been improved when the permeance coefficient is
set so that (3) is satisfied. The efficiency of designed
ultra-high speed motor has been achieved 77.3 %. In
addition, it is found through 3D FEM analysis that theeddy-current loss of the rotor magnet is reduced by 47.6 %when the magnet is separated into six parts.
Feasibility of the analysis results described above isrequired, using a prototype of the real machine. The
prototype shown in Figure 8 will be examined in detail in
the future works.
References[1] Noguchi, T., Takata, Y., Yamashita, Y., and Ibaraki,S., 2005. 160,000-r/min 2.7-kW Electric Drive ofSupercharger for Automobiles. The Sixth International Conference on Power Electronics and Drive
Systems, pp. 1380-1385.
[2] Noguchi, T., Takata, Y., Yamashita, Y., Komatsu, Y.,
and Ibaraki, S., 2005. 220,000-r/min 2-kW Permanent
Magnet Motor Drive for Turbocharger. IEEJ Trans. on
Industry Applications, Vol. 125, No. 9, pp. 854-861.
[3] Okawa, M., 1989. Design Manual of Magnetic Circuitand PM motor. Sogo Research Co., Ltd., Japan.