The 2005 International Power Electronics Conference

220,000-r/min, 2-kW Permanent Magnet Motor Drive for Turbocharger
Toshihiko Noguchi, Yosuke Takata *Yukio Yamashita, Yoshimi Komatsu, and Seiichi Ibaraki **This paper describes an ultra high-speed permanent-magnet synchronous motor drive, which is embedded in a turbochargerof an internal-combustion engine. The electric drive makes it possible to enhance output power of the turbocharger in a
motoring mode and to retrieve combustion energy from exhaust gas in a regenerating mode. The rotor is composed of amechanically reinforced high-performance Nd-Fe-B permanent magnet and its diameter is designed to be considerably small
to prevent huge centrifugal force caused by the ultra high-speed rotation. The stator consists of a laminated iron-core withextremely thin (0.15 mm) electromagnetic steel plates to minimize the iron-core losses. Two types of the stator
configurations, i.e., 3-slot and 6-slot stator designs are discussed from the viewpoint of magnetic field and losses analysis.
Consequently, the latter design is found to be remarkably effective to suppress temperature rise of the rotor due to an
eddy-current loss in the rotor permanent magnet. Also, it is indispensable to reduce the motor inductance less than 10 (µH)
because dc bus voltage of an inverter is limited to 72 (V). A pseudo current-source inverter, which has a current controlled
chopper across the dc bus, is employed to drive the motor with a 120-deg. conduction pattern because conventional sinusoidal
PWM techniques are unable to regulate the motor current in such an ultra high-speed range. Computer simulations andexperimental tests are conducted to examine various operation characteristics of a prototype. The experimental data
demonstrate 220,000-r/min operation at 2.2-kW inverter output power, which agree with the simulation results well and provefeasibility of the proposed system.
Keywords: ultra high-speed PM motor, turbocharger, magnetic field analysis, losses analysis, pseudo current-source inverter

1.

Introduction

Turbochargers are auxiliary machines of gasoline
engines and diesel engines mounted on automobiles,
vessels and so forth. Their principal objectives are
improvement of combustion efficiency, output power andresponse of the engines over wide speed ranges. The
turbochargers consist of a compressor, which compress
vaporized fuel/air mixture, and a turbine, which rotates at
an extremely high-speed with exhaust gas power. Since
the compressor and the turbine are mechanically anddirectly coupled with a shaft, the conventional
turbochargers have less controllability in terms of speed,
torque and power flow. For example, it is difficult to
avoid response delay, i.e., turbo lag, because compressionof the vaporized fuel/air mixture does not begin without
starting the turbine with the exhaust gas. In general, it israther insufficient to overcharge the compressed fuel/air
mixture into the engine cylinders at a low-speed range.
Furthermore, although combustion energy is huge when the*

Nagaoka University of Technology, tnoguchi@vos.nagaokaut.ac.jp
1603-1 Kamitomioka, Nagaoka 940-2188, JAPAN** Mitsubishi Heavy Industries, Ltd., yukio_yamashita@mhi.co.jp
Nagasaki Research & Development Center
5-717-1 Fukahori, Nagasaki 851-0392, JAPANThis work has been supported by Grant-in-Aid for Scientific
Research (B)(2) 16360137 from Ministry of Education, Culture,
Sports, Science and Technology, Japan.

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engine rotates under high-speed and low-load conditions, it
is hardly possible to retrieve combustion energy from theexhaust gas. All of these difficulties come from a puremechanical structure of the conventional systems.
This paper proposes a novel hybrid system, which
introduces power electronics, i.e., an ultra high-speed
permanent-magnet synchronous motor (PMSM) drive, into
the turbochargers, and describes computer simulation andexperimental results of the adjustable speed drive followed
by a motor design guideline on the basis of electromagnetic
and losses analyses. The most difficult problem in thisproject is how to achieve an ultra high-speed operation suchas 220,000 r/min at over 2-kW inverter output with the
PMSM and how to raise power density of the motor
embedded in the turbocharger. There are many othertechnical issues to surmount, e.g., bearings and mechanicalshaft vibration, but this paper focuses on an electricalaspect of the motor and its adjustable speed drive.

2. Outline of Novel Hybrid Turbocharger
Fig. 1 (a) shows a conventional turbocharger. As
described above, output pressure or a rotating speed of thecompressor cannot easily be controlled because itmechanically depends on combustion energy of the exhaustgas from the engine. It is rather difficult to obtainsufficient compression of the fuel/air mixture at low-speed
operation of the engine because the combustion energy ofthe exhaust gas retrieved by the turbine is relatively low.
On the other hand, the exhaust gas is emitted to the open air

The 2005 International Power Electronics Conference

25

Compressor

110

Engine cylinder

29 MaxTurbine(a) Conventional turbocharger.

(a) 3-slot motor.

25

Waste gate
valve

Compressor

Ultra high speed
Turbine
PM motor

29 Max(b) Proposed hybrid turbocharger.
Fig. 1. Overviews of conventional and proposed turbochargers.

(b) 6-slot motor.
Fig. 2. Principal dimensions of 3-slot and 6-slot motors.

Table 1. Principal specifications of ultra high-speed PMSM drive.
Rated power (Cont.)
2 (kW)
Rated torque (Cont.)
0.159 (Nm)Rated speed (Cont.)
120,000 (r/min)Maximum speed
220,000 (r/min)
Overload
200 %-2 (s)through a waste gate valve, and a large amount of theexhaust energy is disposed at high-speed and low-load
operation of the engine. This occasion usually happens
when the vehicle cruises at constant speeds on expressways.
The hybrid turbocharger proposed in this paper isdepicted in Fig. 1 (b). An ultra high-speed PMSM ismounted between the compressor and the turbine on theshaft of the turbocharger, and is embedded in it. Motoringof the PMSM gives assisting torque to the compressor,
which allows higher controllability of the compressed
fuel/air mixture at any rotation of the engine. Also, theturbo lag is dramatically improved with the assisting torqueof the motor. Furthermore, the exhaust energy can be
retrieved by operating the PMSM as a generator, and can be
stored in batteries as co-generated electric energy. This
operation contributes improvement of the total efficiency
from fuel to mechanical and electrical output power of theengine system.
3.

110Engine cylinder

Ultra High-Speed PMSM

3. 1 Specifications of Ultra High-Speed PMSM
A technical target of the motor design is discussed in thissection, which is needed in the above hybrid turbocharger.
According to several matching simulation results of an2281

automobile engine and a turbocharger tested at NagasakiLab., Mitsubishi Heavy Industries, it has been inferred thatthe combustion efficiency and engine output can be
improved by 8 % and 150 % by applying 1-kW assisting
torque to the turbocharger, respectively. In addition, 12 %and 200 % improvement of them can be expected by giving
2-kW assisting torque to the turbocharger. From theseestimation results, an ultra high-speed PMSM drive of
which specifications are listed in Table 1 has beendeveloped for the purpose of enhancement of combustion
efficiency by 10 % and engine output by 200 %.
3. 2 Stator and Rotor Configuration
In order to optimize the motor configuration, two typesof stators, i.e., a 3-slot stator and a 6-slot stator, along with
one type of a rotor were investigated throughelectromagnetic analyses. Both types of the motors are
two-pole machines to satisfy an extremely high-speed
rotation and have concentrated stator windings to simplify
the configuration and to reduce leakage inductance.
The number of turns per pole is expressed by thefollowing equation;E, ··················································· (1)N=
4.44k d k pφ mwhere E is an induced voltage per pole, k d is a
distributed winding factor, k p is a short pitch coefficient,and φ m is a total magnetic flux.
Therefore, thedistributed winding factors and the short pitch coefficientsof the 3-slot machine and the 6-slot machine are derived asfollows, respectively;

Eddy-current loss
in rotor (W)The 2005 International Power Electronics Conference
200
150
100
50
0

20

40

60
80
100
× 103 (r/min)
Motor speed

120

20

40

60
80
100
3
Motor speed ×10 (r/min)

120

20

40

60
80
100
3
Motor speed ×10 (r/min)

120

20

40

60
80 3
100
Motor speed ×10 (r/min)

120

20

40

60
80 3
100
Motor speed ×10 (r/min)

120

20

40

60
80
100
3
Motor speed ×10 (r/min)

120

Core loss (W)

200
150
100
50
0

(a) Flux density of 3-slot motor.
Total loss (W)

400
300
200
100
0

Eddy-current loss
in rotor (W)(a) Losses of 3-slot motor.

(b) Flux density of 6-slot motor.
Fig. 3. Flux density distributions of 3-slot and 6-slot motors.

200
150
100
50
0

Core loss (W)200

k d 3 = k d 6 = 1 ; ·························································· (2)⎛2π ⎞k p 3 = sin ⎜⎟ = 0.866 ; ········································· (3)
⎝3 2⎠

100
50
0
400

············································ (4)As shown in the above equations, the number of turns of
the 3-slot machine is 0.58 times of that of the 6-slot one,which implies that the space factor of the former is superior
to that of the latter. The PMSM to be designed in thispaper can inherently be miniaturized because its
fundamental operation frequency is approximately 3.67
(kHz), and further miniaturization is required to make themotor embedded in the turbocharger. In other words, it isindispensable to enlarge the power density of the machinefrom ordinary values of 0.8-1.4 (W/cm3) to 14 (W/cm3).
Therefore, a great attention must be paid to losses of the
motor as well as a cooling system design. Since the 3-slot
machine has larger spatial harmonics than the 6-slot
machine has, a large variation of the permeance possibly
causes an eddy current loss in the rotor magnet. The eddy
current loss results in considerable temperature rise andfatal demagnetization of the permanent magnet.
3. 3 Electromagnetic Analyses Result
Losses analyses of the 3-slot motor and the 6-slot motor
were conducted with a boundary element method and afinite element method. Fig. 2 (a) and (b) show principal
dimensions of both motors. The outer diameters of thestator and the rotor are 110 (mm) and 25 (mm), respectively.
The motors have a wide air gap of 5 (mm), which is almost

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Total loss (W)⎛1 π ⎞
k p 6 = sin ⎜⎟ = 0.5 .
⎝3 2 ⎠

150

300
200
100
0

(b) Losses of 6-slot motor.
Fig. 4. Losses analysis results of 3-slot and 6-slot motors.

same as thickness of the permanent magnet on the rotor.
A Nd-Fe-B high-performance permanent magnet of
39-MGOe maximum energy product is employed on therotor, and 0.15-mm thick electromagnetic silicon steel
plates are used to compose the laminated stator core.
Fig. 3 and Fig. 4 show electromagnetic analysis results
and losses analysis results at 2-kW output power,
respectively. It can be seen that the 3-slot motor generates
higher eddy current loss in the rotor permanent magnet,
which is approximately five times of that of the 6-slot
motor. This eddy current is caused by the spatial
harmonics associated with the flux variation as describedabove. However, there are not significant differences
between the two motors in terms of a cupper loss and aniron loss of the stator. Consequently, it can be found thatthe 6-slot motor has great advantage over the 3-slot motor
in reducing the resultant total losses, which is

The 2005 International Power Electronics Conference

Magnet rotor

High-efficiency
compressor

High-stability bearing

Carbon fiber
Hold ringHigh-efficiency
turbinFig. 6. Axial configuration of hybrid turbocharger.

DSC1(a) 6-slot stator of prototype.

Vin
72 (V)LC CT ILS1 S2 S3PM
Motor

C + SC2

Buck and Boost
chopper

UVW
S4 S5 S6 Phase voltages
InverterFig. 7. Schematic diagram of pseudo current-source inverter.
(b) Rotor of prototype.
Fig. 5. Photographs of prototype machine.

approximately 50 % of the 3-slot motor’s losses.
3. 4 Implementation of Prototype Machine
Fig. 5 shows photographs of a prototype machine, where
the 6-slot stator with the concentrated windings and themechanically reinforced permanent-magnet rotor can be
seen. As is described in the computer analyses, theconcentrated stator windings are employed to reduce
winding resistance and leakage inductance, of which wires
are 3.5-mm2 PEW. The stator is composed of a laminatedcore of which each steel plate thickness is 0.15 (mm). In
general, the most important design parameters of ultra
high-speed machines are the stator inductance andresistance, and they are only 9 (µH) and 5.2 (mΩ) in thisprototype, respectively. On the other hand, a Nd-Fe-B
permanent magnet of which maximum energy product isfrom 39 to 43 (MGOe) is mounted on the rotor with anadhesive, and is magnetized to hold a sinusoidal MMF
distribution. The rotor is physically reinforced by carbon
fiber and some chemicals to prevent the permanent magnet
from scattering by huge centrifugal force. The air gap is
as wide as 5 (mm), but is good enough to hold appropriate
flux density level. Fig. 6 illustrates a rotating part andbearings configuration of the hybrid turbocharger. Highly
stable slip bearings are employed to support the shaft, andare located between the turbine and the rotor. Thisoverhung structure is effective to simplify the wholeassembly of the turbocharger.
4.

Psudo Current-Source Inverter

In general, a voltage-source PWM inverter with current
minor loops is often used to drive a PMSM. Also, vector

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control is adopted in the microcomputer-based system,
which requires coordinate transforms and sinusoidal current
regulation. However, since the fundamental frequency ofthe motor reaches as high as 3.67 (kHz) at the maximum
operating speed, it is hardly possible to regulate the motor
currents with an IGBT or FET based PWM inverter. Thusa pseudo current-source inverter shown in Fig. 7, which has
a current controlled chopper across the dc bus, is employed
to drive the motor with a 120-deg. conduction pattern
because conventional sinusoidal PWM techniques are notsuitable to regulate the motor current in such an ultra
high-speed range.
The motor current amplitude iscontrolled with a pulse amplitude modulation (PAM)
technique by controlling the chopper duty ratio. Since aswitching frequency of the chopper is as high as 30 (kHz),
it is possible to reduce inductance of the dc bus reactor andto improve response of the dc bus current. At every
moment of commutation in the 120-deg conduction inverter,
surge voltages are caused by the winding inductance of the
motor. These surge voltages are clamped by the dc bus
voltage through a bypass diode of the chopper and body
diodes in the FETs of the inverter. Therefore, it is notnecessary to implement blocking diodes series with the
FETs like most conventional current-source inverters
require. In regenerating mode of the motor, kinematicenergy is retrieved to the dc bus by operating the inverter as
a 120-deg conduction synchronous switched mode rectifier.
Commutation of the inverter is carried out on the basis ofan e.m.f. based mechanical sensorless control algorithm of
the PMSM. The 120-deg conduction pattern is simply
generated from the detected terminal voltages by using
integrators to calculate the flux linkage and several logic
ICs. Indeed, an initial flux position cannot be detectedwith this algorithm because there is no e.m.f. at the initial

Motor phase voltage
10(V/div)

Motor phase voltage
10 (V/div)

The 2005 International Power Electronics Conference00Time 100(µs/div)
Motor current
25(A/div)

Motort current
20 (A/div)

Time 100 (µs/div)00Time 100 (µs/div)

Time 100(µs/div)

Fig. 9. Phase voltage and current waveforms at 220,000 (r/min) and 2.2kW inverter output (experimental result).
1.0
Total power factor

Motor phase voltage
20(V/div)(a) Phase voltage and current waveforms at 220,000 (r/min) and 2.2-kW
inverter output.

0Time 100(µsec/div)

0.9
0.8
0.7
0.6
0.5

0

Time 100(µsec/div)(b) Phase voltage and current waveforms at 120,000 (r/min) and 200-%
overload.
Fig. 8. Operating waveforms (simulation results).

stand-still state; hence, open loop control is adopted to startup the motor until the operating speed reaches 10,000(r/min). After reaching this speed threshold, the 120-deg
conduction pattern is switched over to that generated fromthe e.m.f. based algorithm.
5.

60

80

100 120 140 160 180 200 220 240
3Motor speed ×10 (r/min)

60

80

100 120 140 1603 180 200 220 240
Motor speed ×10 (r/min)

2200
Inverter output power (W)Motor current
50(A/div)

0.0
0Operation Characteristics Examined throughComputer Simulation

Prior to experimental tests, several computer simulations
were conducted to confirm the basic operation
characteristics of the proposed system. The test motor has
identical specification as listed in Table 1, and the pseudo
current-source inverter has 72-V dc bus voltage and0.15-mH reactor in the chopper.
Fig. 8 (a) shows a simulation result of operation at the
maximum speed of 220,000 (r/min) and 2.2-kW inverter
output. As can be seen in this figure, the pulse voltage
waveforms are observed at every instance of thecommutation, and are clamped by the dc bus voltage.
Since the rising and falling rates of the current during thecommutation are restricted by the clamp voltage, reducing
the total inductance of the motor is significantly important.
Otherwise it is impossible to raise the fundamental
operation frequency up to a few kHz with a proper 120-deg
conduction pattern.
Moreover, high fundamental
displacement power factor operation is confirmed because
the motor current is in phase with the phase voltage.

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2000
1800
1600
1400
1200
1000
0.000

Fig. 10. Total power factor and inverter output.

Fig. 8 (b) depicts another simulation result at 200-%
overload condition at 120,000 (r/min). As describedabove, the commutation takes a longer time as the current
amplitude is enlarged with the load because the clamp
voltage limits the rising and falling rates of the current
during the commutation. The situation shown in thisfigure is a limitation of the pseudo current-source inverter,and it is apparently impossible to enlarge the current
amplitude higher than this condition, i.e., 200-% load.
6.

Experimental Setup and Results

Several experimental tests were conducted using a test
hybrid turbocharger and a test pseudo current-source
inverter. The first test was carried out to examine
motoring operation, where the turbine was rotated by an
external air flow source and giving assisting torque with theultra high-speed PMSM to the compressor. During alltime in this test, temperature of the stator windings was
carefully watched, using thermo couplers.
Fig. 9 shows an experimental result at the maximum
speed of 220,000 (r/min) and 2.2-kW inverter output. As
can be measured in this test, the motor current is almost inphase with the phase voltage, and the both waveforms agree
well with those of the simulation result. The total power

The 2005 International Power Electronics Conference
factor was 0.84 because both of the voltage and the current
include a large amount of harmonics, while the excellent
fundamental displacement power factor can be confirmed in
the figure. The temperature of the stator windings rose to120°C at continuous operation of the 120,000-r/min rated
speed and the 2.2-kW inverter output. Therefore, it isindispensable to lower the temperature to prevent
demagnetization of the rotor and hazardous overheat in the
future work, not only improving the whole configuration of
the turbocharger but also refining the motor design. In
addition, it was confirmed that the 1.3-kW regenerative
power was obtained at the rated speed.
Fig. 10 shows the total power factor and the inverter
output power with respect to the motor speed. As can beseen in this figure, the total power factor did not exceed
0.84 because of the current waveform distortion caused by
the 120-deg conduction waveforms. However, a constanttorque operation and a constant power operation weresatisfactory achieved below and over the rated speed of
120,000 (r/min), respectively.
7.

Conclusion

This paper focused on a hybrid system that consists of
the ultra high-speed PMSM embedded in the turbocharger,
and described the motor design and adjustable speed drive
characteristics of the hybrid system through experimental
tests as well as computer simulations. According to theelectromagnetic analyses of a 3-slot machine and a 6-slot
machine, the former has disadvantage in terms of an eddy
current loss in the rotor magnet, which is caused by thespatial harmonics of the 3-slot stator, but superior to thelatter from the viewpoint of the space factor and heatemission of the stator. To the contrary, the 6-slot motor isable to reduce the eddy current loss of the rotor, which
results in nearly 50-% total loss reduction.
A prototype system of the hybrid turbocharger, which
consists of a two-pole and 6-slot PMSM and a pseudo
current-source inverter, was developed on the basis of theabove electromagnetic and losses analyses. Computer
simulation results and experimental results agreed very well,and it was confirmed that continuous operation wasachieved at 2.2-kW inverter output and at 120,000-r/min
rated speed as well as at 220,000-r/min maximum speed.

Acknowledgment
The authors wish to express their sincere thanks tocolleagues and technicians who supported testing of theexperimental setup at Nagasaki Lab., Mitsubishi Heavy
Industries.

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Ueno, "The Novel Approach of Coupled Analysis for Small Size and Ultra-High
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(2) Mitsukichi Okawa, "Design Manual of Magnetic Circuit and PM Motor," Sogo
Research, 1989 (in Japanese).
(3) Takehisa Koganezawa, Isao Takahashi, and Kazunobu Oyama, "Sensorless Speed
Control of a PM Motor by a Quasi-Current Source Inverter," IEE-Japan Proc.
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2285(4) Yosuke Takata, Toshihiko Noguchi, Yukio Yamashita, Yoshimi Komatsu, andSeiichi Ibaraki, "220000r/min, 2-kW PM Motor Drive for Turbocharger",
IEE-Japan Proc. IAS Annual Conference, no. 9, p. 155, 2004 (in Japanese).
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