Efficiency improvement of a bearingless motor with powder iron core
D. Kanehara*, J. Asama*, T. Oiwa, A. Chiba** (*Shizuoka University, Japan **Tokyo Institute of Technology, Japan)

Introduction

Iron loss reduction□A motor which combines magnetic bearing function
and rotation in the same unit
◎ No wear particles, Lubricant-free□Application：Chemical pumps, Blood pumps, etc□Powder iron core is material which compresses
insulated iron powder□ High electrical resistance ⇒ Reduced eddy current⇒
Reducedironloss
Resin binder
Iron powder
×Low permeability than bulk
(φ 100μm)⇒ Torque and force are reduced?
Insulating⇒
Investigation
of
motor
layer

□Proposed a compact bearingless motor (fig.1)□Succeeded in stable suspension and rotation□Problem：Motor loss is large ⇒ Low efficiency (fig.3) fig.4 Powder iron core
□Improvement of efficiency with powder iron core

□Use powder iron core for stator core of proposed
bearingless motor□Stator core is molded with epoxy resin⇐ Because powder iron core is very brittle□Rotor motion range in radial direction is ±0.25mm

Proposed bearingless motor□Rotor：Two iron disks (with PM) and a PM ring
□Stator：C-shape ⇒ Reduction motor height□Two kind of windings：Suspension and motor are combined

Stator

Rotor

Stator core
(Powder iron)

φ 45

Radial PM

Iron disk
Magnetized
direction

performance with powder iron coreGapsensorRotor(a) Real view

C-shaped
stator core
Windings

Epoxy
resin

10
Frame

Thrust PM ringSensortargetThrustPM ring

Radial
PM

Iron
disk

(b) Schematic viewfig.5 Fabricated test machine

StatorPowder iron core performanceFig.1 Proposed structureySuspension force

Suspension flux
PM flux(a) Radial direction

Powdercore
1000 2000 3000 4000
Rotatinal
回転速度[rpm]
speed[rpm]10Bulk core

Copper loss

Iron loss

Motor Total

No loadFrictionless

Ptotal = Poutput+ Pmech+ Pcopper+ Piron

Suspension
圧粉Increase01000 1500 2000 2500 3000
Rotational
speed[rpm]回転速度[rpm]

Piron = Physteresis+ Peddy current

fig.3 Power consumption-85％

Powdercore

501000 2000 3000 4000
Rotational speed[rpm](b) Only motor□Torque (kT[mNm/A]) at 3000rpm ： Powder ＞ Bulk
⇐ Eddy current works as brakes□Efficiency：25％ improvement (at 3000rpm)(c) Axial direction

1000rpm

25

Torque[mNm]□Motor loss is 70% or more of total loss
□Iron loss is 60% of motor loss ⇐ Eddy current
Power consumption[W]

0

-74％

Copper loss
Iron loss

fig.6 Measured power consumption

Test machine loss (with bulk core)

55

Restoring force

fig.2 Suspension principle

10

Bulk core

Motor

15

(a) Total(b) Tilting direction

Total

Suspension

10x

Restoring torque15

15

2000rpm3000rpm
Powder core
Bulk core20

15

Efficiency[%]zPower consumption[W]□Radial direction：Suspension force is generated bysuperposition of PM flux and suspension flux
□Radial position (suspension force) is actively regulated□Axial/tilting direction：Passively stable with magnetic
attractive force

Power consumption[W]□Total：74％ reduction (Motor loss reduction is large)□Iron loss：85％ reduction ⇒ Eddy current is reduced

kT[mNm/A]
Rotational
speed[rpm] Powder Bulk
core
core
1000
20.1
20.8
2000
19.4
17.6
3000
18.6
13.1

10
5

0
0

0.5

1 1.5 2 2.5 3
Torque current iq[A]3.5

fig.7 Measured torque4

1000rpm

50
40
30
20
10
0

2000rpm

Powder core

3000rpm
Bulk core

+25％

0

0.5
1
1.5
Torque current iq[A]2

fig.8 Efficiency

Conclusion□It is effective to use powder iron core for proposed
bearingless motor to improve efficiency

Y. Yamamoto1, J. Asama1,
1 Shizuoka University, Japan.

BACKGROUND AND OBJECTIVE
Rotor

FINAL GOAL OF RESEARCH
Realization of precise positioning bearingless motor
Application possibility: Micro endmilll cutting
axial runout
Problem of conventional cutting
Heterogeneous cutting force
Bearingless motor

Position of endmill
Measurement and control
Cutting force

Stator

Z,θZ

Y,θY

X,θX

Work

Fig.1 Concept of application

Realization of Precision cutting

PROPOSED STRUCTURE
 8-pole homo-polar bearingless motor
Permanent magnet-free (on rotor)

Thrust control stator
Motor and radial
control stator

Thrust rotorcore
4-pole rotor core

BelM_Unit1

Shaft
Thrust permanent
magnetz

BelM_Unit2
Thrust magnetic bearing

Realization of Robust rotor 5 DOF control
BelM_Unit1 and BelM_Unit2: X, Y, θX, θYyThrust control bearing(TMB): Zx  Distributed winding
Sine wave MMF
 24 slots

Fig.2 Proposed structure of homo-polar bearingless motor

PRINCIPLE OF RADIAL SUSPENSION FORCE AND DIRECTION ERROR
 Undersirable Force Interference (Fig.3)
Rotor(a) At a symmetrical position
Suspension force FxY

Suspension flux

Radial control
stator(b) At an asymmetrical position

Suspension
force

Fyx

Error angle

Y

F

θe

X

Suspension force Fx and Fyx

X

Fx
Dense

SparseAffects the control system
Fyx)
Error angle  e  tan 1 (Fx

Suspension force

Bias flux(a) Symmetrical(b) AsymmetricalFig.3 Principle of radial suspension force

OBJECTIVE OF THIS PRESENTATION
Investigation of the optimal two-pole suspension winding configuration forminiaturization of an error angle (Fig.3).

INVESTIGATION OF OPTIMAL SUSPENSION WINDING CONFIGURATIONTHEORETICAL CALCULATION

 Calculated error angle θe
Calculated magnetomotive force distribution A(φ)

 0 Rl 

A ( ) cos( )d
2g  
2
0

Fyx 

2



 0 Rl 

μ : the permeability of free space

A2 ( ) sin( )d g0: the air gap length



0
2g
2
0

R: the mean air-gap radiusl : the stack length

1 turn M turn
 Single-layer winding modelU
V
Slot No.2,3: 1 turn1 2 3 41
4
3V
Slot No.1,4: M turnW2
3
2
(M=0, 0.1, 0.2, …, 0.9, 1)
41U-phase1AV-phase -0.5 A
W-phase -0.5 A

4
3
V2

1 turnM turn W

14

1 2 3 4

1
2
3W

5

U
1 turnRMS value of error angle °Fx Fx, Fyx ,θe

4

2
1
0
0

M turnU-phase1AV-phase -0.5 A
W-phase -0.5 A

V
2

1

0.6

U
1 2 3 44

0.8

1Minimum value(K-0.4, L=0.1)

1 turn

3

786
55
68
2
7
7
8
1 U
5
6
6
5
1
4
8
7
8
2
75
3
6
5786
W
V 2
3
4
1
1 2 3 4
L turnW
K turn
3
4

0.02
0.01

1
0.8

0.6

0

0.4

0

0.2

L-

0.2

0.4

K -0.60.8

01

Fig.7 RMS value of error angle
2
RMS 1.11°

Error angle °Slot No.1~4: 1 turn
Slot No.5,6: K turn
Slot No.7,8: L turn(K,L=0, 0.1, 0.2, …, 0.9, 1)

0.4

Fig.5 RMS value of error angle

RMS value of error
angle °

1 turn0.2

Winding ratio M -U
Fig.4 Single-layer winding (Model 1)

 Double-layer winding model (Model 2)

Optimal winding ratio (M=1)

31Model 1 (M=1)-1Model 2(K=0.4, L=0.1)-2

0

Fig.6 Double-layer winding (Model 2)

99.5%

RMS 0.005°

0

15

30

45

60

75

Rotation angle °90

Fig.8 Comparison of Model 1 and Model 2

RMS value of Model 2: 95.5% decreased against RMS value of Model 1
6

 Analyze error angle θe

Table.1 RMS value of Theoretical calculation
and FEM analysis

Model 1Model 2

Calculation

FEM

1.11°
0.005°

2.22°
0.300°

Certified validity of theoretical calculation
CONCLUSIONS
 Suspension winding configuration of Model 2 is optimal.

Model 1(theoretical calculation)
Model 1(FEM analysis)

4

Model 2(theoretical calculation)

Error angle °3D-FEM ANALYSIS

2

Model 2(FEM analysis)

0

-2
-4
0

15

75

Fig.9 Comparison of theoretical
calculation and FEM analysis

 3D-FEM analysis results agree well with theoretical calculated results.

 Production test model in near future.

30
45
60
Rotation angle °90

Design and Test Resultsof a One-DOF Actively Positioned Bearingless Motor
D.Watanabe*, J.Asama*, T.Oiwa*, A.Chiba***Shizuoka University, Japan**Tokyo Institute of Technology, Japan

Electromagnetic Analysis

Rotor

Z

Magnetic Bearing

Fig.1 One-DOF actively positioned
magnetic bearing motor(Reference:Tokyo Institute of Technology)

×Large size

15
30
45
60
Rotation angle (˚)
Fig.6 Analysis result of suspension force

Motor

S

N

Z

Cooling fan(Fig.3,4)

N

55mm

N

43.8mmCoreless
Coil

Displacement sensor(Position Control)

PM

Lorenz force

Current
direction
V

Coil
X

W

W

V

U

U

Y
Z
X
Current
Magnetic flux
V
W
《Motor U-phase》《Motor PM》 《Coreless coil》
Fig.3 Principle of position control

Rotation
Torque id  0 If  iq  1  , thenθ  0 iu  0  iv  1 2 
i   1 2 
w


Z

r

X

Restoring
force

W

W

V

U

U

S S

S

S

Z
NN

Y

X

N

N

《Radial direction》《Tilting direction》
Fig.5 Principle of passive suspension

0.40.40.20.2
0.0 0

00

1000
2000
1000
2000
Rotation
speed rpm
(rpm)
回転速度

3000
3000

・2250rpm
Increase of oscillation
Friction loss
Increase
Copper loss
・3000rpm：0.14W

Conclusions

PMB
N

Resonance

PMB

Restoring
torqueS

0.60.6U

V
W
Lorenz force Current Magnetic flux
Torque《Motor PM》 《Coreless coil》《Motor W-phase》 《Motor V-phase》
Fig.4 Principle of rotation(Fig.5)・Radial direction(X,Y) ，Tilting direction(θX,θY)

Axial direction (Z)Radial direction (Y)4040

Tilting direction (θX)

3030
2020

1010
00
00

1000
2000
1000
2000
Rotation
speed
(rpm)
回転速度
rpm

3000
3000

Fig.12 Tilting oscillation(Fig.13)YZ

0.80.8

Fig.11 Radial and axial oscillation

V

θZ

U

Y

Z

Fig.10 Stator(Fig.11,12)・2250rpm ： Resonance of radial direction
・3000rpm ： Axial
4μm
Tilting 4.9mrad
Radial 0.07mm

 iu  2 3  iv   1 6 
i   1 6 w

Magnetization
direction

Yoke

PMB
MagnetDrive test (No load)

3σ (mm)Suspension force

Corelesscoil

Fig.8 Prototype

amplitude
Oscillation 3σ
振動振幅mm

Position control

Displacement sensor(Vibration measurement)

Rotor

Fig.2 Proposed model0 
 iu 
 1
cos θ  sin θ id 
 i   2  1 2

32 
v
 
  sin θ cos θ  iq 3
iw 
 1 2  3 2 id  1 If  iq  0  , thenθ  0

PM

Fig.9 Rotor (Reverse)

・Position control(Z) ：d-axis current (id)・Rotation(θz)：q-axis current (iq)Transformation formula

Frame

N

Permanent
X Magnet (PM)

Y

49mm

70mm

S

Stator15
30
45
60
Rotation angle (˚)
Fig.7 Analysis result of torque

PMB
Magnet

26mm

Rotor

0

・Axial length：26mm (Fig.8)
・Rotor diameter ：49mm (Fig.9)
・Magnetic gap ：1.0mm

S・One three-phase inverter○Small size
・6-pole, 9-slot
・Coreless motor
Application

Passive Magnetic Bearing
(PMB)
Yoke

0

Test Model

・Realize of one-DOF actively positioned bearingless motor
only one three-phase inverter.

(Fig.2)0.0

Ave. 10mNm

50Magnetic bearing + Motor

Configuration ＆ Drive Principle

Ave. 0.9N

0.5

1021.5mmMotor

1.0

3σ (mrad)・One three-phase inverter
for motor

X

15

amplitude
Oscillation3σ
振動振幅
mrad

Y

Stator

1.5《Proposed model》

2.0
2.0

Total
Copper loss

1.5
1.5

消費電力 W・One single-phase inverter
for magnetic bearing

Coil

Torque (mNm)

Permanent
Magnet (PM)Power Consumption (W)・Position control : Z direction
One-DOF actively positioned
・Required number of inverter(Fig.1)・Rated current：0.25A・Rated suspension force ：0.9N(Fig.6)・Rated torque：10mNm (Fig.7)
Suspention force (N)Introduction

1.0

1.0

0.14W

0.5

0.5
0.0

0

00

500

1000
1500
2000
2500
3000
1000
2000
3000
回転速度 rpm
Rotation speed (rpm)Fig.13 Power consumption

・One-DOF actively positioned bearingless motor for cooling fan
is proposed.
・Rated suspension force is 0.9N, and rated torque is 10mNm.
・Axial, radial and tilting oscillation amplitude at 3000rpm are
4μm, 0.07mm, and 4.9mrad.

Fabrication of a three-phase votage source inverter and its drive
Y. Fujii, J. Asama(Shizuoka University, Japan)

Amplitude of modulation wave

Introduction

ES

Ec

Conventional AC motor control
Supply of AC by commercial power
×Variable frequency , ×Variable voltage

t
Vu

E/2

t

0
－E/2

Fig.5. small amplitude of modulation wave

Solution

Smaller amplitude of modulation wave than Fig.4.
Amplitude of approximation sinusoidal wave
Proportion to amplitude of modulation wave

Inverter control by PWM(pulse width modulation) control
Variable voltage Variable frequency(VVVF) control
Performance of inverter
Important fabrication of drive circuit

Principle of Drive Circuit

Goal of ResearchControl Circuit

IsolationPWM Signal

5V
0V

Fabrication of three-phase voltage source inverter
and its drive circuit

390

GND2

Outline of Inverter

1
2
3
4

8
0.1F
V 7
V 6
GND 5

Tr3

Tr5

Tr4

iw

Tr6

Drive Circuit

PWM Signal

Fig.1. Outline of Inverter

Tr2

Tr4

E/2

C

G

0

10k

VGE

E

GND1

 15V

7
V0 6
GND 5
V0

AMP

Interior of TLP250

OFF Voltage

IGBT ON

IGBT OFF

ⅰ. PWM ON signal
ⅱ. 6 and 7 pin Vo=15V
ⅲ. VGE=15V→IGBT ON
ⅳ. Start ICE

ⅰ. PWM OFF signal
ⅱ. 6 and 7 pin Vo=-15V
ⅲ. VGE=-15V→IGBT OFF
ⅳ. Stop ICE

Control Circuit

ON Voltage

IsolationPWM Signal

5V
0V

390

GND2

1
2
3
4

 15V

8
0.1F
V 7
V 6
GND 5

Out

DC-DC
Converter

VCC

5V
In

iCE
RG

0

Gate resistance0

Photocoupler
TLP250PWM(Pulse width modulation) Control

E/2

Gate resistance

VCC 8

1
IGBT
2
3
4

OFF Voltage

C

G

16

LS

VGE
VL

10

Wiring inductance

10F

Stabilization
Power

iL

6V

VGE E

10k

 15V

Stabilizaion
power

Voltage V

Forward Convert Circuit: rectifier from AC to DC
Reverse Convert Circuit：from DC to AC
Control Circuit :transmission of PWM signal
Drive Circuit：switching semiconductor device

Tr5

Stabilizaion
power

Experiment of chopper circuit

Principle of behavior

Tr3

In

Control circuit: low electric system
Isolation by photocoupler
Drive circuit: high electric system
15V or-15V output by DC-DC converter

iu
iv

Tr2

DC-DC
Converter

Fig.6. Principle of drive circuit

Reverse convert circuit
DC→AC

Tr1

Out

iCE

0

Photocoupler
TLP250Forward convert circuit
AC→DC

Tr1

 15V

5V

VCC

Inverter Main CircuitControl Circuit

ON Voltage

4
-2
-8
-14

GND1

off
Load

VL

-20
-0.15

Fig.7. Experiment of chopper circuit

-0.1

-0.05

0

Time ms

0.05

Fig.8. Response of VL<Example>Confirmed response of VL(Fig.8)
Vu
focus
on
U-phaseOptimizationofSurge
voltageandswitchspeediuivTr1=ON Vu=E/2
iw
Tr2=ONVu=-E/2
RG=15.6Ω,Shorter
lengthof
L
S,Insertion of capacitance
Vv
Vw
Vu

Tr6

E/2

Tr1ON Tr2ON

Tr1ON

0

Fig.2.Variable Load voltage by switching

Tr2ON

Fabrication of Drive circuit

Tr1ON

t

Fabrication of drive circuit by optimized value ofchopper experiment(Fig.9)
Check of fabricated inverter-E/2

Approximation sinusoidal wave by PWMComparison of carrier wave and modulation wave incontrol circuitIn case of Modulation wave =Sinusoidal modulation wave
Output of approximation sinusoidal wave
Sinusoidal wave es

Load current A

＋

0.15

PWM signal

Sinusoidal modulation wave esComparator

Carrier wave ec

0.1

0.05
0
-0.05

-0.1
-0.15
0

Carrier wave ec

Ec

－

ES

t

Fig.3. Carrier wave comparisones>ec→Vu=E/2
es<ec→Vu=-E/2

Vu

E/2

t
0
－E/2

Approximation sinusoidal wave

Fig.4. Principle of PWM control

Fig.9. Fabricated Drive Circuit

0.01

0.02

Time s0.03

0.04

Fig.10. Confirmed Three Phase Wave

Conclusion
Fabrication of inverter and its drive circuitConfirmed Three-phase wave

0.1