Novel Single-Drive Bearingless Motor with Wide Magnetic Gap

and High Passive Stiffness

Hiroya Sugimoto Seiyu Tanaka Akira Chiba

Tokyo Institute of Technology

1

14PESGM2609 Wednesday, July, 30, 2014

Advantages of the bearingless motors

No lubricant Maintenance-free No wear

No contact

Highly pure water pump in semiconductor industries

Non polluting

Possible Applications

Thus, cost reduction is strongly required.

High speed motor such as a compressor

Cooling fan requiring long life-time

Reducing number of active positioning axes is the most effective.

Magnetic suspension cost is high!

http://www.levitronix.com/

Significant issues for industry application

1-1. Background

Bearingless pump

1

1-2. A number of active positioning axes

One-axis actively positioned bearingless motor can realize to reduce four active positioning axes.

xy

5-axis active positioning

2-axis active positioning

1-axis active positioning

z

x1

y 1

x2

y 2

Thrust magnetic bearing

Bearingless motor units

z

Bearingless motor

Passive magnetic bearing

2

Bearingless motor

A single-drive bearingless motor is focused.

Single-drive bearingless motor needs only one three-phase inverter and one displacement sensor for the suspension and motor regulations.

The number of active positioning

axes

5 2 1 : Thrust magnetic

bearing motor

1 : Single-drive bearingless

motor

Required number of

units

six-IGBT inverter

Sus.:2, Mot.:1

Sus.:1, Mot.:1

Mot.:1 Mot. & Sus.:1

four-IGBT inverter

Sus.:1 0 Sus.:1 0

displacement sensor 5 2 1 1

PM cost ratio $6 $6 $13 $13

Cost ratio 1 0.46 0.35 0.28

1-3. Advantages of 1-axis bearingless motor 3

1-axis magnetic bearing motor

1-axis magnetic bearing motor

z

3-phase Inverter

Single-drive bearingless motor (SDBM)

Single-drive bearingless motor (SDBM)

Two functions of the motor and the thrust magnetic bearing are magnetically integrated in the single motor.

1-phase Inverter

Thrust magnetic bearing

Fz T

Controller

3-phase Inverter

Controller

Fz

T

z

Motor Beaingless motor with thrust magnetic bearing function

1-4. Single-drive bearingless motor concept 4

Important issues in the single-drive bearingless motor are presented.

A novel single-drive bearingless motor with wide magnetic gap and high passive stiffness is proposed.

The 3D-FEM analyses and experiment results are presented.

2. Purpose 5

High passive stiffness for reducing vibrations in the radial and tilting directions

Wide magnetic gap for centrifugal pump applications

Univ. of São Paulo-USP

Toyama Univ.

Proposed SDBM

National Taipei Univ. of Tech.

Shizuoka Univ.

Johannes Kepler Univ.

CalculatedMeasured

Gap factor, g / R0.0 0.1 0.2

20

10

0

30

Rad

ial

stif

fnes

s, k

r(N

/mm

)

40

0

10

20

30

40

0 0.1 0.2

Rad

ial

stif

fnes

s, k

r(N

/mm

)

Gap factor, g/r

in 2014

in 2011 in 2013

in 2011 in 2013

in 2009

3-1. Important research issues 6

Increasing permanent magnets and the number of layers are effective to enhance the passive stiffness.

Stator Rotor PMWinding Displacement sensor

RPMB RPMBSDBM

Controller

3-phase Inverter

Hall sensor

z

x

y

id*, iq

*

z

q

3-2. Proposed structure

N N S S

N N S S N N S S

N N S S

7

Stator Rotor permanent

magnet

Winding Displacement sensor

Passive magnetic bearing

Passive magnetic bearing

Single-drive bearingless motor

Inlet

Outlet

Impeller

Controller

3-phase Inverter

Hall sensor

z

x

y

8 3-3. Requirement of the wide magnetic gap Plastic can must be installed on the rotor and stator surfaces to avoid the damage from chemical fluid.

Rotor can

Stator Rotor PMWinding Displacement sensor

RPMB RPMBSDBM

Controller

3-phase Inverter

Hall sensor

z

x

y

Active axial force to overcome the unstable thrust force must be generated for stable magnetic suspension.

3-4. Unstable thrust force 9

Current direction

Coil-end

Permanent magnet flux

z

S

Fz,L

f1

0

U

x

U’

N

V’

WV

W’

y

S

N

28

Active axial force is generated in one side of the coil-end by a shifting rotor in the axial direction.

3-5. Initial idea of the SDBM structure 10

Structure (a)

0

5

10

15

0 1 2 3 4 5

Ax

ial

susp

ensi

on

fo

rce,

Fz

(N)

d axis current, id (A)

Target value 7 N

0d axis current, id (A)

Axia

l su

spen

sio

n f

orc

e,

Fz

(N)

1 2 3 4 50

5

10

15

(a)

Coil ends

Rotor PM

N

S

zFz

3-6. Active axial force in the initial structure 11

Unfortunately, the axial force of structure (a) is quite low.

Several structures have been investigated.

Target value 6.28 N

Act

ive

axia

l fo

rce,

Fzi

(N

)

3-7. Investigation of several structures 12

Rotor

Stator core

Winding

Current direction

d-axis

q-axis

x

y

U-coil

U-coil

S

N

z N

S

Fz,L

S

NS

U

U’

W’ V’

V W

N

x

y

A BN

S

FzM

N

S

FzL

Suspension flux

Permanent magnet flux

Rotor

Stator bars

z z

Stator yokes

Current direction

Front

Structure (b)

Structure (c)

0

5

10

15

0 1 2 3 4 5

Ax

ial

susp

ensi

on

fo

rce,

Fz

(N)

d axis current, id (A)

0d axis current, id (A)

Ax

ial

susp

ensi

on

fo

rce

Fzi

(N)

1 2 3 4 50

5

10

15

(a)(b)

(c)

(d)

Target value 6.28 NC

on

tin

uou

s ra

tin

g

Tw

ice

13 3-8. Principle of active axial force generation

The active axial force is increased although structures (a) – (c) can’t achieve the target axial force.

A novel structure without magnetic saturation is proposed.

Act

ive

axia

l fo

rce,

Fzi

(N

)

The stator core is constructed with six bars. The rotor is two-pole cylindrical permanent magnet.

The permanent magnet flux is distributed sinusoidal wave between the rotor and stator cores.

RotorStator core

WindingCurrent

direction

d-axis

q-axis

x

y

S N

3-9. Proposed stator and rotor structures 14

N

S

Fz

Suspension flux

Permanent magnet flux

Rotor

Stator

Flux strengthening

Flux weakening

Flux strengthening

Flux weakening

z

When the suspension flux is superimposed on the PM flux, flux density in the air-gap in positive z-direction is high compared with that in negative z-direction.

3-10. Principle of active axial force generation 15

Stator core

Windingy

x

d-axis

q-axisCurrent

directions

it

UU

V

VW

W

V-shaped winding structure

izit

Stator core

Winding

izit

fz

Virtual two-pole concentrated winding is generated by novel V-shaped winding structure.

Fold angle f is key parameter for torque generation.

3-11. Principle of torque generation 16

Rotor PMRPMB RPMB

Unit : cm

Displacement sensor

Windings

Housing

4-1. Test machine 17

z

20°

RPMB RPMBTwo-pole

PM

PM for detecting angular position

Prototype machine with the rotor and divided cores

Rotor One of the divided cores with V-shaped winding

Rotor

shaft

4-2. Demonstration movie 18

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.00 0.02 0.04 0.06 0.08 0.10

Axia

l dis

pla

cem

ent,

z

(mm

)

Time (s)

-4

-2

0

2

4

0.00 0.02 0.04 0.06 0.08 0.10Curr

ent,

id

and i

q(A

)

Time (s)

Axia

l d

isp

lace

men

t,

z (m

m)

-0.2

0.2

0.1

0.0

-0.1

0.00 0.02

0.3

-2

-4

0

2

4-0.3

Cu

rren

t,

i d, i q

(A)

0.04 0.06 0.08 0.10Time (s)

id

iq

Suspension control is activated

Touch down length ±0.2 mm

id= -2.98 A

The rotor is served to the reference position after the d-axis current is excited.

4-3. Test machine 19

-3

0

3

0.0 1.0 2.0 3.0

-0.2

0.0

0.2

0.0 1.0 2.0 3.0

0

2000

4000

0.0 1.0 2.0 3.00 1 2 30

0

0.0

2

4

3-0.2

0.2

w(1

03

r/m

in)

i d(A

)z

(mm

)

3600 r/min

± 0.95 A

± 22 mm

Time (s)

-3

Static magnetic suspension Acceleration up to 3600 r/min

The regulations of the axial position and the rotational speed are realized in the experiment.

4-4. Test machine 20

A novel single-drive bearingless motor with wide magnetic gap and high passive stiffness is proposed.

Design process of the proposed single-drive bearingless motor is shown.

In the experiments, stable magnetic suspension and speed regulations are confirmed.

5. Conclusion 21