DESIGN OF AN OUTER-ROTOR

BRUSHLESS DC MOTOR FOR CONTROL

MOMENT GYROSCOPE APPLICATIONS

MIDDLE EAST TECHNICAL UNIVERSITY

ELECTRICAL AND ELECTRONICS

ENGINEERING DEPARTMENT

MS THESIS PRESENTATION

06/02/2015

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OUTLINE

ACTUATING SYSTEMS IN LEO SATELLITES AND

TYPICAL ELECTRICAL MOTORS

PROBLEM DEFINITON AND MOTOR SELECTION

DESIGN PROCEDURE

ANALYTICAL RESULTS

FINITE ELEMENT METHOD RESULTS

CONCLUSION AND FUTURE WORK

2

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REVIEW OF THE THESIS

AIM: Replacing current BLDC electric motor

with an optimum motor topology

METHOD: Classical Electrical machine

design procedure

PROBLEM: Mass, volume and efficiency

3

2

ACTUATING SYSTEMS IN LEO

SATELLITES AND TYPICAL

ELECTRICAL MOTORS

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LOW-EARTH ORBIT SATELLITES

LEO satellites are used for taking images of the

Earth ground.

In order to take proper images, these systems must have a

sensitive attitude control system.

5

5

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ACTUATING SYSTEMS

Three main actuating systems in LEO satellites

o Reaction wheels

o Momentum wheels

o Control moment gyroscopes (CMG)

Control Moment Gyroscope 6

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CONTROL MOMENT GYROSCOPES

Two motors are used

o Wheel motor

o Gimbal motor

Motor rotation axes are

perpendicular to each

other

Necessary torque is

produced along the third

axis

Design of the motor for

wheel axis is investigated7

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MOTOR TYPES FOR DIFFERENT

APPLICATIONS

8

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BLDC MOTORS

Excited with balanced three-phase windings

Coils wound in stator

Rotor with PM

Torque produced due to interaction of coils and

magnetic field

9

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BLDC MOTORS – CLASSIFICATION (1)

Permanent Magnet structure

Surface-mounted Interior-mounted Buried magnet

10

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BLDC MOTORS – CLASSIFICATION (2)

Flux Direction

Radial Flux Axial Flux

11

Stator

Winding

Rotor

Permanent Magnet

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BLDC MOTORS – CLASSIFICATION (3)

Excitations

Squarewave Sinusoidal

12

0 5 10 15 20 25 30 35 40-1

-0.5

0

0.5

1

Time (s)

Curr

ent

(A)

Typical Current Waveforms in Sinusoidal Excitation

Phase A

Phase B

Phase C

0 5 10 15 20 25 30 35 40-1

-0.5

0

0.5

1

Time (s)

Curr

ent

(A)

Typical Current Waveforms in Squarewave Excitation

Phase A

Phase B

Phase C

13

PROBLEM DEFINITION AND MOTOR

SELECTION

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PROBLEMS IN EXISTING SYSTEM

2 main problems arise:

Compatibility issues

• Over-safe and over-designed motors for CMG’s

Sausage-type commercial designs

• Low inertia contribution to wheel, high inertia

contribution to gimbal

• External coupling

• 4 Bearings – Balance problems

14

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FUNCTIONAL REQUIREMENTS

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TORQUE – SPEED CHARACTERISTICS

16

Torque-speed characteristics is taken from Yılmaz’s thesis in

order to provide a comparison.

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FUNDAMENTAL CONCEPTS

Magnetic circuit

Puts forward the characteristics of the motor

Shows the operating point

Can be best characterized by the concentration

factor and permeance coefficient

17

Change the magnet or the air gap so that desired air gap flux

density is obtained

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MATERIAL SELECTION -

FERROMAGNETIC MATERIALS

Ferromagnetic Materials;

Material used in construction of stator and rotor

Characterized by its B-H curve

Loss characteristics are crucial

• Hysteresis losses

• Eddy current losses

18

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SELECTED FERROMAGNETIC

MATERIAL

COGENT Power No.12

19

Wide operating temperatures up to 230 °C

Operate at high frequencies

Manufacturable in thin steels

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MATERIAL SELECTION

Permanent magnets (PM)

Characterized by demagnetization curve

Three main magnetic parameters:

• Remenant flux density

• Coercivity

• Recoil Relative Permeability

Selection criteria:

• Wide operating temperature

• Resistance against corrosion

• Radiation sensitivity20

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MATERIAL SELECTION - PM

VACOMAX 225 HR (Samarium - Cobalt)

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22

DESIGN PROCEDURE

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OUTER-ROTOR BLDC MOTOR

23

Operate at constant speed : 10000 rpm

Steady – state torque: 32 mN-m

Electrical loading :6000 A-t/m

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OUTER-ROTOR BLDC MOTOR

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Main Topologies

Sinusoidal Squarewave

2-pole 2-pole

6-pole 6-pole

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DESIGN PROCEDURE

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TORQUE EQUATION DEPENDING ON THE EXCITATION IN TERMS OF MAIN SIZES

OPTIMUM MOTOR DIMENSIONS

WINDING DESIGN

EQUIVALENT CIRCUIT PARAMETERS

PERFORMANCE EVALUATION

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BASIC EQUATIONS

26

Torque expressions obtained for design

process, for both RF and AF topologies

AF motor torque expression shown for

completeness

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DESIGN PROCEDURE

27

Torque equation is taken depending on

excitation

Electrical loading is defined

Three main unknowns: ℎ𝑠 slot depth, 𝐷𝑖 inner

diameter and L axial length.

𝑅𝐷𝐿 =𝐷𝑖

𝐿ratio is taken as a parameter

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DESIGN PROCEDURE

28

𝑅𝐷𝐿 =𝐷𝑖𝐿

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DESIGN OUTPUTS

Script codes are prepared in MATLAB

Ratio changed from 0.7 to 20 with uniform steps

Parameters are extracted

Changes depending on pole numbers and

excitations are stated

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COMPARISONS BETWEEN POLE

NUMBERS - VOLUME

30

0 2 4 6 8 10 12 14 16 18 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1x 10

-4

Ratio

Volu

me (

m3)

Sine 2-pole

Sine 6-pole

0 2 4 6 8 10 12 14 16 18 201

2

3

4

5

6

7x 10

-5

RatioV

olu

me (

m3)

Square 2-pole

Square 6-pole

Both 6-pole designs have smaller volume

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COMPARISONS BETWEEN POLE

NUMBERS - MASS

31

Both 6-pole designs have smaller mass

0 2 4 6 8 10 12 14 16 18 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ratio

Mass (

kg)

Sine 2-pole

Sine 6-pole

0 2 4 6 8 10 12 14 16 18 200.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Ratio

Mass (

kg)

Square 2-pole

Square 6-pole

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COMPARISONS BETWEEN POLE

NUMBERS - INERTIA

32

Both 6-pole designs have smaller inertia

0 2 4 6 8 10 12 14 16 18 200

0.5

1

1.5

2

2.5

3

3.5x 10

-4

Ratio

Inert

ia (

kg.m

2)

Sine 2-pole

Sine 6-pole

0 2 4 6 8 10 12 14 16 18 200

1

x 10-4

RatioIn

ert

ia (

kg.m

2)

Square 2-pole

Square 6-pole

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COMPARISONS BETWEEN POLE

NUMBERS - EFFICIENCY

33

Both 6-pole designs show greater efficiency

characterictics

0 2 4 6 8 10 12 14 16 18 200.89

0.9

0.91

0.92

0.93

0.94

0.95

Ratio

Eff

icie

ncy

Sine 2-pole

Sine 6-pole

0 2 4 6 8 10 12 14 16 18 200.88

0.89

0.9

0.91

0.92

0.93

0.94

0.95

Ratio

Eff

icie

ncy

Square 2-pole

Square 6-pole

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COMPARISONS BETWEEN

EXCITATIONS - VOLUME

34

Sinusoidal larger volume in 2-pole, squarewave larger

volume in 6-pole

0 2 4 6 8 10 12 14 16 18 202

3

4

5

6

7

8

9

10x 10

-5

Ratio

Volu

me (

m3)

Sine 2-pole

Square 2-pole

0 2 4 6 8 10 12 14 16 18 200.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6x 10

-5

RatioV

olu

me (

m3)

Sine 6-pole

Square 6-pole

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COMPARISONS BETWEEN

EXCITATIONS - MASS

35

Sinusoidal heavier in mass in 2-pole, squarewave heavier

in mass in 6-pole

0 2 4 6 8 10 12 14 16 18 20

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Ratio

Mass (

kg)

Sine 2-pole

Square 2-pole

0 2 4 6 8 10 12 14 16 18 200.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

RatioM

ass (

kg)

Sine 6-pole

Square 6-pole

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COMPARISONS BETWEEN

EXCITATIONS - INERTIA

36

Sinusoidal larger inertia in 2-pole, squarewave larger inertia in 6-pole

0 2 4 6 8 10 12 14 16 18 201.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2x 10

-4

Ratio

Inert

ia (

kg.m

2)

Sine 2-pole

Square 2-pole

0 2 4 6 8 10 12 14 16 18 201

1.5

2

2.5

3

3.5

4

4.5x 10

-5

RatioIn

ert

ia (

kg.m

2)

Sine 6-pole

Square 6-pole

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COMPARISONS BETWEEN

EXCITATIONS - EFFICIENCY

37

Sine is efficient in 2-pole, square is efficient in 6-pole

0 2 4 6 8 10 12 14 16 18 200.88

0.89

0.9

0.91

0.92

0.93

0.94

Ratio

Effic

iency

Sine 2-pole

Square 2-pole

0 2 4 6 8 10 12 14 16 18 200.89

0.9

0.91

0.92

0.93

0.94

0.95

Ratio

Effic

iency

Sine 6-pole

Square 6-pole

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COMMON FINDINGS

As the ratio increases;

Reduce in mass, volume and inertia

Reduce in inertia contribution

Increase in phase resistance and phase

inductance

Almost constant RMS current

Increase in electrical loading38

39

ANALYTICAL RESULTS

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ANALYTICAL RESULTS

Designs are chosen according to the ratio at which

both magnetic loading and electrical loading

become 0.43T, and 6000 A-t/m, respectively, for

ease of performance comparison

Performance parameters are presented.

40

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ANALYTICAL RESULTS

41

0 2 4 6 8 10 12 14 16 18 202000

4000

6000

8000

10000

12000

14000

Ratio (RDL

)

q (

A-t

/m)

sin 2-pole

sin 6-pole

square 2-pole

square 6-pole

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ANALYTICAL RESULTS

42

R_DL 17 0,95 10 1,5

D_i (mm) 57,39 22 47,33 25,03

L (mm) 3,38 23,16 4,73 16,69

D_o (mm) 107,07 32,62 85,97 39,33

g (mm) 0,75 0,75 0,75 0,75

h_s (mm) 4,66 5,75 4,70 5,50

l_m (mm) 2,55 1,81 2,10 1,88

h_1 (mm) 1,00 1,00 1,00 1,00

h_2 (mm) 1 1 1 1

w_1 (mm) 0,75 0,75 0,75 0,75

w_2 (mm) 4,20 0,92 3,31 1,22

h_sbc (mm) 21,54 2,75 16,46 4,52

t_1 (mm) 9,27 3,09 7,51 3,62

t_2 (mm) 4,19 0,92 3,31 1,22

J (g.m 2̂) 0,29 0,01 0,17 0,02

V (cm 3̂) 30,39 19,36 27,47 20,27

M_total (g) 210,33 97,11 186,12 113,22

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ANALYTICAL RESULTS –

SINE 2-POLE

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ANALYTICAL RESULTS –

SINE 6-POLE

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ANALYTICAL RESULTS –

SQUARE 2-POLE

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ANALYTICAL RESULTS –

SQUARE 6-POLE

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ANALYTICAL RESULTS –

OVERALL PERFORMANCE

47

Parameter Sine 2-pole Sine 6-pole Square2-pole Square6-pole

N_phase 68 26 56 30

R_ph (mOhm) 327,45 82,73 238,38 80,31

L_g (uH) 142,25 181,02 113,31 191,20

L_end (uH) 36,07 1,28 41,22 2,08

L_leak (uH) 110,27 242,58 114,38 193,52

L_ph (uH) 288,58 424,88 268,92 386,79

I_rms (A) 2,65 2,63 2,62 2,63

q (A.t/m) 5993,90 5945,20 5929,94 6010,52

Efficiency 90,32 93,40 90,30 93,39

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FINITE-ELEMENT METHOD RESULTS

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SIMULATIONS

Simulations are carried out in ANSYS MAXWELL

Meshes are automatically assigned by MAXWELL

Boundaries are assigned so that no flux out of motor.

Models have been run in transient manner

All materials are assigned considering nonlinear

characteristics (B-H curves etc.)

Results are presented based on steady-state

characteristics 49

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SINUSOIDAL 2-POLE

50

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SINUSOIDAL 6-POLE

51

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SQUAREWAVE 2-POLE

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SQUAREWAVE 6-POLE

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FEM RESULTS - OVERALL

54

Sine 2-pole Sine 6-pole Square2-pole Square6-pole

ToothFlux

Density1.5 T 1.5 T 1.7 T 1.4 T

Torque 32 mN-m 42 mN-m 42 mN-m 31.8 mN-m

Winding Current 4.25 A 2.06 A 2.94 A 2.00 A

InducedVoltage 17 V 16.3 V 13 V 15 V

Torque Ripple 20 mN-m 20 mN-m 5 mN-m 25 mN-m

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COMPARISON WITH AF EQUIVALENT

55

AF 6-pole BLDC Motor designed by Yılmaz.

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COMPARISON WITH AF EQUIVALENT

56

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FEM RESULTS - OVERALL

57

Sine 2-pole Sine 6-pole Square2-pole Square6-pole AF 6-pole

ToothFlux

Density1.5 T 1.5 T 1.7 T 1.4 T 1.4 T

Torque 32 mN-m 42 mN-m 42 mN-m 31.8 mN-m 32 mN-m

Winding Current 4.25 A 2.06 A 2.94 A 2.00 A 2.00 A

InducedVoltage 17 V 16.3 V 13 V 15 V 15 V

Torque Ripple 20 mN-m 20 mN-m 5 mN-m 25 mN-m 13 mN-m

58

CONCLUSION AND FUTURE WORK

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OVERALL RESULTS

Mass Results

59

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

0.05

0.1

0.15

0.2

0.25

Mass (

kg)

(*) AF design is 0.1 kg

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OVERALL RESULTS

Torque/ Mass Results

60

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

0.1

0.2

0.3

0.4

0.5

0.6

0.7T

orq

ue (

N.m

/kg)

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OVERALL RESULTS

Volume

61

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

0.5

1

1.5

2

2.5

3

3.5x 10

-5V

olu

me (

m3)

(*) AF design is 1.89e-5 m^2

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OVERALL RESULTS

Torque/Volume Results

62

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

500

1000

1500

2000

2500

3000

Torq

ue (

N.m

/m3)

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OVERALL RESULTS

Efficiency Results

63Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole

0

10

20

30

40

50

60

70

80

90

100

Eff

icie

ncy in %

(*) AF design is 0.92

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OVERALL RESULTS

Inertia results

64

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

1

2

x 10-4

Inert

ia (

kg.m

2)

(*) AF design is 0.3e-4 kg-m^2

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OVERALL RESULTS

Inertia contributon results

65

Sine 2-pole Sine 6-pole Square 2-pole Square 6-pole0

10

20

30

40

50

60

70In

ert

ia C

ontr

ibution in %

(*) AF design is about 5.6%

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FUTURE WORK

Temperature characteristics and cooling techniques

must be studied

Proper integration to the system must be studied

Mechanical considerations must be taken into

account

Studies can be carried out in order to further

minimize the torque ripples

66

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THANK YOU FOR LISTENING

QUESTIONS

67

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COMMERCIAL

CONTROL MOMENT GYROSCOPE

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SIMULATIONS

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