Motor Generator p25

Glenn Research Center

HIGH SPEED PERMANENT MAGNET SYNCHRONOUS MOTOR / GENERATOR DESIGN FOR FLYWHEEL APPLICATIONS

Aleksandr Nagorny, Ph.D. National Research Council


• • • • • • • • • • • • • • • •

Outline Introduction Sel elec ecti tion on of the Rat ated ed Poi oint nt The maj major or requi requirem rement ents s for for flyw flywhee heell M/G M/G in spac space e applic applicati ations ons Mee eeti ting ng des desiign req requi uire reme ment nts s M/G M/ G Ma Main in Ma Mate teri rial als s Se Sele lect ctio ion n Lami La mina nati tion on Th Thic ickn knes ess s Sel Selec ecti tion on Modeling tools M/G M/ G Pre Preli limi mina nary ry De Desi sign gn Pr Proc oces ess s Perm Pe rman anen entt Magn Magnet et Mat Mater eria iall Sele Select ctio ion n The Th e RMxp RMxprt rt Ana Analy lyti tica call Des Desig ign n Outp Output ut Armatu Arm ature re Rea Reacti ction on and Dem Demagn agneti etizat zation ion Cal Calcul culati ation on M/G M/ G 2D Fi Fini nite te El Elem emen entt Mo Mode deli ling ng M/G M/ G 3D Fi Fini nite te El Elem emen entt Mo Mode deli ling ng Conc Co nclu ludi ding ng Obse Observ rvat atio ions ns and and Reco Recomm mmen enda dati tion ons s References Acknowledgements


Introduction

•

The mot The motor or / ge gene nerat rator or des desig ign n is a part part of wor work k performed at NASA Glenn Research Center devoted to the development of flywheel modules for use in satellite energy storage and attitude control systems.

•

The ben benefi efits ts of fly flywhe wheel el mod module ule as an ene energy rgy st stora orage ge device in spacecraft application compared to the chemical batteries are the following: higher energy and power densities, deeper depth of discharge, broader operating temperature range.

•

Flywhe Flyw heel el can can be be used used as as a sou sourc rce e of mom momen entu tum m for for attitude control, giving the opportunity to combine two satellite subsystems into one and reduce the overall volume and mass.


Selection of the Rated Point

•

Determination of the output power

ü

The output power determination is based on the load profiles during the load cycles.

ü

From t=0 to t=60 min it is a charge (motor mode), from t=60 to t=90 min it is a discharge (generator mode). In each point the net torque of M/G is equal

Flywheel Combined Energy Storage and Attitude Control Output Power Determination

1

6000 4000

0.5 2000 m 0 N , 0 e u q r o T -0.5

20

40

60

80

-4000

-1 -6000

Tnet=TES+T AC Where TES is the torque component required for the energy storage, and T AC is the component required for the attitude control.

W , r e 0 w 100 o P t -2000 u p t u O

-1.5

-8000 Time of Cycle, min

Torque Tes

Net Torque

Torque ac

Output Power


The major requirements for flywheel M/G in space applications

•

Relatively high electrical frequency of voltages and currents

•

High specific power

•

High efficiency, low total losses

•

Low THD at the back emf waveform

•

Low cogging torque values

•

Low rotor losses

•

High thermal endurance, ability to operate in vacuum without intensive cooling


Meeting design requirements

• • •

• • •

High specific power: Right selection of the M/G configuration Application of high magnetic energy permanent magnet materials Application of high permeability core lamination materials High efficiency, low total losses: Choice of an AC permanent magnet synchronous machine with the zero fundamental frequency rotor magnetic and conductive losses Application of core lamination materials with low specific losses Application of thin diameter stranded wires for the stator armature conductors to reduce the high frequency skin effect losses


Meeting design requirements (cont’d)

Low THD at the back emf waveform and low cogging torque value. Could be achieved by the reducing the high frequency spatial mmf harmonics by the following methods: • Make the magnet pole arcs short pitched • Two layer short pitched stator winding • Skewing of the stator core in axial direction • Relatively large non-magnetic gap; • Small value for slot opening to slot pitch ratio • Special shape for the stator teeth (dummy slots) • Lamination of permanent magnets in axial direction


Meeting design requirements (cont’d) Low value of the rotor losses:

•

The main components of the rotor losses :

ü

Back iron loss;

ü

Eddy current loss in permanent magnet material;

ü

M/G carbon fiber sleeve loss;

ü

Windage loss. The first three components of the rotor losses are caused by the high frequency spatial mmf harmonics. The measures to reduce them are the same as described on previous slide. The effective measure against the back iron losses is lamination of the back iron core.


Meeting design requirements (cont’d) High thermal endurance:

•

High thermal endurance can be achieved by using the appropriate materials for the stator and rotor parts:

ü

Stator and rotor core lamination materials

ü

Wire insulation

ü

Slot insulation

ü

Winding parts insulation

ü

Motor leads insulation

ü

Permanent magnet materials

ü

Rotor carbon fiber composite. For the current M/G design all the materials except carbon fiber composite have the rated temperature above 200 °C.


M/G Main Materials Selection

Using of prospective materials The major motor materials that can affect the motor performance are the following •

core ferromagnetic materials

•

permanent magnet materials

•

magnet wires

•

winding insulation Core ferromagnetic materials

•

Two major characteristics of the core ferromagnetic materials can affect the motor performance

ü

the maximum saturation flux density

ü

specific losses


Modeling tools

Ansoft Corporation RMxprt software is used for the preliminary motor design. The advantages of the RMxprt software are the following: •

The ability to get an easy and fast response in a convenient form

•

The output data can be easily exported to other Ansoft software (Maxwell 2D, Simplorer)

•

The program can perform optimization of the input parameters


M/G Preliminary Design Process

The numerous design iterations were completed to meet the motor requirements in motor and generator mode. Different rotor configurations, permanent magnet and core materials and M/G geometry were optimized using the RMxprt parametric analysis mode.

The highest level of the output power in a combination of relatively low back EMF THD level was obtained for the surface mounted arc shaped magnet configuration

Permanent Magnet Material Selection


M/G Output Power in Generator Mode for different PM Materials 9000 7779.15

8000 7000 6075.79

6000

6387.41

5336.96

5000 W 4000 3000 2000 1000 0 SmCo 24

SmCo 28

NdFe 30

NdFe 38

Type of Magnet Material

NdFe group has higher remanent magnetization and energy product. SmCo has a better thermal characteristic.

Lamination Thickness Selection


Specific Iron Loss versus Lamination Thickness for High Saturation Cobalt Iron Alloy at 1200 Hz and 2 T 250

y = 539416x2 + 5783.6x + 13.285

, s 200 s o L g 150 k c / i f i W 100 c e 50 p S

R2 = 1

0

0

0.0025

0.005 0.0075

0.01

0.0125 0.015

Lamination Thickness, Inch

•

At high frequencies (1.2 kHz) and high flux densities (2.0 T) the specific iron loss is proportional to the square of lamination thickness. For the reduction of the iron loss, the lamination thickness should have a low value (in our case 0.004’’).


The RMxprt Analytical Design Output

STATOR DATA Number of Stator Slots:

24

Rated Voltage (V):

61

61

Number of Poles:

4

4

1666.67

1666.67

115

115

THD of Induced Voltage (%):

0.964

0.964

Cogging Torque (N.m):

0.016

0.015

Line Current RMS (A):

72.594

72.016

Efficiency (%):

98.168

98.164

Synchronous Speed (rpm):

50000

50000

0.27

Rated Torque (N.m):

1.452

1.482

SmCo

Total Net Weight (lb):

7.55

7.55

Inner Diameter of Stator (inch):

2.9

Number of Conductors per Slot:

0.728 6

Frequency (Hz): Operating Temperature (C):

ROTOR DATA Non Magnetic Gap, inch

0.105

Inner Diameter (inch):

1.51

Length of Rotor (inch):

0.728

Max. Thickness of Magnet (inch): Type of Magnet:

Generator Mode 7.6

5.75

Rated Output Power (kW):

Motor Mode 7.6

Outer Diameter of Stator (inch):

Length of Stator Core (inch):

M/G parameters at rated point

The RMxprt Analytical Design Output (cont’d)


Cogging Torque

Output Power Versus Torque Angle

0.02

30000

0.015

25000

0.01

20000 W , r 15000 e w10000 o P 5000

m N 0.005 , e 0 u q r -0.005 0 o T -0.01

50

100

150

200

250

300

350

400

0

-0.015

-5000 0

-0.02

25

50

75

Electric Angle, Degrees

100

125

150

175

200

Angle, Degrees

Efficiency Versus Torque Angle

Current Versus Torque Angle

100

600

80 % , y 60 c n e i c i f 40 f E 20

500 A 400 , t n e r 300 r u C 200 100 0

0 0

20

40

60

80

100

120

140

Torque Angle, Degrees

160

180

200

0

20

40

60

80

100

120

Torque Angle, Degrees

140

160

180

200

The RMxprt Analytical Design Output vs FEA


Flux Density Distribution in the air-gap. 0.002" from the surface of mannets FEA

Flux Density Distribution in the Air Gap (RMxprt) 0.8 0.8

0.6 T 0.4 , y t i 0.2 s n 0 e D x -0.2 0 u l F -0.4

0.6 0.4 0.2

50

100

150

200

250

300

350

400

, B

0 -0.2 0

1

-0.6

3

4

-0.6 -0.8

-0.8

Position in the air-gap, inch

Electrical Angle, Degrees

Flux density distribution in the air-gap, 0.002" from the stator bore FEA

Flux density distribution in the air-gap, 0.053" from the surface of magnets FEA

1.2 0.8

0.8 0.6

0.4

0.4

T , B

0.2 , B

2

-0.4

0 -0.2 0

1

2

-0.4

3

4

0 -0.4

0

1

2

3

-0.8

-0.6

-1.2

-0.8 Position in the air gap, inch

Position in the air gap, inch

FEA shows higher flux density in the air-gap than RMxprt

4

The RMxprt Analytical Design Output (cont’d) Glenn Research Center M/G Voltage

M/G Frequency 1800 1600 z 1400 H , 1200 y c 1000 n e 800 u q 600 e r F 400 200 0

70 60 50 V , e 40 g a t l 30 o V 20 10 0

10

20

30

40 50 Time, min

60

70

80

90

100

0

10

20

30

70

80

90

100

100

10

20

30

40

50

60

70

80

90

100

80 W , s 60 s o L 40 20 0 0

10

20

30

Time,min M/G Output Power

4000 3000

98.0 97.5 %97.0 , y 96.5 c n e 96.0 i c 95.5 i f 100 f E95.0 94.5 94.0 93.5

10

20

30

40

50

-3000 -4000

Time, min

60

40

50 60 Time. min

70

80

90

100

70

80

90

100

M/G Efficency 98.5

-5000

50 60 Time, min

120

5000

2000 W 1000 , r e 0 w o -1000 0 P -2000

40

M/G Total Loss

M/G Armature Current 40 30 20 10 A 0 , t-10 0 n-20 e r r-30 u-40 C -50 -60 -70 -80

0

70

80

90

0

10

20

30

40 Time, 50min 60

Study of M/G Characteristics During the Duty Cycle

Armature Reaction and Demagnetization Calculation




α B r



100

Br t = Br s 1 +



 α  Hc t = Hc s 1 + Hc (ϑ t − ϑ s )  100 

(ϑ t − ϑ s ) 

Samarium Cobalt (Sintered) S2769 Demagnetization Lines at Different Temperatures 1.2

1 20°C 200°C 0.8

T , r B

0.6

0.4

0.2

0 -900000

-800000

-700000

-600000

-500000

-400000

-300000

-200000

-100000

0

Hc, A/m

•

Permanent magnet demagnetization curves are sensitive to the temperature

Armature Reaction and Demagnetization Calculation (cont’d)


•

The common method to check the demagnetization of the permanent magnets due to the armature reaction is described in T.J. Miller's book [1]. The disadvantage of this method is the assumption that the permanent magnet pole has uniform saturation. Permanent Magnet Demagnetization Curve h m =.300 inch

Fa

Φm

1.2 1.1

Φr

Φg

ΦL Pm0

0.9043

Rg

0.9 0.8 0.7

0.7512 T , B

R L

1.10

1

0.6 0.5 0.4 0.3 0.2 0.1 0.00 -900000

-800000

0 -700000

-600000

-500000

-400000

-300000

-200000

0.0000 0

-100000

0

H,A/m

R g =

Φ r = Br A M PC = µ rec

1 + P r 1 R g P m 0 R g

Bma =

µ0 µrec F dem l m

g ′ µ0 A g

Bm =

Bload = Bm − Bma

1 + P r 1 R g (1 + P m R g )

Br

− H m =

B g =

Br − Bm µ0 µ rec

C Φ (1 + P m R g )

P mo =

C Φ =

Br

µ0 µrec Am l m

Am A g



A more accurate way to check the demagnetization is FEA. •

Using Maxwell 2D Transient solver, the solution for the rated load should be determined. The relative rotor-stator position and instantaneous values of the phase currents are determined.

•

Then, using position and values obtained by transient solver, the Maxwell 2D Magnetostatic model can be created and the flux density distribution in permanent magnets can be determined.



Flux lines in the motor and flux density vectors without armature reaction Ia=0


Armature Reaction and Demagnetization Calculation (cont’d) Ia=387 A

Due to the armature reaction the level of demagnetization in the corner of the magnet pole is higher than in the other areas of the magnet


Armature Reaction and Demagnetization Calculation (cont’d) Ia=581A Permanent Magnet Flux Density versus Armature Current 1 Bm av 20°C B m corn 20°C

0.8

0.6

0.4 T , B

0.2

0 0

100

200

300

400

500

600

700

-0.2

-0.4

Phase A Current, A

The average value of flux density in the magnet is still much more than zero, but the corner area of the magnet is demagnetized

800


M/G 2D Finite Element Modeling •

A more accurate solution of M/G characteristics can be found by using finite element analysis. Thus, the accuracy of the results obtained by RMxprt software could be verified.

•

The transient finite element model includes a rotor and stator core, conductors, shaft, magnets, spacers between magnets and a carbon fiber ring. Thus, all components of the rotor losses can be determined with a good accuracy.


M/G 2D Finite Element Modeling (cont’d) •

•

•

•

•

Maxwell 2D transient mode solver with the time-stepping approach was employed. The effects of saturation, eddy currents, slotting, rotor position and space harmonics are taken into account. Because of using two-layer short pitched winding, where two layers of the winding have the angular displacement, there is no mirror symmetry in this machine. Thus there was no opportunity to use the master slave symmetry

Two different approaches were applied to investigate the M/G characteristics: sinusoidal voltage sources and sinusoidal current sources. Both techniques show similar results for the motor characteristics. After transient solution the Magnetostatic solver was used to check the value of the torque, developed for the specified current values.

M/G 2D Finite Element Modeling (cont’d) Glenn Research Center

Tav=1.6045 Nm

M/G 2D Finite Element Modeling (cont’d) Glenn Research Center

Pav=8.1 W

Eddy current losses were calculated for permanent magnets, carbon fiber ring and spacers between magnets. P = l

1 σ

∫ J ⋅ JdA A

Where σ is the conductivity of material, l is the depth of an eddy current loop in Z direction, A is the surface area, J is a current density. Centers of eddy current losses

Could be noticed, that the peaks of eddy current losses are located under slot openings. The value of local eddy current density depends on instantaneous value of the current in the closest slot.


M/G 3D Finite Element Modeling

The stator slot skewing causes a force in axial direction, that can affect the operation of magnetic bearings. The following technique was employed to determine the value of this force. 1) From Maxwell 2D Transient solver, the relative rotor-stator position and instantaneous values of phase currents are found. 2) Using these position and currents values the Maxwell 3D Magnetostatic model was created and the force values versus phase currents were determined, taking effect of skewing into account.


M/G 3D Finite Element Modeling (cont’d) Force in Axial Direction Caused by Slot Skewing vs Phase Current 12 10 8 N , e 6 c r 4 o F 2 0 -2 0

20

40

60

80

100

120

Current, A

3D model takes 2 layer winding and slot skewing into account

140


G3 Flywheel M/G Design

Based on the design results presented, M/G as a part of new flywheel G3 module was fully designed and the prototype is planned to be fabricated this year.


Concluding Observations and Recommendations

•

Ansoft RMxprt, Maxwell 2D and 3D are powerful tools for electrical machine design.

•

Some suggestions to make the use of these programs more convenient are given below

1. It would be useful as a part of the RMxprt outputs for the synchronous PM motors and generators to show the demagnetizing curve of the magnets (with the load and no-load lines), the energy product line, and the phasor diagram of the rated point of the machine. 2. The simulation of high frequency synchronous PM machine in transient mode requires relatively small time steps. Because of this it takes a lot of computing time to get steady state results for the transient analysis in 2D and especially in a 3D simulation.


References

1. J. R., Jr Hendershot, T. J. E. Miller, “Design of brushless permanent-magnet motors,” Oxford University Press, 1996

Motor Generator p25

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