motor generator p25

Glenn Research Center

HIGH SPEED PERMANENT MAGNET

SYNCHRONOUS MOTOR / GENERATOR DESIGN

FOR FLYWHEEL APPLICATIONS

Aleksandr Nagorny, Ph.D.National Research Council

Glenn Research Center Outline

• Introduction• Selection of the Rated Point• The major requirements for flywheel M/G in space applications• Meeting design requirements• M/G Main Materials Selection• Lamination Thickness Selection• Modeling tools• M/G Preliminary Design Process• Permanent Magnet Material Selection• The RMxprt Analytical Design Output• Armature Reaction and Demagnetization Calculation• M/G 2D Finite Element Modeling• M/G 3D Finite Element Modeling• Concluding Observations and Recommendations• References• Acknowledgements

Glenn Research Center Introduction

• The motor / generator design is a part of work performed at NASA Glenn Research Center devoted to the development of flywheel modules for use in satellite energy storage and attitude control systems.

• The benefits of flywheel module as an energy storage 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.

• Flywheel can be used as a source of momentum for attitude control, giving the opportunity to combine two satellite subsystems into one and reduce the overall volume and mass.

Glenn Research CenterSelection 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

Tnet=TES+TAC

Where TES is the torque component required for the energy storage, and TAC is the component required for the attitude control.

Flywheel Combined Energy Storage and Attitude Control Output Power Determination

-1.5

-1

-0.5

0

0.5

1

0 20 40 60 80 100

Time of Cycle, min

To

rqu

e, N

m

-8000

-6000

-4000

-2000

0

2000

4000

6000

Out

put P

ower

, W

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

Glenn Research Center 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


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 effectivemeasure against the back iron losses is lamination of the back iron core.


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 materialsü Wire insulationü Slot insulationü Winding parts insulationü Motor leads insulationü Permanent magnet materialsü Rotor carbon fiber composite.

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

Glenn Research CenterM/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

Glenn Research Center 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

Glenn Research Center 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

Glenn Research Center Permanent Magnet Material Selection

NdFe group has higher remanent magnetization and energy product.

SmCo has a better thermal characteristic.

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

5336.96

6075.796387.41

7779.15

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

SmCo 24 SmCo 28 NdFe 30 NdFe 38

Type of Magnet Material

W

Glenn Research Center Lamination Thickness Selection

• 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’’).

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

y = 539416x2 + 5783.6x + 13.285R2 = 1

0

50

100

150

200

250

0 0.0025 0.005 0.0075 0.01 0.0125 0.015

Lamination Thickness, Inch

Spe

cifi

c Lo

ss,

W/k

g

Glenn Research Center The RMxprt Analytical Design Output

6Number of Conductors per Slot:

0.728Length of Stator Core (inch):

2.9Inner Diameter of Stator (inch):

5.75Outer Diameter of Stator (inch):

24Number of Stator Slots:

STATOR DATA

SmCoType of Magnet:

0.27Max. Thickness of Magnet (inch):

0.728Length of Rotor (inch):

1.51Inner Diameter (inch):

0.105Non Magnetic Gap, inch

ROTOR DATA

7.557.55Total Net Weight (lb):

1.4821.452Rated Torque (N.m):

5000050000Synchronous Speed (rpm):

98.16498.168Efficiency (%):

72.01672.594Line Current RMS (A):

0.0150.016Cogging Torque (N.m):

0.9640.964THD of Induced Voltage (%):

115115Operating Temperature (C):

1666.671666.67Frequency (Hz):

44Number of Poles:

6161Rated Voltage (V):

7.67.6Rated Output Power (kW):

GeneratorMode

MotorModeM/G parameters at rated point


Cogging Torque

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150 200 250 300 350 400

Electric Angle, Degrees

To

rqu

e, N

m

Output Power Versus Torque Angle

-5000

0

5000

10000

15000

20000

25000

30000

0 25 50 75 100 125 150 175 200

Angle, Degrees

Po

wer

, W

Efficiency Versus Torque Angle

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160 180 200

Torque Angle, Degrees

Eff

icie

ncy

, %

Current Versus Torque Angle

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180 200

Torque Angle, Degrees

Cu

rren

t, A

The RMxprt Analytical Design Output (cont’d)


Flux Density Distribution in the Air Gap (RMxprt)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 50 100 150 200 250 300 350 400

Electrical Angle, Degrees

Flu

x D

ensi

ty, T

The RMxprt Analytical Design Output vs FEA

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

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 1 2 3 4

Position in the air-gap, inch

B, T

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

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 1 2 3 4

Position in the air gap, inch

B, T

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

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

0 1 2 3 4

Position in the air gap, inch

B, T

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


Study of M/G Characteristics During the Duty Cycle

M/G Frequency

0200400600800

10001200140016001800

0 10 20 30 40 50 60 70 80 90 100Time, min

Fre

qu

ency

, Hz

M/G Voltage

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100Time, min

Vo

ltag

e, V

M/G Armature Current

-80-70-60-50-40-30-20-10

010203040

0 10 20 30 40 50 60 70 80 90 100

Time,min

Cur

rent

, A

M/G Total Loss

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90 100Time. min

Lo

ss, W

M/G Output Power

-5000-4000-3000-2000-1000

010002000300040005000

0 10 20 30 40 50 60 70 80 90 100

Time, min

Po

wer

, W

M/G Efficency

93.594.094.595.095.596.096.597.097.598.098.5

0 10 20 30 40 50 60 70 80 90 100Time, min

Eff

icie

ncy,

%

The RMxprt Analytical Design Output (cont’d)


Armature Reaction and Demagnetization Calculation

( )

−+= st

Hcst HcHc ϑϑ

α100

1

Samarium Cobalt (Sintered) S2769 Demagnetization Lines at Different Temperatures

0

0.2

0.4

0.6

0.8

1

1.2

-900000 -800000 -700000 -600000 -500000 -400000 -300000 -200000 -100000 0

Hc, A/m

Br,

T

20°C200°C

• Permanent magnet demagnetization curves are sensitive to the temperature

( )

−+= st

Bst

rBrBr ϑϑα

1001


• 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 thepermanent magnet pole has uniform saturation.

Φr

Rg

Pm0

Fa

RL

Φm

ΦgΦL

Mrr AB=Φ

m

mrecmo l

AP

µµ0=

gg A

gR

0µ′

=g

m

AA

C =Φrgm

g BRP

CB

)1( += Φ

rgm

grm B

RP

RPB

)1(

1 1

+

+=

rec

mrm

BBH

µµ0

−=−

gm

grrec RP

RPPC

0

11 += µ

m

demrecma l

FB

µµ0=mamload BBB −=

Permanent Magnet Demagnetization Curve hm=.300 inch

0.00

1.10

0.0000

0.9043

0.7512

0 00.10.2

0.30.40.50.6

0.70.80.9

1

1.11.2

-900000 -800000 -700000 -600000 -500000 -400000 -300000 -200000 -100000 0

H,A/m

B,T

Armature Reaction and Demagnetization Calculation (cont’d)


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.


Glenn Research CenterArmature Reaction and Demagnetization

Calculation (cont’d)

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



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

Glenn Research CenterArmature Reaction and Demagnetization

Calculation (cont’d)

Ia=581A

Permanent Magnet Flux Density versus Armature Current

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600 700 800

Phase A Current, A

B, T

Bm av 20°C

B m corn 20°C

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

Glenn Research Center 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.

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

• 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.

• 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


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

Tav=1.6045 Nm


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

∫ ⋅=A

JdAJlPσ1

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

Pav=8.1 W

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.

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.

Centers of eddy current losses

Glenn Research Center 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.

Glenn Research CenterM/G 3D Finite Element Modeling (cont’d)

Force in Axial Direction Caused by Slot Skewing vs Phase Current

-2

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Current, A

Forc

e, N

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

Glenn Research Center 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.

Glenn Research Center 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.

Glenn Research CenterReferences

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

Glenn Research Center Acknowledgements

This work was performed while the author held a National Research Council Associateship Award at NASA Glenn Research Center Cleveland Ohio

motor generator p25

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