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Modular PM Machines Based on Soft Magnetic Composite (SMC)
Prof. T.A. LipoWen Ouyang (Ph.D Candidate)
University of Wisconsin-Madison
April, 2006
Introduction
• This study is to investigate the benefits of soft magnetic composite (SMC) material in electrical machine design for a typical drive system.
• A modular machine structure is adopted for flexible machine geometry benefiting from the SMC fabrication process.
• For each SMC module, concentric winding is designed to minimize the end winding section and simplify the fabrication process.
• The machine performance is discussed briefly with illustratioin of a prototype Surface PM machine design.
• Healthy and faulted operations are also briefly investigated.
Soft Magnetic Composite (SMC)
• Soft Magnetic Composites (SMC) are composed of surface-insulated iron powder particles.
• SMC can be compressed to form uniform isotropic components with complex shapes in a single step.
• SMC makes it possible to define a magnetic field in three dimensions, thereby permitting the designer to build an electric motor beyond the restrictions set by the traditional lamination technology.
Electrically Insulated Fe-powder Particles
Typical SMC micro-structure
SMC Parts Manufacturing
Technology improvement narrows the gap between steel and SMC.
Soft Magnetic Composite (SMC) Magnetic Property
SMC (Somaloy500) Material Properties
Compressive Strength
340 Mpa
Fatigue Strength 23 Mpa
Young’s modulus
117 Gpa
Poisson’s Ratio 0.18
Impact Energy 1 J
Damping Factor (1/Q)
1.1E-3
MechanicalDensity 7.37 g/cm3
Specific heat 450 J/kg*K
Thermal expansion 11E-6 m/m*K
Resistivity 70 uΩ*m
Physical
B@4000A/m 1.26 T
B@10000A/m 1.51 T
Hc 270 A/m
u-max 500 V*s/A*m
Magnetic
SMC (Somaloy500) Material Properties
Core Loss (W/kg) Measurement from Ring Sample (OD55 ID45 H5 mm)
50 60 100 200 300 400 500 600 700 800 900 1000
0.5 1.8 2.2 3.7 7.5 11.5 15.7 20.1 24.6 29.4 34.3 39.3 44.6
1.0 6.1 7.3 12.3 25.4 39.2 53.7 69.0 84.9 101.6 119.1 137.2 156.1
1.5 12.1 14.6 24.7 51.7 81.1 112.7 146.6 182.8 221.4 262.2 305.3 350.8
(according to CEI/IEC 60404-6)
Tesla
Hz
SMC Iron Loss Characteristics (1)
SMC Iron Loss Characteristics (2)
Although the hysteresis loss of SMC is higher than conventional lamination, better eddy characteristics makes it suitable for applications with high frequency excitation.
2
2
)( )(2 dt
dBkfBkP eB
mhcm
EfDfBkBk
f
Pme
Bba
mhc mhh 2
• Compact winding structure with the minimum end winding section.
• Higher slot fill with simpler winding and less insulation.
• 3D structure provides extra flux path by the extension of pole tip.
• Concentrated winding further reduces the machine volume.
• Machine can be assembled by the SMC modules, which simplifies the machine fabrication process.
Module segment
SMC Module Benefits
Additional Advantages• Reduced copper volume as a result of increased fill factor and reduced end winding length,• Reduced copper loss as a result of the reduced copper volume,• Unity iron stacking factor,• Reduced high frequency tooth ripple losses since the SMC has essentially no eddy current losses,• The above bulleted items suggest a potential increase in overall efficiency,• Potential for reduced air gap length as a result of the tight tolerances maintained in manufacturing SMC
material,• Reduced axial length-over-end-winding dimension as a result of the compact end winding,• Absence of phase insulation as a result of using non-overlapping windings,• Potential elimination of the ground wall insulation since the SMC stator itself acts as an insulator,• No need to stress relieve the stator lamination after punching and assembling the stack, a relatively
costly and time consuming task, (stress relief is, however, included as part of the manufacture of the SMC part),
• Reduced conducted EMI when machine is used with inverter supplies since the stator SMC body acts as an insulator and does not conduct current to ground,
• Reduced bearing currents in the presence of PWM waveforms again because of the use of SMC which acts as insulation against this type of current flow,
• Modular construction allows the possibility of easy removal of an individual modular unit for quick repair or replacement,
• Stator is easily recyclable since the stator can again be compressed back into powered form with pressure and the copper windings readily removed.
Disadvantages
• Relatively high hysteresis loss (low frequency loss),
• Slight penalty a result of smaller saturation flux density,
• Relatively brittle material,
• Producibility of structures to meet close specs not yet mastered,
• Size of producible structures are limited.
• Lower relative permaability (700 vs roughly 3000)
Module Shape Analysis (Trapezoidal)
)2
sin()4
)2(cos()
2sin(
82
nnn
n
BB ma
an
a b -a -b
a b c a
aveB
3
2
aveB
ave
B
a
a b -a -b
aveB
aveB
2
Two phase (virtual 4 phase) Three phase
)2
sin()6
)34(sin()cos(
42
nnn
n
BB ma
an
,...3,2,1
)]}3/2(cos[)3/2cos()]3/2(cos[)3/2cos(
)cos(){cos()6
)34(sin()
2sin()cos(
3
44),(
12
n
ntnt
ntnn
ng
IN
ntB
n
g
,...3,2,1)],sin()sin()4
)2(sin(
)cos()cos()4
)2([cos()
2sin()
2sin(
8),(
12
nntn
ntnnn
g
IN
ntB
n
g
Module Shape Parameter (γ) Dependancy Analysis
Two Phase Three Phase
With higher γ, which means larger slot opening, the fundamental suffers with increased harmonic components.
Module Shape Parameter (χ) Dependancy Analysis
Two Phase Three PhaseWith a rectangular design (χ=0), the fundamental reaches maximum while the same occurs for the harmonic components.
Module Shape Space Spectrum Analysis
1
2
2
B
BTHD n
n
p
Two Phase Three Phase
Note: scale is different
Module Shape Analysis Comments (Trapezoidal)
Comparisons of two phase and three phase design with trapezoidal pole shape
Trapezoid Pole Shape Profile
Two Phase (virtual four
phase)Three Phase
Modules Number per pole pair 4 3
Ampere-turns per module NI (4/3)×NI
Air gap flux fundamental peak value
0.997 T 0.768 T
THDp (χ= π/9, γ=π/18) 39.0% 56.9%
Preferred γ for better THD Small Small
Preferred χ for better THD Large Large
Module Shape Analysis (Sinusoidal)
)()sin()cos(2
dtrLg
NIud m
g
a
)()cos()sin(2
dtrLg
NIud m
g
b
)()sin(2
)(2
dtrL
L
g
NIu
L
Lddd
s
mg
s
mba
0 2
sL
mL
a -a a
-b b -b b
)sin(2
2
2
tL
L
g
NIu
drL
d
dA
dB
s
mg
s
Purely Sinusoidal!
Two Phase
0
sL
mL
a c b
c b a c
a
b
c
0
0
2
2
2
2
1
3
8
g
INB gma
)3
cos()cos(169
642
n
nn
BB ma
an
,...3,2,1
)]}3/2(cos[)3/2cos()]3/2(cos[)3/2cos(
)cos(){cos()3
cos()cos(169
6
3
44),(
12
n
ntnt
ntn
nng
INtB
n
g
Three Phase
Stator Assembly with Sinusoidal Shaped Poles
Module Shape Comparison (Trapezoidal and Sinudoidal)
Comparisons of two/three phase design with trapezoidal/sinusoidal pole shape
Pole Shape Profile
Comparisons
Trapezoidal (χ= π/9, γ=π/18)
Sinusoidal
Two Phase
Three Phase
Two Phase Three Phase
Modules Number per
pole pair4 3 4 3
Ampere-turns per module
NI (4/3)×NI 2×NI (8/3)×NI
Air gap flux fundamental
peak0.997 T 0.768 T 0.792 T 0.864 T
THDp 39.0% 56.9% 0 11.97%
Axial Flux Version Having Sinusoidal Pole Shape
Machine Design Equations
• The general-purpose sizing equations have been developed and takes the form of:
• Or
• With the definitions of the variables in both equations listed below:PR rated output power
K = Ar /As, ratio of electric loading on rotor and stator. In a machine topology without a rotor
winding, K =0.
m number of phases of the machine
m1 the number of phases of each stator (if there is more than one stator, each stator has the same m1).
Ke emf factor that incorporates the winding distribution factor Kw and the ratio between the area spanned by the (salient) poles and the total air gap area.
30
30max
1 21
1D
p
fABKKKK
m
m
KP gLpwieR
egpwieR LDp
fABKKK
m
m
KP 2
02
0max1 21
1
Machine Design Equations (continued)
2/1
0
2
max
max ))(
(1
T
phrms
phi dt
I
ti
TI
IK
T
ie
T
phpkpw dttftf
Tdt
IE
tite
TK
00 max
)()(1)()(1
g
e
D
L
o
g
D
D
to
R
LD
P2
4
Ki a current waveform factor in order to indicate the effect of the current waveform, where:
the current i(t) and Iphmax are the phase current and the peak phase current, Irms is the rms current. Kpw electrical power waveform factor,
where fe(t)=e(t)/Epk and fi(t)=i(t)/Iphmax are the expressions for the normalized emf and current waveforms. e(t) and Epk are the phase air gap EMF and its peak value. T is the period of one cycle of the emf. KL , defined as the aspect ratio coefficient
o , the diameter ratio the machine efficiency, Bgmax flux density in the air gapA total electric loading, including stator and rotor loading Nt the number of turns per phase,f the power supply frequencyp number of machine pole pairs Finally, the machine power density for the total volume can be defined as:where Lt is the total length of the machine including the stack length and the protrusion of the end winding from the iron stack in the axial direction.
Machine Design Equations (Power Density)
tottot
Rden
LD
PP
2
4
tottot
Rden
LD
TT
2
4
Permanent Magnetic Material Improvement
Machine assembly and module profiles
Five Phase Machine Concept Structure
• Five phase machine design offers independent phase control of each module with the integration of switching devices of each module.
• Fault tolerant capability ( up to 2 phase fault ) makes it a potential candidate for applications with critical requirements.
• Higher torque density can be achieved compared with typical induction machine.
• Modular design makes it possible to replace fault modules conveniently when necessary.
Machine Structure Details
1sR
2sR
3sR
1rR
2rR
pmR
pW
pm
p
pmHtH
Machine Magnetic Circuit Model
s
p
1x2xxy
0
N
S N
S
N
S
pm2pm
2pR
rR
0pR
yR
0lkR
0gR
yR
0lkR2pm
rR
1pR
2gR 1gR
2lkR1lkR
Machine Magnetic Leakage Models
NS N
S
Rotor Wg/2 Wg/2
0 xStator
g
Hpm
)2/()1ln(0
000 g
g
pmpm
lk wgH
gLdx
xH
LP
s
p
1x2xxy
0
N
S N
S
1
112
12
20 2)(ln
2 x
xxx
xx
LxP r
lks
s
p
1xxy
0
NS N
S
sr
lktS
gR
0
1
x
y
0
PM
Statortooth
Ht
g
g
gHL
gy
dyLP trHt r
lkf2
2ln
2
2
0
0
0
Machine Magnetic Circuit Network Model
0gR2gR 1gR
2lkR1lkR
cF
inR2
cF
inR2 cF
inR
4 3
1 2
Flux (Analytical)
0.0224 0.0239
Flux (FEA)
0.0228 0.0238
0
0
2
2
0
0
02
02
4
3
2
1
02202
00111
11
22
c
c
glkgglk
gglkglk
lkinlkinin
lkininlkin
F
F
RRRRR
RRRRR
RRRRR
RRRRR
Machine Design Optimization
Two main design methodologies are applied in this project: 1) Analytical. 2) FEA.
The analytical method is based on a closed form analysis of the machine equations. Advantages: 1) Concise formula for the machine performance. 2) Explicit dependency of machine design parameters. 3) Easy for optimization. Disadvantages:1) Errors associated with nonlinearity and complexity of the structure.
FEA method is based on the numerical method analysis derived from Maxwell equations. Advantages: 1) Very accurate solution for the machine performance. 2) Direct geometry modeling and analysis. Disadvantages: 1) Computation cost, especially for 3D. 2) Difficulty to achieve global optimization.
Machine Optimization Method (FEA)
• Response Surface. ( Design of Experiment ) pros: 1) Simple algorithm. 2) Global optimization. 3) Parameter impact information can be obtained. 4) Practiced quite a lot from aerospace engineering, such as plane wing shape design. But few reports on machine design in IAS since 2000. cons: 1) If the parameter number is 10, the sampling points for the initial solution space will be 3^10=59049, which is 41 hours CPU time if each point FEM simulation takes 1 minute. 2) Data analysis method is necessary to reduce the polynomial error. 3) High number of parameters (over 15) will take too much time on the solution space construction, resulting in an unfeasible approach.
jii j
ijj
k
jjjj
k
jjk xxbxbxbbxxxf
2
11021 ),...,,(
The coefficients are evaluated by regression. The error of the model is less than 1% of the FEM prediction in most cases.
Machine Parameter Extraction
• Does it is necessary to consider all the machine geometry parameters?• Machine main parameters could be controlled under 10 without sacrificing the
effectiveness of analysis (Does the radius of slot corner matter much?).• The main machine parameters: Stator side: (1) Stator outer diameter (2) Stator inner diameter (3) Yoke thickness (4) Slot width (5) Slot opening Rotor side: (1) Rotor outer diameter.(rotor inner diameter does not matter
much) (2) 2~4 parameters for surface PMs or 4~6 parameters for IPM. Air gap: This is a very sensitive parameter, can be fixed based on mechanical suggestion. Thus, the rotor outer diameter is dependent on the stator inner diameter if air gap is selected at the very beginning, which reduces the rotor side parameters!• Thus, it is very practical to control the machine parameters with level of 10
variables
Example of Stator Module Structure
The machine stator module can be defined by six main parameters: 1) Stator out radius. 2) Stator yoke thickness. 3) Stator inner radius. 4) Tooth span angle. 5) Tooth body width. 6) Tooth tip thickness.
1sR2sR
pW
p
tH
3sR
Example of Surface PM Rotor Structure
The PM span and PM thickness are key parameters, with extra 1 or 2 parameters necessary if the PM is not a regular shape.
h
Example of Interior PM Rotor Structure
1
2
1h
2h
3h
The bridge width is fixed due to the saturation and stress consideration.
5 parameters are necessary for the definition of a typical IPM rotor structure.
Machine Design Main Parameters (Surface PM)
Parameters IM (GE/3HP) 5 Phase SPM Motor
OD 190mm 120mm
ID 120mm 72mm
RPM 1750 1800
Machine Length 70mm(iron)/150mm(full) 140mm
Torque (T) 11.87Nm 13.26Nm
Effective Volume 4.2529×10-3 m3 1.5834×10-3 m3
Torque Density 2.791×103 Nm/m3 8.374×103 Nm/m3
Cogging Torque 0 5.5% of rated
Torque Density Ratio 5 phase PM / IM = 3.0
Optimized Results (SPM)
Rs1 60mm Wp 20.2mm
Rs2 52mm Hpm 6.4mm
Rs3 36mm αpm 48.2o
Ht 4.5mm Rr1 33mm
αp 50.5o Rr2 20mm
Optimization for maximum torque and acceptable efficiency
Optimized Results (IPM)
Optimization for limited field weakening capability and torque capability
Variables DE RS
Rso 120mm 120mm
Rsi 72mm 72mm
Wp 19.5mm 21.1mm
Ht 4.3mm 5.2mm
Yt 7.8mm 6.7mm
αp 50.5o 52.3o
h1 2.2mm 2.48mm
h2 3.53mm 4.71mm
h3 4.91mm 4.55mm
θ1 36.9o 39.1o
θ2 20.1o 20.9o
Tmax 6.12 Nm 6.35 Nm
Optimized Results (IPM)
Optimized Results (Torque dependency on parameters)
Optimized Results (Inductance dependency on parameters)
Inductance Dependence on Rotor Position (IPM)
Due to large tooth piece design, the machine inductance is inherently dependent on rotor position, the associated energy variation produces cogging torque.
SPM and IPM Rotor Concept Comparison
• Higher back EMF limits the speed range of SPM rotor design due to the DC bus voltage.
• Optimization of module structure to minimize the back EMF harmonics is one of the optimization objectives.
Back EMF HarmonicsBack EMF waveforms
THD_SPM=9.87%THD_IPM=7.16%
SPM Five Phase SMC Drive System Mathmetical Model
• Terminal voltage equation:
where v, e, i, λ denotes the vector of phase terminal voltage, back emf, current, and flux linkage:
• Torque equation from the idealized energy conversion:
dt
direv
iL
ieT Tme
5554535251
4544434241
3534333231
2524232221
1514131211
][
LLLLL
LLLLL
LLLLL
LLLLL
LLLLL
L
SPM Starting Process Simulation (Open loop I)
• Fixed inverter excitation frequency (20 Hz)• Purely sinusoidal current waveform, light load (up) & heavy load (down)
Speed Response Torque Response
SPM Starting Process (Open loop II)
• Variable inverter excitation frequency (10Hz ~ 90 Hz) in 0~1 sec.
• Sinusoidal current supply, light load (up) & heavy load (down) for fan load.
Speed Response Torque Response
SPM Starting Process (Closed Loop)
• Variable inverter excitation frequency with feed back from rotor position.• Sinusoidal current supply, fan type load simulated.
Speed Response Torque Response
SPM Phase Loss (1 phase out)
• One phase open circuit at t=0.5 sec.• Close loop control assumed (rotor position feed back).• Rotor speed reduced due to the torque loss.• Torque pulsation increases due to unbalanced operation.
Speed Response Torque Response
SPM Phase Loss (2 phases out)
Adjacent phases out
Non-adjacent phases out
Speed Response Torque Response
SPM Torque Simulation (FEA) on Phase Loss
Torque Characteristics Average Torque
• Briefly, the average torque is proportional to the phase number in healthy condition. • Cogging torque and current ripple is the main source of torque pulsation if the
machine is healthy.
SPM One Phase Short Circuit (simplified assumption)
• One phase terminal short circuit induces huge torque ripple.• Fault phase current depends on the turns number involved in the short circuit and
machine speed.
Phase Current Speed & Torque
Prototype (SMC Modules and Stator Assembly)
Prototype (Rotor and Motor Assembly)
Prototype (Drive Circuit and Control Board)
MOSFET Drive Circuit DSP Unit
Rotor Concept for Axial Flux Induction Motor
Summary
• Soft Magnetic Composite (SMC) materials have attractive magnetic characteristics for high frequency (high speed) motor designs
• SMC materials have opened up the door to machine designs with 3 dimensional flux paths.
• The chief limitation is currently the permissible size of SMC components.