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Finite Elements in
Electromagnetics
3. Eddy currents and skin effect
Oszkár Bíró
IGTE, TU Graz
Kopernikusgasse 24Graz, Austria
email: biro@igte.tu-graz.ac.at
Overview
Eddy current problems
Formulations in eddy current free regions
Formulations in eddy current regions
Coupling of formulations
Skin effect problems
Voltage excitation, A,V-A formulation
Current excitation, T,F-F formulation
Wn: nonconducting region
Air
J(r,t) = 0
Coil
J(r,t) known
0
Wc: eddy current
region
J(r,t) unknown
Typical eddy current problem
Maxwell’s equations:
0JH curl ,
0Bdiv in Wn,
JH curl ,
BE jcurl ,
0Jdiv ,
0Bdiv in Wc.
BHHB ,
in Wn and in Wc,
EJ , JE in Wc.
Assumption: 00 JT curl .
Boundary conditions
n: outer boundary of Wn
c: outer boundary of Wc
0nH or 0nB onn,0nH or 0nE onc.
ornTnH 0 or nTnB 0 onn,
nTnH 0 or 0nE onc.
Interface conditions
nc: interface between Wn and Wc
nH and nB are continuous on nc.
Magnetic scalar potential in Wn
F grad0TH ,
en
k
kkt1
NT0
Differential equation:
)()( 0T divgraddiv F in Wn.
Boundary conditions:0F or nTn 0 F grad on n.
Finite element approximation
FFFnn
k
kk
n N1
)(
Galerkin equations:
W W
WWF
n n
dgradNdgradgradN i
n
i 0T )(,
i=1,2,...,nn
Magnetic vector potential in Wn
AB curl .
Differential equation
0TA curlcurlcurl )( in Wn
Boundary conditions:0nA or nTnA 0 curl on n.
Finite element approximation
en
k
kk
n a1
)(NAA
Galerkin equations:
W W
WW
n n
dcurldcurlcurl i
n
i 0TNAN)( ,
i=1,2,...,ne
Positive semidefinite matrix
Magnetic vector potential alone in Wc
*AB curl ,
*AE j .
Differential equation
0AA ** jcurlcurl in Wc.
Boundary conditions:
0nA * or nTnA 0 *curl on c.
Finite element approximation
en
k
kk
n a1
)(**NAA
Galerkin equations:
W W
WW
c c
djdcurlcurl n
i
n
i
)(*)(*ANAN
W
W
c
dcurl i 0TN , i=1,2,...,ne
Nonsingular but ill-conditioned matrix
Magnetic vector and electric
scalar potential in Wc
AB curl , gradVjj AE .
Differential equations:
0AA gradVjjcurlcurl ,
0)( gradVjjdiv A in Wc.
Boundary conditions:
0nA or nTnA 0 curl ,
0VV =constant or 0)( gradVjj An
on c.
Finite element approximation
en
k
kk
n a1
)(NAA ,
nn
k
kk
n NVVV1
)(.
Galerkin equations:
W W
WW
c c
djdcurlcurl n
i
n
i
)()(ANAN
WW
WW
cc
dcurldgradVj i
n
i 0TNN)( ,
i=1,2,...,ne,
W
W
c
dgradNj n
i
)(A
0)( W Wc
dgradVgradNj n
i , i=1,2,...,nn
Singular system but improved conditioning.
Current vector and magnetic
scalar potential in Wc
F gradTTH 0 , TTJ 0 curlcurl .
Differential equations:
F gradjjcurlcurl TT
00 TT jcurlcurl ,
)()( 0TT divjgraddivj F in Wc.
Boundary conditions:
0nT or nTnT 0 curlcurl ,
0FF =constant or 0)( TnTn F grad
on c.
Finite element approximation
en
k
kk
n t1
)(NTT ,
FFFnn
k
kk
n N1
)(.
Galerkin equations:
W W
WW
c c
djdcurlcurl n
i
n
i
)()(TNTN
W
WF
c
dgradj n
i
)(N
WW
WW
cc
djdcurlcurl ii 00 TNTN ,
i=1,2,...,ne,
W
W
c
dgradNj n
i
)(T
W
WF
c
dgradgradNj n
i
)(
W
W
c
dgradNj i 0T , i=1,2,...,nn
Singular system but good conditioning.
Coupling A,V in Wc to A in Wn:
A,V-A formulation
Interface conditions on nc:
Continuity of An nB is continuous
Continuity of nAcurl is a natural interface
condition nH is continuous
Galerkin equations remain unchanged
Coupling T,F in Wc to F in Wn:
T,F-F formulation
Interface conditions on nc:
Continuity of F and 0nT nH is
continuous
Continuity of nT F )( grad is a natural
interface condition nB is continuous
Galerkin equations remain unchanged
Typical skin effect problem
u(t)
i(t)
i(t) J(r,t)
B(r,t) Wn: =0
Wc: >0
E1: 0nE
E2: 0nE
cn: nBnH , cont.
n
n
n
HBEJ ,
,t
curl
BE
Wc:
,JH curl
Wn:
,JH curl
,0Bdiv
HB
Integral quantities, network
parameters
2
),()(
E
dtti nrJ
1
),(
E
dt nrJ
W
W
c
dt
tpv
2),(
)(rJ
)()( 2 titR
W
WW
dHdBtW
nc
tB
m
),(
0
)(
r
t
diLd
di
0
)()()(
dt
tdWtptitutp m
v
)()()()()(
Voltage excitation (1)
AB curl in nc WW and , gradUt
AE in cW
EH curl in cW , 0H curl in nW ,
)(, tuU 0nA on1E , 0, U0nA on
2E ,
0nA or 0nH on )( nc WW
nA and nH are continuous on cn 0 nJ
Voltage excitation (2)
W
W
c
dt
tp
2),(
)(rJ
W
WW
dHdBdt
d
nc
tB ),(
0
r
W
W
c
dtp JE)( WW
W
nc
dt
HB
W
WW nc
dt
HB
WW
W
nc
dt
curl HA
WW
W
nc
dcurlt
HA
0
)(
WW
nc
dt
nHA
W
W
c
dt
JA
Voltage excitation (3)
W
W
c
dt
tp JA
E)( W
W
c
dgradU J
W
W
c
dgradUtp J)(
W
W
cnEEc
dUdUdiv
21
0
nJJ
)()()()(
1
titudtutp
E
nJ
Boundary value problem for A,V (1)
Differential equations:
0A
A
t
Vgrad
tcurlcurl
1
t
VU
0)(
t
Vgrad
tdiv
A
0A
curlcurl
1
in Wc,
in Wn,
Boundary value problem for A,V (2)
Boundary conditions:
t
dutV0
)()(, 0nA on 1E ,
0)(, tV0nA on 2E ,
0nA or 0nA curl
1on )( nc WW .
Interface conditions:
nA and nAcurl
1are continuous on cn .
Current excitation (1)
in cW
incW ,
in nW .
on )( nc WW
and F are continuous and cn
,TTJ 0 curl F gradTTH 0,
F grad0TH
0B
J
tcurl
1 0Bdiv in nW ,
)0( nB0nE or
nTnH 0
nB 0nT on
,
.
Properties of T0
0T0 curl nWin ,
1E
dcurl nT0 2E
dcurl nT0 )(ti
nT0 is continuous on .cn
A possible choice of T0
Solve the static current field in Wc
i(t) 00 TJ curl
Wn: =0 Wc: >0 C
W
C
n
ti3
)(
4
)()()(:
Q
S0
rr
rrdsrHrT
nHnT S0 :cn
0T0
W curlcurlc
1:
0nT0 curlE
1:1
0nT0 curlE
1:2
Boundary value problem for T,F (1)
Differential equations:
in Wc,
in Wn,
0TTTT 00 F
grad
tcurlcurl
1
0)( F
graddiv
tTT0
0)( F
graddiv
t0T
Boundary value problem for T,F (2)
Boundary conditions:
2E
or on )( nc WW .
Interface conditions:
and
on 1E ,
are continuous on cn .
0,1
F nTT0nT 0 gradcurl
0nT oncn .
0F 0nT0 F grad
F nT0 F grad
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