IEC 60205:2026

IEC 60205 ®
Edition 5.0 2026-03
INTERNATIONAL
STANDARD
COMMENTED VERSION
Calculation of the effective parameters of magnetic piece parts
ICS 29.100.10 ISBN 978-2-8327-1175-0
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CONTENTS
FOREWORD . 3
INTRODUCTION .
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Basic rules applicable to this standard . 6
5 Formulae for the various types of cores . 7
5.1 Ring cores . 7
5.1.1 Ring cores in general . 7
5.1.2 For ring cores of rectangular cross-section with sharp corners . 8
5.1.3 For ring cores of rectangular cross-section with an appreciable average
rounding radius r . 8
5.1.4 For ring cores of rectangular cross-section with appreciable chamfer c . 8
5.1.5 For ring cores of trapezoidal cross-section with sharp corners . 8
5.1.6 For ring cores of trapezoidal cross-section with an appreciable average
rounding radius r . 9
5.1.7 For ring cores of cross-section with circular arc frontal sides . 9
5.2 Pair of U-cores . 9
5.2.1 Pair of U-cores of rectangular section . 9
5.2.2 Pair of UR-cores of rounded section . 11
5.2.3 Pair of URS-cores of rectangular-circular sections . 12
5.3 Pair of E-cores of rectangular section . 14
5.4 Pair of ETD/EER-cores . 15
5.5 Pair of pot-cores . 17
5.6 Pair of RM-cores . 20
5.7 Pair of EP-cores . 27
5.8 Pair of PM-cores . 29
5.9 Pair of EL-cores . 31
5.10 Pair of ER-cores (low profile) . 34
5.11 Pair of PQ-cores . 39
5.12 Pair of EFD-cores . 45
5.13 Pair of E planar-cores . 46
5.14 Pair of EC-cores . 49
Bibliography .
List of comments. 53

Figure 1 – Ring cores . 7
Figure 2 – Pair of U-cores of the rectangular section . 10
Figure 3 – Pair of UR-cores of rounded section . 11
Figure 4 – Pair of URS-cores of rectangular-circular sections . 12
Figure 5 – Pair of E-cores of rectangular section . 14
Figure 6 – Pair of ETD/EER-cores . 15
Figure 7 – Pair of pot-cores . 17
Figure 8 – Pair of RM-cores . 24
Figure 9 – Pair of EP-cores . 27
Figure 10 – Pair of PM-cores . 29
Figure 11 – Pair of EL-cores . 32
Figure 12 – PLT(plate)-cores . 32
Figure 13 – Pair of ER-cores (low profile) . 36
Figure 14 – PLT (plate)-cores . 36
Figure 15 – Pair of PQ-cores . 40
Figure 16 – PQ-cores . 41
Figure 17 – PLT(plate)-cores . 41
Figure 18 – Pair of EFD-cores. 45
Figure 19 – Pair of E planar-cores . 47
Figure 20 – PLT(plate)-cores . 47
Figure 21 – Pair of EC-cores . 50

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Calculation of the effective parameters of magnetic piece parts

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of a patent.
IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in respect
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This commented version (CMV) of the official standard IEC 60205:2026 edition 5.0 allows the
user to identify the changes made to the previous IEC 60205:2016 edition 4.0. Furthermore,
comments from IEC TC 51 experts are provided to explain the reasons of the most relevant
changes, or to clarify any part of the content.
A vertical bar appears in the margin wherever a change has been made. Additions are in green
text, deletions are in strikethrough red text. Experts' comments are identified by a blue-
background number. Mouse over a number to display a pop-up note with the comment.
This publication contains the CMV and the official standard. The full list of comments is available
at the end of the CMV.
IEC 60205 has been prepared by IEC technical committee 51: Magnetic components, ferrite
and magnetic powder materials. It is an International Standard.
This fifth edition cancels and replaces the fourth edition published in 2016. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) addition, in 5.2, of the drawing and the formulae of pair of URS-cores of rectangular-circular
section;
b) using, in 5.9, 5.10, 5.11 and 5.13, the conventional calculation formula that includes "B -D"
is limited for the x-x cores (x is EL, ER, PQ or E) and addition new formulae for x-PLT cores
that replaces "B -D" with "(B -D+B )/2";
1 1 2
c) addition, in 5.9, 5.10, 5.11 and 5.13, of formulae of l and l for x-PLT cores (x is EL, ER,
1 3
PQ or E) which is different from the l and l of x-x cores;
1 3
d) addition of formula A in each subclause from 5.2.1 to 5.14.
min
The text of this standard is based on the following documents:
Draft Report on voting
51/1592/FDIS 51/1607/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
The purpose of this revision is to provide formulae by which everybody can reach the same
effective parameter values. Firstly, it is necessary to have a sufficient number of significant
figures when figures are rounded off in the process of calculation. Additionally, some of the
calculation formulae have been changed to get closer to the actual shape.
In this revision, the basic idea of calculation has not been changed. Recently, analysis of the
magnetic field in the core has been considerably improved, so that, based on these ideas,
development of new approaches and formulae can be expected.
Furthermore, the new “EC-cores” have been added.
The parameters in the existing IEC standards will be revised with the outcome from the formulae
of this document.
1 Scope
This document specifies uniform rules for the calculation of the effective parameters of closed
circuits of ferromagnetic material.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Basic rules applicable to this standard
4.1 All results shall be expressed in units based on millimetres, shall be accurate to three
significant figures, but to derive l , A and V the values of C and C shall be calculated to five
e e e 1 2
significant figures. It is recommended that the intermediate calculation values used to derive l ,
e
A , V , C and C have at least 10 decimal places 1. Finally calculated l , A , V and A shall
e e 1 2 e e e min
be rounded to three significant figures, and C and C shall be rounded to five significant figures.
1 2
All angles are in radians.
NOTE The purpose of specifying this degree of accuracy is only to ensure that parameters calculated at different
establishments are identical and it is not intended to imply that the parameters are capable of being determined to
this accuracy.
4.2 A is the nominal value of the smallest cross-section. A is the geometrical cross-
min g
section of a ring core with rectangular shape. All the dimensions used to calculate A shall be
min
the mean values between the tolerance limits quoted on the appropriate piece part drawing. All
results shall be expressed in units based on millimetres and shall be accurate to three
significant figures.
The minimum physical cross-section area A is given as: A = min (A )
min min i
NOTE A to be is used for the measurement of the saturation flux density B B on ring cores with rectangular
g max s
cross-section.
4.3 Calculations are only applicable to the component parts of a closed magnetic circuit.
4.4 All dimensions used for the purpose of calculations shall be the mean value within the
tolerance limits quoted on the appropriate piece part drawing.
4.5 All irregularities in the outline of the core, such as small cut-outs, notches, chamfers, etc.
shall be ignored, unless otherwise described in this document.
4.6 When the calculation involves the sharp corner of a piece part, then the mean length of
flux path for that corner shall be taken as the mean circular path joining the centres of area of
the two adjacent uniform sections, and the cross-sectional area associated with that length shall
be taken as the average area of the two adjacent uniform sections.
The effective parameters l , A and V can be calculated as:
e e e
2 32
l = CC A = CC V lA C C
e 12 e 12 e ee 1 2
where
l is the effective magnetic length of the core (mm);
e
A is the effective cross-sectional area (mm );
e
V is the effective volume (mm );
e
−1
C is the core constant (mm );
−3
C is the core constant (mm ).
5 Formulae for the various types of cores
5.1 Ring cores
5.1.1 Ring cores in general
Drawings of ring cores are shown in Figure 1.

Figure 1 – Ring cores
==
2π
C =
h ln dd/
( )
e 12
4π 1/ dd−1/
( )
C =
h ln dd/
( )
e 12
5.1.2 For ring cores of rectangular cross-section with sharp corners
The formula is as follows:
hh=
e
The geometrical cross-section of a ring core with rectangular shape A is given as:
g
d − d
Ah=
g
5.1.3 For ring cores of rectangular cross-section with an appreciable average
rounding radius r
The formula is as follows:
1,7168r
k =
hh(1− k)
e1 1
hd − d
( )
5.1.4 For ring cores of rectangular cross-section with appreciable chamfer c
The formula is as follows:
4c
k =
hh 1− k
( )
e3
hd − d
( )
The geometrical cross-section of a ring core with appreciable chamfer shape A is given as:
g
d − d
12 2
Ah − 2c
g0
5.1.5 For ring cores of trapezoidal cross-section with sharp corners
The formula is as follows:
h(tanα+ tanβ)
hh(1− k ) k =
e 2 2
d − d
1 2
=
=
=
=
5.1.6 For ring cores of trapezoidal cross-section with an appreciable average
rounding radius r
The formula is as follows:
h h 1− k− k
( )
e 12
5.1.7 For ring cores of cross-section with circular arc frontal sides
The formula is as follows:
d − d
φ sinφφ
hh=− 2sin −−
e 

4sin φ / 2
( )
d − d
φ= 2arcsin
4r
When the winding is uniformly distributed over a ring core, it may can be expected that, at all
points inside the ring core, the flux lines will be parallel to its surface.
No leakage flux will therefore leave or enter the ring core. This justifies the use of a theoretically
more correct derivation of the effective parameters, which does not make use of the assumption
that the flux is uniformly distributed over the cross-section.
5.2 Pair of U-cores
5.2.1 Pair of U-cores of rectangular section
Drawings of a pair of U-cores of the rectangular section are shown in Figure 2.
=
q
l1
l″ l′
4 4
X
A
Area
h
Y Y
A
Area
A
Area
l″ l′
5 5
X
Y-Y
l3
X-X
IEC
Figure 2 – Pair of U-cores of the rectangular section
A = pq
A = hp
A = ps
Length of flux path associated with area A :
l ll′+ ′′
2 22
Mean length of flux path at corners:
π
′ ′′ ( )
l = l + l = p+ h
4 4 4
π
l ll′′+ ′= qh+
( )
4 4 4
q
l″
l′
s p
=
=
π
′′′
l ll+ = sh+
( )
5 5 5
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A =
5 5
l
l
i
i
C = C =
1 ∑
2 ∑
A
A
i
i=1
i=1 i
5.2.2 Pair of UR-cores of rounded section
Drawings of a pair of UR-cores with the rounded section are shown in Figure 3.

Figure 3 – Pair of UR-cores of rounded section
In calculating A , ignore any ridges introduced for the purpose of facilitating manufacture.
π
Ap=
A = hs
π
As=
=
Length of flux path associated with area A :
′ ′′
l ll+
2 22
Mean length of flux path at corners:
π
′ ′′
l= l+=l ph+
( )
4 44
π
l ll′′+ ′= sh+
( )
5 5 5
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A =
5 5
l l
i i
C = C =
1 ∑ 2 ∑
A
A
i
i=1 i=1
i
5.2.3 Pair of URS-cores of rectangular-circular sections 2
Drawings of a pair of URS-cores with the rectangular-circular sections are shown in Figure 4.

Figure 4 – Pair of URS-cores of rectangular-circular sections
=
=
Length of flux path associated with area A :
′ ′′
l ll+
2 22
Mean length of flux path at corners:
π
l ll′+ ′′= qh+
( )
4 4 4
π
l ll′+ ′′= sh+
( )
5 5 5
In calculating A any rounding of the interior back wall edges introduced for the purpose of
2,
facilitating manufacture is ignored.
A = pq
A = hs
π
As=
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A
=
5 5
l l
i i
C = C =
1 ∑ 2 ∑
A
i A
i=1 i=1 i
A = min A 3
( )
min i
=
=
=
5.3 Pair of E-cores of rectangular section
Drawings of a pair of E-cores of the rectangular section are shown in Figure 5.

Figure 5 – Pair of E-cores of rectangular section
Area of half the centre limb leg: A
Mean length of flux paths at corners:
π
l ( ph+ )
π d 
lh+
5  
 
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A =
5 5
l l
i i
C =
C =
1 ∑ ∑
A
2A
i
i=1 i=1
i
A min A× 2 3
( )
min i
=
=
=
5.4 Pair of ETD/EER-cores
Drawings of a pair of ETD/EER-cores are shown in Figure 6.
b
l
X 1
l4
A
c
Area
Y Y
A′3
Area
A″
l3
Area
h
l
A3
Area
A2
Area
A
Area
Y-Y
X
X-X
IEC
Figure 6 – Pair of ETD/EER-cores
 1 
A is equal to the rectangle b ac− less the cap or segment A
1   c
 
11b
2 22
A d arcsin−−bd b

c2 2
44d
2
b
l
a
d
c
d /2
S
1 p
=
11 1 b
22 2
A ab− b d−−b d arcsin

1 22
24 4 d
2
Mean length of flux path at back walls:
d
22 3
l d+ db−−

2 22
42
dd− cd− / 2
NOTE l is taken from the mean value of ( ) and ( ) .
23 3
Area of half the centre limb pole:
A AA′+ ′′
3 33
The condition to obtain A' = A" is
3 3
S = 0,2980d
1 3
S = 0,289 01 d
Mean length of flux path at corners:
π
l ( ph+ )
where
a d
p= −−l
π
l 2S+ h
( )
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A =
5 5
l l
i i
C =
C =
1 ∑ ∑
A
2A
i
i=1 i=1
i
=
=
=
=
=
A min A× 2 3
( )
min i
5.5 Pair of pot-cores
Drawings of a pair of pot-cores are shown in Figure 7.
l
1 h
a
l′
A″1
X l″4
Area
A′
Area
A
d1/2
Area
l
d4/2
A
Area
l″ l′
5 5
d2/2
d3/2
A″
Area
A′
Area
X
X-X
IEC
Figure 7 – Pair of pot-cores
b
θ
l″ l″
2 6
S
S
l′ l′
2 6
=
Area of outer ring:
′ ′′
A AA+
1 11
The condition to obtain A' = A" is
1 1
d
S =−+ dd+
1 ( 12 )
Area of centre limb pole:
′ ′′
A AA+
3 33
The condition to obtain A' = A" is
3 3
d 1
S=−+dd
2 ( 34 )
2 8
Area of ring:
An(π− θd) −d
1 ( 12 )
2b
θ= arcsin
dd+
where
b is the slot width;
n is the number of slots.
Core factors associated with l :
l 1 a
= ln
A πhd
l ad−
2 3
=
2 2 2
A π ad h
2 3
Area of centre limb pole:
π
2 2
A = (d − d )
3 3 4
Mean length of flux paths at corners:
π
l = l′ + l′′= (2S + h)
4 4 4 1
=
=
=
π
′ ′′
l = l + l = (2S + h)
5 5 2
Areas associated with l and l :
4 5
A for cores with back-wall slot:
1 h
A (π− nθ)(d−d )+ (πd−nb)
4 12 2
A for cores without back-wall slot:
1 π
A (π− nθ)(d−d )+ dh
4 12 2
Area of A :
π
A d−+d 4dh
( )
5 3 4 3
Core factors associated with l :
l d
6 2
= ln
Anπ− θh a
( )
l da−
6 2
=
A
ad π− nθh
6 ( )
6 6
l
l
i
i
C = C =
∑ ∑
1 2
A
A
i
i=1 i=1 i
A = min A
( ) 3
min i
=
=
=
5.6 Pair of RM-cores
Drawings of a pair of RM-cores Type 1 through Type 4 are shown in Figure 8.
This calculation is also applicable to the core type without a hole.

Type 1 – RM6–S
l′4
A
l″4
X
p
ϕ
lmin
l3
A
A
β
d /2
l″5
l′5
d3/2
l
max
d2/2
e
X
h
l1
c
A /2
X-X
IEC
Type 2 – RM7
A
X
p ϕ
l
min
A
A3
d4/2
β
d /2
l
max
d /2
b
e
X c
A /2
IEC
Type 3 – RM4, RM5, RM8, RM10, RM12, RM14
A8
p
ϕ
lmin A
l′max
d4/2
β
l″ d /2
max 3
d2/2
e
c
A1/2
lmax = l′max + l″max
IEC
α
α
α
a
a
l″
a
l′
Type 4 – RM6–R
A8
X
p
ϕ
l
min
A7
A3
β
d4/2
d /2
l
max
d /2
e
X
c
A /2
IEC
α
a
a) Type 1 – RM6–S
b) Type 2 – RM7
c) Type 3 – RM4, RM5, RM8, RM10, RM12, RM14

d) Type 4 – RM6–R
Figure 8 – Pair of RM-cores
Total area of the outer leg:
1  π  β 1
 
2 2 2
A = a 1+ tanβ− − d − p
 
 
1 2
2 4 2 2
 
 
e
where β=α− arcsin
d
Core factors associated with l :
d
ln f
l d
2 3
=
A Dπh
where
l + l A
min max 7
f = D=
2l A
min 8
,
′ ′′
l = l + l
2 2 2
l (1 d − 1 d )f
2 3 2
=
2 2
A (Dπh)
Type 1, Type 4:
1 1
2 2
l = (d + d )− d d cos(α−β)
max 2 3 2 3
4 2
Type 2:
1  1 b
l = d + d − d d cos(α−β)−
 
max 2 2 3
ϕ
4 2
 
2sin
Type 3:
1 ϕ
l = [e tanβ− c(1− sin )]
max
ϕ
2tanβ⋅ sin
Type 1:
 
1 β 1 1  ϕ π
2 2 2 2
A = d + e tanβ− e tanα− − d
 
7 2 3
4 2 2 2 2 4
 
 
Type 4:
11βφ1 π
2
A d+ dd sinα−β+ cd− tan− d
( ) ( )

7 2 23 3 3
42 2 2 2 4

Type 2:
1β π 1 ϕ 1 
 
2 2 2 2 2
A = d − d + (b − e )tanα− + e tanβ
 
 
7 2 3
4 2 4 2 2 2
 
 
Type 3:
1 β π 1
 
2 2 2
A = d − d + c tan(α−β)
 
7 2 3
4 2 4 2
 
α
2 2
A = (d − d )
8 2 3
Area of centre pole:
π
2 2
A = (d − d )
3 3 4
Mean length of flux paths at corners and mean areas associated with these:
π 1 1 
′ ′′
l = l + l = h+ a− d 
4 4 4 2
4 2 2
 
A = (A + 2βd h)
4 1 2
 
π 1 
2 2
l = l′ + l′′= d + h− (d + d )
 
5 5 5 3 3 4
4 2
 
 
1π 
2 2
A = (d − d )+ 2αd h
5  3 4 3 
2 4
 
5 5
l l
i i
C = C =
1 ∑ 2 ∑
A
i A
i=1 i=1 i
A = min A 3
( )
min i
=
This calculation ignores the effect of spring recesses and stud recesses. These can have some
influence on the outcome of the calculation, especially for smaller cores.
5.7 Pair of EP-cores
Drawings of a pair of EP-cores are shown in Figure 9.

Figure 9 – Pair of EP-cores
As a pair:
lh
=
A
ab−πd /8− d c
l h
=
2 2 2
A (ab−πd /8− d c)
1 11
ld2
= ln
A π−αh −h d
( )( )
2 12 2
4 d − d
l ( )
=
2 22
A
(π−α) (h −h ) dd
1 2 12
4h
l h
2 =
=
A
πd
3 d 2

π


16h
l h 2
3 2
=
=
π d
A
d 2
3 2
π


Areas associated with l and l :
4 5
π d hh− 
1 12
l= ll′+=′′ γ− +
4 44  
22 4
 
(π−α)d + 2 ab−πd / 8− d d / 2
( )
1 1 12
γ=
4 π−α
( )
where y is a hypothetical radius bisecting the cross-sectional area of the ring.

d d h h
1 π 
2 12 1 2
A ab− d− +π−αd −
( )

41 1
2 8 2 22


πd hh−
2 12
l= ll′+=′′ 0,292 89 +
5 55 
2 24


π dd

A= + hh−
( )

5 12
24 2


5 5
l l
i i
C = C =
∑ ∑
1 2
A
A
i=1 i=1
i
i
A = min A 3
( )
min i
=
5.8 Pair of PM-cores
Drawings of a pair of PM-cores are shown in Figure 10.

Figure 10 – Pair of PM-cores
Total area of the leg:
β
A d−−d 2bt
( )
1 12
where
f
βα− arcsin
d
Core factors associated with l :
l ll′+ ′′
2 22
d
ln g
d
l
2 3
=
A Dhπ − h / 2
( )
2 12
where
ll+
min max
g=
2l
min
A
D=
A
l d+−d dd cosαβ−
( )
( )
max 2 3 2 3
11d − dg
l ( )
=
A
Dhπ − h 2
2 { ( ) }
β 1 1  ϕ π
2 2 2 2
A = d + f tanβ− f tanα− − d
7 2 3
8 8 8 2 16
 
α
A dd−
8 ( 23 )
Area of centre pole:
π
A dd−
3 ( 34 )
=
=
=
=
=
=
Mean length of flux paths at corners and mean areas associated with these:
π
′ ′′
l= l+=l hh− + d− d
( )
4 44 1 2 1 2
A= {A+−βd (h h )}
4 1 21 2
π hh− 1
l = l′+ l′′= +− +
{d (dd )}
5 5 5 3 34
4 22
π ()hh−
A ()d−+d αd
5 34 3
l
l
i
i
C = C =
∑
1 2 ∑
A
A
i
i=1 i=1
i
A = min A
( ) 3
min i
5.9 Pair of EL-cores
Drawings of a pair of EL-cores and PLT(plate)-cores are shown in Figure 11 and Figure 12.
EL + PLT (plate)-cores use EL core formulae.
=
C B
l1
F D
l4
l3
l
R
IEC
Figure 11 – Pair of EL-cores
Figure 12 – PLT(plate)-cores
Area of outer leg:

A A− EC− 4 R−πR
( )
1 

F
H
E
A
l
=
Mean length of flux path at outer leg:
EL-EL lD=
D
EL-PLT l =
Area of back wall:
A = (C+(F − F)+πF /2)(B− D)
2 2 1 1
1 πF
EL-EL
A CFF+ −+ B− D
( ) ( )
2  21  1
 
1 πF B −+DB
  
11 2
EL-PLT A CFF+ −+
( )
2  21  
2 22
  
Mean length of flux at back wall:
E F
l −
2 

Area of centre limb leg:

A π+F F− FF
( )

3 1 2 11

Mean length of flux path at centre limb leg:
EL-EL lD=
D
l =
EL-PLT
Area of outside corner:
AA+
1 21
A =
where A = (B − D)C
EL-EL
A (B− DC)
21 1
B −+DB

EL-PLT
AC=


=
=
=
=
=
Mean length of flux path at outside corner:
 
π A E

l =  − +(B− D)
 
 
8 2 2
 
 
πAE
EL-EL
l −+ BD−
( )
41

8 22


 
π AE B −+DB

l −+
EL-PLT  
4  
8 22 2


 
Area of inside corner:
AA+
23 3
A =
where A =((F −F)+πF /2)(B− D)
23 2 1 1
πF
 
EL-EL
A (F− F)+ (BD− )
23  2 1  1
 
 πF B −+DB 
11 2
EL-PLT
A FF− +
( )
23  2 1  
  
Mean length of flux path at inside corner:
π A 
 
l = +(B− D)
 
8 F
 2 
A
π
l +−BD
EL-EL ( )

8 F
2

π A B−+DB

3 12
EL-PLT
l +

5 
8 F 2

2
l
l
i
i
C = C =
∑
1 2 ∑
A
2A
i
i=1 i=1
i
l = CC A = C / C V = CC
e 12 e 12 e 12
A min A× 2 3
( )
min i
5.10 Pair of ER-cores (low profile)
Drawings of a pair of ER-cores (low profile) and PLT(plate)-cores are shown in Figure 13 and
Figure 14.
=
=
=
=
=
=
=
ER + PLT (plate)-cores use ER core formulae.
B
l
C
D
l
l
l5
A
A1
A′3
A′3
β
A″
A″ 3
α
A3
A3
IEC
G
F
E
A
S
S
l
B
Figure 13 – Pair of ER-cores (low profile)

Figure 14 – PLT (plate)-cores
Area of outer leg:

1 αE EG
A CA−−G − sinα
( )

2 44

where
α = arccos (G/E)
Mean length of flux path at outer leg:
ER-ER lD=
D
l = 5
ER-PLT
Area of back wall:
A = C(B − D)
ER-ER
A CB− D
( )
B −+DB

ER-PLT AC=
2 

Mean length of flux path at back wall:
22
l E+ GC+− 2F

4
Area of centre limb pole:
 
A πF
3  
 
Mean length of flux path at centre limb pole:
ER-ER lD=
D
l =
ER-PLT
Area of outside corner:
AA+
A =
=
=
=
=
Mean length of flux path at outside corner:
π
l ph+
( )
where
A E
h= B− D p= −
2 2
AE
hB=−=D    p −
ER-ER
−+
B DB AE
hp  −
ER-PLT
2 22
Area of inside corner:
AA+
A =
Mean length of flux path at inside corner:
π
l 2S+ h
( )
where
ER-ER hB− D
B −+DB
ER-PLT h=
The condition to obtain A′ = A″ is
3 3
SF= 1−=cosβF0, 298 01
( )
5 5
l l
i i
C = C =
1 ∑ 2 ∑
A
2A
i=1 i=1
i i
2 32
l = C / C V = C / C
A = C / C
e 12 e 12 e 12
A min A× 2 3
( )
min i
=
=
=
= =
=
5.11 Pair of PQ-cores
Drawings of a pair of PQ-cores and PLT (plate)-cores are shown in Figure 15, Figure 16 and
Figure 17.
PQ + PLT (plate)-cores use PQ core formulae.
NOTE 1 This calculation ignores the effect of spring recesses.
NOTE 2 The equations below are consistent with those given in IEC 62317-13.

′
″
l
A8 4
X l 4
A
α
β l3
l
max
′
″
l 5
l
A3
L
A /2
X
h
l1
D
C
B
X-X
IEC
d
x
x
IEC
A
l
min
J
G
F
F/2
E
E/2
″
l
′
l
Figure 15 – Pair of PQ-cores
Area of outer leg:
βE 1
A C()A−−G + GI
where
G
 
β= arccos
 
E
 
IE= sinβ
Mean length of flux path at outer leg:
PQ-PQ lD= 2
PQ-PLT lD= 6
=
Core factors associated with l :
For l , A the elemental radius dr shown in Figure 16 is the elemental length of the flux path in
2 2
the integral below. The radius vector extends from F/2 to E/2 for the entire circle. The effective
length l for the section is multiplied by f. The area is the physical area multiplied by K.
2i
Figure 16 – PQ-cores
Figure 17 – PLT(plate)-cores
E
l
f f  E
2i
= dr= ln
 
∫F
A K2πr(B− D) 2πK(B− D) F
 
E E
l 2 fdx 2 f dx 1/ F−1/ E
2 2
= = = f
2 ∫F 2∫F 2 2 2 2
2πx
A [2Kπ(B− D)] x K π (B− D)
{2K [ (B− D)]}
2 2
E
l f fE
PQ-PQ dr ln

∫
F
AK22πr B −−D πK B D F
( ) ( ) 
21 1
EE
l
2 fdx 2 f dx 1/ F−1/ E
f
∫∫
FF
2 2 2 22 2
2πx

A [2Kπ(BD− )] x K π (BD− )
2 11
{2K BD− }
( )


E
l f fE

PQ-PLT dr ln

∫ F
A B −+DB B −+DB F

12 12
Kr22ππK
 
 
EE
l 22fdx f dx 1/ F−1/ E
2 22
f
∫∫
FF
2 2 22 2
Ax
B −+DB
 2πx B −+DB  B −+DB   
2  22
12 12 12
K π
22KKπ

 
 
22 2
  
  
where
AA
K
π
A 22
EF−
( )
A βE−αF+−GL JI
7 ( )
L
α= arctan

J

ll+
min max
f =
2l
min
E +−F 2EF cosαβ−
( )
l =
max
=
==
===
==
===
==
Define the other two physical areas in the flux path at back wall.
A = 2αF(B− D)
A = 2βE(B− D)
PQ-PQ
A 2αF B−D
( )
A 2βE(B−D)
10 1
B −+DB
PQ-PLT
A = 2αF
9 

B −+DB
A = 2βE
10 

The mathematical area A is given as A > A > A .
2 10 2 9
Area of centre limb pole:
A πF
Mean length of flux path at centre limb pole:
PQ-PQ l = 2D
PQ-PLT lD=
Area of outside corner:
1 1
A = (A + A )= [A + 2E(B− D)β]
4 1 10 1
2 2

A= A+ A= A+ 2E B− D β
PQ-PQ ( ) ( )
4 1 10 1 1

B −+DB
11 
A= AA+ = A+ 2E β
PQ-PLT ( )

4 1 10 1 
22 2


Mean length of flux path at outside corner:
π 1 1 
' "
l = l + l = (B− D)+ A− E
 
4 4 4
4 2 2
 
π 11
 
′ ′′
PQ-PQ l l + l BD−+ A− E
( )
4144  
4 22
 
= =
=
=
=
π B −+DB 11 
1 2
′ ′′
l ll+ + AE−
PQ-PLT
4 44  
4 2 22
 
Area of inside corner:
1 π F
 
A = (A + A )=   + F(B− D)α
5 3 9
2 2 2
 
1 π F

A A+ A + FB− D α
PQ-PQ ( ) ( )
5 39  1
2 22

1 π F B −+DB

A AA+ + F α
PQ-PLT ( )
5 39  
2 22 2
 
Mean length of flux path at inside corner:
 
 
π 1
' "
 
 
l = l + l = (B− D)+ 1− F
5 5 5
 
 
 
 
 
π  1 
PQ-PQ ′′′
l ll+ BD−+ 1− F
( )
   
515 5
 
 
 
π B −+DB  1 
l ll′+ ′′ +−1 F
PQ-PLT  
5 5 5  
 
 
l
l
i
i
C =
C =
2 ∑
1 ∑
A
A
i
i=1 i=1
i
C
C
C
l =
V =
A =
e
e
e
C
C
C
The minimum physical cross-section area A is given as:
min
A = min (A , A , A , A , A )
min 1 3 4 5 9
A = min A 3
( )
min i
==
= =
= =
= =
= =
5.12 Pair of EFD-cores
Drawings of a pair of EFD-cores are shown in Figure 18.

Figure 18 – Pair of EFD-cores
Area of outer leg:
CA− E
( )
A =
Mean length of flux path at outer leg:
lD=
Area of back wall:
A CB− D
( )
Mean length of flux at back wall:
EF−
l =
Area of centre limb leg:
FF − 2q
A =
where
q chamfer
=
Mean length of flux path at centre limb leg:
lD=
Area of outside corner:
AA+
( )
A =
Mean length of flux path at outside corner:
π−A E

l +−BD
( )
4 

Area of inside corner:
AA+
A =
Mean length of flux path at inside corner:
2 2
F CF−− 2K
π− BD

l=++
5  

44 2 2



l
l
i
i
C =
C =
2 ∑
1 ∑
A
2A
i
i=1 i=1
i
C
C
C 1
V =
A =
l =
e
e
e
C C
C
2 2
A min A× 2 3
( )
min i
5.13 Pair of E planar-cores
Drawings of a pair of E planar-cores and PLT(plate)-cores are shown in Figure 19 and Figure 20.
E planar + PLT (plate)-cores use E planar core formulae.
=
=
B
C
l
D
l
R1
l3
R2
l5
IEC
Figure 19 – Pair of E planar-cores

Figure 20 – PLT(plate)-cores
F
E
A
l
Area of outer leg:
CA− E
( ) π

A −−4 RR
1 11

Mean length of flux path at outer leg:
E-E lD=
D
l =
E-PLT  7
Area of back wall:
A = C(B− D)
E-E A CB()− D
C
A BB+− D
E-PLT ( )
2 12
Mean length of flux at back wall:
EF−
l =
Area of centre limb leg:
FC π
A= −−2(RR )
3 22
Mean length of flux path at centre limb leg:
E-E
lD=
D
l =
E-PLT
Area of outside corner:
AA+
A =
Mean length of flux path at outside corner:
π A− E
 
l = +(B− D)
 
8 2
 
=
=
=
π−A E

E-E l +−BD
( )
41

π−A E B −+DB
E-PLT
l +
4 
82 2

Area of inside corner:
AA+
( )
A =
Mean length of flux path at inside corner:
π F 
l =  +(B− D)
8 2
 
πF
E-E l +−BD
( )
51

π F B −+DB

E-PLT
l +
5 
82 2

l
l
i
i
C =
C =
∑ 2 ∑
A
2A
i
i=1 i=1
i
C
C
1 C
l =
V =
A =
e e
e
C
C
C
A min A× 2 3
( )
min i
5.14 Pair of EC-cores
Drawings of a pair of EC-cores are shown in Figure 21.
=
=
=
=
=
Figure 21 – Pair of EC-cores

A is equal to the rectangle C Ac− less the segment A and the segment A .
1  c s

11 C
2 22
A E arcsin−−CE C
c  
44E
 
SA()− T− S πS
A +
S
11 1 C SA( − T− S) πS
 
22 2
A AC− C E− C− E arcsin − −
1  
24 4 E 2 8
 
Mean length of flux path at back walls:
1 F
 22
l= E+ EC−−
 
42 
NOTE l is taken from the mean value of and .
EF− c− F / 2
( ) ( )
Area of half the centre limb pole:
A AA′+ ′′
3 33
=
=
=
=
The condition to obtain A' = A" is
3 3
S = 0,2980 F
SF= 0,289 01
Mean length of flux path at corners:
π
l ph+
( )
where
A F
pl= −−
π
l 2S+ h
( )
Mean areas associated with l and l :
4 5
AA+
A =
AA+
A =
l l
i i
C =
C =
1 ∑ ∑
A
2A
i
i=1 i=1
i
A min A× 2 3
( )
min i
=
=
=
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