DOUBLE LAYER
Double layer windings differ from single layer winding mainly on the
following main points:
 Each slot is occupied by the side of two coils and each coil is
arranged to form two layer round stator.
 One layer of the winding lies in the bottom half of the slots
and the other in the top half of slots.
 Unlike the concentric winding double layer winding consists of
identical coils all of the same shape and pitch.
 In a double layer winding, the coil pitch is the distance
between the top and the bottom sides of the coil expressed by
the number of slots spanned or by the coil sides or by the
number of slots occupied by each coil side.
 A coil pitch may be full or fractional. Majority stator windings
use a fractional pitch because
 The amount of copper used in the overhang (end winding) reduced
and hence a saving on copper, and
 The magnitude of certain harmonics in the emf and also mmf is
reduced.
The full pitch is determined by
S
YS 
P
Usually the full pitch is shortened by onesixth i.e. for example if the full pitch is 12 a
fractional will be 10.
Since the coils are wound with a
continuous length of wire there are no
connections between turns.
Double Layer Three phase windings
 The main value characterizing the two layer winding is
the number of slots per pole per phase.
S
q
Pm
 By looking double layer winding externally, it is not possible to
determine q.
 The total number of coils in two layer winding is equal to the
number of slot since each side of a coil occupies one half of a
slot which is equivalent to occupying one full slot per coil.
 In order to avoid making solder joints between coils, several
coils, depending upon slots per pole per phase, are generally
wound from a single length of wire in to full coil group.
 The number of coil groups per phase is a equal to the number
of poles the whole winding. That is
K
K  mP
 P
m
 This is, twice that in a single-layer winding.
Rule for double layer windings
The coil groups should be connected to each
other by joining the leads of like polarity i.e. the
finish of one group to the finish of the next group
and the start of one group to the start of the next
group.
Example # 4: on double layer winding
Given data
S=12; p=2;m=3; a=1; type=Double layer: the pitch shortened by one slot
•
Solution
a)
The number of coil groups, K
K  3 P  3 2  6
b)
c)
i.e. there is two coil groups per phase
The number of slots per pole per phase, q
S
12
q

2
i.e. there are two coils in a group
m  p 3 2
Coil pitch
S 12
YS  
6
Full-Pitch
p 2
Let us shorten the pitch by one slot and make YS = 5.
d)
The electrical angle, 
e)
The angle between adjacent slots, 
  180 P  180 2  360

360
 
 30
S
12
f)
The distance between the beginning of each phase, 
120
120


 4 slots

30
g)
If the beginning of Phase A is slot 1, then the beginning of phase B is slot
1+=5 and the beginning of phase B is slot 1+2=1+8=9
Phase sequence
(Top layer Coil-sides)
A
B’
600
C’
C
B
A’
1
2
3
4
5
6
7
8
9
A
A
C’
C’
B
B
A’
A’
C
10
C
11
B’
12
B’
Connection Diagrams
Phase A
A
Phase B
Phase C
B
C
III
I
1
2
+5
+5
6'
5
7'
6
IV
7
8
A’
+5
+5
+5
+5
V
10'
9
11'
10
VI
12'
11
1'
12
B’
+5
+5
2'
+5
+5
3'
II
4'
3
5'
4
C’
+5
+5
8'
9'
PROCEDURE FOR CONSTRUCTING
OF DOUBLE LAYER WINDINGS

1
Draw 24 vertical lines to represent the two coil sides lying in each of
the 12 slots. For each slot the full line at the left hand side will
represent a top a coil side and broken line at the right hand side a
bottom coil side.
1'
2
2'
3
3'
4
4'
5
5'
6
6' 7
7'
8
8'
9
9' 10 10' 11 11' 12 12'

The top part of slot 1 will be taken as the beginning of the first
phase. According to the selected fractional pitch, the conductor
from slot 1 is now connected to that in the bottom coil side in slot 1
+ 5 = 6’.
1
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
12'

Since q = 2 i.e. each coil group will consists of two coils. The conductor
must therefore leave the bottom of slot 6 to enter the top of slot 2 and
from there the bottom of slot 7. The lead emerging from slot 7 will be
finish of the first coil group.
1
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
12'
Phase A : Coil groups
Phase A
A
I
+5
1
6'
+5
2
7'
IV
7
8
A’
1'
8
1
8
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
12'
1'
+5
+5
12'
1'
Phase B
Phase B: Coil groups
B
III
5
+5
10'
+5
6
11'
IV
11
+5
4'
+5
12
5'
B’
11
8
1
11
12
8
4'
1'
5'
12
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
12'
5'
1'
4'
Phase C
C
Phase C: Coil groups
V
9
10
2'
+5
+5
3'
II
3
4
+5
+5
8'
9'
C’
10
3'
11
12
8
1
11
9
2'
12
8
10
9
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
5'
1'
4'
2'
3'
Current direction
N
S
1-6
7-12
10
3'
11
2'
12
8
1
9
1'
2
2'
3
3'
N
11
9
12
8
10
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
S
5'
1'
4'
2'
3'
Current direction
N
S
1-6
7-12
10
3'
11
2'
12
8
1
9
1'
2
2'
3
3'
N
11
9
12
8
10
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
S
5'
1'
4'
2'
3'
Phase A: Coil groups interconnection
10
3'
11
2'
12
8
1
9
1'
2
2'
3
3'
N
11
9
12
8
10
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
S
5'
1'
4'
2'
3'
Phase B: Coil groups interconnection
10
3'
11
2'
12
8
1
9
1'
2
2'
3
3'
N
11
9
12
8
10
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
S
5'
1'
4'
2'
3'
Phase C: Coil groups interconnection
10
3'
11
2'
12
8
1
9
1'
2
2'
3
3'
N
11
9
12
8
10
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
S
5'
1'
4'
2'
3'
Terminal leads
10
3'
11
2'
12
9
8
1
11
9
1'
2
2'
3
3'
4
4'
5
5'
6
6'
7
7'
8
8'
9
9'
10
10'
11
11'
12
4'
1'
5'
12'
5'
1'
4'
2'
12
8
10
3'
A
C’
B
A’
C
B’