Specifications

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starting end S and finishing end F of a coil. In Fig. 3.1 (a), AD and BC represents the inactive
sides of a coil.
Coil Groups : One or more coils connected in series are called coil groups, as shown in Fig.
3.2. The number of coil groups is equal to the number of poles. In Fig. 3.2, there are four coil
groups, which are equal to four numbers of poles. For AC winding, the total number of coil
groups depends upon the number of poles and the number of phases.
∴ the total number of Coil groups = mP 3.3
Also, total number of coil groups/phase = 3.4
where m = number of phases
P = number of poles.
Example 3.1: Find the total number of coil groups for a 3 phase 6 pole machine.
Solution From equation 3.3, coil groups = mP = 3 x 6 = 18.
Pole Pitch : It is the distance between the centres of two adjacent opposite poles. It is measured
in terms of slots.
One pole pitch = = 180
0
ed
or 180
0
ε 3.5
where S = number of slots
ed = electrical degree
Example 3.2 : Calculate the pole pitch for a three phase 4 pole ac machine having 36 stator
slots.
Solution From equation 3.5, pole pitch = = = 9
Coil Span or coil pitch :
It is the distance between the two active sides of the same coil under
adjacent opposite poles. It is expressed in terms of number of slots per pole or electrical degrees.
Full pitch coil : A coil having a coil span equal to 180
0
ed
is called a full pitch coil, as shown in Fig. 3.3
(a).
Short pitch coil : A coil having a coil span less than 180
0
ed
by an angle â, is called a short pitch
coil, or fractional pitch coil, as shown in Fig. 3.3 (b). It is also called a chorded coil.
á= 0 for full pitch winding 3.6
Fig.3.2 - Coil groups
N S N S N
Number of coil groups
Number of phases
mP
m
=
Number of slots
Number of Poles
S
p
=
S 36
P 4