Page 171 - Electrician - TT (Volume 2)

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Short pitch winding is usually used in almost all machines except variable speed motors. The reasons for adopting
short pitch winding are given below.
1 Winding requires less copper.
2 Copper loss is less.
3 Efficiency of the machine is increased.

4 Winding occupies less space.
5 In alternators, the winding produces uniform sine wave.
Coil group : When you observe the direction of the current flow in a coil, you will see current in the two coil sides
have opposite directions as shown in Fig 4(a).

Fig 3 Fig 4

Accordingly the current in a single coil produces two dissimilar poles. In an ordinary winding, according to the
design, one or more coils may be connected in series to form a group as shown in Fig 4(b).(Three coils form one
group) The total number of coil groups in a winding is equal to the number of phases multiplied by the number of
poles.
Total No.of coil groups = No. of phases x No. of poles
Total No. of coil groups
Coil group per phase
No. of phases

Coil group per phase per pole
Total No. of coil groups

No. of phases x No. of poles
Further the number of coils in a group per phase per pole
Total number of coils
=
No. of phases x No. of poles

Total number of coils
=
Total number of groups
Coil connections : The connection which joins a coil lead of one coil to the other coil lead of the same coil group
is called ‘coil connection’ and is shown in Fig 5.
Pole connection : The connection which joins a coil group of one phase to another coil group of the same phase
of the winding is called pole connection or group connection, and is shown in Fig 5.
Whole-coil winding: A whole coil winding is one in which the number of coils per phase is equal to the number
of poles in the machine. Refer to Fig 6.

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CITS : Power - Electrician & Wireman - Lesson 86-92