Page 184 - Electrician - TT (Volume 2)

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As such there will be only one coil in a group. But concentric winding should have two or more coils in a group.
In this case concentric winding is not possible. Alternatively grouping can be done for half-coil connection, i.e.

No. of coils/phas e/pair of poles
Total No. of coils

No. of phase x No. of pair of poles
12
As per the example 2 coils
3 x 2
i.e. 2 coils/phase/pair of poles.
As per the above example, only half-coil connected concentric winding is possible whereas for the
following example having data 48 slots, 24 coils, 4-pole, 3-phase stator winding both whole coil
and half coil connections are possible. Hence it is necessary to trace the group connection very
carefully before stripping the stator to determine whether the winding connection is whole coil or
half coil.
2 Pitch

1 No . of slots
Pole pitch =
No . of poles
24
As per the example = 6 slots
4
As the winding is concentric, there should be 2 or more pitches normally. According to the above example 2
pitches for half-coil connections are required.
Further it is necessary to have the average pitch equal i.e. to the pole pitch.
(i.e.) coil pitch = pole pitch +1

As per the example coil pitch is 6+1.
Therefore outer coil pitch = 6 + 1 = 7
and inner coil pitch will be = 6 - 1 = 5
(i.e.) Coil throw = 1 - 8 and 1 - 6 In practice it is written as 1 - 8 and 2 - 7.
3 Electrical degrees

i Total electrical degrees = 180º x No. of poles.
As per the example = 180º x 4 = 720º.

180° x 4
ii Slot distance in degrees =
No.of slots
180 ° 4 x

= = 30°
24
4 Phase displacement
i For three-phase winding phase displacement should be equal to 120o

ii Phase displacement in terms of slots
120°
=
slot distance in degrees

120°
As per the example = = 4 slots
30°
5 Winding sequence
As per the example
A phase starts from 1st slot.

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