Page 188 - Electrician - TT (Volume 2)

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2 Coil pitch : Similar to the single layer winding the coil pitch can be short-chorded, long-chorded or equal to the
pole pitch. The pitch of the double layer distributed winding may be odd or even number. As per the example,
the pole pitch is equal to 36/4 = 9 slots and the no. of coils per group is 3. Hence the coil pitch may vary from
9 + 3 that is 6,7 or 8 in the case of short corded winding, 9 in the case of full pitch winding and 10,11 or 12 in
the case of long chorded winding. Hence the possible coil throws can be taken as

1 to 7 and 1 to 8 for short chorded winding
1 to 9 and 1 to 10 for full pitched winding
1 to 11, 1 to 12 and 1 to 13 for long chorded winding.
Normally the winding is designed for either short chorded or full pitch. Occasionally a long chord is used by the
designer in double speed winding. The reason for not using long chorded winding is, it requires more chord length
resulting in the requirement of more copper, and hence, increased heat losses.

3 Coil throw : According to the above example the coil throw for the coil pitch of 8 will be 1-9.
III Electrical degrees :
Total electrical degrees = 180º x No. of poles
[180o distance between poles]
Slot distance in degrees = Total electrical deg rees
No . of slots
180 x No . of poles

No . of slots

180 x 4
As per the example 20
36
IV Phase displacement
i For three-phase winding each phase winding should be displaced by 120 electrical degrees.

ii Phase displacement in terms of slots =
120° ( Electrical )
Slot distance in degrees
120
As per the example 6 slots
20
V Winding sequence : In three-phase winding, the starting end of one phase winding to the starting end of the
second phase winding should have a distance of 120 electrical degrees.
Hence if the ‘A’ phase starts say in the Ist slot then the ‘B’ phase should start from the 1st slot+120o.
Further ‘C’ phase should start from the 1st slot+120o+120o.
As in the example ‘A’ phase starts from, say, 1st slot
‘B’ phase should start from 1+ 6 = 7th slot and
‘C’ phase should start from 1+ 6 + 6 = 13th slot.

VI Placing of the coils in double layer winding: As the winding is double layer, the laying of coils should start
in adjacent slots.
That is the coils should be placed in slot 1, slot 2, slot 3 and so on.

As in the above example the arrangement of coils for the selected pitch 8 will be as given below:

174 175

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