Page 175 - Electrician - TT (Volume 2)

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As in the example

For the example taken, half coil connection is possible for distributed winding by taking full pitch
and placing coil in alternate two slots., but it is not in practice. Hence whole coil connection is taken
as an example.
II Pitch

As in the example, pole pitch = 24/4 = 6 slots.

ii Coil pitch
In AC winding the relation between the coil pitch and the pole pitch is given below.
a Coil pitch = Pole pitch Then the winding is called full pitch winding.
b Coil pitch < Pole pitch Then the winding is called fractional pitch - short chorded winding.
c Coil pitch > Pole pitch Then the winding is called as fractional pitch - long chorded winding.
Further, if the winding is double layer, all the above ‘a’, ‘b’ and ‘c’ are possible. But for single layer distributed
winding as the coils should be placed in alternate slots only, the coil pitch ought to be in odd number.
As in the example, coil pitch = pole pitch = 24/4 = 6 slots.
Here 6 is an even number and winding cannot be of full pitch, so the next alternative is to select a fractional pitch.
Therefore the coil pitch can be taken either as 5 or 7. Normally AC windings should either have full pitch or short
chorded fractional pitch. Hence a suitable pitch is taken of 5 slots.

iii Coil throw
The coil throw for coil pitch ‘5’ as in the example is 1 - 6.
III Electrical degrees
i Total electrical degrees = 180º x No. of poles
(180º is the distance between poles)

ii

As in the example: Slot distance = (180x4)/24 = 30º
IV Phase displacement
i For three-phase winding, displacement between the phases should be 120º .
ii Phase displacement in terms of slots = 120o/slot distance

As in the example, 120º/30º = 4 slots
V Winding sequence
In three-phase winding the distance between the starting end of one phase to the starting end of another phase
should have 120 electrical degrees. Hence we should arrange the winding such that

‘A’ phase starts from say 1st slot
‘B’ phase starts from 1st slot + 120o and
‘C’ phase starts from 1st slot + 120o + 120o.

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