Q. Draw the developed diagram of a simplex progressive 2 layer lap winding for a 4 pole generator with 12 coils clearly indicating the position of brushes.
Sol. No. of commutator segments = no. of coils, C = 12, Twelve coils have 24 coil - sides. For two coil - sides per slots,
Total no. of slots = 24/2 = 12
Back pitch, yb = 2C/P ± K = 24/4 ± 1 = 7 or 5
For progressive lap winding,
Yw = yb - yf = +2
yf = 5 for yb = 7
or yf = 3 for yb = 5
It is desirable that yb + yf/2 should be as nearly equal to pole pitch as it possible. In view of this, yb = 7 and yf = 5 should be chosen for this commutator machine, because 7 +5/2 = pole pitch in terms of coil - sides (= 24/4 coil - sides per pole).
For probressive simplex lap winding, commutator pitch,
Yc = +1
For drawing the winding diagram in radial form, 12 commutator or segments, 12 slots and 24 coils sides are first drawn. Now segment 1 is connected to top coil - sides 1 and then to bottam coil - sides 8 (1 + yb = 1 +7). Note that the coil - sides 1 and segment 1 are so numbered tha the coils resemble a lap coil. Since yC = 1, the coil - sides 8 is connected to segment 2 (1 + yC = 1 +1).Bottom couil - side is now connected to top coil - sides 3 (8-yf = 8-5), then to bottom coil - sides 10 (3 +7), then to top coil - side 5 (10-5), then to bottom is helpful in drawing this winding diagram.
+ yb -yf +yb -yf +yb
(1 - → 8) - → (3 - → 10) - → (5 - → 12)- →
(7 → 14) → (9 → 16) → (11 → 18) → (13 → 20) →
(15 → 22) → (17 → 24)(19 → 26 or 26-24, i.e. 2) → (26 → yf or 21 → 21+yb, i.e. 4) → (28 - yf i.e. 23 → 6) → 1
In the winding table (1→8) means that the top coil - side 1 is, connected to bottom coil - side 8 to form one coil. Same is true for all other coils. Note that winding table does not contain any no, greater than 24 ( = total no. of coil - sides). Each coil - sides is used only once in the winding table and the winding closes on itself (starting from 1 and ending on 1)
In this machine with 12 slots and 4 pole, 3 slots are assumed to be under the effect of each pole. For example, 3 slots with coil - side 7 to 12 under the adjacent S pole and so on. Direction of e.m.fs. and current in the coil - sides under N pole are indicated by, cross and those under S pole denoted by dot. Now the directions of currents can be marked by noting that current flows from segments to cross - marked coil - sides and to the segments from dot - marked coil - sides.
A careful examination of winding reveals that current leave at the commutator segments, 1, 7 and the current enter at the commutator segments 4, 10. Thus four brushes A, B, C, D can be placed in contact with segments 1, 4, 7, 10 respectively. Brushes A and C have the same polarity. These can, therefore, be joined together, similarly brushes C and D can be joined. Armature power can be handled at the terminals marked E, F. If E and F are given are negative and positive polarities respectively; the machine works as motor with the direction of rotation anti - clockwise.
From brushes A, one parallel path is through coils a, b, c to brush B and the second parallel path is through coils l, k, j to brush D. From brush C, one parallel path is through coils f, e, d to brush B and the second parallel path through coils g, h, I, o brush D. These parallel path at any instant. There are thus four parallel paths for a 4 - pole machine - in general, for a simplex lap winding, no. of parallel paths A is equal to the no. of poles P, i.e. A = P. Expression ΦZN/60 (P/A) reveals tha the generated e.m.f. in lap connected machine (equal to Φ.ZN) is independent of the no. of poles.