Specifications

28
Thus, mechanical degree in terms of poles 3.13
where θ
md
= mechanical degree.
Mechanical degree in terms of slots 3.14
where â
m
= mechanical degree between adjacent slots, S = number of slots
Consider a 4 pole machine, as shown in Fig. 3.4 (c). Point A is under North pole N1 and
marked as 0
0
ε. Moving clockwise, point B is situated at 90
0
md
from point A. But point B is under
South pole S1, which is magnetically opposite to N1. Hence, point B is marked as 180
0
ε. Now,
point C is under N2, which is 180
0
md
away from point A. Point C has the same magnetic polarity
as that of point A, hence it is marked as 360
0
ε. Point D under S2 is 270
0
md
from point A, and
hence marked as 540
0
ε, ie., 360
0
ε +180
0
ε = 540
0
ε. After one complete encircling, point A is
reached again, and marked as 720
0
ε, ie., 540
0
ε + 180
0
ε = 720
0
ε.
The mechanical and electrical degrees defer one another from the point of reference. Thus,
mechanical and electrical degrees are related by the number of poles, P.
3.15
where θ
ed
= electrical degree.
In terms of slots, 3.16
where â = electrical degree between adjacent slots.
Example 3.7: Find the mechanical and electrical degrees between adjacent poles for an 8 pole
machine.
Solution: From equation 3.6,
From equation 3.8, .
Example 3.8 : Find the mechanical and electrical degrees between adjacent slots for a 6 pole
machine having 54 slots.
Solution : From equation 3.7, β
m
= = = 10
o
From equation 3.9,
ββ
ββ
β =
ββ
ββ
β
m
= 10 = 30
o
εε
εε
ε
Numbering the coil sides in slots: For convenience in laying out the windings, for double layer
windings, the coil sides forming the top layers in the slot are given odd numbers and those
forming the bottom layers are given even numbers. The scheme of numbering the coil sides for
two different double layer windings in slots are shown in Fig. 3.5.
For single layer winding, coil sides are numbered as shown in Fig. 3.6.
∴
360
S
360
6
P
2
6
2
360
P
360
8
θ
md
= = = 45
o
θθ
θθ
θ
ed
=
θθ
θθ
θ
md
= 45
o
= 180
o
εε
εε
ε
P
2
6
2