Specifications

26
180
0
ed
(a)
0
ed
-α
α
(b)
180
180
0
ed
á = xâ for short pitch winding 3.7
Also â = 3.8
where á = short pitch angle or an angle less than 180
0
ε
â = angle between adjacent slots
x = 1, 2, 3, . . . an integer
Example 3.3 : Find the angle between adjacent slots of a 3 phase, 6 pole motor having 36 slots.
Solution : Slots per pole = = 6 From equation 3.8, â = = 30
o
ε
Pitch Factor or coil span factor or chording factor, K
P
: When the two sides of the same coil
are short pitched by an angle á, as shown in Fig. 3.3 (b), the emf induced in the two coil sides
have a phase angle difference of á
0
. Due to phase angle difference, the actual emf is reduced
by a factor and is called pitch factor or coil span factor or chording factor.
Kp = 3.9
Example 3.4 : Find the pitch factor for a 3 phase 4 pole ac machine wound in 36 slots with a coil
span of 140
0
ε.
Solution: From Fig. 3.3 (b), short pitch angle, á = 180
0
– 140
0
= 40
0
ε. From equation 3.9,
K
P
=
Distribution Factor, K
d
: It is defined as the ratio of phasor addition of emfs induced in all the
coils distributed in m slots under one pole region to their arithmetic addition of emfs induced in
all the coils distributed in m slots under one pole region.
3.10
where m = = number of slots per pole per phase 3.11
Example 3.5 : Compute the distribution factor for a 3 phase 4 pole ac machine wound in 36
slots with a coil span of 140
0
ε.
Fig.3.3 - Full Pitch and Short Pitch coils
180
S/p
180
S/p
36
6
cos
α
2
cos
α
2
K
d
=
s
3p