Specifications

40
Pole pitch Pole pitch
12
4
3
56
8
7
910
12
11
R RB Y B Y
From equation 3.5, slots per pole = = 6
From equation 3.8, angle between consecutive slots, â = = 30
0
.
The allotment of slots to the three phases for a pole pair is given in Fig. 4.4, and this
allotment repeats itself for other pole pairs. In this example, the number of pole pairs is one.
Fig.4.4 - Slots per pole per phase
Half coil winding
The coils comprising a pair of poles phase groups under adjacent poles are concentric.
These coils under two adjacent poles form one coil group and thus there is one coil group per
pair of poles.
If the first phase, say R or A phase starts at slot no.1, the Y or B phase should start at 120
0
/30
0
= 4 slots away ie., in slot (1 + 4 = ) 5, and the B or C phase should start at (5 + 4 =) 9.
The winding should be connected between alternate North and South poles. First, R or A
phase winding is started. If one side of the coil is placed in slot no. 1, the other side of the coil
should be placed in slot no. 8, ie., the last coil side of phase R or A phase under the next pole.
The coil side in slot no. 2 should be connected to the last but one coil side of the same phase
under the next pole, ie., slot no. 7, in this case.
Now, the two coils formed by coil sides placed in the slots 1, 8 and 2, 7 belong to the same
phase, and are concentric with each other. They form a coil group.
The rule for connecting together the coils in a coils group is that the end wire of one coil
must be connected to the beginning of the next coil in the group and so on. This is done in order
that the emfs of the coils in a coil group, add. The other coil groups for the remaining phases.
Fig. 4.5 - Coil group connections for half coil winding.
Fig. 4.5 gives the coil group connections for half coil winding for each phases.
12
2
180
slots per pole
180
6
=