13 MET September 2018 Q.8(B)

A 3phase, 4pole 24 slot alternator has its armature coils short pitched by one slot. Find the distribution factor and pitch factor

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Given 

No of pole = 4
No of slot = 24

To find

1. Distribution factor
2. Pitch factor

Solution

No of slot per pole, n = 24 / 4 
                                  = 6

Angular displacement between slots β = 180 / n
                                                              = 180 / 6
                                                              = 30 deg

No of slot/phase/pole, m = 24 / (3 x 4)
                                        = 2

1. Distribution factor Kd = (Sin mβ/2) / (m sin β/2)
                                      = (Sin 2 x 30/2) / (2 x Sin 30/2)
                                      = 0.965

Armature coils are short pitched by one slot. Therefore angle a is same as that of B.

2. Pitch factor Kc = cos α /2
                            = cos 30/2
                            = 0.965