122 



ALTERNATING CURRENTS 



duced to correct for this relative phase displacement. This 

 factor is unity for a concentrated winding and is less than unity 

 for a distributed winding. The table gives values of kb for a few 

 typical windings. 



VALUES OF BREADTH FACTOR kb 



If fractional pitch is used, the electromotive forces in the two 

 coil sides are out of phase, as shown in Fig. 113 (6), page 109. 

 This again reduces the voltage. Correction for this may be made 

 by multiplying the voltage equation by another factor k p , called 

 the pitch factor. 



VALUES OF PITCH FACTOR k v 



The complete equation becomes 



E = 2.22M P Z4>/10- 8 volts. 



(37) 



Example. A six-pole, three-phase, 60-cycle alternator has 12 slots per 

 pole and four conductors per slot. The winding is % pitch. There are 

 2,500,000 lines entering the armature from each north pole and this flux 

 is sinusoidally distributed along the air-gap. The armature coils are all 

 connected in series. The machine is Y-connected. Determine the open- 

 circuit voltage of the generator. 



The total number of slots is equal to 72. 



Therefore the series conductors per phase 



Slots per pole per phase = 72/(6 X 3) =4. k b (from table) = 0.958. k p 

 0.961. 



Therefore the total induced voltage per phase 



E = 2.22 X 0.958 X 0.961 X 96 X 2,500,000 X 60 X 10" 8 = 294 volts. 



As the generator is Y-connected, the terminal voltage is 

 294 \/3 = 510 volts. Ans. 



