100 ELECTRICAL MACHINERY 



No. 13 wire. The total length per turn is 2 feet, of which 

 1 foot is active. Assume the peripheral speed of the arma- 

 ture to be 3000 feet per minute, that 60% of the inductors 

 are active (i.e., lie under the pole face at the same time), 

 and that the flux density in the air gap is 10,000 lines per 

 sq.cm., the pole fringe not considered. It is desired to find: 

 (1) the generated e.m.f.; (2) the safe current capacity; 

 (3) the armature resistance; (4) the full load terminal 

 voltage. 



The first thing to determine is the active length of inductor 

 per path. This must be a two path winding, hence we have : 



Coils per path = 22; 



Turns per path = 176; 



Length of inductor per path = 176X 1 = 176 ft. ; 



As the flux density is given in lines per sq.cm. we will work 

 in the metric system; 



Length of inductor per path = 5370 cm; 



Active length of inductor per path 



= 5370X60% = 3220 cm; 

 The velocity of the inductors is 3000 ft. per minute 



= 1525 cm. per sec.; 



Flux density = 10000 lines per sq.cm; 



Flux cut per second = 3220X1525X10000 

 = 491 X10 8 lines. 



Hence the generated voltage per path = 491 volts, and 

 this is the generated e.m.f. of the machine. 



Allowable Current. In a well ventilated armature it 

 is safe to allow one ampere per 600 circular mils cross- 

 section of the armature conductor. As a No. 13 wire 

 is 5178 cir. mils in section the safe current to allow is about 

 8.63 amperes per path. As there are two paths in parallel 



