418 THE INDUCTION MOTOR. 



Tt happens that the motor gives its rated output 

 when tho stator current 128 amperes ; the line N P 

 in the diagram thus represents 80 b.h.p. and the value 

 of all such lines as this, now that the value of one of 

 them has been found, is definitely known. 



Taking into account the known slip of the motor 

 we could, from the known output, find the value of the 

 torque vector H L, in pound-feet, and having one such 

 value, the values for all the other lines representing 

 torque would be definitely fixed. 



The value of the torque is obtained as follows : 



H.P. developed = ,^,. = say 80 h.p. in the present 

 case. 



The speed at full load is T^A = 576 revs, per 



minute, since there is a slip of 4 per cent. 



27rnr 

 Smce h 'P- = 3^000 



where T = torque in pound feet. 

 and n = revs, per minute. 



T h-P. * 33,000 80 x 33,000 



2 2r x 576 lb -- tt - 



If the line O B had not turned out to be the stator 

 current for exactly 80 h.p. we could have immediately 

 found where the vector for 80 h.p. lies in the diagram 

 with the help of this value. 



Thus if the point B" had been taken instead of B 

 the output corresponding to the line B" O" could be found 

 exactly as described. From this a scale of watts could 

 be plotted corresponding to the length of B" O" and, 

 after finding the length corresponding to 80 h.p. on this 

 scale, the point B would be determined by drawing a 

 horizontal line at the right height and finding B where this 

 line cut the " output " circle. 



The complete performance of the motor is thus given 

 by the diagram for all loads. 



In practice the diagram is accurate enough for all 

 practical purposes, there being a slight error on the right 

 side, i.e., the actual performance of the motor is rather better 

 than that given by the diagram. There are slight errors 



