206 ELECTRICAL ENGINEERING 



The motor output in horse power is 



^- 



Pi 



746 746 



If the torque developed is T ft. Ibs., then the output in horse 

 power is 



2irnT _ 



550 = 746 



and the torque is 



T = --- - - = 0.1177Z4>7-10- 8 ft. Ibs. (218) 

 74o i 



This is the torque equation of a direct-current motor. 



T = 0.1177 X 10- 8 X (z -} X (p$) ft. Ibs., . (219) 



where Z is the sum of all the currents in the armature conductors 



Pi 



and p$ is the sum of the fluxes crossing the air gaps under all the 

 poles. 



The torque equation may be written 



T = K$I ft. Ibs., ...... (220) 



where K = 0.1177 Z 1Q- 8 is a constant. 

 Pi 



The torque of a motor is therefore directly proportional to the 

 armature current and to the flux in the air gap. 



This is the torque developed at the shaft of the motor. The 

 torque output is less due to the iron and friction losses. 



127. Torque Characteristics of Motors. The torque char- 

 acteristic is a curve showing the relation between the torque and 

 the armature current. 



The torque equation is 



T = K$>I. (Art. 126.) 



Shunt Motor. In the shunt motor <f> is almost constant under 

 load, decreasing only by a small percentage due to armature re- 

 action. The torque therefore varies almost directly with the 

 current. (Curve 1, Fig. 176.) 



