80 THE DIRECT-CURRENT MOTOR CH. IV 



Inserting the value of c,, as found above we get 



//.!/, + //.!/ 

 ] 'M*]f h +M*lf.. ( ' 3o)> 



Using the given data, the new speed appears to be 

 1,398 r.p.m., so that when the external torque is removed, 

 the speed is reduced from 1,152 to 1.398 revolution per 

 minute. 



These results may be checked by equating (1) the 

 torques in the two dynamos, (2) the watts from the line 

 and the sum of the heat watts in the two dynamos, (3) the 

 effective tension and the product of the resistance and 

 the current in each dynamo. 



When two dynamos coupled in parallel are both 

 intended to act as motors, the speed of the combination 

 will be determined by the load or torque on their common 

 shaft, or by the total torque when they are connected by 

 some mechanical arrangement, as for instance when two 

 motors are driving one railway car. 



For a given total torque each motor will take its own 

 share of the load ; the amount of the torque taken by each 

 motor can be ascertained from the diagram. Suppose, for 

 example, that the total torque is 273 inch-pounds. Set 

 this off on the curve of combined torque and draw a 

 vertical line to cut the combined speed curve ; this gives 

 the speed. Then draw a horizontal line to cut the torque 

 curves of A and B, the intersections give the current 

 passing through each dynamo ; in this case .1 is taking 5 

 amperes and B is taking 11 amperes. From these inter- 

 sections draw vertical lines to cut the torque curves of A 

 and B, this will give the torques of the two motors. 42 

 inch-pounds and 231 inch-pounds respectively, the sum 

 making up the total torque. The dynamo A, having 



