CHAPTER IV. 



THE OPERATION OF THE DYNAMO AS A MOTOR. 



52. Conditions which determine the direction of running of a 

 motor. Precisely the same conditions which determine the direc- 

 tion in which a self-exciting dynamo with given connections of 

 field to brushes, must be driven, in order that it may build up as 

 a generator, determine the direction in which the machine will 

 run as a motor when it is supplied with current. 



The shunt dynamo, with given connections of field to brushes, 

 runs as a motor in the same direction in which it must be driven 

 in order to build up as a generator. This may be made evident 

 as follows : 



The heavy arrow in Fig. 75 shows the direction of the elec- 

 tromotive force and current in the armature of a shunt generator, 

 and the dotted arrows show the direction of the currents in field 



supply main 



main 



Geld winding 



6eJd winding 



armature 



main 



Fig. 75. Shunt Generator. 



supply main 



Fig. 76. Shunt Motor. 



and in mains ; the heavy arrows in Fig. 76 show the direction of 

 the current delivered to a shunt motor, and the dotted arrows 

 show the direction of the currents in field and armature. Figs.* 

 75 and 76 show at a glance that the current in the armature of a 

 shunt machine is reversed with respect to the field current, when 



* There are four possible combinations of arrows in Figs. 75 and 76, and each 

 combination has the property here specified. 



95 



