THE OPERATION OF THE DYNAMO AS A MOTOR. IOI 



minals or of a constant current flow through the motor. In 

 practice motors are always used under conditions which approxi- 

 mate to constant voltage driving ; constant current driving is 

 never used. Therefore, the following discussion of the behavior 

 of motors is based upon the assumption of constant, or approx- 

 imately constant, voltage between the supply mains. In this 

 discussion the shunt motor is considered first and in detail ; the 

 compound motor is treated as a modification of the shunt motor ; 

 and the series motor is considered last. 



When a motor is delivering mechanical power, it is said to be 

 loaded, and its output of mechanical power is called its load. 

 The rated full load of a motor is the load for which the armature 

 and field currents are as large as permissible without excessive 

 rise of temperature in any of the motor parts, or excessive 

 sparking. 



56. The shunt motor. The essential connections of a shunt 

 motor to constant voltage supply mains are shown in Fig. 79. 

 It is to be particularly noted that Fig. 79 shows the connections of 

 the motor after it has been started _ 

 and brought up to speed. Special 

 connections are made at starting- 

 as will be explained later. 



The shunt field coils are con- 



nected directly to the supply lOOOOOOO 



mains, usually without including shunt &eia winding 



a field rheostat, and the field Fig - 79 - 



current is therefore constant, so that the only variation of the 

 armature flux, 3>, is the slight variation due to the demagnetizing 

 action of the armature current. 



Shunt motor unloaded. When a motor is unloaded the torque 

 necessary to drive it is small, and therefore the armature current 

 is small, according to equation (25). Consequently the part, 

 RJa> f tne impressed voltage which is used to overcome the 

 resistance of the armature, is negligibly small, since both 7 a and R a 



supply mains 



armature 



) 



