Il6 ELEMENTS OF ELECTRICAL ENGINEERING. 



slowly to the running position where it is held by the retaining 

 electromagnet. The connections of this starting box are shown 



in Fig. 84, and the discussion of that 

 figure gives the details of its action. 



Precautions to be observed in the op- 

 eration of a shunt motor. Always 

 start cautiously. If the motor does 

 not start promptly with the first move 

 of the arm, it is likely that the field 

 circuit is broken, or that the load on 

 the motor is excessive. Do not allow 

 the motor to run long with the rheo- 

 stat arm not in the running position, 



as the resistance in a starting rheostat overheats if it is left per- 

 manently in circuit with the armature. To stop the motor, open 

 the switch and leave the starter arm to set itself automatically at 

 the starting position. Further instructions for the operation of 

 generators and motors are given in Chapter VII. 



60. The compound motor. There are two arrangements of 

 the compound motor, according to whether the current in the 

 series winding opposes the magnetizing action of the shunt wind- 

 ing, or helps it. The first arrangement is called the differential 

 compound winding, and the second arrangement is called the 

 cumulative compound winding. 



The differential compound motor. When a differential com- 

 pound motor is loaded, the current through the armature and the 

 series field winding increases, and, by opposing the magnetizing 

 action of the current in the shunt field winding, reduces the arma- 

 ture flux <I>, and thus decreases the counter electromotive force 

 Z r n, if the speed n remains constant. If this decrease of <3> 

 is just sufficient to give the requisite decrease of counter electro- 

 motive force to permit enough current to flow through the arma- 

 ture to enable the motor to carry its load, then the motor speed 

 will not fall off with increase of load, at least not until the load 



