114 



ELEMENTS OF ELECTRICAL ENGINEERING. 



the field winding as they are cut out of the armature circuit, but 

 this has but little effect upon the field excitation, inasmuch as the 

 resistances RR are very small in comparison with the resistance 



supply wains 



Fig. 84. 



of the shunt field winding, and the resistances RR are cut out 

 of the field circuit when the arm C touches the contact point B. 



When the rheostat arm C is pushed to the right in starting the 

 motor, it stretches a spring and the rheostat arm is held in the 

 running position by an electromagnet, of which the winding D is 

 in series with the field winding F of the motor. When the lines 

 become dead, current ceases to flow through D and F t the elec- 

 tromagnet releases the rheostat arm, and the stretched spring 

 pulls the arm back to the starting position. 



The overload release is actuated by the electromagnet 0, of 

 which the winding is in series with the motor armature. The 

 armature of this electromagnet is held back by a spring, the 

 tension of which is so adjusted that the lever may be moved by 

 a prescribed value of current through 0. When this value of 

 current is reached, the movement of the armature of actuates 

 a small switch s which short-circuits the winding D. This causes 

 the electromagnet D to lose its magnetism, and release the arm 

 C as before. 



