THE OPERATION OF THE DYNAMO AS A MOTOR. 113 



Starting rheostat with automatic overload release and dead line * 

 release. The most frequent causes of trouble with a shunt motor 

 are as follows : (a) The motor may be running normally when, 

 for some reason or other, the supply of current fails, the line wires 

 becoming "dead." When this happens the motor of course 

 stops, but the connections to the supply mains remain intact, with 

 all of the resistance cut out of the starting rheostat. When the 

 supply of current is again established under these conditions, an 

 excessive amount of current flows through the motor armature and 

 it is likely to be burned out. In service, a motor should be pro- 

 tected from damage in this way by a dead line release which auto- 

 matically throws the arm of the starting rheostat back to the 

 starting position when the supply of current fails. 



(U) The motor may be greatly overloaded. When this happens 

 the motor takes excessive current from the supply mains, and the 

 motor may be overheated or even burned out. In service, a motor 

 should be protected from damage in this way by an overload 

 release which automatically throws the arm of the starting rheostat 

 to the starting position when an excessive current flows through 

 the motor armature. 



The electrical connections of a shunt motor starting box with 

 automatic dead line release and overload release, are shown in 

 Fig. 84. The motor armature is represented by A, and the shunt 

 field winding by F. To start the motor, the switch is closed, 

 and the rheostat arm C is slowly moved in the direction of the 

 dotted arrow until it reaches B. The first move of the arm C 

 connects the shunt field winding directly to the mains, and at the 

 same time it connects the armature to the mains through the 

 resistances RR. The continued movement of the arm cuts the 

 resistance out of the armature circuit. In the particular arrange- 

 ment shown in Fig. 84, the resistances RR are left in series with 



* The name applied to this device by the manufacturers is the "starting rheostat 

 with overload and no-load release." This name is, however, misleading, inasmuch as 

 what is called the " no-load release " operates only when the line becomes, dead, but 

 does not operate when the load on the motor is reduced to zero. 

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