OPERATION AND CARE OF ELECTRIC MACHINERY 447 



across the line. If the fuse is blown, of course, the lamp 

 will not burn. 



25. Either the field or the armature circuit may be 

 open. The field circuit may be tested by holding the 

 starting rheostat lever on the first contact button and 

 with the armature stationary, testing for magnetism on 

 the pole shoes with a knife, keys, or similar article. If 

 the field has no magnetism, it shows that the field circuit 

 is open (provided the line is alive) and this open circuit 

 must be located and removed. 



The armature circuit may be open in the armature 

 itself or in the starting box. The open circuit may be 

 found with a bell-ringing magneto, by trying to ring 

 through the different parts of the circuit. 



26. If all of the resistance is " cut in " on the field 

 rheostat, it may be that the torque the motor can develop 

 with the weak field is not sufficient to start the load. The 

 field rheostat should be turned to the " all out " position. 



27. It may be in some cases, that the starting torque 

 demanded of a motor is greater than the motor can exert. 

 On factory loads, where a great deal of belting is used, this 

 is especially likely to be true. In such a case some of the 

 belting must be disconnected from the motor, by a releasing 

 clutch or similar device, until the motor is up to its normal 

 speed. If this is not possible, a larger motor must be 

 installed. 



143. Location of Armature Faults. The three faults 

 which are most likely to occur in an armature circuit are 

 due to short circuits, open circuits, or grounds. An armature 

 winding consists of a series of similar coils, all joined in 

 > -i -ies and insulated from the armature core. If the winding 

 becomes electrically connected to the core at any place, 

 due to abrasion or the cutting of the insulation on the 

 coils, the winding is said to be grounded. One ground in 

 an armature winding does not interfere with the electrical 

 operation of the machine, but the iron frame of the machine, 



