446 ELECTRICAL MACHINERY 



example, the station meters would all deflect backward 

 and the polarity of trolley and ground would be reversed 

 Such a condition would necessitate a complete change of 

 the meter connections or of the connection of the generator 

 to the bus-bar. 



If it is necessary to reverse the residual magnetism, a 

 few dry cells will generally suffice. The shunt field is dis- 

 connected from the armature, at one terminal, and the 

 dry cells (perhaps half a dozen) are connected to the field 

 circuit for a few moments. Then the cells may be dis- 

 connected, the field connected again to the armature, and 

 the generator may start to build up alright. In case it 

 does not, the operation should be repeated, connecting 

 the cells to the field, however, with polarity opposite to 

 that first used. 



In carrying out this operation, it is, of course, necessary 

 to disconnect the armature from the shunt field as the 

 armature would otherwise form practically a short circuit 

 (as it has such a low resistance) for the shunt field; the 

 current from the dry cells would nearly all go through the 

 armature, and so be useless for magnetizing the field. 



22. If a generator is running at a speed very much lower 

 than rated speed, it will very likely refuse to " build up." 

 The speed should be increased until the rated value is 

 reached. 



23. When a motor fails to start, the first thing to deter- 

 mine is whether or not the supply line is alive. This is 

 readily tested by a voltmeter or by an incandescent lamp 

 on a 110- or 220- volt circuit. On a 500- volt circuit several 

 incandescent lamps may be connected in series with each 

 other and then connected across the line to test for 

 voltage. 



24. All motor circuits should be fused and it may, of 

 course, be that the fuses are blown. This may be deter- 

 mined by an inspection of the indicator on an enclosed fuse 

 or by testing the fuse in series with an incandescent lamp 



