450 ELECTRICAL MACHINERY 



bad that no insulation at all exists between the armature 

 circuit and the core. 



To Locate a Grounded Coil. To locate the grounded 

 coil of an armature all brushes are removed but one pair, 

 as for the previous test. The dry cell is connected to the 

 brushes as before, and one terminal of the voltmeter is 

 connected to the shaft of the armature and the other is 

 connected in turn to all of the commutator segments, as 

 indicated in Fig. 288. If there is no ground, the readings 

 will all be alike, while if a ground exists, the voltmeter will 

 give continually varying readings, it being practically zero 



FIG. 288. Connections for Locating a Grounded Coil. 



on one of the segments. This segment is connected directly 

 to the grounded coil. 



Real Ground and Phantom Ground. Now it will be 

 found that each path indicates a grounded coil; one of them 

 is called a phantom ground. The two paths in the armature 

 being in parallel, if there is a point in one path which has 

 the same potential as the armature shaft (indicated by a 

 zero reading of the voltmeter), there must be in the other 

 path a corresponding point of zero potential which also will 

 give a zero reading on the voltmeter. 



The two segments giving zero reading are marked 

 with chalk and then the armature is rotated a fraction of 



