DIRECT CURRENT GENERATORS 61 



to flow from the stronger toward the weaker pole pieces, 

 which slightly demagnetize the former and magnetize 

 the latter, thus equalizing the voltage at the brushes. 

 Not only do the rings prevent an interchange of heavy 

 cross currents between brushes, but they also com- 

 pensate for inequalities in magnetic pull at the pole 

 pieces, tending to bend the shaft or overheat the bearings. 

 The tester should examine these rings to see that the 

 taps are equally spaced and all connections tight. 



If a machine has been correctly connected, and there 

 are no open circuits or reversed spools in the field, the 

 machine should build up when the field switch is closed 

 and all resistance cut out of the field. If it does not, 

 the resistance of the field should be checked with that 

 of a similar machine of the same size and voltage 

 as a 500 volt machine may sometimes be assembled 

 with a 250 volt field. 



When difficulty is had in building up the voltage of 

 a machine, it will usually be found that the current 

 does not flow through the field in the right direction 

 to build up the residual magnetism. If, with the field 

 switch open, the residual flux gives a few volts on the 

 armature and upon closing the switch the voltage drops 

 to nearly zero, the field terminals are connected to the 

 wrong brushes. To remedy, either reverse the field 

 or shift the brushes over one pole. 



In locating the no-load electrical neutral on commu- 

 tating pole machines, the fibre brush method may be 

 used. A fiber brush, provided with two contacts and 

 terminals separated from one another by a distance 

 equal to the thickness of one bar, is placed in a brush- 

 holder on one stud. The brush is then shifted until 

 zero voltage is read between the two terminals. The 

 position of the rocker arm is marked at this point. 

 The fiber brush is then placed on the next stud and the 

 brushes shifted again until zero is obtained, this position 

 of the rocker arm being also marked. This operation 

 is repeated for each of the studs, the rocker arm being 

 finally set on the mean of the positions previously 

 marked. This setting locates the electrical neutral at no 

 load, which should have the same position at full load. 



