FINISHING AND TESTING 



333 



In accordance with another method, instead of adding weight, 

 some of the metal is taken away at the heaviest part. If the 

 spider and end flanges permit of this, a good way is to drill out 

 countersinks with the point of a twist drill until the right amount 

 of weight has been taken off. This is, of course, not permissible 

 if the balance is bad and requires a lot of adjusting. 



Testing. When finished, the armature is tested with a pressure 

 of at least twice the normal working voltage. For street car 

 and railway motors the test pressure is more usually of the order 

 of five times the working voltage. 1 



FIG. 414. Balancing Table for large Armatures (Westinghouse Co.). 



For high-pressure alternators up to 10,000 volts the test 

 pressure is at least double the normal voltage. These testing 

 voltages should be applied across the insulation between the 

 windings and frame, and between electrically independent windings. 



After the high pressure test the armature insulation may be 

 measured with an ohmmeter or testing set for purposes of record. 



Testing of Armatures during Winding. The above-noted in- 

 sulation tests are usually applied to continuous-current armatures 

 immediately before connecting up the winding to the commutator 



1 See Insulation of Electric Machines, Chap, xxii., "Specification for In- 

 sulation." 



