6 COMMERCIAL ELECTRICAL TESTING 



is passed through the armature winding from a com- 

 mutator bar to one adjacent to it. A connection is 

 made from one side of a galvanometer "to ground", 

 the lead from the other terminal of the galvanometer 

 being placed on one of the commutator bars. The 

 supply leads and galvanometer lead are then passed 

 from segment to segment, until first a full deflection 

 is obtained and then zero reading when the leads are 

 removed one segment further. The grounded coil then 

 lies between the bars for which full deflection was 

 obtained. 



A bar to bar test is usually made to disclose short 

 circuits, open circuits, and other similar faults. For 

 this test the windings connected to two adjacent com- 

 mutator segments have their resistance measured by 



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Fig. 2. Testing for Open Circuit 



the "drop of potential method," as indicated in Fig. 

 2. Storage batteries and a special electro-magnetic 

 d'Arsonval galvanometer should be used. With this 

 arrangement readings can be obtained rapidly, as the 

 instrument is "dead beat." 



Measuring the ohmic resistance of the winding will 

 sometimes reveal a wrong connection, which, on a bar 

 to bar measurement, would give a uniform deflection 

 all around the commutator. Series or wave windings 

 may sometimes have all the conductors joined in series, 

 but in the wrong order, so that the armature is inoper- 

 ative. In the case of multiple or lap windings, double, 

 triple or even quadruple spiral re-entrant windings are 

 possible, whereas a single spiral is what is wanted. In 

 taking a resistance measurement for brush to brush, 



