OPEN-CIRCUITS, SHORT-CIRCUITS, ETC. 7 



or running resistance of the armature, see that the 

 measurement is made from the proper commutator 

 segments. For multiple or lap windings, the resistance 

 measured from diametrically opposite points divided 

 by half the number of poles squared will give the true 

 running resistance, while with a series or wave winding 

 the resistance should always be taken at points 180 

 electrical degrees apart. For example, take a four-pole 

 armature with a lap winding and 360 commutator 

 segments. This armature should have its resistance 

 measured between bars No. 1 and No. 181. This 

 resistance divided by four will give the running resist- 

 ance. With a wave winding on the same armature, 

 the resistance measurement should be taken between 

 bars No. 1 and No. 91, this resistance being the true 

 running resistance. 



Alternating current armatures and fields are similarly 

 tested for grounds, short circuits, open circuits, wrong 

 connections, polarity, etc. In testing for grounds the 

 same methods and similar apparatus are used as for 

 direct current machines, except that alternating current 

 machines are usually designed for higher voltages, and 

 consequently testing voltages are correspondingly higher, 

 and greater care must be taken in testing. All high 

 potential tests must be made with carefully calibrated 

 electrostatic voltmeters that have been checked with 

 a spark gap. The testing equipment should be as near 

 the apparatus as possible, since the additional capacity 

 of testing lines may raise the voltage at the receiving 

 end much above that at the generating end. Unless 

 this precaution is taken, excessive voltages may be 

 applied which may damage the insulation. In case a 

 ground develops, a resistance measurement will generally 

 locate the point at which it occurs, unless each phase 

 has two or more multiple circuits. In the latter case 

 it may be more readily located by opening one or more 

 cable joints and separating the circuits. 



A measurement may be taken in the following manner: 

 First, measure the total resistance of the grounded 

 circuit or phase; second, measure the resistance of the 

 winding from one terminal to ground, by connecting 



