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Armature Test

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following the manufacturer's instructions in using the ohmmeter,  field  coil  tests  can  be  made  more  quickly and accurately than by using a test lamp circuit. ARMATURE TEST.—There    are    two    practical tests  for  locating  shorts,  opens,  and  grounds  in armatures—the growler test and the bar-to-bar test. To test for short circuits, place the armature on the V-block of the growler and turn on the current. With a thin metal strip (hacksaw blade is good) held over the core,  as  shown  in  figure  2-17,  rotate  the  armature slowly  through  a  complete  revolution.  If  a  short  is present,  the  steel  strip  will  become  magnetized  and vibrate. To find out whether the armature coils of the commutator   are   short-circuited,   clean   between   the commutator  segments  and  repeat  the  test.  Should  the thin  metal  strip  still  vibrate,  the  armature  is  short- circuited internally and must be replaced. Not all armatures can be tested for short circuits by the  method  just  described.  These  armatures  can  be identified by excessive vibration of the saw blade all around  the  armature  during  the  test.  With  these armatures,  test  for  short  circuits  by  using  the milliampere contacts on an ac millimeter, as shown in figure 2-18. In doing so, keep the armature stationary in  the  V-block  and  move  the  contacts  around  the commutator until the highest reading is obtained. Then turn the armature to bring each pair of segments under Figure 2-17.—Using an armature growler. the  contacts  and  read  the  milliammeter  at  the  same time. The readings should be nearly the same for each pair  of  adjacent  bars.  If  a  coil  is  short-circuited,  the milliammeter reading will drop to almost zero. Test  the  armature  for  grounds  by  using  the  test light circuit, which is a part of most modern factory- built  growlers  (fig.  2-19).  Place  the  armature  on  the V-block  and  touch  one  of  the  test  probes  to  the Figure 2-18.—Testing an armature for a short circuit with a milliammeter. Figure 2-19.—Testing an armature for grounds. 2-16



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