DC armatures are the most time-consuming components to test, but they are also the easiest. There are three basic methods that can be used.
With trending, all measurements are used; however, a bar-to-bar impedance measurement is used for assembled and disassembled testing.
When testing an assembled DC armature, the best method is to perform a bar-to-bar test using the motor brushes. If the motor has two brushes, they don’t need to be raised. If there are four or more sets of brushes, all but two sets 90 degrees from each other need to be raised, which takes them out of the testing circuit. Good contact is maintained by ensuring more than 90% of the brush is in contact with the commutator bars, which should be clean.
Once set, mark the position of one bar on the commutator, then bring the bar to a position where it is just under the leading edge of one of the brushes. In the assembled test, you will probably be covering at least one and a half bars with the brush. Perform an impedance test, mark down the reading, and move the armature so that the leading edge of the brush is over the next commutator bar. Take the next impedance reading and continue until each bar has been tested.
A good result will show a consistent pattern, while an inconsistent pattern will identify a poor armature.
In disassembled bar-to-bar testing, the tester will use an armature fixture or test leads to connect from bar-to-bar. The spacing between each impedance reading should be constant and about 90 to 180 degrees from each other. The first bar should be marked and testing should then be continued until one leg of the testing fixture or test lead has made it 360 degrees around the commutator. Mark the impedance for each bar-to-bar test then look to ensure there is a consistent pattern.
Excerpt with permission from Electrical Motor Diagnostics, by Success by Design Publishing, Old Saybrook, Conn. Visit www.motordoc.com.