For a DC Motor Armature, There is a simple method of determining the condition of the Armature.
Drop Test Method: Give a DC Voltage across the commutator Segments for one pole pitch area from a Power supply or Battery. Connect Positive end of the DC power supply at one end and the Negative end at the opposite end.
For example if the total number of commutator segments are say, 40 in the armature to be tested and the total number of poles is 4, then one pole pitch area will be 10 segments.
Now measure with a Milli volt meter say 0 to 10 millivolts range, the Voltage Drop at the center point, that is between 5th and 6th segment. again rotate the Armature Clockwise or Anti clock wise and measure the next set of segments.
Like this complete measurements for all the 40 segments pairs. simultaneously recording the readings.
If there is any defect in the winding, that is shorted or open, it will show in the readings.
If the reading of Milli volt Meter is uniform for the all the 40 segments pairs, than the armature is good. If there is short between winding or the winding coil between one particular pair of segments, the reading will be less drop in millivolts. If there is any loose or open, the reading will be more than normal readings. Thus one can determine the condition of a DC armature for short or lose or open winding.
When testing a DC armature there is a series of tet should that should be done. The first is. Ground insulation test or more commonly known as a mugger test, usually done at 500VDC. If the ground reading is above 1 meg ohm the armature is good to go to the next test which is a bar to bar test. There are 2 pieces of equipment to conduct this test the best. One of these combined with the mugger test will tell you if the armature is satisfactory return to service. The first bar to bar test is conducted with a “DLRO” digital low resistance ohm meter. The meter will circulate about 8-10 amps thru adjacent successive bars and measure the milli ohm resistance of the circuit. If there is more than a 5% variation then the armature is shorted turn to turn. The next tester which is called a high frequency bar to bar tester. The tester has 4 tet points and as you move it around the armature a high frequency voltage is introduced across the pairs of successive windings and the meter will show a variation if there is a shorted turn. If it passes either of these 2 bar to bar test and the ground insulation test then it can be returned to service.