What makes you think the armature is shorted? Any problem with the winding would show up in the form of overcurrent, loss of power, gain/loss in speed, vibration and heavy arcing at the brushes when under load. However before you condemm the winding ensure that no bars are shorted together by conductive debris in the mica insulation between them.
Tests that need to be carried out that would indicate a problem winding would include.
1 Insulation resistance to earth.(Most people would say 1meg as a min)
2 Bar to bar continuity and resistance, this will require the use of a micro-ohmeter or ductor to provide accurate results. (Depending on the winding design you may have a pattern of high and low figures that constantly repeats, for instance hi hi hi low, hi hi hi low etc).
3 Growler test to indicate shorted coils.
4 Volt drop test, AC is induced into the armature and the voltage measured off the bars, this is usually done in conjunction with the growler test.
The test equipment needed is quite specialist, expensive and usually only found in repair shops. You may be able to hire it if you are confident about using it. My suggestion is to get it into a good shop and ask them to do the tests with you present.....then ask lots of questions.
for shorted turns, in addition to the latter, i could suggest a BALANCE IMPEDANCE/SURGE COMPARISON TEST..We've been using this instrument in our repair shop. It has appraising and conclusive results which pays the cost of the equipment.
