An armature is one of two principal components of an electrical machine usually in a motor or a generator. The other component is the field magnet which creates a magnetic field that the armature can act upon. The two roles of the armature are to create a force, either linear or rotational, and to generate an electromotive force. The armature can be used in either direction as a force creator or as a force generator that produces current in response to a mechanical force. Armatures are widely used and there are many ways to tell if they have gone bad.
Loss of Some Power
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Often electronics such as computers will have multiple armatures to keep some power flowing if one fails. This symptom is easy to spot since the machine will not work properly anymore. For example most computer units contain two armatures which supply power together. If one armature fails then the computer will not boot, however the lights and the fans will power on because there is enough power in the functional armature. Many electronics function in this method and if you notice some power, but not full power your armature is most likely damaged.
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Inspection Plug Brushes
Engine starters in cars are an example of armatures. If you are having problems starting your car it may be sign of a bad armature. The engine will first struggle to turn over requiring two to three attempts to get started. Eventually the engine will not start at all. If you check your inspection plugs and notice that the brushes are damaged, then the most likely cause is that the armature destroyed them. You can confirm this for yourself by installing new brushes and checking to see if they get damaged as well.
Growler
A growler is an electrical device that can check motors for shorted coils. The growler creates a magnetic flux that will cause a shorted armature to put a current into the feeler. If you have a bad armature the feeler will vibrate in tune to the current generated.
Amp Draw
Checking the amp draw of your starter is another simple method to test for a bad armature. The specifications may vary from vehicle to vehicle so you will need to check your car manual for the normal amp draw range. If the amp draw is overly high, that is a sign of a bad armature.