Checking Armature

Armature faults are in many cases not externally visible. Checking should include tracing of open circuits as well as winding and earth shorts.


li_fig_8 1. Open circuits are generally recognizable on the commutator by burn spots between two segments. They can also be measured with a sensitive resistance measuring bridge.

2. Winding short circuits between windings of an armature coil can be traced on an armature test meter. The armature is rotated between two test probes, a magic eye then indicates the winding short circuit. Another instrument consists of an AC magnet with two jaws which take the armature. Here the armature is slowly rotated about its axis a thin piece of steel sheet being placed on top of the armature. If there is a short circuit in the winding the sheet of metal will begin to vibrate violently at 2 or more places around the periphery of the armature. With another instrument the armature is rotated in the same way over the jaws of an AC magnet while the tester probes the commutator core with a probe.

The alternating current produced in the coil of the probe as a result of induction from a winding short circuit can be heard in the test head phones as a humming noise.
li_fig_9 3. A short to earth will occur if the armature core is shorted to the winding or if carbon dust ha s penetrated into the winding. The test should be carried out with a test lamp of 40 V between armature iron and commutator. The test lamp should not light up.

4. If the commutator is oval or is rough as a result of burn spots or grooves have occurred as a result of running in of the carbon brushes it must be skimmed and polished which is essential to obtain a satisfactory surface.

The insulators between the segments should be reworked with a commutator saw until they are recessed back about 0.3-0.5 mm from the contact face of the commutator.
Last modified: Sat, 15 Jan 2005


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