Locating Problems in the DC Machine

The following tests locate the problem and recommend the extent of repair to be done. Most of the procedures require dismantling the machine to access the components.

All test information should be recorded for future reference.

Locating Problems in the Armature

Ohmmeter Test for Ground in the Armature Circuit

The leads Al and A2 include the brush holder and the interpoles. Test for a ground from Al to the frame with an ohmmeter. If the reading is low (from 1 to 50 megohms), cleaning and drying may be all that is needed.

It’s normal for the armature to have a low megohm reading to the shaft (1 to 50 megohms). Brush dust, which is highly conductive, accumulates behind the commutator during normal operation. This is why DC power is not grounded.

If a ground develops in a machine (for example, a locomotive), all motors on the machine have a common connection to each other. The first indication of a ground is bearing failure. (Leakage current from the commutator travels through the bearings.)

Use the following procedure to locate the problem: Lift the brushes and check from the commutator to the shaft with the ohmmeter. A solid ground in the armature (as seen in Fig. 2.12) would require rewinding. If no problem is found in the armature, test the brush boxes—both polarities or adjacent brush boxes—to the frame (Fig. 2.13).

If the brush box connected to the interpoles has a low reading, disconnect the interpoles and test again. If a low reading is in the interpoles, cleaning

and drying them should improve the reading. A solid ground requires replacing or reinsulating the coil(s).

If the brush holder has a low reading, clean and/or replace its bushings and insulators.

Heat-Damaged Interpoles

Interpoles, like the series field, are in series with the armature. If the armature has been overloaded or shorted, high amperes have very likely overheated the interpole coils. If the coils are discolored, they should be replaced or reinsulated.

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