Inspection and Maintenance of DC Motors

Use the following procedures to make inspection and maintenance cheeks:

1. Check the operation of the unit driven by the motor in accordance with the instructions covering the specific installation.
2. Check all-wiring, connections, terminals, fuses and switches for general condition and security.
3. Keep motors clean and mounting bolts tight.
4. Check brushes for condition, length and spring tension. Minimum brush lengths, correct spring tension and procedures for replacing brushes are given in the applicable manufacturer's instructions.
5. Inspect commutator for cleanliness, pitting, scoring, roughness, corrosion or burning. Check for high mica (if the copper wears down below the mica the mica will insulate the brushes from the commutator). Clean dirty commutators with a cloth moistened with the recommended cleaning solvent. Polish rough or corroded commutators with fine sandpaper (000 or finer) and blow out with compressed air. Never use emery paper since it contains metallic particles which may cause shorts. Replace the motor if the commutator is burned, badly pitted, grooved, or worn to the extent that the mica insulation is flush with the commutator surface.
6. Inspect all exposed wiring for evidence of overheating. Replace the motor if the insulation on leads or windings is burned, cracked or brittle.
7. Lubricate only if called for by the manufacturer's instructions covering the motor. Most motors used in today's aeroplanes require no lubrication between overhauls.
8. Adjust and lubricate the gearbox or unit which the motor drives, in accordance with the applicable manufacturer's instructions covering the unit.

When trouble develops in a DC motor system, check first to determine the source of the trouble. Replace the motor only when the trouble is due to a defect in the motor itself. In most cases, the failure of a motor to operate is caused by a defect in the external, electrical circuit or by mechanical failure in the mechanism driven by the motor.

Check the external electrical circuit for loose or dirty connections and for improper connection of wiring. Look for open circuits, grounds and shorts by following the applicable manufacturer's circuit-testing procedure. If the fuse is not blown, failure of the motor to operate is usually due to an open circuit. A blown fuse usually indicates an accidental ground or short circuit. The chattering of the relay switch which controls the motor is usually caused by a low battery. When the battery is low, the open-circuit voltage of the battery is sufficient to close the relay. But with the heavy current draw of the motor, the voltage drops below the level required to hold the relay closed. When the relay opens, the voltage in the battery increases enough to close the relay again. This cycle repeats and causes chattering, which is very harmful to the relay switch, due to the heavy current causing an arc which will burn the contacts.

Check the unit driven by the motor for failure of the unit or the mechanism. If the motor has failed as a result of a failure in the driven unit, the fault must be corrected before installing a new motor.

If it has been determined that the fault is in the motor itself (by checking for correct voltage at the motor terminals and for failure of the driven unit), inspect the commutator and brushes. A dirty commutator or defective or binding brushes may result in poor contact between brushes and commutator clean the commutator, brushes, and brush holders with a cloth moistened with the recommended cleaning solvent. If brushes are damaged or worn to the specified minimum length, install new brushes in accordance with the applicable manufacturer's instructions covering the motor. If the motor still fails to operate, replace it with a serviceable motor.