Background

An IMA electrician must be able to repair motors inplace. Large dc motors and generator sets are highly susceptible to accidental damage when moved. Except for rewinding and machining, almost all repairs can be done inplace. Also, it is much easier to move only the parts that require work outside of the installed space.

During this lesson, we will discuss component disassembly, tests to be conducted, repair procedures, reassembly, and testing.

Preparation for disassembly

Ensure that the power supply is secured and tagged out by tended unit personnel. Use a voltmeter to test incoming power leads to determine that power has been secured.

Have the coupling between the motor and the driven unit disconnected, if applicable.

Remove the coupling half from the motor shaft, if installed.

Open the connection box; break and tag connections. Tie back the power cable and insulate leads.

Remove and store holddown bolts and dowel pins if it becomes necessary to reposition the motor.

Notify the rigging shop for assistance in moving large motors and generators.

Lift and reposition the motor as necessary. Mark and stow all shims.

Rope off the area as necessary.

Conduct a shaft radial runout test. Record the results. Take the necessary corrective actions.

Matchmark end bells and the field housing with a prick punch.

Matchmark the brush rigging yoke and commutator end bell with a prick punch or scribe.

Remove bearing caps and conduct a face runout test of the bearing inner and outer rings if the design permits. Record the results. Take the appropriate corrective actions.

Visually inspect the motor. Refer to the manufacturer's technical manual before beginning disassembly to determine bearing configuration, installation of jacking bolt holes, rotor weight, etc.

Do NOT allow the armature to drop down on the field pole faces when the motor is being disassembled. Use jacks, pipe, chainfalls, or another suitable piece of equipment to support the armature while the end bells are being removed.

Take action to support the armature to ensure that the armature does NOT rest on field pole faces at any time during disassembly. Insert packing material into the clearance space between the armature and field pole laminations at the first opportunity.

Release the brush holder spring tension. Lift all brushes from the brush holders. Loosen the brush holders and swing them upward. Tag the brush rigging connection points and leads from the connection box or field winding. Disconnect the leads.

Disassembly

Always remove the armature from the drive end of the motor since the vast majority of armatures are equipped with internal cooling fans on the drive end that will NOT pass between the field pole faces.

Disassemble the motor or generator in accordance with the manufacturer's technical manual and NAVSEA 0900-LP-060-2010, Volume 1, Chapter 6, paras. 6-11 through 6-14. Mark and stow all component parts as they are removed.

Ensure that the brush holders do not drag on the commutator when removing the commutator end bell.

If it was not inserted previously, insert packing material between the armature and field pole faces when end bell removal permits.

If its size permits, place the armature on a workbench. If the armature is too large to place on workbench, move the armature to the nearest satisfactory working area.