Ensure that the power supply is secured and tagged out by tended unit personnel.

Test all incoming power sources with a voltmeter before beginning work.

Have the coupling between motor and the driven unit disconnected, if applicable. Remove the coupling half from the motor shaft, if installed.

Open connection boxes. Break and tag connections. Tie back all cables and insulate the leads. Remove and store holddown bolts, nuts, and dowel pins if it is necessary to reposition the motor generator set.

Notify the rigging shop for assistance in moving a large motor generator set.

Lift and reposition the motor generator set. Mark and store all shims. Rope off the area as necessary.

Conduct a shaft and slip ring radial runout test. Take corrective action.

Matchmark end bells and the stator housing with a prick punch. Matchmark the brush rigging yoke and slip ring end bell with a prick punch or scribe.

Remove bearing caps, if equipped, and conduct a face runout test of the bearing if the design permits.

Visually inspect the motor and refer to the manufacturer's technical manual before disassembly to determine the bearing configuration installation of the jacking bolt holes, the weight of the rotor, end bells, and other associated parts.

Do NOT allow the rotor to drop down on the lamination when the motor is being disassembled. Use jacks, pipe, chainfalls, and another suitable piece of equipment to support the rotor while the end bells are being removed.

Support the rotor to ensure that it does not rest on the stator lamination at any time during disassembly. Insert packing material into the clearance space between the rotor and stator lamination at the first opportunity. Release brush holder spring tension. Lift all brushes from the brush holders. Loosen the brush holders and swing them upward if possible. Tag brush rigging connection points and leads. Disconnect the leads.

Disassembly of the motor generator set

Disassemble the motor generator set 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. Insert packing material between the rotor and the stator lamination when end bell removal permits and if the packing material was not inserted previously.

Ensure that brush holders do not drag on slip rings when removing slip ring end bell.

Place the rotor on a workbench if size permits. If the rotor is too large to place on workbench, move the rotor to the nearest satisfactory working area.

Support the rotor with jack stands, wooden blocks, or another suitable piece of equipment during maintenance. Do NOT allow the rotor to rest on its lamination. Wrap the slip rings with lint-free cloths and tape them up.

Bearing removal and component inspection

Remove the bearings. Clean end bell bearing housings and shaft bearing journals with lint-free cloths and approved solvent. Use steel wool or wire brush to remove any rust, paint, or dirt.

Measure bearing housings with a bore gage. Refer to the table on bearing housing diameter limits and take corrective action for housings that are not within limits.

Measure shaft journals with a snap gage. Record the results. Refer to the table on bearing shaft diameter limits and take corrective action for journals not within limits.

Clean the shaft, end bells, bearing retainers, stator housing mating edges, and other associated equipment with lint-free cloths and approved solvents.

Inspect the shaft, end bells, bearing retainers, and other component parts for any condition requiring replacement, machine shop work, or other necessary maintenance. Take appropriate corrective action.

Inspect all mating surfaces for conditions requiring replacement, machine shop work, hand dressing, or other necessary maintenance. Take appropriate corrective action.

Inspect brush rigging for cleanliness, evidence of arcing or crystallizing of insulators, or loosening of brush holder supports. Clean brush rigging by wiping with a lint-free cloth dipped in an approved solvent or by vacuuming. Signs of arcing, crystallizing, or other insulation breakdown will require replacement.

Inspect brushholders for the condition of the brush arms and springs, signs of arcing, or mechanical damage. Repair or replace the brushholders as necessary.

Inspect the brushes for frayed shunts (pigtails), cracks, chips, or signs of overheating; the condition of hammer clips or riveted shunt connections; the condition of the contact surface; and length. Replace any brushes that are mechanically damaged, have frayed or loose shunts, or have faulty hammer clips.

Brush length replacement criteria

Brushes with riveted connections or hammer clips will be replaced when they are worn to 1/8 inch of the metallic part.

Brushes having tamped connections and without hammer clips will be replaced when worn to within 50 percent of their original length.

Brushes having spring-enclosed shunts will be replaced when they are worn to within 40 percent of their original length.

Rotor and slip ring inspection

Inspect the rotor windings for the presence of carbon dust, airborne dust, grease, oil, or salt deposits. Remove carbon and airborne dust by vacuuming or wiping with lint-free cloths. Remove oil and grease deposits by wiping with lint-free cloths and an approved solvent. Remove salt deposits from the windings by wiping them with fresh warm water and lint-free cloths. Dry the rotor.

After cleaning, inspect the generator field and motor rotor windings for signs of discoloration, overheating, bare conductors, mechanical damage, or other signs that would necessitate maintenance.

Perform a motor and generator rotor winding internal resistance balance test as previously discussed. Readings not within the parameters previously discussed will require delivery of the rotors to the electrical shop for rewinding.

Slip ring inspection and surfacing

Normally, slip rings will be dark brown in color. A slip ring with a black surface usually indicates excessive brush tension or that an improper brush grade was installed. Pitting is caused by not reversing the polarity of the slip ring.

Inspect coil leads where they are connected to the slip rings for signs of overheating or deterioration of sleeving. Inspect the banding for tightness.

Stator winding inspection

Inspect stator windings and the stator housing for the presence of carbon dust, airborne dust, grease, oil, or salt deposits.

Remove carbon and airborne dust by wiping with lint-free cloths or by vacuuming. Remove oil and grease deposits by wiping the windings and housing with lint-free cloths and an approved solvent.

Remove salt deposits by wiping the windings and housing with warm fresh water and lint-free cloths. Dry the winding. Remove the stator housing to the shop for reconditioning or rewind if you are unable to satisfactorily clean and dry it inplace or if you are unable to achieve a satisfactory corrected insulation resistance reading in accordance with values listed for that component in the nomograph presented earlier in this lesson

Conduct a phase resistance balance test of the stator windings as previously discussed. Stator(s) will be rewound if unacceptable phase resistance values are present.

Bearing installation

Inspect the bearings thoroughly upon receipt. Install them.

Tighten the bearing locknuts in accordance with values listed for that component.

Motor generator reassembly

Reassemble the motor generator set in accordance with the manufacturer's technical manual and NAVSEA 0900-LP-060-2010, Volume 1, Chapter 12. Ensure that the brush rigging is properly aligned to the matchmarked position. Reconnect the leads to the brush rigging.

Set brush holders 1/16 to 1/8 inch from the slip ring. Install the brushes after determining that they are of the proper grade and type in accordance with information from the manufacturer's technical manual.

The original brushes may be reinstalled if inspection determines their applicability, condition, and length are satisfactory. Use a spring scale to set brush tension in accordance with information in the manufacturer's technical manual. Set brush tension to 1.5 to 2.5 pounds per square inch (psi) of brush surface if the manufacturer's guidance is not available.

Seating will not be necessary if the original brushes are used, the slip rings are not resurfaced, and previous operation of the motor generator set appears to be satisfactory.

Install new brushes if the original brushes were unsatisfactory. New brushes must be seated.

Brushes may be seated by attaching slightly overlapping sheets or strips of sandpaper to the slip rings and turning the rotor by hand in the normal direction of rotation, if the design permits.

Brushes may be seated individually or in groups by hand sanding if necessary.

Set brush tension after the sandpaper is in place.

Seat the brushes until a 100-percent fit to the slip ring is achieved. Complete the seating procedure with a fine grade of sandpaper.

Vacuum and wipe the slip rings to remove all carbon dust. Lift each brush by its shunt to ensure that it is free in its holder.

Reinstallation of the motor generator set

Ensure that the motor generator set mounting feet and the mounting area of the bedplate are clean. Return the motor generator to its installed location.

Ensure that the shims are installed in their original locations.

Arrange for alignment of the motor and driven unit, if necessary.

Conduct phase resistance balance tests and internal resistance tests as previously described. Record the results. Readings not within limits will require corrective action.

Connect and insulate motor and generator line leads.

No-load motor generator run test

Have the system tagged in and the circuit energized. Start the motor generator set and observe installed meters to determine whether the motor generator set is providing proper output voltage and frequency.

Make necessary adjustments to the motor generator set voltage and frequency regulators.

Check for brush sparking. Stop the test if arcing is present; proceed as follows:

• Have the circuit deenergized and tagged out.
• Inspect the brush surface for 100-percent seating. Reseat as necessary.
• Retest spring tension and brush holder spacing from the slip ring.
• Have the circuit tagged in and energized.
• Restart the motor generator set. Observe it for proper operation.

Do NOT proceed with the test until the motor generator set is operating properly.

Check motor phase current with a clamp-on ammeter. Compare to information in the manufacturer's technical manual or nameplate. Record the results.

Monitor bearing temperature by attaching a suitable thermometer to the bearing housing with putty, duct seal, or another suitable adhesive. Record the results.

Listen for unusual noise. Check for vibration.

Load test

Ensure that the motor generator set output is 450 volts at 400 hz.

If brushes have been replaced and the slip rings turned or cleaned, the slip rings must be reseasoned.

If the brushes are 100 percent seated and the slip rings are properly seasoned, apply load to the generator in four equal increments, 30 minutes apart.

After each load increase, check:

• Motor current: phases A, B, and C
• Generator output voltage: phases A, B, and C
• Generator frequency
• Generator load current: phases A, B, and C
• Brushes for arcing
• Bearing temperature with previously installed thermometer

Record results and compare them with the nameplate or the manufacturer's technical manual. Use installed meters when practical.