Background

After a motor has been carefully machined, fitted with new bearings, and rewound, the electrician must assemble and test it. If the assembly is not performed carefully, the winding or motor may be damaged and repair time will be lost. Repair personnel assembling a rewound motor frequently may be under pressure to rush the job, but still must ensure that assembly and testing procedures are performed properly.

Preparation before reassembly

Ensure that the following conditions are in effect.

• All parts are in the work area.
• All new hardware has been received.
• Bearings on the shaft are properly fitted and greased.
• Bearing housing machine work is complete.
• All defects noted in the receiving and disassembly inspection have been corrected.
• Varnish has been removed from mating surfaces, the laminations, and feet.
• All chainfalls and slings are rigged for handling heavy components.

The following preassembly tests must be performed on the windings and entered in section 5 of the motor data sheet (electrical):

• Insulation-to-ground test
• Resistance balance test with double bridge
• Surge comparison test
• AC high-potential test
• Insulation resistance-to-ground test
• Phase balance test

The winding must pass all tests or the motor cannot be assembled for delivery.

Motor assembly

Wrap the bearings in lint-free material to prevent contamination.

Prepare a stator protector: obtain a sheet of NOMEX, rag paper, or gasket material long enough to extend from one end of the motor winding to the other, and wide enough to cover about one-fourth of the inner circumference of the core. This is to protect the stator from the rubbing of the rotor as it slides into the stator bore.

Place the stator protector inside the stator on the bottom of the bore.

Keep the rotor off the windings while inserting it in the stator.

Ends of the all-thread rods should point towards the end of the rotor.

With the rotor resting in the stator, attach all-thread rods of sufficient length to the bearing retainer bolt holes on the front of the rotor. Place the front end bell adjacent to the front end of the shaft. Position the all-thread rods through the end bell grease retainer bolt holes.

Slide the front end bell on the shaft over the bearing. Guide the all-thread through the retainer holes. Align the matchmarks.

Catch the end bell to frame bolts, but do NOT tighten yet.

Use the all-thread rods to pull the retainer in line. Catch the retainer bolts in the two empty holes, but do NOT tighten.

After the bolts are caught, remove the all-thread rods and install remaining retainer bolts.

Attach all-thread rods of sufficient length to the back bearing inner retainer. Place the back end bell on the back end of the motor shaft and slide it towards the motor frame.

Fit the pipe over the drive shaft, if necessary. Support the shaft on the machinist's jack and/or place wood blocks under the pipe or shaft.

Move the back end bell onto the pipe, if used. Remove the stator protector. Move the back end bell towards motor stator and position the all-thread rods through the retainer bolt holes. Slide the back end bell over the back bearing as close to the frame as possible. Make sure matchmarks are aligned. Catch end bell frame bolts, but do NOT tighten them.

Remove the jack, blocks, and pipe.

Pull the ends of the all-thread rods to position the end bell retainer.

If there is an outside grease cup, it should be one-third full of grease and in position at this time.

Catch the retainer bolts through the holes without all-thread rods in them. Remove the all-thread rods. Catch remaining retainer bolts.

Pull all end bell bolts up evenly (back and front). Alternate from end to end. Tighten on diametric opposites (this is called diagonal sequencing).

Rotate the shaft continuously and slowly while tightening the bolts to detect cocking.

Torque in accordance with specifications.

Pull up the retainer bolts following the above procedures.

When all bolts are torqued, the rotor must rotate freely. Listen for rubbing as rotor moves.

Post assembly testing

Perform insulation resistance-to-ground and phase resistance balance tests to ensure that the winding was not damaged during assembly.

Move the assembled motor to the test area.

Ground the motor frame.

Rest line leads in a special box to ensure lead separation.

Connect the motor to the proper line voltage. Test run the motor for 30 minutes.

Results of a phase current balance test must show current within five percent for all phases, three percent for submarine motors.

Conduct a phase current balance test. Maximum imbalance allowed is 5 percent for surface craft and three percent for submarine motors.

Check bearings for unusual heating or noise.

Use a Strobotac or hand-held tachometer to check motor speed.

Monitor the motor to ensure that it is operating properly during the test run.

Stop the motor and disconnect it from the line.