JQRs 251.35 and 251.44 Coil Cutter and Burnout Oven

Stator Burnout and Stripping

Learning Objectives
You will be able to:

Heat and remove stator coils without damaging the stator in accordance with standards established by NAVSEA 0900-LP-060-2010 Series, Chapter 9, paragraphs 9-12 through 9-25.

Record necessary data on a motor repair identification sheet so that the stator can be rewound properly.

Background

Improper burnout and cleaning of a stator can ruin an otherwise repairable motor. The frame and laminations can warp or break if the job is not done properly. Proper burnout and cleaning will add to the operating life of the motor and help reduce the electrician's workload.

During this lesson, we will discuss the heating of stator windings prior to coil removal by both the burnout oven and radiant heating methods. We will then discuss coil removal by both the hand and power removal methods.

It is necessary to soften the hardened varnish and loosen the insulating materials enough to allow for their removal. Regardless of the methods used, you must remember that, because of heating, there is always a chance of personnel injury. In addition, you may be using a power cutoff saw to remove winding end turns and hydraulic pullers for coil removal. Again, the need to be especially careful cannot be overemphasized.

You will also complete and verify the rewind information that you compiled during the lesson on winding identification and data taking.

Burnout and heating of windings

The burnout oven method will be used for standard and T-frame motors only.

Burnout oven

Preheat oven to a maximum of 700°F for standard motors and 600°F for T-frame motors. The maximum surface temperature of the laminated iron surface should be measured on the iron by thermocouple and must not exceed maximum temperature.

Conduct a core loss test before and after burnout.

Remove the crown of the winding opposite the connection end.

Place the stator in the oven for approximately 12 hours or until all insulating materials have turned to ash.

Always use insulated gloves and eye protection when removing the stator from the oven.

Carefully remove the stator from the oven and allow it to cool in preparation for verification of winding information and coil removal.

Radiant heat

Be sure to operate the cutoff saw in accordance with local operating procedures and safety precautions. Use insulated gloves and a plastic face shield when handling the stator.

End turns from the stator's connection end must be removed before using the radiant heat method. While radiant heat may be used for any type of motor winding, it is the only approved method of winding heating for aluminum-framed and sealed-insulation system motors. The lower temperature (maximum of 400°F) does not damage the laminations.

Use a cutoff saw with rotating turntable to remove the end turns from the stator's connection end. Cut them 1/4 inch above the stator core. Be sure to save the cutoff end turns for verification of winding information.

Be sure to operate the stator heater in accordance with local operating procedures and safety precautions.

Do not allow the heater cables to catch on obstructions while rotating the turntable. The heater leads may be pulled and shorted if the leads are allowed to catch.

Place the stator on a heater ring with the uncut coil ends up. Observe the temperature of the windings frequently during the heating process. Power pulling of coils will commence when the temperature of the windings reaches 400°F.

Click on images to enlarge.

Cutting Off End Turns

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Stator with End Turns Removed

There are two methods of stator coil removal: hand and power.

Hand method of stator coil removal

Slot wedges will be removed with a power hacksaw blade and hammer. Place the blade along the length of the slot wedge and tap the blade with a hammer until the teeth are embedded in the wedge.

Tap the end of the blade with the hammer so that the wedge is driven out in the direction in which the teeth are pointing. Stand the motor on end, connection side up. Verify as much of the winding data as possible. Record the information compiled.

Position the motor so that the bore is horizontal. Cut the end turns of approximately one span of coils. Remove cut coils with pliers.

Be sure to leave one uncut coil to be used as a sample.

Cut and pull other coils in same manner. Verify the coil span as soon as both sides of the same coil are visible. Remove the uncut sample coil.

Click on image to enlarge.

Removing Slot Wedges

After the next step, stow the sample coil in a safe place because you will be using it later in this training.

Count and record the number of conductors in the sample coil. Divide this number by the number in hand, if applicable, to determine the number of turns in the coil. Record this information, if applicable, on the motor repair identification sheet (see example at right).

Power method of stator coil removal

Be sure to operate the coil puller in accordance with local operating and safety precautions. Use insulated gloves and a plastic face shield when handling the stator.

Be sure that the stator is securely clamped to the stator warmer.

Position the power puller, attach the tongs, and then embed them in the end turns. Apply just enough pressure to barely loosen the coils.

Be sure to wear insulated gloves and face protection when moving the stator. Place the heated coils in a metal container as soon as they are removed. Remember, the coils will initially be heated to approximately 400°F.

Remove the tongs. Rotate the turntable and continue to loosen all coils. When all coils are loose, reattach the tongs and begin removing coils.

Determine the coil span as soon as both sides of the same coil can be seen. Record this information on the motor repair identification sheet.

Count and record on the motor repair identification sheet the number of conductors in a sample coil. Divide this number by the number in hand, if applicable, to determine the number of turns in the coil. Record this information, if applicable, on the motor repair identification sheet.

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Motor Repair Identification Sheet

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Coil Puller Setup for Stator Coil Removal

Review Questions
If you can correctly answer the questions below, you have learned the key points in this lesson. Choose the correct answer for each question, then look at your score below the questions. Review the lesson if you miss any questions.