Chuck Yung
EASA Senior Technical Support Specialist

You've just dismantled a special motor for a customer, and the core test indicates the watts loss/pound is excessive.  The high core losses are caused by shorts between the laminations.  This may be the result of a ground failure.  Or excessive temperatures may have caused the deterioration of inter-laminar insulation (called coreplate.)

Whatever the cause, a replacement is 16 weeks away, and your customer wants his motor repaired.  This motor sounds like a prime candidate for a restack, but you are hesitant.  Your company has a reputation for quality, and the finished product has to meet your usual high standards. 

You want to know the best procedure for repairing this core.  Here are some guidelines to help you do the best possible repair. 

Draw it! 
The first step in a restack is to sketch the stator core.  The distance from each end of the stator frame should be documented, as well as the overall core length.  When the core includes vent ducts, the length and location of each section should be recorded. Tip: Reference all dimensions from a common point.  Trying to measure the length of each segment and the width of each vent duct will result in stacked tolerance er rors, leading to confusion when the finished dimensions are compared to the original measurements. 

Organize & coordinate 
When restacking a large stator with several vent ducts, keep the segments in separate stacks.  This is to make sure the finished job is symmetrical from the core center to each end.  When a finished restack has more laminations at one end, the rotor is pulled axially in that direction by magnetic centering forces.  These forces change with load, so the full-load magnetic center will be different from no-load magnetic center.  That can damage sleeve bearing thrust shoulders, perhaps even damage the driven equipment. 

Sanding 
The usual practice is to sand each lamination by hand to remove burrs and charred varnish. Lay the lamination on a flat steel table for this step. Use of an orbital (jitterbug) sander expedites the process. Some service centers only sand the fingers, as this is where most burrs are found. Others sand the entire surface, but only one quick pass to remove high spots & burrs. 
Insulating the laminations 

Coreplate material should be of sufficient temperature rating that the motor can be burned out in the future without requiring another restack.  Use a material designed for this purpose, capable of withstanding burnout oven temperatures.  DO NOT rely on the aerosol lamination spray sold by many suppliers.  It is intended to coat parts (e.g., the stator bore) to prevent rusting.  It is not of suitable temperature rating to withstand a burnout cycle. Some individuals paint one side of each lamination.  Dipping every other lamination is faster. Thin the coreplate material with a suitable solvent, and set up a drying rack.  Work one section at a time to keep them separate.  Hang the laminations on edge, so they drain well.  (String a wire like a clothesline, with plastic sheeting beneath to protect the floor from drips.)  If an alignment notch is present, hang half the dipped laminations with the notch up, half with the notch down.  This minimizes the effect of any taper in the coreplate thickness as it drains. 

Disperse the damage 
Damaged areas can be distributed through the good iron.  Some rebuilders flip alternate laminations, others exchange the damaged laminations with good ones from other sections so that the damaged laminations are distributed through the entire core length.  The second option is best, since the alignment key is not always centered on a slot. Flipping alternate laminations helps, but keeps the damaged laminations concentrated in two areas.  If the slot is skewed and has an alignment notch, it may not be possible to do either.  In those cases, use magnetic wedge material to restore the missing sections of  iron. This only replaces about 65% of the missing iron, so it is not ideal. 

If you do not sand the laminations, do not flip them when restacking!  The inherent burred edge caused by the original punching process will create an excessive gap between laminations and cause the core to appear flared at the teeth. 

Pressing the restacked core 
Stacking pressure is usually not spelled out by manufacturers, and varies according to manufacturer, size and age of the machine.  As a guide, the pressure typically ranges from 75-150 psi (515-1030 kPa), based on the backiron area only (i.e.- do not include the tooth area.)  For a long stack, press at intervals during the restack process to keep things uniform.  With vented stacks, pressing every other section works well. Try to duplicate the original core length as nearly as possible.  When the core seats against a shoulder in the frame, it helps to support the stack when pressing.  Cast-iron frames may crack/break when too much pressure is applied. Using a steel plate burned out to about 1 smaller than the core outside diameter works well.  It lets you apply pressure to the stack and still access the backiron for installing retainer keys or welding.  Tip: When welding, weld a key at each rib, rather than trying to weld directly to the lamination stack.

After restacking, measure the sections and make sure everything is symmetrical from the center. The segment at each end should be the same length, the second section in from each end should be the same length, etc.  Make certain that the sections match the as-found measurements and locations.

It is usually necessary to file the slots after pressing to prevent coils from being cut during insertion.  Use of a stacking fixture helps minimize this.  Some service centers use a piece of keystock milled to the original slot width as a guide during the stacking process.  This works well, especially if the laminations are periodically tapped to seat against one side during the installation process. Use a soft-face mallet to tap the laminations toward the rib closest to the stacking guide key. Tapping after every inch or so of laminations helps keep the bore uniform. 

Quality assurance 
After completing the restack, confirm the effectiveness of the core repair by performing a core test.  Providing your customer with the before-and-after watts-loss values provides objective evidence that the core has been repaired by clearly indicating a reduction in losses.

One final word of caution about restacks: They are labor-intensive!  A few cores are welded full-length down the backiron — in several locations. Others are pressed into the frame and can be removed easily.  Experience is the best teacher when it comes to figuring out how long any particular restack will take.  A restack is usually not cost-effective unless the motor is special or quite large. 

Categories: AC motors, Stators/windings, Laminations, Repair procedures & tips

Tags: Stator core Restack

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