www.easa.

com July 1999

Motor Restack Procedures Provided

By Chuck Yung
EASA Technical Support Specialist
You ve just dismantled a special motor for a most burrs are found.
customer, and the core test indicates the watts- Others sand the entire
loss/pound is excessive. The high core losses are surface, but only one
caused by shorts between the laminations. This quick pass to remove
may be the result of a ground failure. Or exces- high spots & burrs.
sive temperatures may have caused the deteriora-
tion of inter-laminar insulation (called coreplate.) Insulating the
Whatever the cause, a replacement is 16 weeks laminations
away, and your customer wants his motor re- Coreplate material
paired. This motor sounds like a prime candidate should be of sufficient temperature rating that the
for a restack, but you are hesitant. Your company motor can be burned out in the future without
has a reputation for quality, and the finished requiring another restack. Use a material de-
product has to meet your usual high standards. signed for this purpose, capable of withstanding
You want to know the best procedure for burnout oven temperatures. DO NOT rely on the
repairing this core. Here are some guidelines to aerosol lamination spray sold by many suppli-
help you do the best possible repair. ers. It is intended to coat parts (e.g., the stator
bore) to prevent rusting. It is not of suitable
Draw It! temperature rating to withstand a burnout cycle.
The first step in a restack is to sketch the stator Some individuals paint one side of each lamina-
core. The distance from each end of the stator tion. Dipping every other lamination is faster.
frame should be documented, as well as the Thin the coreplate material with a suitable solvent,
overall core length. When the core includes vent and set up a drying rack. Work one section at a
ducts, the length and location of each section time to keep them separate. Hang the laminations
should be recorded. Tip: Reference all dimensions on edge, so they drain well. (String a wire like a
from a common point. Trying to measure the clothesline, with plastic sheeting beneath to
length of each segment and the width of each vent protect the floor from drips.) If an alignment
duct will result in stacked tolerance er rors, notch is present, hang half the dipped laminations
leading to confusion when the finished dimensions with the notch up, half with the notch down. This
are compared to the original measurements. minimizes the effect of any taper in the coreplate
thickness as it drains.
Organize & Coordinate
When restacking a large stator with several vent Disperse The Damage
ducts, keep the segments in separate stacks. This Damaged areas can be distributed through the
is to make sure the finished job is symmetrical good iron. Some rebuilders flip alternate lamina-
from the core center to each end. When a finished tions, others exchange the damaged laminations
restack has more laminations at one end, the rotor with good ones from other sections so that the
is pulled axially in that direction by magnetic damaged laminations are distributed through the
centering forces. These forces change with load, entire core length. The second option is best,
so the full-load magnetic center will be different since the alignment key is not always centered on
from no-load magnetic center. That can damage a slot. Flipping alternate laminations helps, but
sleeve bearing thrust shoulders, perhaps even keeps the damaged laminations concentrated in
damage the driven equipment. two areas. If the slot is skewed and has an
alignment notch, it may not be possible to do
Sanding either. In those cases, use magnetic wedge
The usual practice is to sand each lamination by material to restore the missing sections of iron.
hand to remove burrs and charred varnish. Lay the This only replaces about 65% of the missing iron,
lamination on a flat steel table for this step. Use of an so it is not ideal.
orbital (jitterbug) sander expedites the process. Some If you do not sand the laminations, do not flip
service centers only sand the fingers, as this is where them when restacking! The inherent burred edge

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EASA CURRENTS www.easa.com

Motor Restack Procedures Provided . . . Continued From Page 1

caused by the original punching process will that the sections match the as-found measure-
create an excessive gap between laminations and ments and locations.
cause the core to appear flared at the teeth. It is usually necessary to file the slots after
pressing to prevent coils from being cut during
Pressing The Restacked Core insertion. Use of a stacking fixture helps mini-
Stacking pressure is usually not spelled out by mize this. Some service centers use a piece of
manufacturers, and varies according to manufac- keystock milled to the original slot width as a
turer, size and age of the machine. As a guide, guide during the stacking process. This works
the pressure typically ranges from 75-150 psi well, especially if the laminations are periodi-
(515-1030 kPa), based on the backiron area only cally tapped to seat against one side during the
(i.e.- do not include the tooth area.) For a long installation process. Use a soft-face mallet to tap
stack, press at intervals during the restack the laminations toward the rib closest to the
process to keep things uniform. With vented stacking guide key. Tapping after every inch or
stacks, pressing every other section works well. so of laminations helps keep the bore uniform.
Try to duplicate the original core length as
nearly as possible. When the core seats against a Quality Assurance
shoulder in the frame, it helps to support the After completing the restack, confirm the
stack when pressing. Cast-iron frames may effectiveness of the core repair by performing a
crack/break when too much pressure is applied. core test. Providing your customer with the
Using a steel plate burned out to about 1 before-and-after watts-loss values provides
smaller than the core outside diameter works objective evidence that the core has been repaired
well. It lets you apply pressure to the stack and by clearly indicating a reduction in losses.
still access the backiron for installing retainer One final word of caution about restacks: They
keys or welding. Tip: When welding, weld a key are labor-intensive! A few cores are welded full-
at each rib, rather than trying to weld directly to length down the backiron — in several locations.
the lamination stack. Others are pressed into the frame and can be
After restacking, measure the sections and removed easily. Experience is the best teacher
make sure everything is symmetrical from the when it comes to figuring out how long any
center. The segment at each end should be the particular restack will take. A restack is usually
same length, the second section in from each end not cost-effective unless the motor is special or
should be the same length, etc. Make certain quite large.