Chuck Yung 
EASA Technical Support Specialist 

There are times when a winding cannot be processed through the burn­out oven, so it must be removed “cold.” The bond strength of most resins is approximately 8-10 psi (55-70 kPa), which means that a fairly large coil might have nearly 3,000 pounds (1350 kg) of bonding force with the slot. 

In those cases, there are some use­ful tips that can be used to reduce the difficulty in removing the coils. Many of the techniques in this article can be adapted for open slot wound rotors and armatures. 

Wedge removal 
The first step is to remove the wedges from the stator. Sometimes, a wedge-removing tool made from a power-hacksaw blade can be used. See Figure 1. A coarse-cut blade of 6-8 threads per inch (4-6 mm) works well. Place the teeth against the wedge, tap the tool down to seat the teeth into the wedge, and drive on the end of the tool (at a slight downward angle) to drive the wedge out of the core. 

When a winding has been VPI processed, the wedge is bonded to the coil top and too tight to drive out. The next alternative is to use a circular saw (a hand-held, rotary blade saw used primarily for carpentry). Set the cut depth to the wedge thickness, and slit each wedge the full length of the core. This requires a steady hand, as it is essential to avoid striking the core. 
Next use a hammer and chisel, or a screwdriver, to drive each section of wedge out of its wedge groove. 

Coil removal 
Depending on the core length, there are several methods for removing the coils; the shorter the core, the easier this task usually is. For most of the methods described below, a custom pry bar is helpful. See Figure 2.A length of flat bar, with thickness less than the slot width, is needed. The length of the tool is based on practical considerations; approximately 3 feet (1 meter) works well. The longer the tool, the more lever­age can be applied. However, longer tools are unwieldy. In most cases, this job goes faster with two people working together, one on each end. 

For large diameter machines with a short core length, insert the pry bar near the bend at the coil top side, using the adjacent coil as the initial fulcrum. Apply enough force to move the coil, and work the pry bar in further with each applica­tion of force. 

Many winders cut each coil at one knuckle, as they are lifted, until the coil span is exposed. Once the bottom coil side is revealed, use the same technique to pry the coil bottom out of the slot.

So the coils are removed like this: Coil top side is lifted, bot­tom of the same coil is lifted, and the coil set aside. Repeat with the next coil, and so on. 

For longer coils, or coils that are more difficult to remove, there are some other useful tips. Place a block of wood on the bore, wrap a short length of cable or rope around the coil, and use the pry bar (resting the tip of the pry bar on the wooden block) to lift the coil end. Once the end is lifted a bit, it is easer to work the pry bar under the coil side and lever it out. 

For very large machines, especially 2-pole windings with a long coil exten­sion, there is another tip that may war­rant the extra effort. Use an industrial reciprocating saw to cut off the coil extension in one piece. Leave a couple of inches of coil protruding past each end of the core. Use the cable or rope method described above. It is some­times easier to place a heavy bar or pipe through the bore of the stator, and use the cable to attach the pipe to each coil end. Use the pipe as a spreader bar, using an overhead crane to apply uniform force and remove each coil. 

An alternative to the cable de­scribed above is to fabricate a steel shoe, or box slightly larger than the coil periphery, which can be slid over one coil end. See Figure 3. Weld an eye to the top of the shoe, and use the overhead crane to expedite the coil removal. 

For rewinding a large machine in place, this method works very well. When the machine is a low-speed synchronous, with a large bore diam­eter and short core length, we often jack the stator back from the rotor and have limited working space. Use the machine’s shaft as a pulley/pivot with the crane, so that each coil is pulled nearly straight relative to its slot. 

An air hoist is preferable to a conventional crane for this, given its variable torque and speed. 

When rewinding a large stator “in­house,” there is a benefit to having a large machining capability. Use a large vertical turret lathe (VTL) and mount a saw on the tool post, place the stator in the VTL with the bore vertical, and literally cut off the coil extensions. Hint: saw off the end that is down first, so the coil extension drops rather than pinching the saw blade. Flip the stator and remove the opposite end. 

It is sometimes practical to preheat the entire stator in the bake oven to soften the resin. How well this method works depends on several factors, including the type of resin used. The drawbacks to preheating the stator include risk of burns to person­nel as well as the time required to heat the stator and the probability that you will have to stop and reheat the stator several times before the entire winding is successfully removed. 

For long cores, the task can be made easier by carefully cutting off the connection, exposing the coil ends, and using a DC supply such as a welding machine to energize and heat each coil as you go. That method is time-con­suming, so it is only practical for long cores with tightly bonded coils. 

High-pressure water blasting is another method some use to literally blast the insulation out of each slot. This method in­cludes some serious potential risks and should only be used by qualified personnel. 

Tools 
A pry bar length of ap­proximately 3 feet (1 meter) is practical, with a thickness approximately three quar­ters of the slot width, and one side of one end tapered. The tapered end should be smooth to avoid nicking of the slots. The end should not come to a sharp point, but rather be blunt with a round­ed point. A sharp point is physically too weak to be a useful lever and is more likely to damage the core. 

If you work on a lot of large sta­tors and have to cold strip them, use two lengths of channel iron and a length of heavy-wall pipe. Place one piece of channel iron directly across each end of the stator, dissecting the stator, and punch/drill/burn holes in line with the end bracket bolt-holes. Bolt the channel iron to the frame. Machine or burn a hole in both pieces of channel, in line with the centerline of the stator bore, large enough to slide the pipe through. 

Fabricate a bracket so that a small hoist can be suspended from the pipe, and use the cable or shoe technique described above, with a short spreader bar, to make the job easier. 

Cautions 
Never use a torch to apply direct heat to the core. The risk of damaging the interlaminar insulation (coreplate) is too great. Always keep in mind that, even though no heat is applied to the core, the coreplate can still deteriorate due to vibration. 

For larger, vintage machines, decades of vibration during normal service can loosen the stack through-bolts and lead to chafing of the coreplate. A core test, both before and after winding removal, is essential for every rewind. 

When it is necessary to restack a core, always keep in mind that the geometry of the finished core is criti­cal. The ribs and structure supporting the core are often not concentric to the bore id and should not be relied upon to align the laminations during a restack. Instead, the bearing bore should be referenced. In nearly all cases, the rabbet fits are concentric to the bore. 

Remember, too, always use safe work practices and appropriate pro­tective equipment! 

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

Tags: Form wound Winding removal

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