Chuck Yung
EASA Senior Technical Support Specialist
We all know that stator cores should be burned at a controlled temperature to prevent lamination deterioration that can lead to harmful eddycurrent losses. But what about armatures? While that DC machine is energized by direct current, it is also true that the armature itself sees alternating current as the current in each coil reverses while passing from pole to pole.
A temperature-controlled burnout oven permits us to cremate a stator without worry, but an armature is another story. Because the commutator is integral to the armature, and cannot be easily removed, some repairers resort to a hand-stripping operation. Careful use of a torch to warm the windings accelerates the stripping job, but controlling the core temperature can be difficult. And stripping a large armature without heat is all but impossible by conventional methods.
Protect the core
Here is a great procedure used by a few members to protect the core and save labor while doing so. It sounds too good to be true, doesn’t it? The first time I saw this method employed I was amazed not only at the efficiency of the whole process, but also at the great condition of the armature core after stripping. I thought back on the armatures I’ve stripped and wound over the years, and wished that I had known this method then!
The procedure is simple. Use an old lathe as a dedicated armature stripping/ banding lathe. A dead center works best on the tailstock end. Situate the lathe under a jib crane fitted with an air hoist. The variable torque of an air hoist makes it preferable to electric for this application. If the armature is tig-welded, machine the risers as usual. Mark the slots and commutator bars as you normally would, then cut through the windings just behind the risers. A cutoff saw mounted to the tool post works well for this step. The Plexiglas guard should be large enough to allow the operator to safely see what he is doing. Cut just deep enough to get through the bottom leads. That permits inspection of equalizers, if any.
Remove the wedges. These can often be driven out using a wedge-driver. For large armatures, it may be easier to cut the wedges lengthwise instead of knocking them out. If so, a Skilsaw works well, with the blade depth adjusted to the wedge thickness.
Next, lift a coil by prying. Use the air hoist to pull the top of the coil from the slot. Repeat with the next coils until the span has been lifted. For the remaining coils, pull the top and the bottom in one continuous operation per coil.
Ways to grip the coil
Is there a good way to grip the coil? There are several methods that work well. One is to use a pinch-clamp like those used for lifting steel plates by the edge. Some winders half hitch a strap around each coil. Others use a pair of vise grips to clamp the coil, and attach the hoist hook beneath the pliers. Experiment to see what works best for you.
To remove the leads from the risers, use pliers. If the armature was soft-soldered, use a small torch tip and heat the risers just enough to melt the solder, and pull the wires as you go. For armatures that were tig-welded, shorten the blade of a kitchen knife to make a handy chisel. Blow the armature out, and it is ready to test. No carbon, very little cleaning required, and no worries about the core being damaged by a careless stripper.
It isn’t often that we find a tip that improves quality and saves labor!
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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