David Sattler
L&S Electric, Inc.
Unless great care is taken, pulling magnet wire from a motor stator often bends or splays the lamination’s end teeth. Bent teeth, or teeth that have been splayed outward at the ends of the core stack, will likely compromise the quality of the repair job. Studies1 show that motor efficiency may be reduced by splaying end teeth. Even if that reduction in efficiency is slight, any reduction in efficiency results in higher costs and wasted energy.
Even though these performance reductions are seldom noticed by customers, our goal in motor repair is always to produce the highest quality rewind possible. Therefore, we have designed and implemented the use of disc clamps to hold the stator tooth tips in place while pulling magnet wire from the slots. The clamping fixtures described in the photos have helped ensure that we avoid damaging the stator teeth during the stripping process.
Photo 1 shows machined clamps we use when we have many units of the same size coming through the shop. Note the step in the clamp plate. The smaller outside diameter (OD) is the pilot, used for centering, and the step to the larger OD is the clamp surface.
Photo 2 shows the clamp installed in a stator core. The clamp secures the teeth to keep them from bending outward as the magnet wire is pulled in the stripping process. The clamping plates can be snugged down to the stator tooth tips using a bar across the opposite end of the stator frame as shown in photo 3, a threaded rod protruding from a table or base plate on which the stator bore is centered or another clamp plate at the opposite end of the core. Before installing the clamp, make a reference mark or record a reference measurement, so you can verify the core did not move in the frame due to clamping force. Although you should not need to use significant pressure to secure the clamps, some cores are secured to their frame by only a light interference fit or other minimal means, so shifting of the core during clamping or while pulling the magnet wire is possible. If shifting does occur, reposition and secure the core assembly within the frame according to your reference measurement before installing insulation and coils into the laminations.
For general rewind work where core diameters vary, we make a simplified version of the clamp plate. Photo 4 shows a clamp we made in a few minutes using a scrap of diamond pattern floor plate. Note how the screws are used for centering rather than the step in OD in the machined example (Photo 1). To make this clamp, we scribed a circle of the required diameter around a center screw in the plate and used a vertical band saw to cut it. We finished the edge of the circle using a vertical belt sander. Scribing another circle of appropriate smaller diameter provides a reference for adding optional screws for centering as shown in Photo 4. By using this process, we can easily make customized clamps for individual jobs. Computer numerical control (CNC) machines could also be used to make customized clamps or to create sets of clamps of incremental diameters.
Photos 5, 6, and 7 show a fixture for clamping cooling fan stators with external teeth. In this example, the baseplate is fitted with a centering fixture to accommodate a bore in the center of the stator frame (Photo 5). The clamp plate is secured by four threaded rods to the baseplate. The clamp plate has a step turned in its bore to ensure the clamp will be centered over the core.
While these clamps provide excellent support for the teeth, technicians must still use care when removing magnet wire. Uneven core stacks will cause some teeth to be held tighter than other teeth leaving those tooth tips more susceptible to pulling free from the clamps. Technicians should always inspect the clamped stator before stripping magnet wire to determine if any areas warrant extra caution when pulling wire.
If your goal in motor repair is to produce the highest quality product possible, then taking care not to damage the core is an important part of the process. We credit our EASA Accreditation auditor for consistently making the prevention of “splayed teeth” an important issue and for encouraging us to share our method of using tooth clamps to alleviate the problem for the broader motor service center community.
1 The Effect of Repair/Rewinding on Premium Efficiency/ IE3 Motors Copyright ©2021, EASA, Inc. (Version 0121), 2-8, Stray loss analysis
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ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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