The Art of the Rewind

The Art of the Rewind

Using rewound stators and armatures is the norm in the maintenance industry -- yet few offer the service

By Greg Napert

October 1999

Many accessory shops in the country provide the service of overhauling and repairing aircraft generators and alternators, but few offer the service of armature and stator rewind. The armature and stator assemblies are the hearts of generator and starter assemblies.

The armature is a relatively complex component built with copper wire and metal laminations around a single shaft. The stator is not quite so complex in design, yet the labor involved in rewinding it makes it impractical in most cases. Many shops can easily handle light repairs to the armatures and stators. These repairs include machining the commutator or undercutting mica — possibly even recoating exposed wires with varnish, but complete overhaul is beyond the scope of their capabilities. Instead, the armature is replaced with a new or rebuilt unit. Because of these challenges, there are only a handful of shops in the country that overhaul armatures. One of those is a facility in Ft. Lauderdale, Florida by the name of Thrust-Tech Aviation, Inc. (TTA). Thrust-Tech is an FAA Repair Station with a Limited Accessory and Limited Specialized Services certificate.

Armando Leighton, Jr., owner and founder of Thrust-Tech explains that "The data for rewinding armatures and stators is difficult to come by. The manufacturer in most cases does not offer an overhaul manual with part numbers and specific wire gauges. Data is a sacred commodity. Much of the data that Thrust-Tech and other rewind shops in the country have developed is a result of reverse engineering."

The company basically takes one of the units, disassembles it and takes the exact dimensions of the wire. It then counts the number of turns of wire on the factory units to determine precise resistance. The procedure involves using the same or better insulating materials, verifying that the output of the overhauled unit is the same as the original, and is within specifications. The whole process is repeated and has to be approved by the FAA under a process specification. This reverse engineering and approval process (which can take upwards of 6 months), may also weigh heavily into the reluctance of shops to get into the rewind business.

Skill required
"Rewinding an armature isn't as simple as you might think. It's a fairly labor-intensive procedure — where everything is assembled and bent into shape using fixtures and jigs," says Leighton. "We have managed to automate some of the sub components such as insulators and finger plates, varnishes and finishes, etc, but much of the process still has to be done by hand. Also, the windings all have to be the exact dimensions required to fit into the stator. Everything is calculated — each bend, each winding, the number of windings, the gauge of the wire, etc. It's really quite an art, a craft, and can have a learning curve of up to two years for a technician to master the rewind. And, it gets more difficult with the higher-powered armatures as the higher the amperage, the thicker the gauge of wire, and the thicker gauges become more difficult to bend and shape by hand."

Technicians actually develop a signature way of doing their own armature rewind by using their skills in the procedures. Further, the build up requires a great deal of testing and inspection during the rebuild. For instance, an armature needs to be tested for shorts at various intervals during the build-up process. Technicians must keep checking for shorts and make sure it is insulated properly or they will go through a great deal of labor only to find out they have to take it apart and start all over again.