Reverse Engineering an Obsolete Military Power Transformer

Written by Mike Horgan
Engineering and Quality Manager at Butler Winding

Butler winding recently completed an electronic reverse engineering effort for the Tobyhanna Army Depot. This was quite a challenge of reverse engineering. We began work on, what we initially thought, was a multi-tapped power transformer in an encapsulated box with connectors. We were given a non-functioning assembly of an EMP Electronics Inc. part (see photo 1) and some system level drawings but nothing specific about the assembly. When we disassembled the unit, we realized this was more than just one transformer. There was also a wound inductor connected across two of the leads of the transformer. We were able to determine the transformer size based on physical measurements and concluded it was an EI-65 lamination and made of 12 mil thick silicon-iron. By unwinding the transformer we determined the turns ratios of this four winding transformer.

Unfortunately the wound toroidal inductor broke into several pieces during disassembly (see photo 2). We could physically count the turns that were on it and had to get a material analysis done on one of the broken core pieces to determine the core material. It turned out be a 60 permeability MPP core, Magnetics 55894-A2.

Using both a resistance meter and an inductance meter we were able to determine the schematic and connection points.
We built one prototype (without encapsulation) for our military customer to electrically evaluate (see photos 3, 4, and 5). It performed as needed in a military static inverter power system. We can now proceed to encapsulate the prototype and build the remaining units.

Other military and research institutions Butler Winding has worked with recently include the Naval Undersea Warfare Center, NAVAIR or the Naval Air Systems Command, NASA€™s Columbia Scientific Balloon Facility, Sandia National Laboratories, Applied Research Laboratory at the Pennsylvania State University, NASA€™s Goddard Space Flight Center, Woods Hole Oceanographic Institution, and the Applied Research Lab of the University of Texas at Austin.

Photo 1: This is the non-functioning assembly provided by Tobyhanna Army Depot. We disassembled it to determine what components were in it and how the components functioned. During the reverse engineering, we discovered that it had both a transformer and an inductor inside of it – possibly created this way to make replacement simple.

Photo 2: The wound electrical toroidal inductor broke into several pieces during disassembly. You can see part of the winding – covered with epoxy.

Photo 3: This is the final custom assembly, prior to encapsulation. It includes a laminated transformer and toroidal inductor encased in one box.

Photo 4: This is the final custom assembly, prior to encapsulation, of the reverse engineered power transformer.

Photo 5: This is the final custom assembly, prior to encapsulation.

If you need assistance with your design, please contact Butler Winding and ask for Engineering.