The Challenge of Reverse Engineering an Obsolete Military Power Transformer" Article by Mike Horgan

Unfortunately the wound
toroidal inductor broke into
several pieces during disassembly.
Read about our challenge

Our Engineering & Quality Manager, Mike Horgan, recently wrote an article on a reverse engineering project we completed.  The article is titled, "Reverse Engineering an Obsolete Military Power Transformer".  This project was challenging since there was incomplete and outdated documentation for us to work from including a lack of detail concerning the current.  Tobyhanna Army Depot worked to find all of the old documentation (possibly 30 years old) to help us custom design and recreate this transformer assembly.  Many necessary details were left out - making this quite the challenge.

Here is an excerpt from the article Mike wrote:

"...Butler winding recently completed a reverse engineering effort for the Tobyhanna Army Depot 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."   Read more...

Practical Considerations of Common Mode Inductors

Common mode inductors, more generally known as chokes, are used to filter unwanted electrical noise usually caused by the switching action of switch mode power supplies.  Specifically it filters out common mode noise or noise that is common to both the positive and negative outputs of a dc power supply, for example.  The main idea behind a common mode inductor is that you put two symmetrical windings on a magnetic core, typically a ferrite toroid core, and mount it a plastic header, see figure 1. The windings are connected so the current flow will be in equal but in opposite directions.  This keeps the core from saturating since it will have two equal but opposite magnetic fields and the full inductance of the core will help attenuate the switching noise common to both lines, see chokes/common mode inductors.

The most common core shape used in a common mode inductor is the toroid. It has two big advantages compared to other shapes. First, if you think of it as a winding on one half of a toroid opposing a winding on another half of a toroid you have two very symmetrical shapes which helps prevent core saturation. Secondly, a toroid has no mating surface compared to other core shapes so it will have more inductance and is usually 35% higher in inductance compared to a similar sized two piece core shape. The two windings on the toroid need to be as symmetrical as possible since leakage inductance will be created by the current flow. Leakage inductance can start to saturate the core which will cause the common mode inductance to decrease. An unequal number of turns on the two windings or an unsymmetrical winding shape can increase leakage inductance resulting in reduced inductance leading to reduced filtering of the noise. For more details see the theory of common mode inductors.

Typically common mode inductors handle currents ranging from tens of mA to about 20A. This means wire gauges from #38 to #12 are commonly used. Currents much above 20A are usually not wound on a toroid since the core can break due to winding stress associated with heavy gauge wire. Other core shapes and unique windings are usually used above 20A. Also multiple inductors in parallel could be used. High permeability ferrite materials are very susceptible to winding stress and inductances can be down to 50% of the expected inductance due to the winding stress on the core. Usually heating the wound cores slowly relaxes the stress and brings the inductance back up. I emphasize that the heating should be ramped up and down slowly since ferrite is a ceramic and can crack due to rapid temperature changes. 

One of the more surprising aspects of toroidal common mode inductors, or toroids in general, is that they are wound automatically, not by hand on toroidal winding machines.

written by Mike Horgan
Engineering Manager
at Butler Winding

Clamp on current transformers using permanent magnets

A clamp on current transformer, or a current clamp, is a common piece of test equipment used to measure current flow in a primary cable without making physical contact with the cable. A toroidal shaped current transformer is generally regarded as the best choice for a core shape, but a frequent dilemma is how to connect the transformer without disconnecting the cable. A spring loaded current transformer with a cut toroid shape is often employed to solve this problem. Another approach is to use two permanent magnets to clamp the split toroid back together.

We recently manufactured some split core current transformers using this idea. First an epoxy coated silicon iron tape wound core is cut in half, permanent magnets are glued to the core mating surfaces along with an alignment bracket, and a plastic hinge is taped on the toroid halves, see figure

1. Two secondary windings are wound onto the core, see figure

2. The windings are connected in series so the current from the two windings adds together. A final inductance test is performed at 100mV/1kHz to ensure the windings are connected correctly. Inductance measures about 15.5mH on properly connected parts and 9.9mH on parts with the connections reversed. A test limit of 12.5mH minimum is used. More insulation is added to help isolate the primary and secondary windings and a power cord is also assembled, see figure

3. These current transformers were coated bright yellow to make them easier to locate, see figure

4. The inside diameter of this current transformer is greater than 4.5” so current flow in very large cables can be measured.

Written by Mike Horgan
Engineering Manager
Butler Winding

Testing Transformers used in Switch Mode Power Supplies

Most electrical equipment is powered by a switch mode power supply. Switching the power at high frequency, 25kHz to 250kHz typically, reduces the size and cost of the power supply. One of the key components in a switching power supply is the transformer. The transformer consists of a ferrite core set, two or more coils wound on a coil former or bobbin, lead wires or printed circuit pins, and a clip or some other means of holding everything together. Butler Winding manufactures custom high frequency transformers and tests 100% of them to make sure they meet all design criteria.

Almost all our transformers are tested on a Voltech AT3600 transformer tester which is calibrated yearly. A four wire Kelvin connections is made to make certain that the voltage and current measurements are made as close as possible to the device under test commonly referred to as the DUT, see figure 1. The first test performed on the DUT is either continuity or resistance. Continuity makes sure the DUT is connected correctly and test fixturing or test leads are correct. Resistance testing additionally measures winding resistance which ensures the correct wire gauge has been used.

Figure 1: A Voltech AT3600 automated transformer tester with a 200W, 50kHz ferrite
EE core transformer under test connected with Kelvin flying test leads.

The second test on the DUT is series inductance. This makes sure the correct type of ferrite material has been used and the correct number of turns has been wound on the bobbin. The test conditions this measurement is made at must be carefully selected. The voltage must be chosen so that the resulting magnetic field density within the ferrite core is between 5 – 10 Gauss. The test frequency should be relatively low, like 1kHz – 10kHz, so you are well below the self resonance frequency of the DUT.
Based on their application, some transformers have an intentional air-gap in the ferrite core. It is common to have an air-gap on the center leg of a ferrite E core set to ensure the core does not saturate if it is operated in a unidirectional application like a switching flyback transformer. If the DUT has an air-gap then leakage inductance is commonly measured. This makes sure the gap is the correct size, 0.001” to 0.040” typically, and that the windings are positioned correctly.

Turns ratio testing is performed on the DUT by applying a signal to one winding and measuring the transformed signals on all the other windings. Polarity is also tested to make sure the windings were wound is the correct direction; this is commonly referred to as the transformer dot convention.

Since isolation is often a transformer requirement, HIPOT testing either AC or DC is performed on the DUT. Voltages range from 100V to 5kV. The high voltage is applied across winding to winding or winding to core and the resultant leakage current is measured. A maximum current limit is set commonly 250uA.

Figure 2: EFD15 size test fixture to which facilitates fast transformer testing. The operator inserts the transformer, clamps it down, and pushes one button to test it. A green/red light switch indicates the pass/fail results.

Test fixturing is used for rapid 100% testing of all transformers. Both thru-hole and surface mount transformers are tested. See figure 2 for our recently tooled EFD15 surface mount test fixture. All test data is saved under its part number and manufacturing date. Special tests beyond what was discussed here can be made. Contact Butler winding to discuss further.

Written by Mike Horgan
Engineering and Quality Manager at Butler Winding