Abstract

There is a problem with testing torroidal or encapsulated transformers for their DWV characteristics in that the core is normally inaccessible. For high voltage transformers, knowing whether or not the core is sufficiently insulated to prevent breakdown is crucial to the component reliability. If there is a lack of dielectric material between any winding set and the core, it is impossible to determine potential failure susceptibility by the normal DWV testing means. Using MILT- 27’s “Corona discharge” test circuits for this determination requires special equipment, “corona free” capacitors, and has other issues. I have developed a screening method using a Fluorescent Lamp Supply “drive box” with a sinusoidal output from 2 to 7KVpp at or above 25 KHz that will expose corona in transformers when directly back driven into the secondary windings. Transformers that do not have sufficient insulation between winding sets and windings to core can develop corona discharge which can be seen on an oscilloscope via a commercially available isolated current probe as high frequency spikes modulating the original sine wave, heard on an AM radio the same as lightning or static discharge, or cause complete collapse of the voltage at test voltages within seconds when tested up to the rated voltage of the particular winding set. A sample set of 80 transformers was tested, and a 30 percent failure rate was discovered.

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