[Amps] Transformer question

[Jeffrey Madore]

I have a transformer question that someone might be able to shed some light on.

Last fall I picked up 3 similar power supplies at the flea market at MIT. I believe they were designed to support some type of robotics. The fact that they were brand new and full of great parts is what really caught my interest.

One of the components in these power supplies is a power transformer with a 240v primary. On the schematic diagram it shows another winding on the primary side which is connected to an 8uf 660v oil filled cap. The secondary is connected to a block bridge rectifier with capacitor filter. The transformer core measures about 5"x6"x2". My plan was to build a bench supply using all three transformers / rectifiers / filter caps.

On testing, with the oil filled cap, bridge rectifier and filter C connected, I obtained the following values using a 75 watt lightbulb for a load as indicated:

120v pri - 60 vdc @ no load 
120v pri - 60 vdc @  loaded

240v pri - 60 vdc @ no load
240v pri - 60 vdc @  loaded


I then disconnected the oil filled cap and repeated the above:

120v pri - 30 vdc @ no load
120v pri - 23 vdc @ loaded

240v pri - 60 vdc @ no load
240v pri - 50 vdc @ loaded


I realize that the load used is small for this transformer but it was convenient for a quick test. My question is: what is the purpose for the additional winding and oil filled capacitor. With this capacitor in the circuit, the secondary voltage seems unaffected by drastic changes in primary voltage. It will be interesting to see it's behavior at higher loads. Without the oil filled cap, the voltage regulation is fairly poor at even a small load. The voltage across the oil filled cap was c. 600vac, and the DC resistance of the windings is: 2 ohm pri, 0.3 ohm sec, 4.3 ohm oil filled cap winding. These readings were taken with a DMM thus are approximate.

I understand basic transformer theory and have a handle on primary field, secondary field, and the action of each on both windings. Now, entering the picture, is another winding with a capacitive load. Is the field of this winding somehow affecting primary XL? Could it be that the higher Epri  resulting in higher Ic, causes a field that opposes the primary field to some degree, thus lowering XLpri?  I'm getting lost...  Is this a method of improved regulation?  Has anyone experience with a transformer such as this?

Thanks you for any information, theories, practical experience, etc

73 de
Jeff - K1LE - CT ><>............ar




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