Its a "ferro-resonant" transformer. Its designed to be a
regulating transformer and under little or no load
actually puts out a square wave. The core operates in
a saturated mode, these transformers are quite inefficient
if under loaded, typically they should have a constant load of
about 70-80% of capacity.
Mark WB8JKR
On Wed, 7 May 2003 11:56:54 -0400 "Jeffrey Madore" <K1LE@ARRL.NET>
writes:
> 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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