Hi Jeff,
What you have is a ferro-resonant transformer. That was used quite
extensively in battery chargers for the marine industry. It provides a
fairly good regulation with wide variations in load and input voltage.
However if you measure the primary current you will find it quite high.
They are not very efficient. Even with no load on the secondary the
primary current is not low.
73
Gary K4FMX
Jeffrey Madore wrote:
> 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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