I believe you have answered your own question, Jeff. Transformers of this
general type, with a tuned third winding were built by Sola, Raytheon and
probablly others. Usually they were used to regulate a varying line voltage
and supply a more constant voltage to equipment. Sometimes the units had
other than a 120v or 240v secondary on them. 73, Dan, N5AR
----- Original Message -----
From: Jeffrey Madore <K1LE@ARRL.NET>
To: <AMPS@contesting.com>
Sent: Wednesday, May 07, 2003 8:56 AM
Subject: [Amps] Transformer question
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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