Constant Voltage Transformer Operation?

n.

It seems to be be a ferro-resonant constant voltage transformer. The third winding and the capacitor apparently set up a resonant tank circuit.

The core is apparently intended to saturate, so the maximum flux in the tank circuit is limited and more or less constant, and thus the peak current and voltage. Because it is part of a a resonant tank, the waveform on the output winding stays more or less sinusoidal.

Bill Sloman, Nijmegen

Apparently you're unsure of yourself, more or less!

Jamie

The transformer has two sections: a core section around the primary winding, and a section around the secondary. These are coupled somewhat looser than a regular transformer, so (without the capacitor) the secondary voltage would be lower and "squishier" than a regular transformer (i.e., higher series inductance). Adding the correct value capacitor cancels the series inductance, forming a resonant tank. But the increased voltage difference across the barrier forces the transformer deeper into saturation, thus limiting voltage.

The transformer needs to be designed so that, over the design range of input voltage (usually +/-10%) and load (0-100% current), the transformer must remain in saturation, without overheating. Finally, because the secondary voltage is generally lower (in terms of volts/turn), a lot more copper is needed. Iron operated in saturation also has high core losses. This makes these transformers particularly large and low in effeciency.

One upside: the effective LC filter between primary and secondary isolates harmonics and transients; although the input and output are still a bit distorted, they make reasonable sine waves, so it doesn't matter much if your primary waveform is part square wave, or that your load has similar behavior (like a capacitor-input rectifier).

Tim

Current in the regulatiing winding saturates the core and decouples the primary and secondary windings.

the primary starts off poorly coupled and is rated for higher current than in an ordinary transformer to survive the saturation.

n.

I've not played with them, so it's all book-learning ...

-- Bill Sloman, Nijmegen

Bill is right. The primary is wound to saturate the core. They often have a special core design, with more than 1 section and air gap between the two. The secondary voltage would normally be flat topped with a saturating primary, but a third winding forms a resonant circuit to filter the harmonics. The output regulation is usually very good and it's far simpler and more robust than an electronic regulator.

Power factor is not good though...

Chris

Ya, added that to make it *clear that the short was a little long! I removed the 0.7 ohm meter lead resistance and still had a 1.5 ohm short.

Mikek

• apparently I didn't make it clear. :-)