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In Reply to: RE: Backward Power Transformer posted by Triode_Kingdom on September 21, 2017 at 21:05:07
As you have seen using a transformer backwards works OK as long as you stay within the limits to not let the magic smoke out.Great way to learn more about transformers.. As a teenage kid with meager resources, I used a filament transformer driven in reverse to generate the plate high voltage supply for a small, single tube 12AX7 auto amp. Worked for several years before it eventually let the smoke out.
A close ham radio friend who is a master of old time tube high power RF amplifiers is using a power line distribution transformer (somehow they got the nick name "pole pig", google that term...) running it in reverse with a variac to create a 3000 volt plate supply for a kw RF amp. Forget the spec's on the transformer but i recall he is running it considerabilty below its rating.
To what extent this can work without letting the magic smoke out, focuses a lot on the design of the magnetic core and of course the power handling capability of the individual windings.
As simple as the magnetic core of a transformer seems, their design has considerable complexity. But for simplicity sake and understanding, those properties can be considered somewhat analogous to Ohm's law.
Loose analogy for the energy in a magnetic core
Magnetizing force (created by the driven winding) = flux density (intensity of the magnetic field over the cross section of the core) x permeability (the ability of the core material to be magnetized)Got cha's in magnetics
The permeability of the core material is a non linear function (see nasty sharp knee in permeability curves, hysteresis properties in URL to document below.Once the magnetizing force creates a flux density beyond the cores saturation point, (core magnetized to the maximum extent possible), the excess energy just creates heat.
Once the magnetic core is in saturation, the winding driving it now behaves as if the magnetic core has become an air core and the current in the winding skyrockets.
When using the transformer as originally designed (driving it through the primary) all of the above was worked out by the designer for the transformer to operate as spec'd.
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Caveat:
Unless the transformer was also designed (design of the magnetic core) to handle driving secondary currents, and you are operating within those limits.. OKBut, if you drive the magnetic core into saturation (by increasing the load the transformer beyond it's unknown (unless spec'd) limit, current in the secondary winding being driven will skyrocket, magnetic temperatures escalate, and you are on the path to a very fast burnout of the transformer.
For further information the document below starting in the section titled "Fundamental Characteristics of a Magnetic Core" goes into the gory details :<)
Edits: 09/27/17 09/27/17 09/27/17 09/27/17Follow Ups:
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