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This Post Has Been Edited by the Author
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 to a degree.
To what extent involves the design of the magnetic core. There is a somewhat analogous law of magnetics to ohm's law formulia for magnetic cores.
Ohms Law:
E (voltage) = I2 (current squared) x R (Resistance)
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 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.. OK
But, 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.
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