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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
Losses are the reason.. Need to think of the primary winding, the magnetic core and the secondary winding as a set of series circuits so far as losses are concerned, which will affect the voltage output as you have seen. A part of those losses are there when used in normal direction, and the design off course took that into consideration . Driving it in reverse presents a different design limit situation for many reasons as discussed below.As you have already seen using a transformer backwards works OK as long as you stay within the limits to not let the magic smoke out.
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 fine for several years.
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 considerably below its rating.
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To get into the specifics, requires diving into the properties of magnetic core and of course the power handling capability of the individual windings. Driven secondary to primary losses can be different compared to driven primary to secondary and may also exceed the magnetic core's ability to transfer the same amount of energy.
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 the nasty sharp knee in permeability curves, hysteresis losses 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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Caveats:
The secondary windings, especially if multiple secondaries and, intended application could likely have been designed to handle only some fraction of the transformers overall power rating. Driving the secondary windings may present more of a problem with the core being driving into saturation.If you drive the secondary coil itself beyond it power handling capability or, the magnetic core into saturation (by increasing the load on the transformer beyond it's unknown (unless spec'd) limit in this configuration, 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 :<)
btw the limitations magnetic cores present can also be used to good effect as a very effective surge suppression system. I have a ferro-resonant design uninterruptible power system (UPS) that has a 2000:1 surge suppression capability. That is with a 2000 volt spike on the power input only 1 volt will be seen on the protected output.
Edits: 09/27/17 09/27/17 09/27/17 09/27/17 09/27/17 09/27/17 09/27/17 09/27/17 09/27/17Follow Ups:
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