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I experimented with a power transformer this evening by wiring it backwards. It's the first time I've tried this with a small transformer of this type. The transformer is a Hammond 186C120 with dual 115V primaries and a 120V CT secondary. The diagram is below:I was hoping to use this by applying line voltage across the whole CT secondary. I assumed the two primary windings in series would then develop about 230V, and I would be able to draw 50 mA at that voltage.
For the test, I connected a 5K resistor across the series primary windings, then applied line voltage to the secondary. Line voltage was 123V. Instead of 230V however, all I measured was 182V. Just to be sure the transformer isn't defective, I also connected it in the usual way, with the two primaries in parallel. When I applied 123V to the primaries, the secondary produced about 127V with a 1.15K load (about 110mA). That's the correct voltage, relative to the 115V input that Hammond specifies.
Can someone explain why this transformer doesn't develop the same voltage ratio forward and backward?
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Buy Chinese. Bury freedom.
Edits: 09/21/17Follow Ups:
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 winding also could present more of a problem with the core being driven into saturation.If you drive the secondary coil itself beyond its 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 09/27/17 09/27/17
It only works that way if no current is drawn. If current is drawn, then the resistance of the windings consumes some of the voltage. This is why PSUD wants to know both the open-circuit (no load) voltages AND the winding resistances. Just another one of those things that are not obvious until they are. :^)
Incidentally, it works the same with audio output transformers.
the secondary's rated voltage is specified under full rated secondary current, with voltage drop in secondary's resistance and core loss factored in. As a result, turns ratio is not 1+1:1.04, but rather something like 1+1:1.35. That's why when turned backwards, the transformer puts out 91+91V.
"... turns ratio is not 1+1:1.04, but rather something like 1+1:1.35. That's why when turned backwards, the transformer puts out 91+91V."
Thanks! I spend a lot of time designing high-Q RF transformers, and their behavior is always bidirectional. The fact that a power transformer might be different in this respect just didn't occur to me.
As it turns out, 182V will probably fit my project better than the voltage I was expecting. I need ~250V DC at 40mA, and I think this will just get there. Now I can continue with the layout. Yay!
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Buy Chinese. Bury freedom.
Yes, the exact same behavior is observed if you reverse connect either the Triad N-77U or Triad N-68X .
The desired result can be obtained by adding a boost trafo on the AC mains side of things.
Eli D.
Ah, good to know!
So yes, figure the backwards-wired 115V+115V:115V transformer isn't 1+1:1.04 at all. Losses involved. 1+1:1.35?
So it's probably best to use the N-68X wired as normal, with voltage doubling DC rectification. So you get half the current output. With an N-68X I should get 120*2 = 240VAC, rectified to 240*1.3(ish) = 312VDC under load. Correct?
312V*0.1A = 31.2VA, which is within the 50VA rating.
But 312V*0.2A = 62.4VA, and that's too far over the 50VA rating. Gotta bump up to the N-77U for a small stereo power amp. Oh well...
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