Hey, cut me a break. It was after 1:00 AM! 
You are, of course, correct that the curve should not only be square (in this case, square refers to the vertical sides of the curve) , but narrow as well. I did gloss over over that and I shouldn’t have. Thanks for taking the time to write out that explanation.
Q.
Who has designed transformers - but not recently.
Jinx, to cut to the chase for and ME. Think of a transformer as a set of gears.
That is a superb analogy.
Well to start with, yes the equations in my posts describe an ideal transformer but I covered reality with this paragraph. “For an actual transformer these relationships for an ideal transformer are inserted into an equivalent circuit that involves resistive losses to account for winding resistance and core losses. [bold added] If the frequency response of the transformer is important, such as for audio transformers, the inductance and capacitance of the device is also included in the equivalent circuit.”
For the non-technologists on the board, this is the usual practice. Start by assuming ideal conditions in order to begin solving the problem. Then add the imperfections such a friction, hysteresis, component tolerances and so on as you get more detailed. In fact, a Bachelor’s degree allows you to do the ideal case. Add some experience and you can add in the imperfections. That’s a big advantage of advanced degrees. You get the canned experience of others in a deeper study of your field before you ever leave school.
There’s a lot of truth about the comic description of the meaning of the academic degrees.
“What do BS, MS and PhD stand for?”
“Your know what BS stands for, don’t you?”
“Well, MS is More of the Same and PhD is Piled higher and Deeper.”
Others have explained the workings of a transformer, but your confusion comes from the notion that ‘the amps can be whatever you want’. The amps delivered by the transformer secondary (even in a short circuit) is limited by the energy supplied to the primary side.
I think that the source of the confusion here is that there are two different voltages which are relevant here. First, you’ve got the potential difference between one wire and the other. For long-distance power transmission, this is typically thousands or millions of volts, and this is the high voltage that the transformer outputted for you. But this is not the voltage you should use in V = IR to determine the current through the cables: There, V is the voltage difference between one end of the cable and the other (potentially thousands of kilometers away).
To illustrate, suppose that through some technological revolution, we started making long-distance power lines out of superconductors. At the power plant, we step up our power to a million volts (there wouldn’t actually be a reason to do this with superconducting wire, but bear with me here). So if you hooked up one lead from a voltmeter to each of the wires, you’d read 1 million. The current is also determined at this stage, and is whatever it needs to be to give the same power output as input. Now, with V = IR, and R = 0 (because we’re using superconductors), we find that V is zero. This is the voltage drop along the length of the wire. So one of the voltages we’re working with here is 1 million, and one of them is zero.
Exactly. Using the gear analogy, saying “the amps can be whatever you want” is the same as saying if you use a big gear to drive a small one you can increase the output velocity way up there and “the output torque can be whatever you want.” No way. The output torque is limited by how much your prime mover can put into the big gear. And the output torque is transferred back to the input gear by the square of the gear ratio.
Sorry, I was referring to these later posts:
It was this pair that jnglmassiv appeared to be replying to.
I’m not really hip to all that engineering talk, but transformers lose a lot of energy through heat. I knew guys at work who cooked their lunches by setting them on transformers. If you can cook a pork chop, you don’t have conservation of energy.
True, but the energy lost through heat is a small fraction of the total energy being transported through the transformer. Large transformers like those on power poles around town are roughly 95% efficient.
As has been said (several times), transformers are analyzed using an ideal, lossless model placed into an equivalent circuit that accounts for the losses.
And in fact you do have conservation of energy. All of the input energy appears as output energy in one form or another. Some of it as electrical energy in the secondary and some of it as heat that you can use to cook a meal or warm the room.
Transformer losses come in three varieties. One is the resistance of the windings, another is the hysteresis of the core and the third is incidental eddy currents in the core material. The first is made small by using wire as big as possible.
Hysteresis is a result of it being necessary to do work on the molecular magnets to align them with the applied field. In case of an alternating applied field the molecular magnets are first aligned one direction and then the other resulting in some of the input energy being unavailable as output energy. This is made as small as possible by using soft, easily magnitized and de-magnitized material for the core.
Eddy currents in the core are made small by using a laminated core with the laminations being insulated from each other, usually by an oxide layer on the surface.
While I’m not familiar with the efficiency of HVAC power-line transformers (I would assume it is fairly high), common transformers and the associated linear power supplies used in wall warts (those big black bricks that plug directly in the outlet to provide dc to some appliance you own) are notorious for their inefficiency. Try plugging one in without the appliance attached i.e. with the DC end hanging free; I was amazed to discover that some still get rather hot!
My understanding is that many newer wall warts are using switching power supplies, which reduces current on the primary side when the secondary demands less. That indicates to me the inefficiency of the transformer is accepted as a given.
No argument. That’s why I said “large transformers like those on power poles around town.”
Small transformers, small motors and such are notoriously inefficient.
Actually, my whole answer was predicated on the misunderstanding of the conservation of energy displayed by the statement about transformers being hot enough to “cook a pork chop.” The law of conservation of energy doesn’t imply that all of the energy that goes into a device emerges as useable energy at the output.
And the “cook a pork chop” is also questionable. A common specification for temperature rise in electrical components like a transformer is 40 C above ambient. For an ambient of 80 F this gives a maximum transformer temperature of 184 F. Hot, but not frying temperature. And few designers would size a transformer so that it ran constantly at maximum allowable temperature.
[QUOTE=David Simmons]
No argument. That’s why I said “large transformers like those on power poles around town.”
Small transformers, small motors and such are notoriously inefficient.
Actually, my whole answer was predicated on the misunderstanding of the conservation of energy displayed by the statement about transformers being hot enough to “cook a pork chop.” The law of conservation of energy doesn’t imply that all of the energy that goes into a device emerges as useable energy at the output.
And the “cook a pork chop” is also questionable. A common specification for temperature rise in electrical components like a transformer is 40 C above ambient. For an ambient of 80 F this gives a maximum transformer temperature of 184 F. Hot, but not frying temperature. And few designers would size a transformer so that it ran constantly at maximum allowable temperature.[/QUOTE
Has anyone considered the ‘wattless power’ loss,aka VARs,in having in service an energized trransformer, of any range of load capability?
VAR meters measure and indicate the portion of the indicated load that is non-productive.
Of course there’re many more var sources than transformers but those are the subject of this series of exchanges------or did I miss a portion?
Maybe my angular displacement is to great for being in phase!
Just a thought