ECN Forum Forums General Topics of the Trade General Discussion Area Transformers: mind-boggling, mysterious beasts

Spark,
The Load impedance of a transformer's secondary is reflected back to the primary by the square of the turns ratio. Let's take an example of a 12V, 10A transformer. It should have a 10:1 turns ratio if the primary is 120V. To get a 10A current flow in the secondary, the load resistamce/reactance has to be 1.2 ohms. 1.2 ohms load impedance X 10^2 = 120 ohms reflected back to the primary. 120 V/120 ohms = 1A. The numbers work out and all is good in the neighborhood.

Experience tells us that if you have your TX plugged in, unloaded, and only get 12 volts out, it probably is not considered a 12 volt TX. The voltage out should be what it puts out at rated load.
Joe

[This message has been edited by JoeTestingEngr (edited 12-21-2005).]

lets not forget that spark master already knows about transfromers,

and that he assumes ones name has bearing on thier heritage.

Guys, let's please leave personal barbs out.

Bill

VA in = VA out. But please note that there are ironlosses ( eddycurrents and hysteresis losses in the iron core )and copperlosses from the windings to be added to the primary power absorbed from the mains. the copperlosses increase with the load I²R, the ironlosses remain almost constant

Transformers are normally rated in VA's. At UPF power factor you can call it Watts but generally its lagging at around 0.95 PF hence VA's are used.

Winding ratios are:
Up*Ns = Us*Np where:
U is voltage, N is winding turns, p is primary winding, s is secondary winding.

Current ratio's Ip*Up = Is*Us. In your case
Ip*120 = 3*12 » Ip = 36/120 = 0.3 Amps.

Personally I don't like to rely on the resistance values measured across the primary or secondary windings because of the different gauges of wires used and the transformer is an AC device. It's Ok to identify a primary or secondary winding.

By unknown transformers, to test I usually put a 200 Watts lightbulb in series during testing which will absorb the fault current if it was accidently the wrong primary voltage. In NZ we have 230 V. and there are a lot of trannys out here with 2 * 115 volts primary windings.

I hope this explanation may help out a little.

The transformer makes this infinitely safer, not because of current or voltage limiting, but because it provides electrical isolation.

A Variac (a variable autotransformer) has only a single winding, and provides no DC isolation between input and output. Shock hazard.

Hey Joe,

Thanks for your response. The TX in question might have been 13.8 volts or something unloaded, I was just rounding it off to 12 for illustration purposes.

RODALCO, thanks too. It just seems like if the NEC tells us that, for example, 12 gauge wire is for 20 amp circuits, something should tell us the amp rating of an unmarked transformer, going by the wire thickness. How will a transformer fail anyway? I don't know, but would assume that it would be the wire burning in half and/or shorting out due to over-amping. JWhite? Any opinions or advice from the master?

SMF

Hey Larry,

Define "electrical isolation" if I can still get grounded and severely amp-loaded. If I'm getting electrocuted, how is electrical isolation helping me? How exactly is my entire electrocution experience getting all warm and fuzzy and comfortable with this 2nd tranny?

SMF

SMF,

Again, let's please leave the personal barbs out.

My apologies to all for anything I said that might have offended. Thanks to webmaster for running a tight ship to prevent things like that.

I love electrical work, it's all so fascinating. The more I learn, the more I'm blown away by things electrical. Nobody knows it all, right? Some people are experts in some areas, some are master electricians who may have only dabbled in some aspect of electrical once in their career, while by chance leaning toward certain aspects of electrical work, becoming highly experienced in those areas.

People who are highly educated in one particular area might pass through here and be able to really change my life, turning me in the right direction quickly so I can finish this project sooner and do it first class.

I've got almost 4 years of experience in commercial electrical, and I work on other projects on the side, like building a hot wire cutter or a gyroscope, building world-record-breaking machinery, signs, building networks, fixing all kinds of electrical and electronic devices, and right now I'm thirsting for certain answers to my questions about transformers.

I tend to look at the resistance of a nichrome wire, for example, at 1.5 ohms; then figure how many amps it would draw at 12 volts, which would seem to be 8 amps. Now I'm told that the resistance changes, so I don't know what to think exactly.

I have a device I bought at a garage sale called a Fido-Shock, it's a device that's wired up to an electric fence to keep an animal in the yard. The Fido-shock has a transformer that steps 120 volts up to 700 volts or something like that, but I guess that increase in voltage reduces the amperage. Somehow it's supposed to be safe because of the reduction in amperage. Can somebody tell me how a step-up transformer, going to 700 volts, can be safe enough to use on Fido because of reduced amperage, while a step-down transformer, dropping 120 to 12 volts, can be safer because of reduced voltage (while the amps can go super high)? What's more dangerous? What would cause more damage to somebody who became part of the ground path? Would it simply be a matter of VA, regardless of the voltage?

SMF

Sparkmaster,

Please do not take this as a personal barb: You _don't_ know how transformers work.

You know the basic concept, but you don't know the details. As an analogy, you know that a car works because gasoline gets used inside it, but you don't know the details of how an engine works, and you are asking why cars go at particular speeds.

But that is okay! This is a place for learning. Just please be open to changing your basic understanding of how transformers work, as you better understand the details. Also please understand that most if not all science and science _teaching_ is a continuous train of more refined ideas, with each new idea expanding and _correcting_ the previous concept. Very often we use one of the earlier understandings of physics because it is accurate enough for the application, even though we _know_ that there is a better approximation that we could use.

Newton described a few laws of motion, eg. F=M*A, and the laws of Newtonian mechanics are still taught in school. Newtonian mechanics is still used for designing machines and building buildings and pretty much all of our lives. But we _know_ that Newtonian mechanics is wrong. Einstein's 'Relativity' theories provide more accurate laws of motion, a better picture of the universe. And Einsteinian mechanics is also taught in college physics. But for most of every day life, Newtonian mechanics gives answers that are almost equal to Einsteinian mechanics, with the 'error' being smaller than the measurement errors in the tools that we use. In other words, for most of everyday life, you can't tell the two apart.

I bring up that little example because when people describe complex beasties like transformers, they _knowingly_ use mathematical descriptions that have known errors, and only bother with more complex mathematical descriptions when the magnitude of the errors actually impacts the operation of the devices that they are building. Usually most of the details of a transformer are simply ignored because those details are down in the noise of the production process.

Okay. Someone said 'back to the books'. Here is a lovely little 'book' free on the web: http://www.ibiblio.org/obp/electricCircuits/AC/index.html

This text will take you through the concepts of AC reactance, mutual inductance, transformer operation, 'reflected impedance', and other concepts. For a deeper understanding, you should also look at the separate section on DC circuits, to get the concepts of galvanic isolation, etc.

In a nutshell:

1) Whenever an electric current moves through a wire, you will get a magnetic field.

2) Whenever you have a wire in a _changing_ magnetic field, you will get a voltage induced in the wire.

3) The wire interacts with its _own_ magnetic field. In other words, when you apply a voltage to a wire, current will start to flow, creating a magnetic field. But this means that the magnetic field must be _changing_ since we changed from no magnetic field to having one.

4) The magnetic field created by a wire will always act _against_ any change in the current flowing in the wire. It won't be able to _prevent_ the current flow, but it presents a drag to the change in current flow. Think 'mechanical inertia'; when you push on an object, it pushes back even as it starts to move.

5) If you coil a wire up and add an iron core, you amplify the magnetic field, and thus amplify the voltage produced. You increase the electromagnetic inertia of the system. By coiling up a wire, you have produced an inductor or 'choke'.

6) When you apply AC voltage to an inductor, an interesting thing happens. The applied voltage is constantly changing. The current flowing is constantly changing. The magnetic field is constantly changing. You will find that the current that actually does flow is that amount which will produce a magnetic field which will balance the applied voltage. Increase the voltage, and more current flows, so that you get more magnetic flux, and thus more induced 'coil produced counter' voltage.

7) In a transformer, you have _two_ coils interacting with the same magnetic field. The changing magnetic field in the core induces voltage is _both_ coils. With no load connected to the secondary coil, no current flows in the secondary coil. The changing magnetic field induces voltage in the primary coil, which acts to balance out the applied voltage. A bit of current flows in the primary coil, just enough to produce the magnetic field.

8) When you connect a load to the secondary coil, current flows in the secondary coil. This current also interacts with the magnetic field...by _weakening_ it. But now we have a weaker magnetic field, so the counter voltage induced in the primary coil is reduced. Which means that more current will flow in the primary, bringing up the magnetic field strength, so that the voltage induced in the primary coil balances the voltage applied to the primary coil. What this means is that you don't simply have the primary coil 'producing' a magnetic field and casting it out on the waves; instead the primary and the secondary are mutually interacting with the magnetic field. When you draw current from the secondary coil, you are changing how the magnetic field responds to the current flow in the primary, and causing more current to flow in the primary. The primary circuit sees what is happening on the secondary circuit, by the _mutual_ induction of the transformer.

9) You can think of each turn of each coil in the transformer as separately interacting with the magnetic field, and that any turns of wire electrically connected together will have their interactions added up. If the primary and secondary coils have different numbers of turns, then they will have different voltages and currents needed to be balanced with the magnetic field, but they will _both_ be balanced with the magnetic field.

10) Because the primary coil 'sees' the load on the secondary coil, you can come up with equations for the how loads on the secondary side 'appear' on the primary side. You will see the term 'reflected impedance' used to describe this. In your hot wire cutter example, you might have 6A flowing in the secondary, with 12 turns in the secondary, for a total of 72 ampere-turns. The primary will have 120 turns, and must supply the 72 ampere-turns, so you would have 0.6A flowing in the primary. 12V, 6A, Ohm's law: 2 ohms. On the primary side: 120V, 0.6A, and thus 200 ohms. The impedance 'seen' on the primary side of the circuit is (turns ratio)^2 times the impedance connected to the secondary side.

I hope that this gets you started. I strongly suggest that you read through the texts mentioned above, because this will provide you with essential background that we would simply be repeating here...see how large the above 'nutshell' has become. Feel free to ask questions about the points that you don't understand, but please do the reading. Transformers are complex beasties, and can't be dumbed down if you still want answers to your questions

-Jon