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Old 10-10-2009, 02:50 PM   #1
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Why don't transformers and electromagnents spark up and trip the breaker?


I've always wondered, since transformers and electromagnets are nothing but a coil resulting in a short circuit, why is it that they work, and don't short out and trip the breaker or burn out whatever powers them?

I've magnetized screw drivers by wrapping copper wire around it and hooking it up to a 12v source but it normally gets very hot and if I was to leave it, it would burn out. If I tried with 120 I'm sure it would just short out and throw sparks at me. AC is not really good for magnetizing stuff anyway so never tried.

Do they normally add a big resistor or something in the mix? I'm mostly curious about the huge 1MV transformers, a short circuit at that voltage would not be pretty.

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Old 10-10-2009, 03:40 PM   #2
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Why don't transformers and electromagnents spark up and trip the breaker?


First off, the wire used to wrap the iron cores in transformers has resistance, and there is a lot of wire, so immediately there is significant resistance, hence no short circuit. But also, there is considerable inductance due to the iron core, and since you are dealing with AC current, the effect of the inductance is to induce resistance to current flow, hence the effective resistance is greater than the simple resistance of the copper wire. This is a bit complex, to really understand what is going on, you need to study the theory of AC circuits, in particular the interaction of inductance, resistance and capacitance. By the way, the same thing applies to electric motors, and any other device which has inductance, capacitance or both.

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Old 10-10-2009, 04:10 PM   #3
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Why don't transformers and electromagnents spark up and trip the breaker?


With DC, the only thing that opposes current flow is resistance.

With AC, in addition to resistance, you also have inductive reactance and capacitive reactance to oppose current flow. Transformers have minimal resistance and high inductive reactance. That is why you might get a low ohmic reading, yet the transformer draws very little current. Same holds true for an electromagnet. In order to have an effiecient DC electromagnet, you need many turns of fine wire to create enough resistance to have lower current flow and less heat. That is why contactors' and relays' coils are rated for AC or DC, and not both, because they are wound differently. (more turns of small wire vs. less turns of larger wire) If you run a coil designed for AC on DC, you can plan on it heating up in short order. Entire books are devoted to this stuff.

What is a "huge 1MV transformer?"
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Old 10-10-2009, 04:33 PM   #4
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Why don't transformers and electromagnents spark up and trip the breaker?


If you short the secondary of a transformer, sufficient current may flow so as to burn out that coil. Alternatively, since the impedance of the primary goes down and the primary current increases as the load (connected to the secondary) increases, the primary might burn out first.

Transformers may be designed (with more turns of wire) so that the current draw can never reach levels that result in burning out even if the secondary is shorted. In general these transformers have small load capacities. The term used to describe these is "impedance protected".

The lower the AC frequency, the lower the impedance for a given transformer. So if you use a 60 Hz transformer on 50 Hz, the load level where burnout might occur is smaller, or depending on the way it is made, the transformer (primary) might even burn out with no (secondary) load.
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Old 10-10-2009, 07:04 PM   #5
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Why don't transformers and electromagnents spark up and trip the breaker?


Ahh I see, makes sense now. I sort of had an idea it might be resistance but did not figure even a tiny orange wire had enough to be significant. The induction I never even thought of. Seems quite advanced. Making a transformer is probably beyond DIY unless it's for a lower voltage application. :D

And by 1MV transformer I meant bad boys like this:



Maybe not quite 1 mega volt, but probably close.
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Old 10-10-2009, 07:19 PM   #6
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Why don't transformers and electromagnents spark up and trip the breaker?


By the way, that transformer pic you posted is probably 1 MVA or larger. Just like small transformers are rated VA, medium to larger transfromers are rated KVA. The big ones used in substations are rated in MVAs. (Mega-volt-amperes) 1000 KVA is equal to 1 MVA.

I actually wound a transformer myself, yes it takes a lot of work but it was quite a learning experience.
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Old 10-10-2009, 09:01 PM   #7
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Why don't transformers and electromagnents spark up and trip the breaker?


those big transformers have very low resistance because they use bus-bars as their "wire". the efficiency is 99.98% usually. they still have 2 fans that come on at different temp. settings.

they can take an incredible amount of heat too.. and run best at around 100C - 150C (oil circulation).

i'm not sure what you mean by run in short circuit. they are definately not run in short circuit. the magnetic field built up by the inductance slows down the current and limits it (usually NOT the resistance) within the 60Hz cycle. the fact that it is AC source is what allows it to work.

they are pretty useful piece of equipment invented many many many years ago and STILL the preferred device for voltage level translation in grid networks. must have been invented by some pretty smart "people" to have such lasting and widespread use.

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Old 10-10-2009, 09:03 PM   #8
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Why don't transformers and electromagnents spark up and trip the breaker?


Yeah I've learned quite a while back while messing around as a kid that transformers wont work at all with DC. You will get an initial energy on the other side but it wont last long, maybe one second.

Also bad idea to put a charged capacitor on a step up transformer and touch the other end. Why I did these things, I don't know.
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Old 10-10-2009, 10:30 PM   #9
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Why don't transformers and electromagnents spark up and trip the breaker?


One note, if you use an AC coil to magnetize screw drivers you need to use a momentary switch, other wise it magnetizes, demagnetizes, and magnetizes in the opposite direction 60 times a second.
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Old 10-10-2009, 11:01 PM   #10
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Why don't transformers and electromagnents spark up and trip the breaker?


Quote:
Originally Posted by hayewe farm View Post
One note, if you use an AC coil to magnetize screw drivers you need to use a momentary switch, other wise it magnetizes, demagnetizes, and magnetizes in the opposite direction 60 times a second.
Yeah, I use DC current, and just wrap wire around it. Have not done it in a while though. I usually just use an AC adapter (converts to DC as well as produce a voltage that can handle the short, but it does get pretty hot). Though knowing that what plays a role in DC is resistance, I should probably use that small orange wire next time. Maybe even build an actual "screw driver charger" :p

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