# Shorted Secondary

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heres an interesting one.

Imagine a transformer pluged into the main without a load on the secondary, it barely gets warm and will happily stay that way for days/weeks etc...

now short out the secondary and it get quite hot quite fast, maybe even to the destruction of the device.

now split the transformer into 2 halves, we have our primary plugged into the mains and its quite happy acting as a simple AC electromagnet

why then should it get hot if I bring the other half (the shorted secondary coil) near to it?

How does it "Know"? )

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heres an interesting one.

Imagine a transformer pluged into the main without a load on the secondary' date=' it barely gets warm and will happily stay that way for days/weeks etc...

now short out the secondary and it get quite hot quite fast, maybe even to the destruction of the device.

now split the transformer into 2 halves, we have our primary plugged into the mains and its quite happy acting as a simple AC electromagnet

why then should it get hot if I bring the other half (the shorted secondary coil) near to it?

How does it "Know"? )[/quote']

This is the famous "open circuit" and "short circuit" test of the transformer.

When you have no secondary, it is equivalent to an open circuit, there is only leakage current in the primary, no heating whatsoever.

When the secondary is shorted, the current in primary goes up like crazy (when you perform short circuit test, you actually apply only that much voltage which gives you a current saturation in secondary, on calculation this turns out typically to be less than 20V) Even with voltages around 15V, you will get huge currents in secondary, depending upon the constants of the transistor. If you apply 220V or 110V in a short circuit, you are bound to burn out the device, too much current, too much heat.

You can analyse the equivalent circuit of coupled inductances to have a clearer picturer of "how it knows".

When we did short circuit tests, we were strictly told to restrict ourselves to less than 10V at input and even then we were getting huge current ratings. Also, the test was to be performed in minimum time possible , hopefully 4 to 5 minutes to avoid excessive heating and damage to the device and to ourselves.

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so how does the Primary "Know" whats going on in the Secondary or Where it is? )

(I love this puzzel)

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Thats equivalent to asking how does the battery know wether there is an open circuit or not.

As I mentioned, looking at it as an equivalent circuit with no gaps between elements (such as the one between the primary and secondary coils) and replacing them with approprite impedences, it becomes exactly equivalent to the battery situation.

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not quite, you see the transformer is physicaly Split. the primary is powered so it basicly and electromagnet, the closed loop secondary is on the other side of the room.

in effect the closer I bring the secondary to the primary, the more current will be drawn from the primary (and the secondary connected to nothing, in my hand may even start to warm up as well!).

what Manor of sorcery is at play here?

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Manner of sorcery === Magnetic field lol

I still don't see too much of a problem here.

The further you go the less the mutual inductance, and again the equivalent circuit incorporates this factor.

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and so any AC Electro magnet is subject to "feedback", by any incidental device that may even cause its destruction?

edit, theres stil no answer provided yet!

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I think its that the primary coil normaly has current flowing according to its resitance and self attenuation from its own inductance. When the closed secondry is brought in the back emf in it as the polarity switches lasts long enough for its magnetic flux to induce a current in the primary in the same direction as the reversing current from the mains and this is a higher current than would normaly flow. This higher current would then induce a bigger flow in the secondry and the effect would cascade to the limits of the circuit.

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and so any AC Electro magnet is subject to "feedback"' date=' by any incidental device that may even cause its destruction?

edit, there`s stil no answer provided yet! [/quote']

If you subject an electrical device to an electric field (voltage), it will get damagded.

If you subject an electro magnetic device to magnetic fields it has the same effect - the magnetic vector potential will rise and have an effect analogous to electric current. I still see no sorcery at work )

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why then should it get hot if I bring the other half (the shorted secondary coil) near to it?

How does it "Know"? )

the amount of heat that's dissapated is a function of the coupling coefficient.

So if the secondary is across the room, coupling is infinitesimally small. The closer the secondary gets to the primary, the coeficient goes up

If you want the formula for "coupling coeficient" I'll give it to you.

PULKIT......there IS heating in the primary (with the secondary open), there's an AC voltage across the primary, the primary has an impedance that's a function frequency, thus current flows, thus heat is dissapated.

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PULKIT......there IS heating in the primary (with the secondary open), there's an AC voltage across the primary, the primary has an impedance that's a function frequency, thus current flows, thus heat is dissapated.

Do not disagree one bit.

Done calculations theoretically as well as seen it practically )

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