Power Transfer in Transformer 4

[prc]

I have a query -How the power is transferred from the primary winding to secondary winding in a transformer? General answer is through electromagnetic field, electric to magnetic and then to electric. But how? Can we explain this clearly so even a layman can grasp.

 

[Power0020]

generating a flux linkage due to excitation of the primary winding, the linkage is then "intercepted" by secondary winding and induces voltage proportional to the flux, number of turn and frequency.

the secondary winding is loaded so current flows, the primary winding now sees a demand on the magnetic circuit to current flows in the primary winding as well in order to keep the working flux almost constant "it depends on the applied voltage, which is almost constant".

so, current in, current out, power in is equal to power out, preserving the energy flow.

The magnetic induction gives galvanic isolation, but two circuits are linked together, the reason is that the magnetic flux/field affects the electrons on metal atoms and give them energy to move and create electromotive force "e.m.f." then if the secondary winding is closed, a current flow.

 

[HamburgerHelper]

Read up on Faraday's Law. All transformers with cores or shells operate using Faraday's law. Remember in middle school when you passed a magnet through a hoop, it induced a current and voltage in the hoop. Same thing but instead of passing a magnet, you are using the changing magnetic field produced by the currents in one of the windings to induce a voltage in the other winding and instead of one winding, you have I think thousands. A lot of hoops instead of just one.

https://en.wikipedia.org/wiki/Faraday's_law_of_induction

Auto-transformers operate without relying on induction. It is just one big winding with a tap or taps. They often though have a stabilizing delta, which relies on induction.

https://en.wikipedia.org/wiki/Autotransformer

 

[davidbeach]

If autotransformers operate without induction the how can they step up voltage? It's very much induction, just between different parts of the same winding.

 

[HamburgerHelper]

David,

Yeah, you are right. It is all self induction.

 

[Skogsgurra]

Or, rather, mutual induction.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

Or is it self plus mutual induction? Let us take a simple two winding transformer with concentric HV and LV windings. When we energize HV with LV open, a magnetic flux is generated in the core that induces a voltage in LV. But HV generates a small leakage flux between HV &LV windings, linking with itself and with LV also. This is similar to the flux that we see in a solenoid coil and has nothing to do with core or the core flux. Because this flux quantity remains the same whether you design the core with a flux density of 0.5,1.0 or 1.5 T. But due to this term, many engineers consider this as a "leaking" flux from core as water leaking out from a municipal town pipe line. It seems Steinmetz used this term, but to me it should be called as additive flux, generated by the current in windings going to core adding or subtracting the flux there. Its value increases linearly with current and the flux jumps in to core limb or yoke wherever it comes nearer to it. When current flows in LV, it also generates a flux linking with it and also with HV. Most of flux from HV current closes path by flowing through tank walls heating it, in turn. So voltage is transferred to secondary through the core flux. But it seems current is not transferred through core flux.

 

[prc]

Let us consider a two limbed core with HV and LV separately in the two limbs ,unlike concentric arrangement, the normal norm. When we energize HV, voltage will be developed at LV terminals, but when we try to extract current from LV, the terminal voltage will drop to zero due to extremely high leakage impedance between HV and LV winding. So current will not be transferred to LV even with full induction in core. It seems current is getting transferred by the leakage flux. (mutual induction?)

Then a question will be raised why the core size is increasing with transformer MVA? Consider 1, 10,100,1000 MVA 132/11 kV transformers. A few hundred kg for 1 MVA to 200 tons for 1000 MVA. The only difference (apart from more copper area to carry more current from extra MVA) in these units is number of turns provided for the same rated voltage. As MVA goes up, number of turns is reduced as square root of the ratio of MVA increase. So when number of turns is reduced, area has to be increased to remain with the flux density limit. E= 4.44 x frequency x flux x number of turns. The number of turns are reduced with increasing MVA to reduce the leakage flux ( that depends on NI, current x no of turns) so that leakage impedance can be reduced ( impedance varies as square of turn number) with in manageable limits(4-18 %) to avoid unacceptable voltage drops at full loading.

 

[prc]

"Yeah, you are right. It is all self induction" I am not sure. Let us take a 100 MVA 220/132 kV auto transformer. Only 40 MVA {(220-132/220) x100 } will be transferred from HV to LV by transformer action balance 60 MVA will be directly moving to LV through the galvanic connection between HV &LV windings. So the size and losses in a 100 MVA auto will be almost same as that in a 40 MVA two winding transformer, the reason for using auto connection in transformers. Of course the common (132kV) and series(220-132) windings will act as primary and secondary of a transformer, moving 40 MVA by transformer action.