Resulting magnetic flux in the core of a transformer

In summary, a transformer creates a magnetic field that opposes the applied voltage, and the resulting flux is what powers the transformer. The flux is generated by the current through the primary coil, and the flux is kept balanced by the current through the secondary coil. If you change the voltage applied to the transformer, you will see a change in the flux, and this change is used to power the transformer.
  • #1
HeleneFR
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In a transformer, let's say we have:
I1, I2 - currents through the primary and secondary winding
V1, V2 - voltages
N1, N2 - number of turns
F1, F2 - magnetic fluxes through core, produced by the currents I1 and I2 (they are opposing...)
R - the reluctance of the core

We have V1/V2 = N1/N2 = I2/I1.
F1=N1*I1/R; F2=N2*I2/R
I think I1 and I2 are in phase too.
That means F1 and F2 are practically equal.
How can we have a nonzero resulting flux through the core? F=F1-F2
If the flux through the iron core is zero, then the cause that produces V2 (variation of the flux) does not exist, then V2=0?
I know I'm wrong, but i don't know where is the mistake...
 
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  • #2
Hello helene, :welcome:

As you say, the driving force for induction emf is not the flux itself, but the change in the flux. To keep up counteracting F1 with F2, the amplitude of V2 has to be nonzero.
 
  • #3
HeleneFR said:
How can we have a nonzero resulting flux through the core? F=F1-F2
Say that the impedance in the primary coil = Z1. Then the transformer will balance the flux through N1 so that

( Vsupply - dψv/dt ) / Z1 = I1 as for the primary coil when the transformer is unloaded. ( ψv = Flux * N1 ).

I1 is the current needed for magnetizing the core so that this balance is achieved.

Say that I1 is too small, ψv will be too small → the back emf will be too small → the voltage difference ( the parentheses above ) will be to high, so that I1 will increase.

If you load the transformer, I2 will decrease the flux so that I1 must increase to keep up balance.
 
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  • #4
HeleneFR said:
We have V1/V2 = N1/N2 = I2/I1.
F1=N1*I1/R; F2=N2*I2/R
I think I1 and I2 are in phase too.

realize that I1/I2 isn't exactly equal to turns ratio.
There is small magnetizing current to produce flux enough to oppose applied voltage. It flows in primary winding only and is a loss.
But in a power transformer it's quite small compared to load current so is generally ignored. In the golden era of slide rule accuracy(3 digits) it was a quite reasonable approximation.

Have you studied equivalent circuit of a transformer ?
https://en.wikipedia.org/wiki/Transformer

Do you have a lab at school where you could apply variable voltage to a transformer and plot magnetizing current ?

old jim
 

1. How does the number of turns in the transformer core affect the resulting magnetic flux?

The number of turns in the transformer core directly affects the resulting magnetic flux. The more turns there are, the stronger the magnetic flux will be. This is because each turn of the core adds to the overall magnetic field and increases the flux density.

2. Does the material of the transformer core impact the resulting magnetic flux?

Yes, the material of the transformer core can greatly impact the resulting magnetic flux. Materials with high magnetic permeability, such as iron, are able to create a stronger magnetic field and thus result in a higher magnetic flux.

3. How does the input voltage affect the resulting magnetic flux in the transformer core?

The input voltage has a direct relationship with the resulting magnetic flux in the transformer core. As the input voltage increases, the resulting magnetic flux also increases. This is because a higher voltage will create a stronger magnetic field and thus result in a higher magnetic flux.

4. What is the role of the frequency of the input voltage in determining the resulting magnetic flux?

The frequency of the input voltage also plays a role in determining the resulting magnetic flux in the transformer core. An increase in frequency will result in a higher magnetic flux, while a decrease in frequency will result in a lower magnetic flux. This is because the frequency affects the rate at which the magnetic field is changing, which in turn affects the resulting magnetic flux.

5. Can the resulting magnetic flux in the transformer core be controlled?

Yes, the resulting magnetic flux in the transformer core can be controlled by adjusting the number of turns in the core, the material of the core, and the input voltage and frequency. By manipulating these factors, the resulting magnetic flux can be increased or decreased to meet the specific needs of the transformer.

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