Magnetic flux and current directions of transformer

In summary, the magnetic flux in an electrical transformer is a sum of all the induced magnetic fields, which are multiplied by a cross-section of the magnetic core. The current directions in the attached picture is wrong, as currents must be in phase or opposite in directions in order for them to be in-phase.
  • #1
goodphy
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Hello.

Let's take a transformer picture first.

Transformer3d_col3.svg.png


The basic equation explaining how the transformer work is the equation of Faraday's law of induction.
[tex]\begin{array}{*{20}{c}}
{{V_P} = - {N_P}\frac{{d{{\rm{\Phi }}_B}}}{{dt}}\;\;\;\;\left( 1 \right).}\\
{{V_S} = - {N_S}\frac{{d{{\rm{\Phi }}_B}}}{{dt}}\;\;\;\;\left( 2 \right).}
\end{array}[/tex] A current in the primary winding IP1 generates B-field (magnetic field) B1 which induces a current in the secondary winding IS1 . IS1 induces another B-field B2 which induces a new current in the primary IP2. IP2 also induces another B-field B3 which induces a new current in the secondary IS2 and so on.

So...I think that the magnetic flux Φ in above equations is a sum of all induced magnetic field B1 + B2 + B3 ... multiplied by a cross-section of the magnetic core. Could you tell me whether or not I'm right on this?

And when I think about Lenz's law, current directions in the attached picture is wrong. The currents must be out of phase or opposite in directions in this picture. In order for currents to be in phase, one of the winding direction must be inverted. For example, in the primary winding, the winding should be done from back side to front side of the core (front side is the side facing reader) to achieve in-phase currents. Could you also confirm my opinion?

Thanks for reading my post and I'm waiting for any replies.
 
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  • #2
goodphy said:
A current in the primary winding IP1 generates B-field (magnetic field) B1 which induces a current in the secondary winding IS1
Read Faradays law again:

n/dt does not induce a current, but a voltage.
Say that the secondary winding is unloaded, no current at all will pass the secondary winding.
 
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  • #3
Current makes magnetomotive force MMF, amp turns .
Flux B that flows around the core is MMF/Reluctance of the core.
Flux induces voltage not current, as Hesch points out.

Currents in both Primary and Secondary windings make MMF's. Flux is ΣMMF's / Reluctance of core.

Some authors pretend there is more than one flux flowing and use sum of those fluxes instead, and that is their choice but they should define their method and terms up front.
I prefer to think in terms of summed MMF's because it's easier to account for nonlinear reluctance of the core(and other magnetic effects) in your thinking..

old jim
 
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  • #4
goodphy said:
So...I think that the magnetic flux Φ in above equations is a sum of all induced magnetic field B1 + B2 + B3 ... multiplied by a cross-section of the magnetic core.
No.
goodphy said:
A current in the primary winding IP1 generates B-field (magnetic field) B1 which induces a current in the secondary winding IS1 .
As Hesch said earlier, primary magnetic flux induces voltage and not current.
goodphy said:
IS1 induces another B-field B2 which induces a new current in the primary IP2.
Right and it is called the 'reflected load current'. This reflected current Ip2flows such that Np*Ip2=Ns*Is.i.e. mmf due to Ip2 exactly cancels out the mmf due to secondary current and this keeps the flux in the core constant. There is no B3, B4 and so on.

goodphy said:
A current in the primary winding IP1 generates B-field (magnetic field) B1
This current is called as 'magnetizing current'. As long as primary voltage (rms) and frequency are constant, this magnetizing current is constant irrespective of the load on the secondary.

Edit: The great old Jim got there before me!
 
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  • #5
goodphy said:
And when I think about Lenz's law, current directions in the attached picture is wrong. The currents must be out of phase or opposite in directions in this picture.

Are you sure about that ?
Looks to me like currents flowing in directions shown create opposing primary and secondary MMF's, both up by right hand rule,
which is how transformers work.

Current is pushed into primary by source
current is pushed out of secondary into load.
 
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  • #6
Sorry cnh, had i known you were on it i'd have held back.
 
  • #7
jim hardy said:
Sorry cnh, had i known you were on it i'd have held back.
Please don't say that!
Your explanations are way better than mine (and full of interesting anecdotes). It is a great experience to learn from an expert like you!
 
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  • #8
There's a time for old guys to step aside and let youth run ahead.
It makes us happy . Like maybe we helped a little.

Keep up your good work !
 
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1. What is magnetic flux?

Magnetic flux is the measure of the strength of a magnetic field passing through a given area. It is represented by the symbol Φ and is measured in units of webers (Wb).

2. What is the direction of magnetic flux in a transformer?

The direction of magnetic flux in a transformer depends on the direction of the current flow in the primary winding. The flux will flow in the opposite direction in the secondary winding, according to Faraday's law of electromagnetic induction.

3. How does the direction of current affect the magnetic flux in a transformer?

The direction of current determines the direction of the magnetic field created, which in turn affects the direction of magnetic flux. If the current flows in one direction, the flux will flow in the opposite direction, and vice versa.

4. How does the direction of magnetic flux affect the output of a transformer?

The direction of magnetic flux determines the direction and magnitude of the induced voltage in the secondary winding. If the flux is increasing, the induced voltage will be in one direction, and if it is decreasing, the induced voltage will be in the opposite direction.

5. Can the direction of magnetic flux be changed in a transformer?

Yes, the direction of magnetic flux can be changed by reversing the direction of current flow in the primary winding. This will result in a corresponding change in the direction of flux and the induced voltage in the secondary winding.

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