I think I got this but am not sure. I try to read about it but I find everything magnetics very hard written. So let me explain my understanding if I can in a simple way. Then I'd love for one of you who understands this to say yes or no go back to school...
Lest say we have a perfect transformer, the secondary 100% coupled to the primary. The secondary will set up a magnetic field that completely nulls the primary's field so the core is not loaded at all due to load current.
What does load the core is the magnitization current due to the finite value of the inductance in the primary coil. Since the primary's reactance goes down with frequency, if the voltage level is the same as the frequency is lowered, the core will saturate only because the magnitization current becomes too high. The secondary does not create opposite EMF to null the magnitization current.
So the core's max flux density and saturation current is the same whether at DC or at 1MHz...
Edit... I know, the current thru an inductor increases forever if DC is applied over it. So I mean very low frequency, not really DC. But the current level, if one could stop at the saturation level at DC, is the same if very low frequency or very high frequency.
In a real transformer the secondary is not fully coupled to the primary, and does not set up an equal but opposite EMF, so now the core will saturate due to both the magnitization current and the load current. But there is some back EMF from the secondary so still it is the magnitization current that dominates saturation.
?
Thanks for any clarifications...
Lest say we have a perfect transformer, the secondary 100% coupled to the primary. The secondary will set up a magnetic field that completely nulls the primary's field so the core is not loaded at all due to load current.
What does load the core is the magnitization current due to the finite value of the inductance in the primary coil. Since the primary's reactance goes down with frequency, if the voltage level is the same as the frequency is lowered, the core will saturate only because the magnitization current becomes too high. The secondary does not create opposite EMF to null the magnitization current.
So the core's max flux density and saturation current is the same whether at DC or at 1MHz...
Edit... I know, the current thru an inductor increases forever if DC is applied over it. So I mean very low frequency, not really DC. But the current level, if one could stop at the saturation level at DC, is the same if very low frequency or very high frequency.
In a real transformer the secondary is not fully coupled to the primary, and does not set up an equal but opposite EMF, so now the core will saturate due to both the magnitization current and the load current. But there is some back EMF from the secondary so still it is the magnitization current that dominates saturation.
?
Thanks for any clarifications...
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You should distinguish primary magnetizing current from primary reflected load current. Primary magnetizing current is max without secondary load. With secondary load, the primary winding resistance comes into play. It reduces the effective winding voltage and so magnetization is reduced with increasing load.
The saturation limit may be explained by a primary magnetizing current or an equivalent primary Vsec product as well.
The latter explains best the low frequency border.
The saturation limit may be explained by a primary magnetizing current or an equivalent primary Vsec product as well.
The latter explains best the low frequency border.
The magnetic induction B is proportional to the excitation H, which is proportional to the instantaneous current in the primary.
The parameter time only appears in the impedance of the winding, but for the rest the saturation current will be the same at any frequency (neglecting second order effects)
The leakage inductance has nothing to do with the core induction and saturation: Bprimary depends only on the primary current, and the presence of a secondary, leaky or not changes nothing.
That is reflected in the design formula's, which only take the primary parameters into account
Thanks.
So if a core saturates at 2amps, it does so at 1Hz and 1MHz...? It's just that at 1Hz, the primary inductance needs to be very high or else the magnitization current alone will exceed 2A and the core is saturated even before signal is applied. There is no opposing flux from the secondary to cancel the magnetization current, since mag-current is not coupled to secondary.
Current transformers may be an extreme case, the primary being very low inductance (often only one turn) and the secondary often 100 turns more than primary. The secondary is heavily loaded, almost shorting out the primary. The opposing flux generated by the secondary (back EMF) cancels out the flux from the primary, therefore a current transformer can be very small compared to the current it handles. This must be because the secondary is so much larger than primary and therefore manages to create such opposinf flux. How else?
My thoughts used to be to design the transformer core size in accordance with primary currents due to load current + magnitization current. So lets say the primary current from load is 2A, the magnitization current due to inductance and frequency is no more than 100mA, the core must handle 2.1A. But my transformers are way too big this way. And I think the reason is because I should not take the current due to load, and this is because the load creates the opposing flux due to secondary - Lenz' law.
It is this I am trying to confirm or understand better.
Yes, Isat would be independent of frequency with the same coil, and an ideal core.
As impedance at 1MHz equals 1000000x impedance of 1Hz it takes a voltage multplyed by 1000000 to impose the same magnetizing current.
I cannot exactly figure out where your understanding ends...
Yes, that's why low frequency transformers are bulky
For regular, voltage-mode transformers, the effect of the secondary is a spurious issue, as long as it does not load the primary AC source to the point of causing it to cave in.
That is such a case: the primary voltage collapses completely thanks to the effect of the secondary, making the core to operate at ~zero-flux
Once again, the design equations for the number of turns do not take into account the effect of the secondary (for voltage transformers).
In general, you do not base your design on the magnetizing current: it is dependent on the variable and non-linear µ of the core, resulting in a very poor and inaccurate design.
The regular method relies on the voltage applied, which implicitly includes the magnetizing current and corrects the µ variations.
For a sine signal, the formula is n= V/(4.44*B*A).
For a squarewave, the 4.44 factor becomes 4.
For flyback transformer, you need to take the inductance into account, because it is essentially an inductor-based design, the transformer function being accessory, and the formula is n=L*I/B*A
Saturation in a core is when the flux density in the core reaches a value that the particular material used can not longer handle. For example for an 80% nickel core it is around 7400 (0.74 Tesla) gauss while for something like M3 steel it is 24000 (2.4T) gauss.
The flux density is dependent on the core cross section, the number of turns around the core, the peak voltage signal (rms) that the coil will see and the lowest frequency at which that peak signal occurs. This all assumes no DC is flowing in the coil.
Of course there is an equation that links all these parameters but I usually cheat by using an on line calculator like the one one the Daycounter website.
The flux density is dependent on the core cross section, the number of turns around the core, the peak voltage signal (rms) that the coil will see and the lowest frequency at which that peak signal occurs. This all assumes no DC is flowing in the coil.
Of course there is an equation that links all these parameters but I usually cheat by using an on line calculator like the one one the Daycounter website.
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