transformer
Regarding the following excerpt from a text:
The text mentions that at RF air core transformers are adequate, but for 50/60Hz low freq. applications iron core needed to obtain a low reluctance.
The text does not explain the reason. What is the relation between the frequency being low and the need for low reluctance medium? Low reluctance intensifies the magnetic field but why particularly at low frequencies it is needed?
Rest assured you may safely assume that the magnetic permeability of varnish is close enough to that of air or vacuum (\$\mu_r \approx 1\$), and yes, the gap size will typically dominate many of the characteristics.
See for example this page, especially the non magnetic materials at the end of the table. Unless your varnish is loaded with metal or ferrite powder you should be fine.
why does it rise with 5Amax and in (2) at 14Amax isn't the magnetic field only dependent from the product of Iprimâ‹…N1 and the saturation of the material constant for a fixed frequency? Is there a load dependency of the magnetic field in the core?
Magnetizing current (and the resultant magnetic field in the core) is created by voltage across the primary winding's magnetizing inductance (Xm in the transformer equivalent circuit below). Since current in an inductor rises at a rate proportional to voltage and time, increasing primary voltage or reducing frequency will increase magnetizing current, driving the core into saturation if the voltage is too high and/or frequency too low.
Putting a load on the secondary makes the primary draw more current to feed it, but this transformed current is separate from the magnetizing current and so does not directly affect saturation. However the increased primary current does cause a voltage drop across the primary winding's resistance (Rp) and leakage inductance (Xp). This voltage drop subtracts from the voltage across the magnetizing inductance, so a higher voltage and/or lower frequency is required to drive the transformer into saturation.
When you put a short circuit across the secondary the primary current becomes very high, causing a significant voltage drop in the primary. Putting two output windings in series causes higher current draw for a particular input voltage and frequency, increasing voltage drop and so requiring higher voltage and/or lower frequency (and therefore even higher current) to reach saturation.
Best Answer
At high frequencies, hysterisis and eddy current losses come into play if a ferromagnetic material like iron is used a core. Hence Air core is preferred at high frequencies, despite its higher reluctance. But at low frequencies, hysterisis and eddy current losses are nullified and hence Iron core is a better choice. Way better flux-linkage and better efficiency than air core ones.