Toroidal Core Calculation

Question

Few days ago I got sample of toroidal core from my friend. I don't know any technical details of core just know OD-10cm, ID-6cm, height- 5cm and wet about 1.845 kg(M4 grade CRGO). I want to make toroidal transformer using this core but how to calculate core area and maximum VA capacity of core.I take some photos and attach.

Answer 1

Saturation of the core is a major limiting factor that may dictate more turns on the primary. More turns = much more primary inductance (L is proportional to turns squared) and this means a lower magnetization current for a given sinusoidal voltage on the primary.

Lower mag current means lower saturation (note that saturation is NOT due to VA being passed through the transformer).

So, a smaller core needs more turns to avoid saturation at a given primary voltage and, of course, more turns means more copper losses. This is why bigger cores are better for VA ratings - not only can you use fewer turns (to obtain the same primary inductance) but the H field is smaller. H is ampere-turns per metre where the "per metre" part is the nominal average length around the toroid (or core).

Yes, for a bigger core, each turn is a bit longer (more resistance losses) but the net effect is a decrease in copper loss and this means more VA can pass through the core.

But the problem here is that you know nothing about the core material hence you cannot predict where on the BH curve saturation might become a problem. This means you cannot safely predict magnetization current and you don't really know how much inductance the core produces per turn (because permeability of the core is unknown).

All this leads me to say is that if I were in your position I'd throw it in the garbage and buy something that that was defined in data sheets.

Answer 2

Vt=NAB and Urms=4.44fNAB for sinusodial waveforms. A is your cross section area of the core (shold be (0.1-0.06)*0.05 = 0.002 m^2). B is 1 or just below it. Check the specs for the core material.

The VA rating depends how much wire you can squeeze in. Thinn wire = more turns, more voltage but less current before overheating it and vice versa. If you have isolation requirement, that will steal area from you as well so less VA than theoretical max.

Answer 3

Is this intended for a power circuit, or for small signal propagation? Also, what frequency range will this see? For a power circuit, indeed a spec sheet with defined parameters would definitely make me feel better. If this is for small signal propagation like interstage coupling, safety may be far less of an issue.

If you have the patience and are feeling adventurous and if you have an oscilloscope and a couple of amp meters, you could optimize this for the frequency and voltage amplitude that you plan to run this at (i.e. wind the primary with progressively more turns such that you reduce the copper loss, while monitoring the wave form fidelity on the secondary, checking to assure you reach a point where you have maximum voltage transferred before the high frequency response begins to suffer as a result of the increasing inductance with the increased turns, and then back the turns back off until you reach that happy point. For small cores like this, the applications are almost surely for much higher frequencies than 60hz. The lower the frequency that you are propagating, the more mass you need in your core for a given number of ampere turns, else you will have insufficient inductance to hold back all that current created by the DC resistance of the copper wire. Also if this is ferite core, that's usually ideal for running high frequencies with the intent to reduce eddy currents. I can't remember the last time I saw a small torroid core used for power supply, that wasn't being used as a "switching power supply", or otherwise where the operating frequency was at least well into the tens of kilohertz or perhaps much higher.