Use of an inductor as a current limiter

If I have an iron core transformer primary winding with a resistance of about about 2.4R and an iron core inductor with an inductance of about 3000uH, how do you go about calculating the AC voltage that will appear across the primary windings of the transformer with an input voltage of 240V?

The transformer input current (and respective inductor voltage drop) can't be determined from the transformer winding resistance. You ask for the no-load or loaded input current.

Do determine the transformer input voltage for known current, AC network calculation methods have to be applied (considering magnitude and phase).

FvM said:

The transformer input current (and respective inductor voltage drop) can't be determined from the transformer winding resistance. You ask for the no-load or loaded input current.

Do determine the transformer input voltage for known current, AC network calculation methods have to be applied (considering magnitude and phase).

Click to expand...

Can you point me in the direction of a good tutorial site so I can try and learn how to do this?

I pulled apart a microwave oven and extracted the HV transformer, but also a small transformer, with only a primary winding, that was connected in series with the primary of the HV transformer.

So presumably the manufacture has used the same means of current limiting in the primary as many folks do when using a MOTs for tesla coils. I.E. the get a second MOT, remove the secondary winding and connect the primary winding in series with the primary winding of an unaltered MOT that delivers the HV to the tesla coil.

Since I have this iron core choke, I may as well see if I can use it rather than a dimmer switch to control the primary current.

I have removed the HV secondary winding by the way and am intending to try making a step down transformer with it.

A 3 mH inductor won't make much sense for current limiting purposes, according to a reactive impedance of about 1 ohm. How did you derive the number?

As you know, magnetron transformers are usually designed as stray field transformers, isn't it the case with this transformer?

FvM said:

A 3 mH inductor won't make much sense for current limiting purposes, according to a reactive impedance of about 1 ohm. How did you derive the number?

As you know, magnetron transformers are usually designed as stray field transformers, isn't it the case with this transformer?

Click to expand...

I measured the inductance of the iron core choke with my multimeter.

I know a bit about stray field transformers but my modified MOT is no longer one. I have removed the secondary windings and the magnetic shunts and it now consists of the iron core and the primary windings.

Apart from the fact all the E laminations and all the I laminations are welded together, rather than being alternated, it is now a standard transformer. Not withstanding the fact that it is probably a bit less efficient due to the E and I laminations not being alternated and that the weld lines undermine the electrical insulation of the individual laminations a little.

What would you suggest would be an ball park inductance value adequate for current limiting purposes?

And how do you do the calculations that leads you to suggest that 3mH is insufficient?

Quite simple, if the transformer acts as a short circuit, the current can be calculated as I=V/XL, XL being the reactive impedance of the inductor, XL=2*pi*f*L.

You are asking about elementary AC circuit calculations. A good introduction is e.g. https://www.ibiblio.org/kuphaldt/electricCircuits/AC/index.html

FvM said:

Quite simple, if the transformer acts as a short circuit, the current can be calculated as I=V/XL, XL being the reactive impedance of the inductor, XL=2*pi*f*L.

You are asking about elementary AC circuit calculations. A good introduction is e.g. https://www.ibiblio.org/kuphaldt/electricCircuits/AC/index.html

Click to expand...

Thankyou very much FvM - that web site is the sort of thing I was after.

I did previously find that formula for impedance but I wasn't really sure how to apply it in an AC circuit. Turn out it was much simpler than I had assumed.

- - - Updated - - -

I have a transformer out of a plug pack that has an inductance of 1710mH. So If I pull that apart, remove the secondary and reduce the primary windings I should be able to get the inductance down to around 37mH which, combined in series with my transformer primary winding, should limit the AC current to between 10A and 16A (max for AWG 18 magnet wire).

According to the formula you provided.

To make a power inductor (either for AC or DC), it's not sufficient to achieve an inductance xx. The inductor must be also designed in a way that it doesn't saturate at the rated current. In case of iron core inductors, this is hardly possible without an air gap.

As another point, due to non-linear magnetizing characteristic, the inductance at higher current will be considerably different from the value measured with a LCR meter. See https://www.edaboard.com/threads/302652/

FvM said:

To make a power inductor (either for AC or DC), it's not sufficient to achieve an inductance xx. The inductor must be also designed in a way that it doesn't saturate at the rated current. In case of iron core inductors, this is hardly possible without an air gap.

As another point, due to non-linear magnetizing characteristic, the inductance at higher current will be considerably different from the value measured with a LCR meter. See https://www.edaboard.com/threads/302652/

Click to expand...

I extracted a ferrite toroid from a tv circuit board with twin windings, i.e from the 'live' part of the board. The magnet wire looks as though it is AWG 18.

In series these twin windings have an inductance of about 50mH.

With my modified MOT primary the total inductance is about 80mH.

According to those formulas that should limit the AC current to about 10A. What exactly would be the consequences of the the toroid saturating?

The fuse in the live part of tv circuit board, from which I obtained this toroid (and most tv circuit boards), is 3.15A.

So there is a good chance that the toroid will be saturated at around 10A.

Perhaps I should play it safe by adding the second toroid and limiting the current in my primary coil to 3A or less.

I just checked and, if my calculations are correct, even with a primary current of 3A, I should still get well over my target secondary current. 240 * 3 / 13 where 13 is my target secondary voltage.

I could afford to add a third twin toroid and limit the primary current to even less than 3A as a generous safety margin.

For the time being, I assume that your "twin windings" inductor is a current compensated common mode choke. It's saturation current in series circuit will be very low.

In saturation, the inductance will drop to a small fraction of the initial value, you should place a fuse to protect your circuit.

FvM said:

For the time being, I assume that your "twin windings" inductor is a current compensated common mode choke. It's saturation current in series circuit will be very low.

In saturation, the inductance will drop to a small fraction of the initial value, you should place a fuse to protect your circuit.

Click to expand...

Ball park figure for a current that would saturate a common mode choke FvM?

Are we talking 1A, 100mA or 10mA? Judging by the gauge of the wire on the chokes and the fact that the power supply fuse was 3.15A, I would have guestimate some where around 1A.

And yes I was intending to put a fuse on both the primary, and the secondary for that matter.

- - - Updated - - -

The inductor looks like one of these:



So thanks for that - now I know what they are exactly.

- - - Updated - - -

Just reading this: http://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_choke.

The saturation current can be calculated if you know the core data. My estimation is something around 100 mA for a common mode choke with the said data.

FvM said:

The saturation current can be calculated if you know the core data. My estimation is something around 100 mA for a common mode choke with the said data.

Click to expand...

Oh OK, so they are really not suitable for my purposes.

Well then I will go back to using a small laminated iron core transformer that I salvaged. It was the power source for a set of Christmas lights.

It has an inductance of about 258mH with its secondary shorted and, combined with the transformer proper primary, the total inductance is around 300mH.

I calculate that the primary current should be limited to below 3A which seem fairly safe....with a 3.15A fuse.