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Modeling Cascade Current Transformers

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LAN_403

HV Enthusiast

Tue May 28 2013, 07:58PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Just wondering if anyone took a stab at modeling cascaded current transformers (i.e. PSPICE for example)???

Actually, all the parameters are available.

Just not sure what kind of coupling it would be between the primary conductor and the winding itself.
Or maybe its so low, it really doesn't matter much.

Ben Solon

Wed May 29 2013, 01:25AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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I did something of the sort a couple days ago. Because the primary lead goes through the torroid you still have a pretty high k value, maybe not as high as I entered, but in the end it really doesnâ€™t matter. I modelled Steve Ward's 1:1089 via two 1:33 cascaded transformers and got very nice results with k.98 in ltspice. Because the current is a sinusoid, there are no harmonics of parasitics that are of real importance.

The situation is rather ideal, but you can add or change values to make it more or less realistic. Mine of coarse is the bare minimum as i was just getting a working simulation for a problem Iâ€™ve been having with my induction heater.

1369790714 3900 FT154352 Cascade Inverter

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Wed May 29 2013, 02:26AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Thanks for the info! Yeah, i didn't think the coupling was as high as .98. When you think about just a single wire going through the middle of the core, you think that thing is really loosely coupled.

I suppose i could measure the coupling, but too lazy to do that right now!

BTW, i always wondered by the cascaded CTs were always in a 33:33 ratio for 1:1089.

Isn't 32:32 ratio for 1:1024 closer to the 1000:1 ratio you ideally want? Unless i'm missing something which definately could be possible.

Physics Junkie

Wed May 29 2013, 02:43AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
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EasternVoltageResearch wrote ...

Isn't 32:32 ratio for 1:1024 closer to the 1000:1 ratio you ideally want? Unless i'm missing something which definately could be possible.

Good question. There's got to be a reason for the 33:33 ratio working better because, for the record, I have tried 32:32 and other ratios too - it just doesn't work the same. A 32:32 ratio for a feedback transformer severely messes up the feedback signal which in turn drove my coil with really weird spark output. When using it as a current monitor instead, the resulting waveform was very messy.

Steve Conner

Wed May 29 2013, 08:13AM

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A single wire going through the middle of the core might look loosely coupled, but remember the core has a really high permeability. Say it has mu=1000, then the flux from the wire will find the core 1000 times more attractive than the surrounding air. So probably all but 1/1000 of it will go into the core.

Since all of the flux that passes through the core must also link the secondary, it follows that the coupling will be roughly 1-(1/1000) = 0.999.

I've always used a single 1:33 CT with a burden composed of an 0.33 ohm resistor in series with two back-to-back UF5408s. I use the voltage from the resistor alone to drive my OCD circuit, and the voltage across both to drive the feedback circuit.

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Wed May 29 2013, 01:19PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Physics Junkie wrote ...

EasternVoltageResearch wrote ...

Isn't 32:32 ratio for 1:1024 closer to the 1000:1 ratio you ideally want? Unless i'm missing something which definately could be possible.

That doesn't sound right. 1:1089 isn't really much different than 1:1024. As long as you select the proper burden resistor, all you are stepping down a current and inducing a voltage across a burden resistor.

Of course, if you are already using it with say Steve's UD, then perhaps his UD is already optimized for the 33:33 ratio and maybe thats why you are having issues.

Physics Junkie

Wed May 29 2013, 04:06PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
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Yeah I don't know, it doesn't sound right at all. That's why I tried it in the first place because I figured a 1024 winding would be more accurate or something. Could have also been a mistake in the winding process such as very uneven spaces or an overlap or something. I might actually try this again in the near future to see if the results are the same

Ben Solon

Wed May 29 2013, 05:27PM

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PJ:

As Steve points out, as long as the wire passes through the core, little else matters. That's why we use torroids rather than EE, EI or UI cores. Interwinding capacitance also plays little to no role in this type of transformer. There aren't enough turns and the voltage isn't nearly high enough for capacitance to have a significant effect. Gaps and overlaps won't have any effect. I'd look at your burden or the input sections or the driver if you're having trouble there.

EVR: I agree with Steve. As I mentioned, passing through the core gives it a theoretical k of 1. That's because there are no flaws in the path of the flux. A two part transformer loses k through the joint between core halves. To be fair though, anywhere from .9 to .95 will give you much of the same results.

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Wed May 29 2013, 06:33PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Thanks guys!

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