Calculating flux density in a flat conductor

[Al (astral highway)]

Originally Posted by trobbins

We haven't seen the circuit to which the braid is used to pass current..

It is a self-excited power oscillator. The braid will pass the current from each of two switching transistors in a half-bridge, to the tank circuit. Overcurrent will be clocked by one current transformer and its circuitry, while the phasing will be controlled by feedback from another.

Originally Posted by trobbins

I'd be thinking most such power stages conducting in excess of 100A have pretty low on-resistance

...yes, really low.

Originally Posted by trobbins

I don't think we know the pulse current frequency yet, or if it mainly sinusoidal or square'ish.

Fres is likely to be between 220 and 270KHz. The pulses into the phase control transformer will be sinusoidal. After the burden resistor, they will be clipped by a suitable Zener diode, with a fast switching diode in series, to square pulses with slouchy shoulders. These will be squared up a to perfectly square waverorm with a Hex Schmitt trigger and then go through a bit more logic.

You can see from the photo that I posted in post no14 that we're getting 160nS rise time from the overcurrent monitoring current transformer and passive circuitry. This is 30 times faster than the shut-down window, if we call this just 5uS for safety. Ok, so there's an op amp and another IC as active components after this but the whole shutdown cycle will be less than 0.5us

[trobbins]

Al, for that test waveform in #14, did you gauge the delay between the squarewave input signal, and your sensed signal ?

Are you able to test the CT somehow (prior to its final application) when the input signal is say a combination of that squarewave and a separate high current signal of the level that a 150A waveform will generate, to see what core saturation may occur?

Sorry, but I'm a bit hazy on the braid location. Is the braid in the tank circuit, or one switch leg of the H-bridge, or combining the currents from the top and bottom legs of the H-bridge?

[Al (astral highway)]

Hello, I won’t be able to find anything like 150A pulses until I’ve built the half bridge and tested it. I might be able to rig up a class E amplifier stage to my square wave generator and get 20A pulses as a compromise, but I’d rather just build the DC bus and half bridge and get involved with the control circuitry.

Saturation is an issue with these designs : thanks for pointing it out . I note it is sometimes handled by having two cascaded CT’s.

I’ll do the delay test that’ you mention in your first paragraph .

The braid (if indeed I use it - I could also use silver-coated Teflon covered wire, 12 gauge ), as I have some) will be in the short conductor to the tank circuit . It will carry pulses from the DC bus, alternating +340 and -340V.

[Al (astral highway)]

Afterthought

Theoretically only, what would happen as the toroid internal diameter became smaller and smaller, contracting around the conductor inside it? Eventually, what if it became the same size as and then smaller than the diameter of the conductor? I visualised this as e.g a copper pipe with the toroid and CT inside it.


It’s only a thought- experiment but I can’t converge on a result.

Kalee20 ‘s post no. 20 is relevant and inspired this Q.

[Craig Sawyers]

The magnetic field in a conducting tube is zero. So the signal picked up by a toroid inside a conducting tube is also zero.

See eg http://www.ittc.ku.edu/~jstiles/220/...%20Current.pdf

[Al (astral highway)]

Originally Posted by trobbins

Al, for that test waveform in #14, did you gauge the delay between the squarewave input signal, and your sensed signal ?

I've now got a squarewave input pulse of 45nS. The output is 160nS, to there's your delay, roughly 100nS.

[kalee20]

Originally Posted by astral highway

Afterthought

Theoretically only, what would happen as the toroid internal diameter became smaller and smaller, contracting around the conductor inside it? Eventually, what if it became the same size as and then smaller than the diameter of the conductor? I visualised this as e.g a copper pipe with the toroid and CT inside it.


It’s only a thought- experiment but I can’t converge on a result.

Kalee20 ‘s post no. 20 is relevant and inspired this Q.

Just as Craig has already said, you'd get nothing from the CT.

The total field around a closed path depends on the current passing through the path. So if all the current is happening outside the toroidal core, with no current going through its hole, then the field in the core will be zero.

It's good to carry out these thought experiments, they serve as a good sanity check!