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Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Kizmo wrote ...
Goodchild wrote ...
FYI Your not really adding any lead with the circuit you added there. The device that squares the chopped sine has to be fast and have a some what narrow window of hysteresis on it usually less than 5mV to 10mV. The 7414's hysteric window at 1V to 1.5V is way to large for phase lead, any lead added with the inductor is basically lost in the large hysteretic window and slow skew rate of the 7414.
Funny how people keep saying it doesnt work and yet still it works just fine. Been there done that and im very happy with results.
Before:
After:
All i have is slighly beefier feedback ct (around 1:500) and adjustable inductor with some resistance as load. Feedback signal is taken normally and fed into 7414
You got something else going wrong there in that first one, the voltage is nearly 90* out of phase with the current. That's not something phase lead can fix.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Goodchild wrote ...
You got something else going wrong there in that first one, the voltage is nearly 90* out of phase with the current. That's not something phase lead can fix.
And still it did fix it.
EDIT: After looking at original high res pic, inverter output is about 1.5µs at 67kHz behind zero crossings which is not that much imo. Pretty typical for what i have seen with these drivers.
FYI Your not really adding any lead with the circuit you added there. The device that squares the chopped sine has to be fast and have a some what narrow window of hysteresis on it usually less than 5mV to 10mV. The 7414's hysteric window at 1V to 1.5V is way to large for phase lead, any lead added with the inductor is basically lost in the large hysteretic window and slow skew rate of the 7414.
I'm using a Magnetek CST206-1a CT. It has a current ratio of 1/100 and an inductance of 14mH. I'm running at about 200kHz. If I have e.g. a primary current of 100A, I should have a secondary current in my CT of 1A. Open ended the CT should put out a voltage of 2*pi*f*L, in my case a whopping 18kV. Of course it is clamped, but shouldn't the hysteresis voltage of the 7414 be seen in relation to the 18kV, i.e. as negligible?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's the other beef I have with this circuit, how is the inductor supposed to generate a phase lead when it's clamped with a square wave across it?
The whole idea of the "Prediktor" is that you shift the zero crossings of a sine wave earlier in time by adding in a bit of its time derivative, which is of course a cosine wave leading it by 90 degrees.
But doing that to a square wave doesn't shift the zero crossing times earlier, it just adds overshoot.
So it seems to me the phase lead should be generated linearly where the voltages and currents are sinusoids, then it should be clamped to a square wave later.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The Steve's agree, the phase lead should be added with a sinusiodal voltage signal before its clamped or has other non-linear transformations applied to it. And you guys can complain about it, but im just looking at it from the perspective of an engineer thats kludged too many things and would rather make circuits that are easily predictable. The fact that Kizmo made it work is somewhat of a curiosity to me, id be interested in seeing the voltage signal out of the CT referenced to a primary current measurement. It might be one of those things that works fine in one system, and needs major re-tweaking with another. My scheme seems to work fine with a wide range of operating frequencies (up to 600khz with my QCW system, and 30khz with Gigantor DRSSTC), using the same phase lead circuit.
The above work was inspired from Finn Hammers "Prediktor", though i felt that i reduced its complexity by using a variable inductor... i have been quite happy with that. Eric Goodchild alerted me to the SLOT TEN variable inductors from coil-craft, so i use those now.
As to the original question, I am in no way affiliated with Eastern Voltage Research.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Steve Ward wrote ...
The above work was inspired from Finn Hammers "Prediktor", though i felt that i reduced its complexity by using a variable inductor... i have been quite happy with that. Eric Goodchild alerted me to the SLOT TEN variable inductors from coil-craft, so i use those now.
And I agree with Ward here, If I was going to rework the Predikter, I would probably use his driver today. I would, however, still insist on having a precision rectifier on the OCD, to facilitate a calibrated current reading right on the box, and for this job I would use the circuit demonstrated by Nicko, due to it's higher degree of linearity in loading the current sensing resistor.
The fact that Kizmo made it work is somewhat of a curiosity to me,
It might be that Kizmo is testing with no clamping at all. He is using a 1/500 CT and loads the CT with a small resistor and inductance. Clamping voltages are around 5V (+ and -). Without clamping the load inductance will definitely cause phase shifts. At larger primary currents loading from the clamps will dominate, which causes the phase shift to go away.
I need to correct my previous post. I was guided by the idea, that a clamped CT works much like an op amp driven into saturation. That picture is wrong so the calculation given is irrelevant. The conclusion, that the hysteresis does not matter is still right, I believe, since the voltage snaps very fast between the clamps. (if there are no loads to the CT other than the clamps)
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