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Re: DRSSTC-3 VCO drive test (long)



Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 06:30 PM 2/5/2005, you wrote:
.........
 But things are looking much
better.  I found that if my CT had much self capacitance at all that i
could really screw up the phase relationship.  So i tried making a new
CT with far less turns, and only a single layer of winding... this
seems to have helped!  I also found that the switching eventually does
get super close to ZCS when i crank up the power.

I noticed that my CT had a lot of delay too at low current but it got very fast at higher currents. Not sure why. Maybe up higher on the hysterysis (sp?) curve or the higher currents force their way past the parasitics and all. The inductance of the CT may also have an effect. I was thinking I would have to wind my own but since it fixes itself at higher current where it counts it is not a problem.


I can tell because
the switching noise on the current waveform starts *right* at zero
:-).  I was running primary feedback today without any problems at all
:-)  Had it running 400A in the primary and about 25" sparks to my
target.  Didnt push it too hard cause i didnt have my metal shielding
to protect the lower level from the sparks... it would be pretty
annoying to have another failure because of that again.

I noticed some interesting things about the primary feedback.  It
seems that with my current tuning, the primary current bounces back
and forth from the upper to lower modes, about 2-3 cycles in each
mode.  It produces some interesting "notch-like" steps in the primary
current.  I noticed this in pspice as well.

Once I learned about all the modes and notches, I seem to see them everywhere too. My model suggest that it will start out very notchey but it will smooth out once the streamer load kicks in. However, I think you can play with the tuning to pretty much change that all around. Of course, compared to secondary feedback, it is a mess ;-)) But the notches in that case are going to hard and non zero current crossing switching. I think primary current feedback is the way to go to keep the IGBTs happy. I am not sure primary feed back likes VCO, PLL and other local oscillator types of control though. With the frequency really jerking around, it is very hard to follow unless the controller is totally fly-by-wire without any predetermined frequency.


I guess Terry is running
his setup sorta like this too.  I also notice that when i get near my
maximum power, the primary current appears to stick with one pole
about half way through the 100uS burst.  I figure let it do whatever
it wants so long as its making sparks and the IGBTs arent exploding
:-))).

I think that is streamer loaded tuning the primary to lower Fo.


My overcurrent detector is working like a charm still... but i wish i
would have used a 10T trimmer rather than a single turn... its a bit
hard to get it *just* right.  But im already designing a new PCB to
replace the old one, and the new PCB will have the overcurrent stuff
built in so i dont have 2 seperate boards on the coil.  I had to use a
little trick (Terry might want to listen up) to make the overcurrent
detect work properly.  Im using an LM311 comparator, and never thought
about the super high input impedance.  Noise was triggering the
detector at first.  Now i just put a 100 ohm resistor across the CTs
input to the "-" input.  This resistor acts as part of the CTs burden
resistor (10 ohms).  This is needed because the burden resistor is on
the OTHER side of the full wave bridge im using to get all + peaks.
Also, its a good idea to filter the heck out of the other input, i put
a 47uF tantalum right on the pins.  Seems quite stable now :-).

Mine really does not have any high impedances in the circuit:

http://drsstc.com/~terrell/schematics/ProtectionCard-01-sch.gif

I added the 1k resistors and reverse diodes to help protect the LM339 but they are right up close to the LM339 and don't pick up much. By using a bipolar CT, I was able to get rid of a negative reference or bridge rectifier thing:

http://drsstc.com/~terrell/schematics/CurrentMeasurement.gif

With just another winding, you have the negative side too but you can still use positive logic. I see a lot of commercial CT or starting to be made center tapped now too.


More work to be done... so little time!

I caught a stupid head cold and have not been able to do anything the last two days &:-p I was thinking of at least playing with a few models tonight, but I'll probably just crawl back into bed... Looks like snowy weather now too :-p Have to make my next DRSSTC primary/secondary system weather proof... So close, yet so far :-(


Cheers,

        Terry


Steve W