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Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Hi. This morning i reversed the ct's and started testing waveforms. I was getting nothing before i flipped the ct's around and now as soon as i start to turn up the variac it seems that the ct's are picking up feedback. I am not sure if the waveforms that are going to the igbt's are correct or not. Here are pictures of the 4 igbt gate signals with no input from the variac. The first pic is channel 1 and 2 and the second pic is channel 3 and 4.
Those signals look correct because all the controller is doing now is trying to start the oscillations but it cant because im not putting any voltage across the bridge. Now as soon as i apply a couple of volts across the bridge, it looks like the feedback ct is picking something up and sending something to the gdt but it doesent look right. Here is a pic of the signal going into igbt 1 and the signal coming from the feedback ct!
The scope is set at 5v/dev .1ms/dev and the top waveform is what is going into the igbt and the bottom waveform is what is coming back from the feedback ct. I really am not sure what is wrong and i would be very glad if you guys can help me. Thanks.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Hmmm, in the last pic (with the ubersmall thumbnail), if the top trace is the gate waveform , then somethings definately not right. Its worth noting that with primary CT driven feedback there is a threshold primary current that must flow before the CT gives a high enough signal to start oscillation. So you will not get a good waveform untill you put enough bus volatge in, sometimes as high as 40v in my experience, "a couple of volts" probably wont do it. Also can you post a pic or two of your board / bridge / CT's ? A pics worth a thousand words :) And also , can you post a trace of the gate waveform where you are driving the feedback input with a signal generator in place of the CT? This will help determine whats going on with that gate waveform, and eliminate the possible problem above.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
I am using Steve Wards new drsstc controller. Here is a pic of the H bridge
Here is a pic of the ct's
Now for the good news. I put a 5vp/p 136khz "resonant frequency of the secondary" on the feedback input and i got some very good results. It looks like the controller is working fine and it is the ct that is my problem Here is a scopeshot of the waveform from one of the gdt outputs.
From what i know that is what it should look like. Let me know if i am correct. If so i think i have dialed down my problem to the ct's.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Yep looks good with the sig gen. ( well looks quite wierd actually if it is as you say from "one" of the gdt outputs, with the dual trace and all, but at least its nice and square, and there may be something wierd with your scope)
================================================
==================================== = I would say you simply need a higher bus voltage to get things oscillating, have you tried around 30-50V yet? = ================================================
====================================
Also what burden resistance if any are you using on your feedback CT? And I assume from the look of them your CTs are approximately 1000:1 ?? ie 33:1:33 ?
Looking at pic2 , I notice you have a lot of extra wire on your GDT leads to do a loop through a ferrite bead , great idea on the bead I do it too, but I would trade that extra lead lenght and just pass through once, also I only pass the primary wires through this bead. But basically I think shorter wires is better than the extra turn. You may also need more turns on the GDT itself, but thats impossible to say from here :)
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Dylan wrote ...
=================================================
=================================== = I would say you simply need a higher bus voltage to get things oscillating, have you tried around 30-50V yet? = ================================================
====================================
Also what burden resistance if any are you using on your feedback CT? And I assume from the look of them your CTs are approximately 1000:1 ?? ie 33:1:33 ?
Looking at pic2 , I notice you have a lot of extra wire on your GDT leads to do a loop through a ferrite bead , great idea on the bead I do it too, but I would trade that extra lead lenght and just pass through once, also I only pass the primary wires through this bead. But basically I think shorter wires is better than the extra turn. You may also need more turns on the GDT itself, but thats impossible to say from here :)
No i haven't tried anything above 10v across the bus so i will play with that tomorrow. As far as the gdt bead, I will leave it as it is because all the wires are twisted pairs but if it is really a problem i will remove it. The gdt's are 33 turns cascaded so yes a roughly 1:1000 ratio. Lastly about the gdt, i am using 5000 permability ferrite material and because i have the 4 "striped" wires paralleled i can get super high coupling so more than 5 turns is really overkill.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Yep, cool, sounds like you have the GDT itself sussed. Any yep, you probably will not get it to start oscilating with only 10V. Time to turn it up . Hope it all goes well! By the way , very nice, neat construction, looks great. Look forward to seeing some pics of it in action.
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260
Yes....i have to agree with Dylan, My coils don't start up until around 25+ volts on the bus line,
Put a pointed nail or such-like on your top load and a grounded rod about an inch or so away and turn up your bus volts to around 30 volts and see if you can get some tiny sparks.....if not try some tuning.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Good news! i cranked up the voltage and at around 10v on the bus the feedback kicked in and gave a excellent gate waveform on the gates. Now the bad news is still no sparks One odd thing i noticed and i am not sure if this is correct or not but when i cranked up the bus voltage past 20v the bus voltage suddenly jumped up to 40-60v depending on my interrupter settings. Last thing i would to ask, is were do i test to view my primary waveform?
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Excellent! As for the bus voltage jumping around , no this is not normal, my first thought is that the power supply/filter cap you are using cannot keep up, a nastier thought is there may be some cross conduction going on in your bridge. But you say the gate waveforms are good, so this may not be the case. Mmmm, not sure. ------- EDIT ---------- Just re-read your post, and it sounds like you have some kind of voltage doubler effect going on in the bridge, and to be honest I've never noticed it on any of my coils, and am not sure at this point what could be going on there. ------------------------
What are you using to provide power? I'd be checking the phasing on your GDT outputs again just to be sure there is not cross conduction going on.
And for the primary(current) waveform , the output of your feedback CT :) OR to get an unclipped waveform, use your OCD CT.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
IT WORKS!!!! But there is a catch. It is running very unhappy and no matter how much tuning i do it draws 8 amps at just 30vdc input! I can only get 2.5" to a grounded object and the arcs are HOT. Here is the primary to secondary waveform that i recorded by putting a 1 turn secondary coil inside the primary. Here is the waveform of the feedback ct. note that my oscilloscope trace is acting wierd and i cant center it onto the screen "time for a new scope" so even though the waveforms are not ringing negative, they really are and it's just were the oscilloscope trace is. All the waveforms are 5us/div. I really have no clue what is wrong.
Edit: sorry about that link. I had never had that happen to me. Anyway it is fixed. Sorry
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DRSSTC help.
. Hope it all goes well!
One odd thing i noticed and i am not sure if this is correct or not but when i cranked up the bus voltage past 20v the bus voltage suddenly jumped up to 40-60v depending on my interrupter settings. Last thing i would to ask, is were do i test to view my primary waveform?
