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Building my first DRSSTC - Updated thread...

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Mads Barnkob

Sat Jun 04 2016, 12:14PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

nzoomed wrote ...

Mads Barnkob wrote ...

That looks perfectly fine, running a transformer unloaded will ofc create overshoot and oscillations in a much higher frequency as it oscillates with the parasitic capacitance of your probes.

You could experiment with different gate resistor values to get a faster rising edge or dampen the overshoot even more.

Yes loneoceans also confirmed to me that its normal, so i hope i can test everything out tomorrow with a UPS transformer and adjust the phase lead initially at low voltage, and hope to perhaps see some short sparks from the secondary.

Im using 5.1 ohm currently for the gate resistors.
How high can you typically go? I take it that overshoot is undesired, but i expect its difficult to remove completley?

It is a trade-off between larger amplitude ringing with lower value gate resistors and higher switching losses with higher gate resistors. There is no optimal value, it is all up to your design criteria, most industrial designs only have one key parameter, temperature. We do not really have that as a limit, so adjust gate resistors for best possible inverter switching output, you do not want to switch too fast or too slow, just right in-between.

nzoomed

Sun Jun 05 2016, 01:46AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Thanks for that info.
At present im concerned about my tuning the coil.

I thought 10 turns on the primary would have been plenty, JavaTC gives me a resonant frequency of around 53.78 KHz for the primary and 58.22 for the secondary.

If i click on auto tune on Java TC, it tells me that 9.29 turns should be enough to tune it.

Im using 160mm pipe, secondary is 725 long and using 30AWG wire. Primary is 8mm copper pipe and inner radius around 95mm and outer radius is 225mm. MMC is 0.24uF

Toroid is 670 outside diameter made from 150mm semi rigid ducting.
The scope was showing the resonant frequency at just under 53KHz.

When i scoped the primary, i cant tune it any lower than 56 KHz so there is a 3KHz difference between the primary and secondary.

I could easily squash the ducting to make a smaller toroid to raise the frequency.

Im a bit lost here, as my initial calculations with javaTC were almost 10KHz higher.

Any ideas?
Will i have to add a larger primary at some point?

Mads Barnkob

Sun Jun 05 2016, 06:00AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

If you have a primary coil already made, adding some more capacitance to the MMC is the "easiest" solution, maybe not the cheapest :)

I plotted your numbers in JavaTC and that is quite different from your measurements, something is not right here in the numbers you gave. Did you mistake some of the inputs and gave diameter instead of radius?

nzoomed

Sun Jun 05 2016, 06:58AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Mads Barnkob wrote ...

If you have a primary coil already made, adding some more capacitance to the MMC is the "easiest" solution, maybe not the cheapest :)

I plotted your numbers in JavaTC and that is quite different from your measurements, something is not right here in the numbers you gave. Did you mistake some of the inputs and gave diameter instead of radius?

No, everything looks right that you have entered, except you have got way less turns on there than what i worked out. I worked out about 2621 turns, another calculator said around 2400 turns for 30AWG.
I take it you selected auto tune and its set the radius of 16.4 for the primary and around 5 turns?

I last used JavaTC about 12 months ago and it was telling me about 64 KHz, which was what i was going by.
I still have all my notes and re-entered them, so dont know what went wrong, it baffled me, only difference was that my toroid is about 70mm larger in diameter and my MMC is slightly larger at 0.24uF, but i dont think thats enough to put things out of whack.

Here is my last screenshot from JavaTC at that time.

Mads Barnkob

Sun Jun 05 2016, 08:47AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Oops, I looked at wire length and not winding length from my calculator :)

You are right, your numbers are correct.

If I were you, I would double the MMC capacitance to 0.48 uF and that would give you a tuning point around 7 turns.

nzoomed

Sun Jun 05 2016, 09:03AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Mads Barnkob wrote ...

Oops, I looked at wire length and not winding length from my calculator :)

You are right, your numbers are correct.

If I were you, I would double the MMC capacitance to 0.48 uF and that would give you a tuning point around 7 turns.

Thats alot more $$$ ive got to spend on some CDE942 caps! :P
I also dont know where to put them, would be cheaper and easier for me to make the toroid smaller i guess?
Anyway, I did not measure the primary with the secondary in place, so i expect this will raise the frequency somewhat?

Blackcurrant

Sun Jun 05 2016, 10:46PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Try putting the coil in the middle of the room and up a bit ie. on a small table see if the sec coil res goes up a bit.

nzoomed

Sun Jun 05 2016, 11:45PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Blackcurrant wrote ...

Try putting the coil in the middle of the room and up a bit ie. on a small table see if the sec coil res goes up a bit.

Ill try that, might have to do it outside, the toroid will hit the ceiling in my workshop.

I am going to shortly check the resonant frequency of the primary with the secondary and toroid in place, this should hopefully raise it a bit.

In the meantime i have been scoping the bus with 15VAC fed into the voltage doubler.
Its too low voltage on the primary to give me a signal from the CT so am unable to tune the phase lead until i put around 100v there, but here is a scope shot of the bus(yellow) and gates(blue)

below

nzoomed

Wed Jun 08 2016, 03:25AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Got my first sparks after doing low voltage tests on the bus (15VAC doubled and rectified to 40V)

What is the correct way of connecting the scope to the bus and gates?

I had initially connected the probe to one of the gates and the ground clip to the corresponding emitter and i put both ends of the other lead across the bridge.
I was told that both ground clips should be connected to the negative rail on the bus.
This fixed the buzzing sound the scope was making, but im still having trouble getting a image to lock in to the scope, everything sweeps across the screen.

Is there anything i need to do with these digital scopes to lock in the waveform?
Im also sure with the phasing, if it matters what gate i connect the scope probe up to.

I still cant drive the coil without feeding the signal generator into the driver board, i expect this is because im running at such a low bus voltage. I was getting a slight tingle on my hand if i touched the earth terminal of the scope, is this a concern? I dont know why this is possible with a 40V bus voltage.

I want to adjust the phase lead now, but will need to feed in 100V or more i expect.
My main concern is if this will damage the IGBT's if the phase lead is not set correctly, but i need to power up the coil to set the phase lead, so i expect this is not too much concern.
Anyway, below is an image of the coil operating currently.

Thanks for everyone's help :)

1465356315 54503 FT169902 13321947 891745584267234 6949388223842216318 N

orac12

Wed Jun 08 2016, 04:24AM

Registered Member #9879 Joined: Tue Jan 29 2013, 05:00AM
Location: New Zealand
Posts: 37

Adjust your trigger level on the scope to try and get a better lock. Just scope one thing at a time, less chance of messing things up that way.

Remember the phasing of your CT has to be correct for it to run, you either change the polarity or the jumpers on your driver board. I think I had to run around 70 volts on the bus to get enough before it would start oscillating it self. Just run short on times (keep current low) and enough voltage to get it oscillating to check phase lead, even if its way off, at low voltage and low on times you shouldn't damage anything.

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