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Tuning for max power

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Graham Armitage

Sat Jan 17 2015, 10:01PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Well, I finally fired up the new coil today, and after some basic tuning and low power tests, I opened her up. Sadly to say the output was well below expectations. The design should handle 700A primary current easily, but the OCD is set at 450A and only trips with a ground strike. Initial arc length was around 28" max but with beefed up primary connections and finer tuning I got it up to 36". She won't go any higher.

I am running 340v (120v voltage doubler) on the DC bus, with the CM300 IGBT H-Bridge and 0.1875uF (5 strings of 4 0.15uF 2kv caps). Coil is oscillating at 96kHz and the whole system is running really cool, so no heating heating concerns. Wave form on the primary looks great with 100us burst length.

The dimensions/specs of the coil are very similar to other coils on the web producing 4'-5' arcs, so I know it can do better. I got similar results with a much smaller coil before, so not sure what is up. Would welcome any ideas on where to focus my debugging.

One thing I just noticed is that increasing the burst length does not extend the ring-up envelop and hence the current - it just staets adding a second envelop after ring-down. Still without any OCD trip. That seems like a tuning issue correct?

Andy

Sat Jan 17 2015, 10:16PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Sad what thats good ,just invert some inputs and you will invert the ouput.
What current flow throught the double were you achiving.
Based on the last better your chocked for input, which addeds to the one above.

Edit

Graham Armitage

Sun Jan 18 2015, 03:17AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

I did some more testing. The part that is most confusing is why after 100uS the primary starts to ring down, presumably before the current can rise to it's full potential.

I am starting to suspect something may be happening with the interrupter. The interrupter is designed to reduce the burst length if the duty cycle exceeds 5%. Although I don't believe the duty cycle is that high (I am running at low bps). Not sure what else could be turning off the oscillation if the OCD has not tripped and burst length is set to be over 300uS.

The Total input current is only at 4A, which should be MUCH higher - expecting closer to 8-9A at resonance.

Uspring

Sun Jan 18 2015, 12:14PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

It's not unusual for the primary current to drop after having risen to some value. That can be caused by arc loading. The peak current depends on the tuning. I'd try to increase primary L somewhat in order to lower primary fres. You should be able to find a point, where the OCD is just not hit.

Kizmo

Sun Jan 18 2015, 12:27PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

This is interesting subject.

Some coilers seem to be very interested about the notching of primary current, and how to tune the coil so that primary current reaches its highest value at first notch, and then cut the drive before second ring up takes place.

But from what i have seen from my big coil, if I run the coil at high power (say more than 15kW DC power), it will not form this notch no matter what, primary current just stops rising and that point seem to be the spot where driving any further doesnt help much at all. At lower powers, below 10kW or so, i can see some heavy notching depending how i tune the primary.

Right now the coil seem to hit its performance peak at burst lengths of about 150 to 190Âµs at 55kHz.

Antonio

Sun Jan 18 2015, 12:46PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

A primary current that first increases and then decreases is normal and correct. This means that the energy coming from the driver is being transferred to the secondary, and not just accumulating in the primary circuit. If you end the burst at the first minimum of the primary current envelope, the system is working as efficiently as possible. If you are using primary current feedback the sistem tends to fall in this situation, but if the resulting burst is short no much energy is transferred. There are several solutions: a) Decrease the primary circuit impedance (more capacitance, less inductance). b) Increase the duration of the "beat", by lowering the coupling. c) Move the excitation frequency to one of the resonances (feedback will move it back). d) Add a breakout point, so the lowered quality factor smooths out the beats.

Kizmo

Sun Jan 18 2015, 12:52PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

If I remember right, expert i talked to said that disappearing notches at heavy spark loads is to be expected too, i just cant remember what the mechanism and theory behind this was.

Graham Armitage

Sun Jan 18 2015, 02:29PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Thanks for the suggestions. Some are easier to try than others. The part I am not understanding is why the ring-up stops at 150uS regardless of the burst duration (in this case 300uS). Looking at the primary current running at max power (pic below) this is what seems to be happening. I am using primary current feedback and ending the burst at or after the ring-down. Increasing the burst duration does not extend the envelope amplitude, it just distorts it or starts adding another envelope at the end of the burst. Does the ring-up stop because energy is being transferred from the secondary via the arc?

I do have a break-out and the torroid is a standard 4"x17" duct torroid - if that influences anything. I will try reduce the coupling first as this is easiest.

The primary current is around 375A and time div is 50uS.

So I tried reducing the coupling and it did extend the envelope a little, but it just flattened out and did not keep ringing up. Here are the specs on the primary coil.
C - 0.1875uF
L - 12.7uH
Z - 8.23ohm

Antonio

Sun Jan 18 2015, 04:42PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I am not liking the appearance of the primary current. The envelope should be more rounded at the top and almost symmetrical. Look at the voltage waveform at the bridge output.

Graham Armitage

Sun Jan 18 2015, 05:11PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Antonio, if I increase the burst length it does become more rounded at the top, but no increase in amplitude. I can't run at full power with my isolation transformer (not big enough) and so don't want my scope probe near the bridge output. Here is a pic of the Bridge output and primary current during a lower power test with the isolation transformer and after ZCS tuning. This looks normal to me.

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