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PIRANHA-III Power control?

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Terry Fritz

Sun Dec 17 2006, 02:16AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I am working on PIRANHA III issues today. A dual MOT system that runs off a 120VAC 20A circuit with ease. It is in the 8 foot arc to ground range... About 2kW input at ~90% efficiency.

These coils like to run with the MOT input voltage at about 95-105 volts to stay out of the MOT saturation range which just waists precious current. Normally that is done with a 15 -20 amp variac...

Variacs are very nice, but they are real heavy and not real "modern" or solid state. The charging circuit (MOT / Primary Cap loop) is resonant too to drive up to a 15000 volt firing voltage... But the waveforms are pretty "tame" and the power factor is excellent without any fiddling.

It would be super cool to use a beefy lamp dimmer circuit or some similar cheap but very reliable solid state thing to control power other than the variac. The PIRANHA input section is very forgiving of sloppy input wave forms and all so no big deal there. Ceiling fan controllers are made for inductive loads (low power though)...

I don't know much about dimmer circuits and such so I can't guess at what to do and feed the computer models well. 0:-|

If anyone knows what to do to make a cheap hardy dimmer type circuit that could run two MOTs (~2kW) I am all ears

Cheers,

Terry

...

Sun Dec 17 2006, 04:23AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Could you copy steves design but only use a full wave rectifier (as opposed to doubler) arrangement?

Marko

Sun Dec 17 2006, 12:53PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I tried a simple dimmer based upon this:

And a lashup looked like this:

(isolation transformer used).

And seemed to work nicely, driving inductive and pseudo-capacitive (rectifier+cap) loads without probs.

I only have it lashed up for now and I haven't driven anything bigger than small 300VA OBIT (just a bit more than an amp compared to your 15amps), but I feel like it might work since circuit follows the same point as Steve's dimmer (wich may not be in spirit of simplistic SiSG design).

'Pulse-train' oscillator is here substituted by simple diac-based relaxation oscillator.

Power factor is although, assured to go crazy, and you can't use PFC caps on MOT's because they will blow the triac up.

This is the secondary waveform I got from a small transformer at about 50% setting:

But, it never goes 'above' supply voltage and triac never turns off during the cycle.

This is resistive load for comparison:

With a dual MOT supply, triac must also stand the magnetizing current of enormous core so you might need something like 25..30amp triac.

I think it will work but surely not as nicely as a traditional variac...

davovinch

Sun Dec 17 2006, 01:01PM

Registered Member #293 Joined: Tue Mar 07 2006, 11:57AM
Location:
Posts: 10

Hi Terry, Have you thought about a Router Speed Control. You can get them rated at 20 amps.
Regards,
Dave

Terry Fritz

Sun Dec 17 2006, 06:26PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

Thanks Dave!!! And many thanks to all for the responses

)

Figures 7 and 8 of ST's AN308 describe "pulse train" Triac firing which seems to be very good for really screwed up loads.

The circuit is simple and cheap

) Steve Conner used a similar idea for his OLTC II supply but with much more control circuitry:

The best part is apparently the ST circuit is appearing in devices called "Router Speed Controls".

Some versions are less than $20 with free shipping

)

I was about to get one till I saw this

))))) $12.50 is a fair price I guess even if the router is "sold separately"

)

I am not "sure" of the circuits inside them, so I would have to get one and take it apart. Also would disable the "full" switch setting which to too easy to hit be accident. They are all obviously of the same design with different cases. Everyone is dropping the prices on them since they are obviously very "cheap". Some might be "too cheap"

) But others like the Grizzly one are probably made pretty well. Very convenient fuse location too. Probably easy to "enhance" the fuse value if needed :o)))

So I'll see if I can pick one up today and let you know what's in it. They "might" be a good "variac replacement" in general. I would think the PIRANHA would be a far more pleasant load than a router so it might work fine. If so, I will use the 15 amp version on PIRANHA-II

If they work, this was all waaaaay too easy

))

Cheers,

Terry

Steve Conner

Sun Dec 17 2006, 11:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I suggested to use that ST application circuit Firkragg found, a while back. My own one should work: I was able to dim transformers and motors with it by removing the two SCRs from the doubler, connecting them in inverse parallel, and putting that in series with the hot line. The two gate drives are isolated, so the SCRs can be connected any way you like. But it's really a bit complex. Hopefully the router speed controller should do the same thing, I'm looking forward to see if it works!

I imagine the main problem would be spikes from the "spark gap" feeding back through the MOTs, and the high dv/dt of these spikes causing the triac in the controller to self-trigger. Hopefully the leakage inductance of the MOTs (quite a lot if you leave the shunts in place) should filter that out, though.

Terry Fritz

Mon Dec 18 2006, 02:31AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

The Router Controller is just a plain dumb dimmer circuit with heavy duty parts. I little report is here:

The internal circuit is a standard simple Triac dimmer type circuit with minimal parts.

I t appears they are "all the same" since the external layout of various version is identical.

A company in China probably turns out a million an hour

I was a little surprised to see the "ST" triac in it, but realizing that in China "any part" can be "labeled as you wish"

However, it might be simple to "add" a few parts to it for the ST pulse design... It has most of the "stuff" all there ready to go as is...

Hmmmmmm...

The MOTs I used are not modified.

Cheers,

Terry

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