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A mongo SS Variac?

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Finn Hammer

Mon Jun 08 2009, 06:01PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

First light with "Thumper" is approaching and I hope you can help me, as the folks at
United Automation
(

)
are slow to respond. Not responding, really.

What I have is on the picture,

a FC36M controller board,

and a 3pcs. Semikron SKKT132/14E SCR block.

What I want is a 3phase delta SS variable voltage DC supply with preset current draw. It should have a remote box where I can set the desired output voltage, and it should also have adjustable current draw from mains, so that I can get the most out of available power, without blowing fuses. A 16A, a 32A and a 64A setting would come in handy.

If it is possible to configure it as an input current limited variable voltage supply, things would fly for me.

Since the Ghetto Blaster is designed to "perhaps" tolerate higher then 1/2 power device voltage rating, it would be nice if a voltage doubler could be included too.
I have a crate of 3300ÂµF/420V lytics to make it.
The load is, by chance, 3300ÂµF/840V shunted by the "Thumper".

I really don't have any idea where to start with this, so all help is highly appreciated.

Cheers, Finn Hammer

Sulaiman

Mon Jun 08 2009, 07:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

At the simplest level, producing (near) 0V to Vrectified
I think that you need a 'dc choke' (inductor)
which is likely to be as large as but heavier than your assembly!

I can't remember the calculations, but if you can (for design ease)
specify maximum and minimum loads not drastically different (5:1 power?)
I can have a go at it.
(We sometimes scrap such things where I work, but postage would be horrendous)

Capacitive loads such as a doubler would need some inductance or else
the step-voltage change due to phase-angle control would cause enormous C.dV/dT currents.

Looks like remote control will be easy! (a battery and a potentiometer)(set to Voltage input)

The spec sheet is sparse, but since current proection is mentioned I guess you'll need 3 (or 2 if they're a bit cleverer) current transformers.
Have you got a better spec.sheet? A quick look at the site didn't give me much.
You will need the data to use the multi-function RJ45 port.

Finn Hammer

Mon Jun 08 2009, 07:24PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

The spec sheet on the site, the one that I linked to is all that I have, and is what was supplied with the board.

I have no problem with inductors up- and in-stream, but using the erh, whatshammacallit RJ45 port sounds like a challenge I'm not comfortable with.

The max load should be 20kW at the moment, but more later.

))))

I frankly think it would be bad if I cannot turn it down lower than 4kW, so some slack here?

But anything goes if it will substitute for a ridiculously expensive stack of 1256's

Cheers, Finn Hammer

edit: Just found out it is british located company. I will give them a call tomorrow.

tesla500

Mon Jun 08 2009, 07:30PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Hi Finn,

Some questions:
Do you care about power factor?
Does the output need to be mains isolated?
What output voltage range do you want? Right down to 0V, or would > Vmains(pk) be OK?

The answers to these questions will determine what topologies you could use.

I've built a power supply with mains current limiting (A battery charger for an electric car actually, 20A 120/240V input, 0-400Vdc output, non-mains-isolated). A microcontroller is needed to do the mains current limiting, you wouldn't want to try to do the math in analog form.

David

Finn Hammer

Mon Jun 08 2009, 08:01PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

tesla500 wrote ...

Hi Finn,

Some questions:
Do you care about power factor?

to some extent, but being 3-phase should help. Anything better than PF=0.8 is ok with me.

tesla500 wrote ...

Does the output need to be mains isolated?

Nope

tesla500 wrote ...

What output voltage range do you want? Right down to 0V, or would > Vmains(pk) be OK?

Say down to 30-40 Volts errrhhhh a bit lower?? and up to mains rectified x 2 = 1120V or thereabouts.

tesla500 wrote ...

The answers to these questions will determine what topologies you could use.

I've built a power supply with mains current limiting (A battery charger for an electric car actually, 20A 120/240V input, 0-400Vdc output, non-mains-isolated). A microcontroller is needed to do the mains current limiting, you wouldn't want to try to do the math in analog form.

letÂ´s quit the input current limit idea, hell I can keep my eyes glued to an ammeter no problem.

tesla500 wrote ...

David

Cheers, Finn Hammer

tesla500

Mon Jun 08 2009, 08:39PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

The easiest way to do this is probably to double/filter the mains, then use a buck converter to get any voltage lower than the rectified mains. This will also provide current limiting.

Thinking about mains current limiting again, I only needed complex math in mine because I had no sensing of input current, and the current had to be calculated based on input voltage and output power. If you do have input current sensing, a simple feedback loop should be able to regulate the input current to any desired level, by reducing the output current limit if the input current goes too high.

David

Dalus

Mon Jun 08 2009, 09:26PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Reminds me of my own coil. I also wanted to switch to a 3~ solid state psu. Eventually I ended up also choosing a thyristor based controller (still need to build it

) The simplest way to do this is take 3 of Conners solid state variac circuits and use them to drive the thyristors. It's not ideal but the other solutions like a 3~ buck boost converter with PFC are just way too hard to build for us.

Anyway take a look at

and start from there.

Sulaiman

Mon Jun 08 2009, 11:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Go here

for datasheets.
eg X10652 is for your model also.

Make sure you've got a good filter for the controler power
... don't rely only on the onboard filter.

Steve Conner

Tue Jun 09 2009, 10:24AM

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

Hi Finn,

I'll look into this and get back to you. It should be possible to come up with something that works with your board as well as the Mk. 2 single-phase SCR driver that I designed.

Probably the hardest part will be true RMS line current limiting, which is what you really need if your intention is to stop breakers from popping. A naive implementation with two or three CTs on the lines would limit the rectified average current, which isn't the same thing at all, since the RMS-to-average ratio of the line current changes radically with firing angle. The rectified averaged line current is basically the same as the DC bus current (proof by conservation of charge) so that's what we'd be limiting. It might be better to just sense DC bus current directly with a Hall effect CT.

Second hardest part will be isolation for the voltage feedback. I'd be tempted to use a Hall effect VT.

There are dozens of possible 3-phase doubler configurations, but I can't remember any right now. :O

Dalus

Tue Jun 09 2009, 10:34AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Here's a 3~ multiplier that I just found

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