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Solid state Variac, What to use?

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Finn Hammer

Sun Jul 17 2016, 01:35PM

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

All,

Excuse me for being a bit rusty, but I have been off the loop for a couple of years, and need some advice.

For a new coil I am building, I would like to omit the variac that I usually use, and instead employ some sort of Solid State regulation of the buss voltage.
I feel mostly inclined towards using something that I can go out and buy, since the challenge of this coil is mainly a mechanical-dielectric one

What options do I have? there are many scr and triac based controllers out there, and I am quite interested in the type, where I can mount a SCR right beside the IGBT modules and feed the buss right from that triac, But I need the circuit that fires the SCR.

I know this should be within my skill level, but I am pressed for time, and have little space to experiment.

I hope for your understanding and support!

Cheers, Finn Hammer

Edit:

What I have in mind is more or less beefy scr regulators like this one:

But don't want to appear cheap, so prolly this one:

And since they are announced to work with resistive and inductive loads (motor speed controll) my query is this:
May I assume they hold up against a capacitive load, provided I insert a current limiting inductor between the regulator and the buss capacitors ?

Hazmatt_(The Underdog)

Sun Jul 17 2016, 04:28PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

So basically you want any circuit in theory, that uses a pulse transformer to control the SCR's. These usually have a timer attached to them which gives better linear control. The old "lamp dimmer" style has lousy control at the extreme ends.

I found this circuit that is driving the pulse transformers

(the link right to the picture didn't work)

And you still need how the pulse transformers connect to the SCR's

And the last schematic there shows how to connect the SCR's to the pulse transformers.

For timing, the last one is just using a single pulse to "close" the SCR's right, but what I typically see in the power control are the anti-parallel SCR's on the hot-lead (triac mode) and they are controlled by 2 clock pulses 180* out of phase.
So at that point, you could use your TL494 to generate the timing and drive the primary side of the pulse transformers, and then the secondary step-down side fires the SCR's.

You should be able to find one of these guys for $100 all built up, I know that's not the point, but its less hassle.

Matt

klugesmith

Sun Jul 17 2016, 05:02PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Hi Finn. Glad you mentioned wanting to vary the voltage of your high-power DC bus.

Otherwise I would find out about my co-worker's new solid-state AC power supply. It has variable frequency and voltage, but does not do three-phase. Don't know what, if anything, makes it different from a VFD for motors.

My personal experience with high-power DC buses carrying rectified mains current does not exist. Do you need active regulation of the voltage? How much ripple is acceptable to a SSTC driver? I think that induction heaters are OK with 100% ripple, i.e. rectified sinusoid on the bus, as a simple way to approach 100% power factor.

Finn Hammer

Sun Jul 17 2016, 07:11PM

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

Hazmatt_(The Underdog) wrote ...

You should be able to find one of these guys for $100 all built up, I know that's not the point, but its less hassle.

Matt

Matt,

Getting them already built up is very much the point, because with the time constraints, there is hardly time to build myself.

Cheers, Finn Hammer

Steve Ward

Sun Jul 17 2016, 07:19PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

And since they are announced to work with resistive and inductive loads (motor speed controll) my query is this:
May I assume they hold up against a capacitive load, provided I insert a current limiting inductor between the regulator and the buss capacitors ?

The inductor would be pretty crucial i think, though the AC line inductance might be enough to make the circuit work, just watch out if you ever try to rely on it in an industrial setting with "stiff" power feed

. It would look something like a buck converter, where the inductor current is a function of the difference in input to output voltage integrated over time, divided by the inductance. I can't think of any reason why the phase-angle control method wouldn't work here. Just be careful at start up, but i think it should be ok as long as you don't ramp up the phase too fast and get too much charging current

.

I guess a simple resistive pre-charge circuit won't suit your needs? Test on the variac till you are confident, and just always run on full voltage after that?

I'm curious about your project. Good to see you back

Shrad

Sun Jul 17 2016, 09:08PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I have some SCR phase angle switching controller ICs and pulse transformers that I can give away but will need to dig them out... old 80's technology, pure bipolar +15/-15 VDC.. PM me if interested

Hazmatt_(The Underdog)

Sun Jul 17 2016, 10:46PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Hi Finn,

The one you posted is just the cheapo diac-triac circuit we've all seen, RC based timing, with non-linear response at the bottom and top end.

The Ohmitrol from Ohmite was similar with an adjustment pot on the guy so you could "bias" where it started to control the circuit. It gave us nothing but trouble because it was controlling the weld power on a lid welding machine for small hermetic kovar packages.

I also tried to control my drill press induction motor with one of these things, but the trouble with induction motors as everyone knows here already (I have to learn these things as I go along) is the dammed things need a lot of surge current to start, so as soon as you throw a dimmer on one, the thing constantly stalls out and tries to start during the peak of the controlled cycle.

So these little "dimmer" dudes are really only good for two applications in my opinion (whatever that is worth), for lamps and for universal motors. Because all the other applications give you poor control and they are not linear.

Any timer will give you linear reliable control, it's just a bit more work then the RC circuit setup.

Patric

Mon Jul 18 2016, 06:42AM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 254

I use this circuit for ages without trouble:

Finn Hammer

Mon Jul 18 2016, 07:40PM

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

First of all, thanks everybody for your patience, suggestions and kind words. It is this kind of exchange that works so well and stimulating on my mind, while I figure out which method to use.
I think I have found what I want to use:

This is a small nifty shield that bolts on to a SSR, delivers phase angle firing, and accepts all the usual industry standard inputs for control.
This means that I can feed it with a PWM signal via Fiber optic, something that I value highly, just like on the timing controller.
With a soft change option, an inductor upstream, i will try to feed it into a Delon voltage doubler, and get full control of the buss voltage.
Only problem seems to be their restrictive shipping policy, but IÂ´l work that out.

Cheers, Finn Hammer

Erlend^SE

Tue Jul 19 2016, 08:25PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

I would generally go for 3 phase inverter, since it gets closer to the effect of a real variac.

Triacs isn't really a generic variac replacement!

Steve Ward's suggestion about buck-converter seems like a good idea.

You plan a TC only "variac"? or more general use?

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