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2.2kVA variac

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Tonic

Tue Jul 08 2008, 10:11AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

I'm lucky - got a monster variac

I have disassembled it for curiosity and checking if it's fine. Photos:

1215510426 528 FT0 07

1). The DC resistance is 2.2ohm. With inductance reactance, will be impedance enough to hold current at nominal level? The mains current is 230V/50Hz.

2). The lacquer doesn't hold all wires, I can move some of them with my finger. Can I add a layer of resin? I though it would minimise hums and I'm a bit afraid that vibrations can destroy wire insulation (is it even possible?)

3). After reading about variacs, I came to conclusion that inrush current is a common problem for big toroids, so a soft-start is needed. There were few types of them (resistors, triacs, transistors, relays etc.) and many schemes, but I'm interested in this one:

However, I will change relay power supply (R1-R3 and C1) to safer option - a transformer with voltage regulator and capacitors. I'm considering buying 12V 10A relay, since I have a transformer that gives 15.5V (which will be regulated to 12V). Also, a free-wheel 1N4005 will be placed across relay, to return back EMF.

Question #1: from what a inrush current limiting resistors resistance depends? What a wattage would you recommend?

Question #2: with this circuit, can I power variac with load? Or have I to power up variac, wait a while, then plug load?

I haven't tested it with mains, nor lower voltage yet - it's still disassembled and today I'm going to clean it, then assemble everything and test. Well, steel and plastic elements will be washed with detergents, but what about coil? How can I clean it without destroying?

What else can I check before testing? There are no shorts to case. I'm going to place 10A fuses, at input and output.

For tests, I will plug variac to 24V transformer, then check with DDM if it works. The next step will be mains and there I will use light bulb as load and indicator if adjusting output voltage is working.

Steve Conner

Tue Jul 08 2008, 12:44PM

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

The variac looks quite small compared to the cat. Do you also have a monster cat?

I wouldn't change the power supply in that soft-start circuit. The whole point of it is that it's a low-current supply, with current limited by C1, that will charge the 470uF capacitors slowly, which is what makes the time delay.

I would change the limiting resistor to something like 22 ohm, 50 watt wirewound, especially if you want to start with a load connected. The value isn't really critical. If you wanted to be pedantic, you would make it such that the RL time constant is about one-third of the time delay. But you probably don't know the time delay either. (You may need to adjust the capacitor sizes in this circuit to optimize the time delay, if you used a different relay to the guy who designed it.)

BTW, speaking of RL time constants, you are right, the inductive reactance should be much larger than 2.2 ohms, and the current drawn without a load should be low.

aonomus

Tue Jul 08 2008, 02:49PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Inrush current is definitely a problem, I recently bought a benchtop powersupply and it has its own multitapped torroid inside. Every time I turn it on, it momentarily trips my UPS for my computers.

I wouldn't string 4x 5W ceramics together, a 20W wirewound or like Conner said a 50W might be better for the variac since its 2.2kVa, its going to be drawing a fair amount of current under load and smoking resistors is never fun.

As for cleaning, you could probably wipe down the coil sides carefully and make sure the brush isn't highly worn out.

Dr. Dark Current

Tue Jul 08 2008, 04:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Re the inrush limiting resistors, I think it is safe to use something like 5-10 ohms for 230V (almost any breaker will hold 40A for a fraction of second) so it does not heat so much. And 1/2 second is more than enough delay after which to bridge the resistor IMO.

Tonic

Mon Jul 28 2008, 10:59PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Indeed, this variac looks wimpy comparing to cat, perhaps it's just a photo. But for me it was biggest variac I've seen. Now I know there are monsters like as acertain person here has a 50-80A or so variac.

I've made soft-start working. Nothing blew at first plug, it just worked. Weird and surprising. I've used the exactly same components (but with relay for higher current).

Anyway, there's a video where I'm testing variac:

And picture:

1217285652 528 FT49361 Softstart1

I've used 200W as ballast, but recently I bought a 27ohm ceramic resistor, which should handle few dozens of watts. As I upgrade it, I will post.

Fraggle

Tue Jul 29 2008, 01:06AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I use a 10W wirewound in a little tube of oil to ballast 3 MOTs for my MOC charger and it works great. It`s amazing what they handle under oil.

Tonic

Tue Jul 29 2008, 09:12AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Well, I picked up a easier way - just bought resistor. That's how it looks:

1217322658 528 FT49361 Resistor

The overall length is 10cm and diameter is 2.2cm. Ceramic has 6mm thick wall. Anyone could guess the heat dissipation?

Firefox

Tue Jul 29 2008, 09:45PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Looks like a 2.7 ohm 100W based on the writing on the side.

Tonic

Wed Jul 30 2008, 03:48PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Firefox wrote ...

Looks like a 2.7 ohm 100W based on the writing on the side.

It's a resistor I mentioned before, and has 27ohm resistance. Huh, 100W? That's quite much :) Thanks.

Weston

Fri Aug 01 2008, 11:41PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Would this work for a 4.3 kva variac?

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