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Marko

Tue Dec 04 2007, 07:30PM

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

Hi, all,

We have three quite large variacs at school - smaller one is something like 5amps per phase I think, with max output of 500V, and two larger ones are true monsters, 3x 10A 500V outputs (15kVA total!); I can barely lift one of those and require 2 strong men to carry.

All of them kept tripping 16A breakers at random and blowing fuses at random when plugged in without any load connected.
Sometimes only one phase would trip.

We tried powering up the small one gradually through a monstrous 3x200W 0-1800ohm three phase potentiometer - and it worked very well!

Nice gadget to have.

What I simply don't understand is how can an unloaded transformer pull such massive current at startup. It is not shorted since it works well after being started up by a potentiometer.

How can something of that high inductance pull such a current.

I understand that permeability of iron doesn't reach max value until H is high enough, but still, to this extent?

How I never blow fuses when plugging in my arc welder, or microwave oven which is in same power range?

Marko

Dr. Dark Current

Tue Dec 04 2007, 07:58PM

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

I don't really know as to why this happens, but any large transformer will pull huge current the first few cycles after turned on. This current is the largest if the transformer is turned on at zero crossing. (this is why you often hear a "thud" as a transformer is turned on).
Microwaves contain relay delay soft-start circuit- the transformer is turned on with a series resistor which is then shorted by a relay (at least I think they contain one).

Dr. Drone

Tue Dec 04 2007, 09:48PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Marko

Tue Dec 04 2007, 09:57PM

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

I don't understand how can core be ''magnetizing'' for several cycles like this.

As far as I know magnetization takes place through first, and demagnetization on second quarter of cycle and so on.

Where does this energy from first few cycles go? Is it reactive or dissipated as heat somehow?

Dr. Slack

Tue Dec 04 2007, 11:09PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The core is sized so that it almost saturates +ve on one half cycle, and almost saturates -ve on the other half cycle. Any less agressive than this and you are wasting iron, or specification.

An unloaded transformer looks like a high value inductor. In the ideal case, you switch on at the peak of the voltage waveform (yes, zero-volt switching is for lamps and interference, not for inductors). In the following 1/4 cycle, the input voltage goes to zero, and the Volt.second area under the voltage quadrant ramps the core flux up to near saturation. The next 1/4 sees a -ve voltage quadrant, which ramps the flux back to zero, and finishes with peak -ve volts input. The next two 1/4 cycles now ramp the flux to near -ve staturation and back in a mirror image of the first two. This is the normal operating action of the transformer.

Now consider the case when you switch on at zero crossing. The volt.second area under the first quadrant ramps the flux up to near saturation. Now the input volts are at peak. The next quadrant, being of the same polarity, continues to ramp the flux up, this time to near 2x saturation - oops.

The flux is the integral of voltage (neglecting winding resistance). As you know, and integral contains a constant of integration. We get to choose the value of the constant depending on the switch-on phase. If we switch at peak volts, the constant is zero, and the flux goes +/- 90% staturation. If we switch at zero volts, the constant is 90% saturation, and the flux will still go, or try to go, +/- 90% from that, except that once the core saturates, it takes a collosal current to generate the flux. This is why a big enough transformer sometimes goes "thunk" as you turn it on, and the lights dim momentarily.

The L/R time constant of the transformer being shorter than the thermal time constant of the fuse is what comes to the rescue, often transformer inputs will be specified with a time-delay, a high thermal mass, fuse. The initial surge of current decays at L/R.

Shaun

Wed Dec 05 2007, 01:27AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Am I correct in thinking, Neil, that you are saying some sort of large Slo-Blo fuse will solve the problem?
Makes sense to me.

Dr. Slack

Wed Dec 05 2007, 11:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes. It's not quite the evil "put in larger fuse until it stops blowing" method, because it is a slower fuse or longer time delay on the breaker that's required, and it's to cure a well-defined effect. If you can find a slow enough fuse to survive the worst case switch on surge, which is also small enough to protect the wiring, then that's the simplest way to do it.

An alternative method would be to use a triac to switch on at the waveform voltage peak. Most clever triacs and controllers are designed to switch at zero volts, but I vaguely remember seeing one that was specified to switch at peak for inductive loads, though a quick FWSE hasn't turned it up. Otherwise you could build your own controller and use a standard triac.

Yet another method could use a large NTC resistor in series, they are made for inrush control, but they may be expensive or rare these days. This effectively does what winding it up slowly on a variac does.

Both the triac or hot NTC resistor would waste some power if they stayed in circuit all the time, but either could be shorted with a relay after a few seconds once the transformer had been "started".

Dr. Dark Current

Wed Dec 05 2007, 11:25AM

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

I'd connect a few Ohms high wattage (10-20W) resistor in series with the load and a relay that would short out the resistor with little delay. I think this is the siplest method and it doesn't waste any power (except startup).

Steve Conner

Wed Dec 05 2007, 11:48AM

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

That's how I did it in my stereo amp. It had two large toroidal transformers and a big bank of smoothing caps, which made a lot of inrush current. If I switched it straight onto the mains, it tended to make a bit of a thud, dim the lights, and blow the fuse. The fuse problem could be "solved" by using a bigger one, but the loud noise that it made was still annoying.

I just used a big 15 ohm power resistor that got shorted out by a relay powered by the preamp supply. I started out with a smaller 47 ohm resistor, but it exploded after a few "starts".

Some toroidal transformer makers actually specify that a soft-start should be used on their larger models.

Those NTC things are still used for inrush limiting in switched-mode power supplies. Another popular soft-start circuit that I know of is a resistor shorted out by a triac.

Zum Beispiel

Wed Dec 05 2007, 12:21PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Hmm, don't know if this is relevant, but I get the same thing when I plug in my 2.5kVA variac... The lights dim when I plug it in and it draws a large current spike. Of course, the house I live in is old so all the wiring is a "bit" dodgy, so this probably adds to the effect.

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