Autotransformer.

[erasmo0]

Hello, I powered a radio receiver with 140 volts AC from a Variac and the two phase and neutral wires have short-circuited for two seconds ... have I damaged the variac? The power is 800 V/A and life saving did not fire

[kalee20]

Should not have damaged the Variac if it has been properly protected by fuses. The thing most likely to burn out is the carbon brush.

[erasmo0]

in fact I had not put the fuse and I was using two flying wires from its output to the input of a 220/12 V AC transformer, the "variac" works, it seemed a bit hot, as if it had been on for a couple of hours ...Hi kalee��

[Diabolical Artificer]

Should not have damaged the Variac if it has been properly protected by fuses

Fuse didn't blow though. What fuse do you have on the mains side erasmo0? You should have protection for the variac and protection for the DUT.

You might have burnt some of the insulation, give it a good visual inspection.

Andy.

[erasmo0]

on the network side I have the life saver of 25 A~ ths,Andy

[jonnybear]

Originally Posted by erasmo0

on the network side I have the life saver of 25 A~ ths,Andy

Do you mean 2.5a? if you have a 25a you have no saver at all you could power a house with that.
John

[Skywave]

Use an Ohmmeter and make some resistance measurements of the Variac. You'll soon discover if there any O/C windings.

Al. / Apr. 24

[erasmo0]

Originally Posted by jonnybear

Do you mean 2.5a? if you have a 25a you have no saver at all you could power a house with that.
John

in fact I have a single magnetothermic switch from 25 A~

[erasmo0]

Originally Posted by Skywave

Use an Ohmmeter and make some resistance measurements of the Variac. You'll soon discover if there any O/C windings.

Al. / Apr. 24

how do you measure the coil of the variac? isn't it enameled copper wire like in transformers?

[jonnybear]

According to my calculations 800va at 230 volts is 3.47 amps if you are protecting your auto transformer with a 25amp magnetothermic switch (MCB) you have no protection at all even with a 6amp one you would be pushing it. check Resistance across input winding, then from one end of the winding at a time to the wiper which should vary by adjusting the knob.
John

[erasmo0]

but, should the 3.3A fuse be placed at the input of the coil, or at the output? the check on what scale of the ohm meter should be done? ... I only checked the output of the variac, when it is connected to the mains and I did not find any hum, only in one point by rotating the brush, I dropped -2 ​​V Is this test correct to do with the autotransformer on no-load or under load?

[jonnybear]

This is a diagram of a variac, A & C is the input Live Neutral, output is B & C, B being variable C neutral. switch meter to ohms there should be a reading across A & C
then check reading between A & B then B & C by rotating knob these readings should be variable. Transformer should always be fused on the primary input side. The transformer will give you a AC reading weather loaded or not, if you are not getting an output voltage between 0-240 volts and meter resistance shows open circuit you have probably damaged the transformer. some pictures of the unit would be of interest, as it may have internal fuses.
John

[erasmo0]

it has no internal fuses, I made the measurements with the multimeter on a low ohm scale and I have no interruption of the continuity then I gave the mains voltage and I measured the volts: 1.5 V ~ / 230 V ~ with - + 0.5 V it seems to work. .. on 115 V however or noticed a voltage drop of - 2 V ~ can it be the line?! .. when I made the dead short it was at 1/2 scale that is in the middle of the coil, the greatest danger was with brush contact at the center of the coil or at the ends of coil?

[jonnybear]

Originally Posted by erasmo0

it has no internal fuses, I made the measurements with the multimeter on a low ohm scale and I have no interruption of the continuity then I gave the mains voltage and I measured the volts: 1.5 V ~ / 230 V ~ with - + 0.5 V it seems to work. .. on 115 V however or noticed a voltage drop of - 2 V ~ can it be the line?! .. when I made the dead short it was at 1/2 scale that is in the middle of the coil, the greatest danger was with brush contact at the center of the coil or at the ends of coil?

I think things are being lost in translation, If you put a 15 watt 240 volt bulb between B & C on the diagram and mains on A & C by rotating the control the bulb should vary in brightness, never dead short a transformer as you will damage the windings.
JOHN

[erasmo0]

and a good idea: 1 15w light bulb like those of cars or one from abajour?

[Chrispy57]

Do not use a car (12V) bulb - it must be a high voltage (240V) bulb in the variac circuit for safety and correct operation. I use a 60W tungsten lamp as an example of what may be readily available to you in these days of compulsory LED bulbs for domestic use.

Cheers
Chris

[erasmo0]

Hello Chris ah, okay I will connect the tungsten lamp in the points, indicated in the diagram and if the coil has one or two turns in short, the lamp should be brighter, right?

[Chrispy57]

Hi,
I don't see why it shoud be brighter if there is a shorted turn - the short circuit would mean that those two turns are at exactly the same potential, and the brightness would therefore be identical for brush contact with each of the two turns. However, I doubt that the subtle difference in brightness between any two adjacent turns would be noticable to the eye.

When my old variac did actually have shorted turns under the carbon brush position of long term use, I seem to recall that it quickly became hot and emitted smoke from that area whenever used, no matter what position the carbon brush was later in.

If your variac operates normally - no overheating or smoke when used with that 60W tungsten lamp across the points B & C in the diagram, then I would be confident to test it with the type of equipment you intend to use it with.

Cheers
Chris

[Radio Wrangler]

If there is a shorted turn, then though there may be low voltage across the short, there may still be a very large current flowing in the short.

Transformer action will make the whole thing behave as if each section of wire was shorted with an appropriately scaled version of the resistance of the short plus the copper resistance of the shorted turn.

This places a lot of added load on the mains supply and is likely to pull the supply voltage down somewhat.

It really doesn't matter whether a bulb gets brighter or darker, things are not going to last long enough to care. If the only protection on the mains is a 25A breaker, smoke and possibly fire are on the menu.

Copper resistances may limit the primary current to less than the breaker current, and this is more dangerous than more current pulling a breaker cleanly.

David

[erasmo0]

good explanations, OK Chris, I also believe that the variac has not had any damage, but it could have had if the short dead persisted, I am doubly happy both for the variac even if subject to failures, from what you told me, and for the new things that I have been able to learn, David is more pessimistic than us ... but he is right, because it was a bad short albeit short ... I made the necessary measurements as you told me and I am almost convinced that the variac is OK: consumption no-load current about 25mA ~ ... I will try to connect it to a load for a couple of hours ... Ths Sir