Autotransformer 500 V/A ~ 1

[Erasmo0]

Hello,while I was checking another transformer through an autotransformer and some wires in the air I produced a dead short on the output of the autotransformer ... but the thermal switch upstream of the two transformers has not tripped. What tools could I use to check if the first Has the transformer been damaged?

 

[itsmoked]

Certainly help if you drew a napkin schematic for us and told us something about the transformers! Are they the size of your fist or the size of a pickup truck?

If the thermal didn't trip is the short still underway???! What interrupted the short?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Erasmo0]

[img

https://res.cloudinary.com/engineer...0FE911-DE88-4C3B-B1C2-317669B8086F_bahjuf.gif

hi, mr ... I would say a small 3.5A x230 V ~ pik-up ... the short was interrupted by me, because I was holding the contact...for scheme,above the link

 

[itsmoked]

Okay. The only damage I'd expect would be the wiper on the transformer coil. That's the foot that travels down the transformer coils.

I'd open it up and look at the path of bare wire that the wiper runs down. Look for a charred spot or soot. If you don't see anything setup a voltmeter to measure the output and then slew the autotransformer across it's full range very slowly while looking for any drop-outs or opens. If none are seen the transformer is fine.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A fuse on the output will give Better protection for your transformer for overloads.
If your auto-transformer is set for 5% output, it would have to develop 70 Amps output before the primary current reached 3.5 Amps.
The windings won't take that much current without overheating rapidly.
As Keith says, look for damage where the wiper or brush was positioned at the time of the incident.
Also examine the windings on the short side of the brush. Look for discolouration or burning.
When you energize the transformer, be careful.
If some windings have shorted due to overheating, they may be expected to overheat quite rapidly when the transformer is re-energized.
A shorted turn may circulate a lot of current with not enough primary current to trip a 15 Amp circuit breaker, for quite a while.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Erasmo0]

okay, I'll check it visually ... thanks to both, in case I can use the connected oscilloscope instead of the voltmeter (hand voltmeter or numbers?)

 

[itsmoked]

A meter would be better but a scope rated at that high a voltage with probes rated that high could be used. You want to sweep the knob looking for non-linearity or opens. With a scope you will see every tiny glitch including valid ones whereas a meter will average everything blotting out normal glitches. Be aware.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Erasmo0]

okay, i used the meter, and rotating slowly i did not notice anything abnormal except a couple of volts, less, however a voltage that was not stable on the DMM (+ - 0.5V AC) but i believe this is regular

 

[waross]

The two worst cases:
An open; There will be no current and not output voltage.
Shorted turns; There will be higher than normal no-load current and the shorted turns will generate excessive heat.
This condition often escalates to destruction fairly quickly.
But, no smoke when you were testing voltages is a good sign.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Erasmo0]

Already, he hasn't smoked, only the cable at its exit at the point where the phase touched the neutral, but I immediately disconnected, as if I were the switch; who knows if there is a mathematical formula to be applied to the electronics in these cases, even out of simple curiosity? .. A few more seconds and ... I think he would have gone

 

[waross]

I meant my last post as more general information on transformers.
If the cable that touched the neutral was the incoming power lead, it is unlikely that the transformer was damaged.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Erasmo0]

ah, I thought you were referring to the variac, so the transformer I tested with the 1/2 strength variac during the short had V1 / V2 = 0 V1 / V2 ... but instead at the input of the variac what voltage, was present in that instant !?

 

[waross]

Sorry. I misunderstood and thought that you had shorted the input to the Variac (Tradename)

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Erasmo0]

no ... I had short-circuited the output ... but, if instead the short, had been on the opposite side of the coil, that is, what changed at the input!?