Can I use my transformer in a reverse way?

[J_Nichols]

I have a toroid transformer where the input is 230 VAC and it has 2 output each one rated at 40 VAC.
Can I feed the 40VAC part with 230VAC and get >1 Kvolt output?

 

[JimB]

No.
You can feed the 40v winding with 40v and get 230v out of the primary.

If you feed 230v into the 40v winding, something will go BANG.

JimB

 

[BobW]

Just to clarify what JimB said, you cannot go over the rated voltage on any winding of the transformer without damaging it. If you exceed the voltage ratings, the insulation can break down and you'll end up with a short which will destroy the transformer.

However, as long as you stay within the voltage limits then you can drive a transformer backwards. For example, if you had a 40VAC source and wanted to get 230V then you could feed the 40V into the secondary and get 230V out of the primary.

BTW, You also have to stay within the current ratings of each winding.

 

[J_Nichols]

Thanks both for the answer.

 

[crutschow]

The problem with a higher than rated voltage on a transformer winding is not the insulation resistance, it's that the core saturates and the magnetizing current will get very high, causing rapid overheating of the transformer.

 

[dr pepper]

As cruts said.
Commercial transformers are designed close to their saturation limit, quadrupling the voltage will almost certainly saturate the core making it a short circuit, the transformer probably wouldnt even get to 1kv before a fuse or a connecting wire blows.

 

[J_Nichols]

As cruts said.
Commercial transformers are designed close to their saturation limit, quadrupling the voltage will almost certainly saturate the core making it a short circuit, the transformer probably wouldnt even get to 1kv before a fuse or a connecting wire blows.

I know very little about transformers and I have a question.
You has wrote about "saturation". I think saturation is related with the amount of the magnetic flux that the core can support, right? What causes saturation in the core? Voltage?

 

[crutschow]

I know very little about transformers and I have a question.
You has wrote about "saturation". I think saturation is related with the amount of the magnetic flux that the core can support, right? What causes saturation in the core? Voltage?

Yes, it is magnetic saturation of the core that I am referring to.
The saturation is caused by too much magnetizing current, which is determined by the core inductance, and the applied AC voltage and frequency.
This magnetizing current varies directly with the applied voltage and inversely with the applied frequency. Thus for a given frequency the magnetizing current increases with voltage until the core material can support no more flux and it saturates.
This causes the inductance to rapidly decrease, causing a large increase in the current. This will burn out the transformer in a short period of time.
To minimize the amount of core material and costs, transformers are normally run at near the core saturation point for their rated voltage and frequency. Thus it doesn't take a voltage much higher than rated to cause core saturation.

 

[DerStrom8]

To be more precise, sure it may be possible (I've run the 12v winding of a 120:12 transformer with 120v and did get a HV output) but as the others said you'd be risking a lot. Generally the insulation is designed only for a certain voltage, and if you apply too high of a voltage it could short and make a bang, or catch fire.

 

[schmitt trigger]

Just for fun, I have tricked transformers to operate the way you want. I used a variable voltage, variable frequency AC supply.

To sextuple the allowable input voltage, you would have to sextuple the frequency. I assume your AC mains is 50 Hz, so that would mean 300 Hz.

Of course, when you do this two things happen:
-Core losses become sky high and the transformer core starts getting very hot, very quickly.
-The insulation will be hissing and crackling due to corona discharge.

As I said, I did this only for fun and for 10 to 15 seconds at most.

 

[simonbramble]

I wired up a mains transformer the wrong way round when I was first learning electronics. It did indeed go bang - spectacularly well. Very quickly, very loudly

 

[dr pepper]

Your right saturation is the point where the core cannot take any more magnetic field, increasing it past that point makes the trans act like an air core.
If its a iron laminate transformer like most mains trans's are then the max flux is usually 1.2 tesla, more turns on the core reduces the flux, less turns increases it, also contrary to what most think the less the load on the trans the greater the flux.
The calculation for the no. of turns is little more than volt x seconds, so if you change either of these (seconds being frequency) then flux desity is changed.