TRANSFORMER QUESTION

[pfcgreene]

My question: Can someone explain how transformers work. Do they take a voltage and step it up or down. To step down will the current on the secondary increase. I have a 110vac at 1 amp primary and secondary 24vac 4 amps. I want to input 60vac at 1 amp. Will the secondary be 13vac at 4amps.

[DaveW]

If the transformer is a 110 VAC input rated at 24 VAC out then

the answer is yes.  The ratio would be 4.5833, 60 volts in would

give 13.09 volts out(assuming the correct frequency at 60 volts.)

But most transformers are rated to 120 VAC in, which would give a

ratio of 5, or 12 volts out at 60 volts in.  However, to provide a

24. volt rating at 4 amps the unloaded voltage out would be a nominal
    
28. - 32 volts,  so with 60 volts in the unloaded voltage out would
    
    

be as high as 17 volts. This means that when loading the output you

may only be able to draw 3 or 3.5 amps before reaching the 13 volt level.

[ghurd]

Thats about almost right.

Most people figure watts in equal watts out, but there are a lot of losses.

With 110VAC at 1A (=110w), that is 24VAC at 4A (=96W)

Same as 60VAC at 1A (=60W), that's 13VAC at 4A (=52W)

About 87% efficient.

Transformers are very sloppy. Most are not that good or efficient. Most little one I work with are less than 50% efficient.

Most have a very high open circuit voltage.

Higher amp drains decrease the voltage.

Lower amp drains increase the voltage.

Many things can effect the output.

But that's a decent start.

G-

[Nando]

GHURD;

I need to correct his misconception:

>Transformers are very sloppy. Most are not that good or efficient. Most little one I work with are less than 50% efficient.

Transformers properly designed, even the small ones do have better than 90 % efficiency.

Transformers with below 200 watts output may have around 94 % efficiency and higher power start to go up to the 97-98 % efficiency.

All depends on the quality of the transformer ( metal and design parameters).

The so called 50 % efficiency are those Wall plugged transformers that have high impedance behavior, on purpose, to limit the power output with a variable loading.

I am basing this on my work for many years.

New engineers that did not have experience with transformers needed to design one or two under my supervision, learning the core losses, the internal input and output resistance and even characterizing some already built, including calculation of efficiency basing this n the dry parameter testing (No power) and confirming the calculations with the wet parameter testing (with power).

Nando

[finnsawyer]

Transformers can step up or down.  If you just reduce the input voltage and keep everything the same the answer to your second question is no.  Engineers have developed an electrical model for the transformer that involves the resistance of each winding, the self inductance of each winding, and an added inductance term called mutual inductance that relates the output voltage to the input current and the input voltage to the output current.  The transformer is designed to provide the rated output voltage and current into a resistive load without any undue heating.  So, for your example you get 24 volts ac at 4 amps out with the proper load for 110 volts in at 1 amp.  If you reduce the input voltage everything drops by the same ratio according to the model since it involves linear elements.  If you reduce the load resistance you may be able to get 4 amps out but maybe not at one amp input.  It is likely that you will need more than one amp at the input, which can drive the transformer into non-linearity.

[dinges]

Sigh.

I love this board but I'm amazed you're getting answers. These kind of questions can be answered with a quick 'google'-search on 'transformer' and 'explanation' and 'works'. Or go to 'howstuffworks.com':

http://science.howstuffworks.com/inside-transformer1.htm

Sorry mate, but before asking others, at least make an effort yourself.

Peter,

The Netherlands.