How to determine wattage of a transformer by core size?

[gary350]

I need a 120 VAC primary winding and 48 VAC secondary winding with CT. The transformer needs to be 250 watts. I need 160 watts from each 24 VAC winding.

No transformers like this for sale on ebay. I have several old transformers I can rewind. I need to determine the power rating if a transformer by the core size?

So far I have not found any formulas in my book or online, how to calculate power by core size.

I have several used transformers I can rewind. Some are marked and some are not.

I have a 480/240 VAC 500 watt transformer step down to 120 VAC core area is 3.682 sq inches.

I have another 480/240 VAC 500 watt transformer step down to 120 VAC core area is 5.764 sq inches.

I have 7 microwave oven shunt type transformers 1000 watts, core area is 3.781 sq inches. It is impossible to over load a shunt transformer max amp load on the primary should be 8.33 amps.

 

[Grossel]

Becaus the physical size is not the only thing that determine any transformers maximum wattage.

Heat loss in windings, overall surface area (cooling) and iron type is also major factors that may limit the maximum wattage.

On transformers in power grid, fans may be used as cooling to make the transformers capable to deliver slightly more power over time.

 

[The Electrician]

So far I have not found any formulas in my book or online, how to calculate power by core size.

You should read this thread:

https://forum.allaboutcircuits.com/showthread.php?t=38273

 

[gary350]

You should read this thread:

https://forum.allaboutcircuits.com/showthread.php?t=38273

That is some interesting information. It sorta confirms what I already knew but I though sure it would be more scientific than that. 10 years ago when I was building Tesla Coils we use to put a power transformers in series with the power line and use it as a variable choke to limit the current to the Tesla Coil HV power transformer. When a Tesla Coil = (TC) reaches its peak power rating there is no point in putting in more power if you get nothing extra out. A variac or a selector switch to several different load on the secondary will allow different currents to flow in the primary it makes a great variable current choke coil.

I did some experementing with different size transformers. 2 identical transformers both 480/240 VAC on the primary and 120 VAC on the secondary both rated 500 watts each have a different saturation point. The smaller transformer may be able to transfer 1KW to the primary and the larger transformer can transfer 2KW. Both transformers would start to over heat after about 3 minutes. I used a 5KW transformer as my choke with several 250 watt hot water heating elements as the secondary winding load for 12KW on my large 10" TC that produces a 27 foot circle of sparks and arcs in my back yard.

I am not real sure why a transformer core size should be 2X or 3X larger than it really needs to be that just seems to add extra weight. I can see where 3X or 4X would be great in an Audio Transformer it will allow the peaks to transfer to the secondary coil.

A TV speaker should have a variable choke coil used as the volume control then when the advertisements come on 2X louder than the movie the choke will limit the volume = to what the movie sound level was.

 

[MrAl]

Hi,

Another interesting method is to apply a voltage to the primary or maybe even the secondary at the LOWEST desired frequency of operation and look for flat topping. When the flat topping starts to occur, the transformer voltage shouldnt go any higher in voltage than about 80 percent of that no matter what the wattage is. Now we know the voltage.
Looking at the primary (or secondary) wire gauge, we can estimate the max current using 500 or 600 circular mils per amp. Now we know the current.
Power is current times voltage, so we would know the max power the primary (or secondary) can handle and that tells us the power rating of the transformer.
This should work because the designer of a transformer would not want to use a wire gauge that is too much thicker than really required.

Note that the voltage test has to be done at the lowest DESIRED frequency of operation. This means even if the transformer was designed for 50Hz if we want to use it at 25Hz we have to test it at that frequency. This is because the max voltage is related to the max flux density which is related to the core cross sectional area, and that also relates to the frequency of operation.
This also means that we might get more out of it if we run it at a higher frequency as long as the core loss doesnt go up too much.

The window area is related to the power because we can only fit a certain amount of copper inside the window no matter how many turns we have.