Transformer question

[Radioceo]

You have a transformer with 240 volts on the primary with 10 volts on the secondary with a load that draws 10 amps. The transformer secondary is maximum rated for 12 amps at 10 volts. You reduce the primary voltage to 120 volts to give 5 volts on the secondary and you replace the load with a device drawing 15 amps at 5 volts. Is the transformer happy or unhappy with it's new job?

 

[HowardMBurgers]

Depending on the transformer if you reduce the voltage by half, the amount of current through the transformer will probably exceed the design limits, especially if you reduce the primary voltage by half but only reduce the current draw at the secondary by 1/3.

 

[ChiefOperator]

Most likely it's unhappy. The secondary is rated at 12amps; you're exceeding that rating regardless of the voltage differences. The windings on the secondary side *may* overheat based on the winding's rating, especially if it's continuously loaded at 15amps.

Second, the 12amp rating of the transformer does not mean that it should loaded to 12amps continuously. The continuous (3hrs or more) load should be no more that 80% of its rated value.

 

[RadeoEngineer]

It's not unhappy, it's unconscious.

 

[LA_Guy]

Wire size is determined by the CURRENT flowing through the wire, not the voltage. Wire rated for 12 amps intermittent duty is probably 20 guage. For 15 amps the wire size should be 18 guage.

This is why high tension wires run such high voltages. Work is measured in WATTS which is volts multipled by amps. 20 volts at 1 amp produces 20 watts of power. So does 10 volts at 2 amps-or 100 volts at .2 amps.

BUT the 100 volt transformer only needs wire that can carry .2 amps, (30 guage or less) while the 10 volt one needs 22 guage. Since every time you reduce the guage by 2 numbers it gets twice as thick, you'll be using wire that's 16 times thicker then the 100 volt wire. for the 10 volt one.

On the other side, a million watts can be made by running a half million volts at 2 amps or running 10 volts at 10,000 amps. Now you understand why interstate high tension lines are usually a half million volts or more.