Step up and step down transformers

[thomas49th]

Hi, am I right in thinking that step up transformers are used to step up the voltage just before they are sent along the power lines to have a low current, which reduces loss of energy by heat. HOWEVER if you think of the equation Voltage = current x resistance, wouldn't the current have to increase if the voltage decreases (unless the restiance suddenly shot up)?
I don't quite see how it can be?

Thanks

 

[marcusl]

Your question is confusing.

Hi, am I right in thinking that step up transformers are used to step up the voltage just before they are sent along the power lines to have a low current, which reduces loss of energy by heat.

This is correct.

HOWEVER if you think of the equation Voltage = current x resistance, wouldn't the current have to increase if the voltage decreases (unless the restiance suddenly shot up)?
I don't quite see how it can be?

This is the opposite of what you just asked, and of what the utility companies do. Current goes down when voltage is stepped up, so smaller lighter wires can be used. Cross country high tension lines (big metal towers) operate up near 500kV for this reason. Lines on telephone poles operate in the 7 to 30 kV range; the cylindrical transformers you see on the poles then step this down to 220 and 110V for home use.

 

[thomas49th]

but doesn't that disagree with the equation

V=current x resistance

 

[marcusl]

What is your question again?

 

[thomas49th]

I'm asking why does the equation voltage = current x restiance not agree with the fact that when voltage is stepped up after leaving power stations the current goes down. Does this mean that the resistance goes up? If current increases, volts increases and vice versa... so howcome when voltage is stepped up current doesn't go up?

Thanks

 

[marcusl]

Ah, now I understand your question. Customers' power usage remains the same, regardless of the voltage the utility uses on the long distance lines, so the load resistance presented to the line changes depending on the voltage chosen. The transformer on the pole that steps down the line's voltage for household use also steps up the apparent house load resistance, as seen by the line.

 

[iceman_ch]

When the power company is stepping up or stepping down a transformer they aren't necessarily concerned about current. The current goes down making it so that they can use a smaller wire which cost less. The real reason for stepping down is to overcome voltage drop. If you look at the current as a constant because there is a breaker that will pop if the current goes over a certain amount of amps. Then resistance stays the same but the voltage dropped due to the resistance in the wire is less depending on the voltage you use. For example say you have a wire that is 200 feet long and has .06 ohms of resistance on it. The breaker is at 20 amps so at 120 V you would loose 1.2 V which is only 1 percent of the total voltage. Say you pumped that up to 240 V then you would still loose 1.2V but it's only .5 percent. Of course there are other things that go into all of this like the size of wire and the load requirement but, this is the basic part of it. The reason voltage drop is important is because the more voltage drop you have the more current is going to be required by your load. I hope that this explanation was helpful.