Current and voltage relationship

[costanza]

Just a little help if anybody wouldn't mind. I'm currently struggling with a theory.

I was taught that in transmission the greater the voltage, the smaller the current. Hence the high voltage used.

What I can't understand is how this rule applies when transforming a 230v supply to a 110v for use on a building site etc. Surely that would increase the current and make things worse?!

 

[sjm]

For transmission you are talking about transmitting power ie watts, therefore watts=current x voltage. As you say the higher the voltage the lower the current required to transmit the same power.
On a building site 110volts is safer because current=voltage/resistance. Therefore the resistance of a body would draw slightly less than half the current at 110volts as it would draw at 230volts.

Hope this helps.

 

[Thegennyman]

To answer your question regarding building site transformers, voltage and current relationship Formula for transformer voltage and current relationship is

V1/V2 = I2/I1 VI = 220 VOLTS. V2 = 110 VOLTS LETS SAY I1 = 5 AMPS THEN

I2 = VI X I1/V2 = 220 X 5 / 110 I2 THEN = 10 AMPS

half the voltage you double the current. I work on high voltage transmission up to 400kv A 400kv transmission line can handle 3500 amps, operating at 3 phase.
The line can then carry 2400 Megawatts

 

[olly w]

Building site transformers are also centre tapped. So the voltage from live to earth (or neutral to earth ) would only be 55v.

 

[telectrix]

Building site transformers are also centre tapped. So the voltage from live to earth (or neutral to earth ) would only be 55v.

spot on olly. 110v between L&E would be too great for safety on site. the centre tap on the secondary winding is earthed, so you have 55-0-55. and costanza, the current is higher at 110v, but that is not the issue. reducing the voltage is purely a safety issue.

 

[costanza]

Thanks for your help guys.

However there are two differing opinions here if I'm not mistaken.

So current = voltage/resistance...that would make the current smaller in a 110v than a 230v system.

 

[sjm]

In a 110v system the current required to supply the same power would be higher because
power(watts)=current x voltage. However if you have a constant resistance, such as a human body, and you put a voltage across it the current it draws is less as the voltage decreases because current=voltage/resistance.
I hope this explains why 110v, or as the others have pointed out 55v-0v-55v, is safer if you were to have a shock from it.

 

[telectrix]

Thanks for your help guys.

However there are two differing opinions here if I'm not mistaken.

So current = voltage/resistance...that would make the current smaller in a 110v than a 230v system.

no. for the same amount of power (watts). watts = voltsxcurrent, so if you halve the volts, you double the current.

 

[costanza]

Right, sorted. Thanks a lot for your help. You've explained it better between you than my 3 lecturers at college

 

[telectrix]

there are lecturers and lecturers, some good, some mediocre.

 

[Sparky Ninja]

At the college where I lecture part time there are 5 full time lecturers.
1-Only does part time courses
1- Is NVQ EV and so has no time to teach
1-Is an absolute dinosaur who hasn't seen a building site in 40 years
1-Is an electronics obsessive
1-Is actually a photocopier repair man by trade

It's factors like this that make my students prefer and request me the following year.