primary current is ?

[searley]

the voltage supplied to the primary winding of an ideal step down transformer is 1000v and the output voltage is 400v.if the secondary current is 20A the value of the primary current will be ?

the answer is 8 but how do i get this ?

 

[a1]

Below, Is a better way to look at it...

Take a look here, as I have put a lot of Formulas together on my website.

http://www.electricianforum.co.uk/electricalformulas.html

 

[Apache]

the voltage supplied to the primary winding of an ideal step down transformer is 1000v and the output voltage is 400v.if the secondary current is 20A the value of the primary current will be ?the answer is 8 but how do i get this ?

If you look at Admin's formula you will notice that the current is the other way arround to number of turns and voltage.

So if in a transformer voltage goes up, current goes down - an inverse relationship.

If you think of it in terms of 'power' then P=VI

Since power is a function of current and voltage (assuming no losses):

so 1000 X 8 = 8,000W! and 400 X 20 = 8,000W!

So it's the same

Think of the national grid.............

 

[searley]

all these * in formulas will i ever use all this sh-te when qualified ?

 

[seajayess]

The best way to do this is (power in = power out) so secondary power 400x20=8000 Watts so power in is 8000 Watts 8000/1000=8 Amps hope this helps CJS

 

[oliver1234]

all these * in formulas will i ever use all this sh-te when qualified ?

I doubt it and when you do you will have forgotten it anyway.

It's useful to learn it now though because if you ever do need it you will be able to re-learn it quickly.

 

[Andy™]

Once you know how to work them out, you will be able to do them in future. you wont remember the formula for everything, but a quick google will help you (most of your questions get answered after a quick search on the net for the formula...)

 

[green-hornet]

all these * in formulas will i ever use all this sh-te when qualified ?

Depends on how far you want to go, and how much you want to upset the design engineer when you calculate his findings are wrong.

Remember most of what you learn appears to be irrelevent, until you need it!

Then that knowledge sets you above the 5 day wonders, who have no idea what the F==== your talking about.

Stick at it and you will reap the rewards later.

 

[ericmark]

I will give an example where I have used it.

Site with loads of 110 volt cables mainly 1.5mm

 

[steptoe]

makes the difference between being a house basher/5week wonder and actually being a proper spark.

stick at it, you will see the difference it makes in a few years time.!

good luck,