formula for volts on a transformer

[lboogy]

Hi everyone,

 

I've come accross this on the bitesize revision pages and it doesn't make sense to me, I was hoping someone would be able to explain it, the link is :

http://www.bbc.co.uk/school...

 

my problem with is is that the formula for working out the relationship between the voltages on a transformer and the number of turns on a coil is that they seem to change the formula round. First of all it is:

 

Vp/Vs = Np/Ns

 

then they write it as:

 

Vs/Vp = Ns/Np

 

which obviously isn't the same thing unless I'm missing something! They then add in a multiplication when there isn't even one in the formula!!! This is what the question is:

 

"A transformer has 20 turns on the primary and 400 on the secondary. What is the output voltage if the input voltage is 500V?"

 

So following the first formula, I would have thought that this would be: 500/Vs = 20/400, but they do:

 

Vs/Vp = Ns/Np

Vs/500 = 400/20

Vs = 500 x (400/20)

Vs= 10,000 Volts

 

I don't understand that if the formula is Vp/Vs=Np/Ns they can then turn it around completely to Vs/Vp=Ns/Np, and I also don't understnad how they can put a multiplied by 500 in as there is no multiplication in the original formula. Could anyone shed any light on this for me?

 

I hope someone can help!!!!

[Klaynos]

The fomulas are teh same, just reordered:

 

[math]\frac {V_s}{V_p} = \frac {N_s}{N_p}[/math]

 

[math] {N_p} \frac {V_s}{V_p} = {N_s}[/math]

 

[math] {N_p} {V_s} = {N_s}{V_p}[/math]

 

[math] \frac {N_p} {N_s} {V_s} = {V_p}[/math]

 

[math] \frac {N_p} {N_s} = \frac {V_p} {V_s} [/math]

 

If you times the whole thing through by 500, the /500 on the left = 1 so can be ignores so you are left with *500 on the rhs.

[YT2095]

transformers work in Both directions

and yes your calc at 10,000v is correct

 

without going into detail it`s basic ratios in the 400:20 ratio we reduce this to 20:1

 

if that `1` is 500 then reverse the ratio and multiply it by the 20.

[swansont]

I don't understand that if the formula is Vp/Vs=Np/Ns they can then turn it around completely to Vs/Vp=Ns/Np, and I also don't understnad how they can put a multiplied by 500 in as there is no multiplication in the original formula. Could anyone shed any light on this for me?

 

 

This is basic algebra. If it's causing you problems you need to refresh yourself on it. Physics is going to be mighty difficult to do otherwise.

[imp]

Hi everyone,

 

"A transformer has 20 turns on the primary and 400 on the secondary. What is the output voltage if the input voltage is 500V?

 

I hope someone can help!!!!

 

My simpleton mind looks at it this way: First, if primary turns are less than secondary turns, primary voltage must be LESS than secondary voltage. So, if primary voltage is 500V, secondary must be MORE than 500V. How much more? Secondary has 20 times as many turns as primary, (400/20) = 20, so secondary voltage must be 500V. X 20, or 10,000V.

 

Study the method. Then go back to your Algebra I textbook, and see about becoming more proficient in Algebra. EVERYTHING you do in Science and Math. in the future will be VERY VERY dependent on being slick with Algebra!

Believe me! imp