The efficiency of a transformer

[Asmaa Mohammad]

Homework Statement

A step-down transformer has an efficiency of 90%, the voltage of it primary coil is 200V and its current is 0.5A, and its secondary coil has a voltage of 9V and a current of 10A and its number of turns is 90 turns.
Calculate the number of turns of the primary coil.

Homework Equations

η =VsIs/VpIp * 100

Vs/Vp = Ns/Np

Is/Ip = Np/Ns

The Attempt at a Solution

I solved the problem in two slightly different methods, the first one:

And the second one:

The solution in the first method (Np = 1800) is the correct one, but I don't understand why the second one is wrong. I thought that both will lead me to the same answer, but apparently they didn't, could you explain it for me, please?

 

[cnh1995]

The transformer is not ideal.
Hence,

Vs/Vp = Ns/Np

this is not true.

 

[Asmaa Mohammad]

The transformer is not ideal.
Hence,

this is not true.

OK, but why (Is/Ip = Np/Ns) is true?

 

[cnh1995]

OK, but why (Is/Ip = Np/Ns) is true?

It is because of the mmf balance in the core. (Magetizing current is neglected as it is very small compared to the load current).

 

[Asmaa Mohammad]

It is because of the mmf balance in the core. (Magetizing current is neglected as it is very small compared to the load current).

I didn't know about the MMF balance in my textbook, but I googled it and understood it. Many thanks!

 

[cnh1995]

I didn't know about the MMF balance in my textbook, but I googled it and understood it.

Good!

Many thanks!

Thank google!