# Homework Help: Load resistance, dissipated power, transformers. (AC)

1. Apr 7, 2013

### evol_w10lv

1. The problem statement, all variables and given/known data
I have got this scheme:

Alternating voltage V = 120 V,
Total resistance of wires Rw = 0.6 Ω
Current is 100 A, ratio 1:5 and 5:1 (as we can see it in the scheme)

Have to calculate dissipated power in the load (Pl), dissipated power in the wires (Pw), total dissipated power (Pt) and voltage in load (Vl).

2. Relevant equations
Vl = I * Rl
Pl = (5*I)2 * Rl \\5 - because using given ratio
Pw = Iw2 * Rw = 0.6 * Iw2 \\Iw - current true the wires

3. The attempt at a solution
I'm trying to use this equation Pt= V*I = Pw + Pl= 0.6 * Iw2 + (5*I)2 * Rl
But i have got lack of understanding, how exactly is current true the wires Iw, total current It and I guess current in the load is Il = 100A?

2. Apr 7, 2013

### rude man

First, what does the load reistance R_L look like at the input winding of the output transformer?

Last edited: Apr 7, 2013
3. Apr 7, 2013

### evol_w10lv

I don't know how to calculate the value of the load resistance reflected back through the transformer.

4. Apr 7, 2013

### rude man

Then look it up! It's a very simple and extremely important formula based on power out = power in.

5. Apr 7, 2013

### evol_w10lv

I guess, you are speaking about ideal transformer formula P in = P out then first transformer's I1*V1 is the same as second transformer's I2 * V2, but there are wire resistance between them.

6. Apr 7, 2013

### rude man

Assume the wire resistance is all in the secondary winding of the input transformer.

So if the output transformer secondary winding load is R, what is the impedance looking into the primary winding of that transformer?