# Homework Help: Transformer and power question mutiple choice

1. Jul 5, 2016

### Clara Chung

1. The problem statement, all variables and given/known data
The turns ratio of a transformer is 20:1 step down and the primary coil is connected to a 220V a.c. supply. If the secondary coil is connected to a '36W, 15V' light bulb and the efficiency of the transformer is 80%,find the current in the primary coil.

2. Relevant equations
P=IV, Ns/Np=Ip/Is

3. The attempt at a solution
I calculate by 36/0.8 = 45 W, 45/220 = 0.204A, which is the answer of D, but the correct answer is B, which is 0.11A, Am I wrong or the is the question wrong?

2. Jul 5, 2016

### cnh1995

36W is the power rating of the bulb at 15V. The bulb is not getting 15V in this situation, so the power is not 36W. What is the actual operating voltage and resistance of the bulb?

Last edited: Jul 5, 2016
3. Jul 5, 2016

### Clara Chung

That's all the information given. I think the resistance of the bulb can be found by 15^2 / 36= 6.25 ohm

4. Jul 5, 2016

### cnh1995

Right. Now what's the voltage across the bulb?

5. Jul 5, 2016

### Clara Chung

Voltage 220/20 = 11V, but isn't there energy lost?

6. Jul 5, 2016

### cnh1995

Yes. That will reduce the secondary voltage. But I think this is how it will be resolved..
If the secondary is kept open and you applied 220V across the primary, you'll get 11V across the secondary, which is the maximum OC voltage, or secondary induced emf E2. Now, if the secondary is loaded, the load voltage will drop. In practice, load voltage should be equal to the secondary induced voltage. A tap changer is used in the primary which will reduce the number of primary turns and hence, secondary induced emf will be increased such that load voltage remains 11V.

7. Jul 5, 2016

### Clara Chung

thanks a lot with those extra information!!!DDD now I get the answer by 11^2 / 6.25 =19.4 W, 19.4/0.8 =24.2 W ,24.2/220 = 0.11A :D

8. Jul 5, 2016

### cnh1995

I'm not sure if my above reasoning is true for such a small system but in large power systems, this is how the load voltage is stabilized. Maybe this transformer has no copper loss and only iron loss is present. So this will not cause any voltage drops and you'll get 11V across the load.