Transformer Question?

Question

A current of 300 A is required from a 240 V, 15 A supply. A transformer with 200 primary turns in available. How many turns are on the secondary.

I got an answer of 4000, but the book gives me an answer of 10, who is right? I'm assuming it's an ideal transformer.

Thanks

Answer 1

Power is W=VI=240*15=3600 W.

Current required is 300 A=> V=W/I=3600/300=12 V output required.

Ratio turns 200:secondary = ratio of voltages = 240:12

Hence multiply ratios by 200/240

200:secondary=200:12*200/240=200:10

Hence 10 turns on secondary coil.

Answer 2

assuming zero loss, the LOWER the voltage, the more current is required.

300 / 15 gives a ratio of 20:1

200/20 =10 so 10 turns will result in 300 AMPS (at 12 volts).

you were probably thinking about VOLTAGE and multiplied.

you can cross check your answer by looking at WATTS.

input side 240 volts * 15 amps = 3600 watts

output side 12 volts * 300 amps = 3600 watts

Answer 3

We need more current in secondary and hence we need to step down the voltage. So, our step down ratio is given by

primary voltage / secondary voltage = secondary current / primary current = 300 / 15 = 20

So, number of turns in secondary = No. of turns in primary / step down ratio = 200 / 20 = 10 .

The book is right and what you did is multiply with the ratio instead of dividing.

Remember, if the secondary has more turns than the primary, the secondary voltage will be more than the primary and so the current has to be lesser. Conversely if we need higher current in secondary, we need to step down the voltage and that means fewer turns in secondary side.

Such step down transformers are used in welding industry.