Question

A transformer having efficiency of 90% is working on $200\, V$ and $3 \,kW$ power supply. If the current in the secondary coil is $6\, A$, the voltage across the secondary coil and the current in the primary coil respectively are

• A. 300 V, 15 A
• B. 450 V, 15 A
• C. 450 V, 13.5 A
• D. 600 V, 15 A

Answer 1

Answer: (B)

450 V, 15 A

Answer 2

Here,
Efficiency of the transformer, $\eta=90\%$
Input power,$P_{in}=3 \, kW=3\times10^3 \, W=3000 \, W$
Voltage across the primary coil,$V_p=200 \, V$
Current in the secondary coil,$I_s=6 \, A$
As $P_{in}=I_pV_p$
$\therefore$ Current in the primary coil.
$I_p=\frac{P_{in}}{V_p}=\frac{3000\, W}{200 \, V}=15 \, A$
Efficiency of the transformer,
$\eta=\frac{P_{out}}{P_{in}}=\frac{V_sI_s}{V_pI_p}$
$\therefore \frac{90}{100}=\frac{6V_s}{3000} \, or \, V_s=\frac{90\times3000}{100\times6}=450\, V$