First slide

Electrical machines and devices

Question

A step-down transformer is connected to 2400 volts line and 80 amperes of current is found to flow in output load. The ratio of the turns in primary and secondary coil is 20 : 1. If transformer efficiency is 100%, then the current flowing in primary coil will be

Moderate

A

1600 A
B

20 A
C

4 A
D

1.5 A

Solution

$\frac{N_{s}}{N_{p}} = \frac{V_{s}}{V_{p}} \Rightarrow \frac{1}{20} = \frac{V_{s}}{2400} \Rightarrow V_{s} = 120 V$
For 100% efficiency $V_{s} i_{s} = V_{p} i_{p}$
$\Rightarrow 120 \times 80 = 2400 i_{p} \Rightarrow i_{p} = 4 A$

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