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Question

A transformer of efficiency 90% draws an input power of 4kW. An electrical appliance connected across the secondary draws a current of 6A. The impedance of the device is


A

60Ω

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B

50Ω

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C

80Ω

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D

100Ω

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E

120Ω

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Solution

The correct option is D

100Ω


Step1: Given data and assumptions.

Input power, Pin=4kW=4000W

Secondary current, Is=6A

Efficiency, η=90%

Step1: Find the output power.

Formula used:

η=PoutPin

Where, η is the efficiency, Pout is the output power, Pin is the input power.

Now,

90%=Pout4000

90100=Pout4000

Pout=3600W

Step3: Find the impedance of the device.

We know that,

Pout=Zi2

Where Z is the impedance and i is the current.

Now,

Z=Pouti2

Z=360062

Z=100Ω

Hence, option D is correct.


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