SSC JE Electrical Engineering 24 Jan 2018 Morning Paper

Question : 150 of 199

Marks: +1, -0

What will be the reading (in kW) of both the wattmeter used to measure the power of a three–phase three wire system having input of 7 kW and power factor 1?

4, 3
3, 4
3.5, 3.5
4.5, 2.5

Validate

Solution:

For 3ϕ wire system using two wattmeter method.

w=w_1+w_2

and

ϕ=\tan^{-1}({√3(w_1-w_2)}/{w_1+w_2})

Now cos ϕ = 1 and ⇒ ϕ = 0

∴0=\tan^{-1}({√3(w_1-w_2)}/{w_1+w_2})

⇒0={√3(w_1-w_2)}

⇒w_1=w_2

Now

w_1 + w_2 = 7

∴2w_1 = 7

∴w_1=7/2=3.5\kw=w_2

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