Test: Poly Phase AC Circuit - 1


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Test: Poly Phase AC Circuit - 1 - Question 1

Which of the following is not true for a three-phase system compared to 1-φ system?

Test: Poly Phase AC Circuit - 1 - Question 2

In a balanced three-phase star connected system, the phase voltages

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 2

Here, line-to-line voltage leads the respective phase voltage by 30°.

Test: Poly Phase AC Circuit - 1 - Question 3

Three identical resistances are connected in delta against a balanced three-phase voltage supply. If one of the resistances be removed, the reduction in the power will be

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 3

When all the resistance are present,

(Since Vph= VL for Δ connection)
When one of the phase resistance is removed,

Test: Poly Phase AC Circuit - 1 - Question 4

 In a delta connected system, the line current are

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 4

Test: Poly Phase AC Circuit - 1 - Question 5

A balanced Y-connected load is supplied from a balanced 3-phase 400 V system. If the current in each phase is 15 A and lags the phase voltage by 30°, the total power will be

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 5

Test: Poly Phase AC Circuit - 1 - Question 6

 In a balanced three-phase circuit VAN= 1500 ∠20° V and VCN = 1500 ∠-100° V.

The value of VBN will be

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 6

Since VCN lags VAN by 120°, therefore phase sequence will be ACB.
Here, VBN will lead VAN  by 120° but, will have the same magnitude.

so,

Test: Poly Phase AC Circuit - 1 - Question 7

 The value of the rms line voltage VL for the below circuit if the rms line voltage is 100 V will be

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 7

Converting the Δ-connected load in star equivalent, we have
 

So, line to neutral voltage for equivalent Y-load 

∴ Line current,

∴ Line voltage at source 

Test: Poly Phase AC Circuit - 1 - Question 8

Two wattmeters, both have reading of 3 kW when connected for the two-wattmeter method with current coils in lines A and B of a 600 V ABC circuit having a balanced A load. The Δ phase impedance will be

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 8

we have

(Since W1 = W2 = 3 kW)
or, cosθ= 1

Hence, the load will be purely resistive.

Test: Poly Phase AC Circuit - 1 - Question 9

 Three capacitances, each of C/3 Farads are connected in delta. Their equivalent star value for each capacitance is

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 9

We know that,

Test: Poly Phase AC Circuit - 1 - Question 10

The instantaneous value of currents in both phases B and C of a 3-phase balanced system are -20 A. For a phase sequence of ABC, the instantaneous value of current in phase A is

Detailed Solution for Test: Poly Phase AC Circuit - 1 - Question 10

Since the system is balanced, therefore

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