Previous Year Questions- Three-Phase Circuits | Network Theory (Electric Circuits) - Electrical Engineering (EE) PDF Download

Q1: For a two-phase network, the phase voltages V_p and V_q are to be expressed in terms of sequence voltages 

The possible option(s) for matrix S is/are      (2024)
(a)
(b)
(c)
(d)
Ans: (a, d)

Q2: In the circuit shown below, a three-phase star-connected unbalanced load is connected to a balanced three-phase supply of 100√3 with phase sequence  ABC. The star connected load has Z_A = 10Ω and Z_B = 20∠60°. The value of Z_C in Ω, for which the voltage difference across the nodes n and  n′ is zero, is       (2022)
(a) 20∠−30°
(b) 20∠30°
(c) 20∠−60°
(d) 20∠60 °
Ans: (c)
Sol: Given: n & n' are at same potential, therefore,

Q3: A three-phase balanced voltage is applied to the load shown. The phase sequence is RYB. The ratio  is ____________.        (2021)
(a) 1
(b) 2
(c) 3
(d) 4
Ans: (a)
Sol: and ∴

Q4: In the given network  The phasor current i (in Ampere) is      (SET-2  (2015))
(a)$173.2\angle -60\circ$ 173.2 ∠ −60°
(b) 173.2 ∠ 120°
(c) $100.0\angle -60\circ$100.0  ∠ −60°
(d) $100.0\angle 120\circ$100.0 ∠ 120^∘
Ans: (a)
Sol: ′i′ is taken as input,  
Apply KCL at node
Node voltage is 100 ∠ 120^∘

Q5: The line A to neutral voltage is 10 ∠ 15°V for a balanced three phase star connected load with phase sequence ABC . The voltage of line B with respect to line C is given by       (SET-3 (2014))
(a)
(b)
(c)
(d)
Ans: (c)
Sol: Given,
As the system is balanced and phase sequence is ABC, therefore,
∴ voltage of line w.r.t. line C is

Q6: A two-phase load draws the following phase currents :  These currents are balanced if ϕ₁ is equal to.       (2012)
(a) -ϕ₂
(b) ϕ₂
(c) $(𝜋/2-𝜙2)$(π/2−ϕ₂)
(d) (π/2+ϕ₂)
Ans: (d)
Sol: In two phase, current are balanced if phase difference is 90^∘.
Phase difference is 90^∘,

Q7:  For the three-phase circuit shown in the figure, the ratio of the currents I_R :  I_Y : I_B is given by      (2005)
(a) 1:1: √3
(b) 1:1:2
(c) 1:1:0
(d) 1:1:√3/2
Ans: (a)
Sol: Assuming phase sequence to be RYB
Taking V_RY as the reference,
Using KCL,