JNTU
JNTU B.Tech II NETWORK THEORY Previous Question Paper (November/December - 2020 - R13)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
What is the difference between RYB and RBY phase sequence?
Answer
In a three-phase system, the phase sequence denotes the order in which the phase voltages reach their respective maximum positive values. For a standard RYB sequence, the order is R followed by Y, then B, with each phase separated by 120 degrees. In an RBY sequence, the order of the Y and B phases is reversed. This change in sequence reverses the direction of rotation of three-phase induction motors and affects the phase relationships in unbalanced loads, although it does not change the magnitude of the line voltages in a balanced system.
A balanced star connected three phase generator with phase voltage 150 V supplies an unbalanced star connected load with impedances (4+5j), (4-9j) and (3-6j). Determine phase currents.
Answer
Given a balanced star-connected supply with phase voltage Vp = 150 V, we define the phase voltages as:
The load impedances are ZR = 4+5j = 6.40∠51.34° Ω, ZY = 4-9j = 9.85∠-66.04° Ω, and ZB = 3-6j = 6.71∠-63.43° Ω. Since the load is star-connected without a neutral wire, the neutral point of the load (N') is shifted from the supply neutral (N). The displacement voltage VNN' is calculated as:
Calculating individual terms:
- VR/ZR = 150∠0° / 6.40∠51.34° = 23.44∠-51.34° = 14.63 - 18.29j
- VY/ZY = 150∠-120° / 9.85∠-66.04° = 15.23∠-53.96° = 8.97 - 12.31j
- VB/ZB = 150∠-240° / 6.71∠-63.43° = 22.35∠-176.57° = -22.29 - 1.36j
- Sum of currents = 1.31 - 31.96j = 31.99∠-87.65°
- Sum of admittances = (1/6.40∠51.34°) + (1/9.85∠-66.04°) + (1/6.71∠-63.43°) = (0.095 - 0.119j) + (0.041 + 0.092j) + (0.067 + 0.133j) = 0.203 + 0.106j = 0.229∠27.56°
VNN' = 31.99∠-87.65° / 0.229∠27.56° = 139.69∠-115.21° V. The phase currents are then determined by Iph = (Vph - VNN') / Zph:
- IR = (150∠0° - 139.69∠-115.21°) / 6.40∠51.34° = (150 - (-59.45 - 126.45j)) / 6.40∠51.34° = (209.45 + 126.45j) / 6.40∠51.34° = 244.64∠31.13° / 6.40∠51.34° = 38.23∠-20.21° A
- IY = (150∠-120° - 139.69∠-115.21°) / 9.85∠-66.04° = (-75 - 129.9j + 59.45 + 126.45j) / 9.85∠-66.04° = (-15.55 - 3.45j) / 9.85∠-66.04° = 15.93∠-167.49° / 9.85∠-66.04° = 1.62∠-101.45° A
- IB = (150∠-240° - 139.69∠-115.21°) / 6.71∠-63.43° = (-75 + 129.9j + 59.45 + 126.45j) / 6.71∠-63.43° = (-15.55 + 256.35j) / 6.71∠-63.43° = 256.82∠93.48° / 6.71∠-63.43° = 38.27∠156.91° A
What is the relationship between power consumed by balanced star and balanced delta connected loads if the phase impedance is same in both the cases?
Answer
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A three phase three wire power supply is connected to a three phase star connected load with connecting wires. The load per phase is 5∠30° Ω and the current in the phase is given by 12 ∠0° A. If the source is operating with 0.9 power factor, determine the resistance of the connecting wire.
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Derive the expression for the voltage across the capacitor in a series RC circuit (R=5Ω, C=10µF) excited by a sinusoidal voltage of 100V, 50 Hz if the supply is connected at t = 0. Assume zero initial conditions.
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Derive the expression for the current in a series RLC circuit (R=10Ω, L=10 H, C=10µF) excited by a sinusoidal voltage of 100V, 50 Hz if the supply is connected at t = 0. Assume zero initial conditions.
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Comment on the time domain response of a second order system if the poles are complex conjugate having negative real parts.
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What are the necessary conditions for transfer functions? Explain.
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Comment on the time domain response of a second order system if the poles are unequal negative real values.
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What are the properties of driving point function? Explain.
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For the network shown in the figure, determine impedance parameters and using these parameters calculate hybrid parameters.
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What is the main properties of high pass filter?
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Calculate the coefficients in the Fourier series expansion of the following function: f(t) = 10 + 10cos(2t + 45°).
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