This set of Network Theory MCQs focuses on “Voltage, Current and Power in a Delta Connected System”.
1. In a delta connected system, the voltage across the terminals R and Y is 400∠0⁰. Calculate the line voltage VRY. Assume RYB phase sequence.
Explanation: In a balanced delta-connected system we know |VRY |= |VPh|, and it is displaced by 120⁰, therefore the line voltage VRY is VRY = 400∠0⁰V.
2. In the question 1 find the line voltage VYB.
Explanation: As |VYB |= |VPh|, and is displaced by 120⁰, therefore the line voltage VYB is VYB = 400∠-120⁰V. A balanced three phase, three wire, delta connected system is referred to as mesh connection because it forms a closed circuit.
3. In the question 1 find the line voltage VBR.
Explanation: We know, |VBR|= |VPh |, and is displaced by 120⁰, therefore the line voltage VBR is VBR = 400∠-240⁰V. Delta connection is so called because the three branches in the circuit can be arranged in the shape of delta.
4. In delta-connected system, the currents IR , IY , IB are equal in magnitude and they are displaced by _____ from one another.
Explanation: In delta-connected system, the currents IR , IY , IB are equal in magnitude and they are displaced by 120⁰ from one another. From the manner of interconnection of the three phases in the circuit, it may appear that the three phase are short circuited among themselves.
5. In a delta-connected system, the currents IR = IB = IY =?
Explanation: In a delta-connected system, the currents IR = IB = IY = IPh. Since the system is balanced, the sum of the three voltages round the closed mesh is zero; consequently no current can flow around the mesh when the terminals are open.
6. The relation between IL and IPh is in a delta connected system is?
a) IL = IPh
b) IL =√3 IPh
c) IL =3 IPh
d) IL = 3√3IPh
Explanation: The relation between IL and IPh is in a delta connected system is IL =√3 IPh. The arrows placed alongside the voltages of the three phases indicate that the terminals are positive during their positive half cycles.
7. The line currents are ___ behind respective phase currents in a delta connected system is?
Explanation: In a delta connected system, all the line currents are equal in magnitude but displaced by 120⁰ from one another and the line currents are 30⁰ behind the respective phase currents.
8. In a delta connected system, the expression of power (P) is?
a) VLILcosφ W
b) √3 VLILcosφ W
c) 3VLILcosφ W
d) 3√3VLILcosφ W
Explanation: The total power in the delta circuit is the sum of the powers in the three phases. In a delta connected system, the expression of power (P) is P = √3VLILcosφ W.
9. A balanced delta-connected load of (2+j3) Ω per phase is connected to a balanced three-phase 440V supply. The phase current is 10A. Find the total active power.
Explanation: ZPh = √(22+32 )=3.6∠56.3⁰Ω. cosφ = RPh /ZPh = 2/3.6 = 0.55. IL = √3× IPh = 17.32A. Active power = √3 VLILcosφ = √3×440×17.32×0.55= 7259.78W.
10. Find the apparent power in the information provided in the question 9.
a) 10955.67 VAR
b) 10.95567 VAR
c) 109.5567 VAR
d) 1.095567 VAR
Explanation: Sinφ = 0.83. Reactive power = √3 VLILsinφ. VL = 440V, IL = 17.32A. On substituting we get reactive power = √3 x 440 x 17.32 = 10955.67 VAR.
Sanfoundry Global Education & Learning Series – Network Theory.
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