Two Mark Question and Answers set - 4

76. What is resonance ?

Ans: An A.C. circuit comprising of R, L and C is said to be in resonance when the applied voltage and the source current are in phase. Thus, at resonance the power factor of the circuit is unity and the circuit acts as purely resistive.

77. Write the expressions for the instantaneous emfs in a 3 phase circuit.

Ans:

eR = Em Sin ωt,  eY = Em sin (ωt – 2II/3), and

еB = Em Sin (ωt+2II/3)

78. What is phase sequence?

Ans: The phase sequence of the alternating quantities is the order in which they attain their maximum values.

79. Name the two types of 3 phase connections. In each case give the relation between line value and phase value for both voltage and current.

Ans: There are two types of 3 phase connections, namely star connection and delta connection.

80. In a 3-phase circuit, what do you mean by balanced load?

Ans: When the loads in all the phases are identical,it is called balanced load.

81. When is a 3-phase supply system called balanced supply system?

Ans: When all the 3-phase voltages are equal in magnitude and displaced by 120° in space, the supply system is called 3-phase balanced system.

82. List out 3 advantages of 3-phase system over 1-phase system.

Ans: (a) For the same size of an ac generator, output of 3-phase generator is double that of 1- phase generator. ES

(b) 3-Phase motors are self-starting, whereas the 1-phase induction motors are not. It is due to the production of rotating magnetic field in 3-phase supply and only pulsating torque in case of 1- phase supply.

(c) Fault location is easier in 3-phase motor than that in 1-phase motor.

83. In 2 wattmeter method of 3-phase power measurement, one of the meters gave reading after reversal of its current coil connection. What do you infer from this?

Ans: The power factor is definitely less than 0.5

84. In 2 wattmeter method, what do you infer about the power factor when one wattmeter shows zero reading? (Remember that meters are faultless)

Ans: The power factor is definitely 0.5

85. Give the relation connecting the power factor angle with two wattmeter readings.

Ans:

86. What will be the readings of the two wattmeters used for measurement of power in a three- phase circuit at unity P.F. ?

Ans:

i.e., Both wattmeter readings are equal to each other and each will read half the total power.

87. Compare balanced and unbalanced network.

Ans: Let the three-phase circuit consist of loads Z1, Z2 and Z3. If all the loads are equal both in magnitude and phase angle and connected to a balanced supply system, it is called a balanced network.

If all the loads are different, it is called unbalanced network, even when the supply system is balanced.

e.g., for balanced load Z1 = Z2 = Z3 = 10 ∠ 30° Ω

for unbalanced load Z1 = Z2 = Z3 = 10 ∠ 30° and Z3 = 15-30°

88. A balanced star-connected load of (3j4) Ω impedance is connected to 400 volt 3-phase supply. What is the real power consumed by the load?

Ans: For star system, unless specified, the given voltage is taken as line voltage.

89. Explain as how to solve unbalanced neutral isolated three-phase load connected to a balanced supply.

Ans: Step 1: Obtain the unbalanced delta connected load for the given star-connected unbalanced load.

Step 2: Assume the phase sequence to be RYB. For each phase, find the phase current by applying Ohm's law.

Step 3: Calculate the line currents by applying KCL.

90. The two line currents taken by an unbalanced delta connected load are:

I_ɑ = 10 ∠ -120° amp.

I_b = 5 ∠ 150° amp.

What is the line current I_c?

Ans :

91. Explain how the power factor of a balanced three-phase load can be determined by two- wattmeter method.

Ans: The current coils of the wattmeters are connected in two lines. The potential coils are connected to the third line in which current coils are not connected. The three-phase supply is given and wattmeter readings W, and W, are noted. Assume that W₂ > W₁. Then,

tan ϕ = √3 (W₂ - W₁) / (W₁ + W₂ )

From this ϕ is calculated.

Then power factor = cos[tan^-1 √3 (W₂ - W₁) / W₁ + W₂ ]

92. Draw the circuit for two-wattmeter method of measurement of three-phase power.

Ans:

Note: If the load is delta-connected, connect the same load between ABC by removing the star- connected load

93. Write the relations between phase and line qualities in a delta-connected balanced load.

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94. Write the expressions for the power factor in balanced three-phase circuit.

Ans :

Power factor cos ϕ = 

95. A three-phase balanced wye-connected load has 400 V line to line voltage and 10 amperes line current. Determine the line to neutral voltage and phase current.

Ans:

Phase voltage = line voltage / √3

= 400 / √3

= 231 volts

Phase current = line current  10 amperes

96. Draw the characteristic curve of wattmeter ratio and power factor in a three-phase balanced system.

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97. Write the expression for the total power in a three-phase balanced circuit defining each quantity.

Ans :

Total average power = √3 | V_L | I_L |  ϕ

where V_L = line voltage

I_L = line current

cos ϕ = power factor

ϕ = Angle between phase voltage and phase current.

98. State the relation between the line and phase quantities of a balanced three-phase wye connected system.

Ans :

Line voltage = √3 × phase voltage

i.e., = | V_L| = √3 | Vϕ |

Line current = Phase current

| I_L | = | Iϕ |

99. Write the expression for the Wattmeter readings connected to measure the total power in a three-phase balanced circuit.

Ans:

W₁  = | E_L | | I_L | cos (30 + ϕ )

W₂  = | E_L | | I_L | cos (30 - ϕ )

100. Draw the phasor diagram of phase and line voltages and currents in a delta-connected balanced load.

Ans: