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UPRVUNL’21 EE: Daily Practices Quiz 22-July-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.

Q1. In DC motor starters are used
(A) To limit the starting current
(B) To increase the starting torque
(a) Only A
(b) Only B
(c) Both A and B
(d) None of the above

Q2. Four-point starter is used when ______________
(a) Motor field current is varied in narrow range
(b) Motor speed is varied in small range
(c) Motor field current is varied over wide range
(d) Can be used anywhere

Q3. A 3-phase induction motor runs at 3% slip develops mechanical power equal to 10 kW. The air gap power is
(a) 11 kW
(b) 10.03 kW
(c) 9.7 kW
(d) (10/0.97)kW

Q4. In 3-phase induction motor, the maximum torque is:
(a) Proportional to rotor resistance
(b) Proportional to square root of rotor resistance
(c) Independent of rotor resistance
(d) Proportional to square of rotor resistance

Q5. Corona loss can be reduced by the used of hollow conductors because,
(a) The current density is reduced
(b) Eddy current in the conductor is eliminated
(c) For a given cross-section, the radius of the conductor increases
(d) Of better ventilation in the conductor

Q6. Which of the following theorems can be applied to any network linear or non-linear, active or passive, time variant or time invariant?
(a) Norton theorem
(b) Thevenin theorem
(c) Tellegen theorem
(d) Superposition theorem

SOLUTIONS
S1. Ans.(a)
Sol. If DC motor started without starter, then:
a large current flow through the armature during starting. This current is large enough to damage the armature circuit.
very high starting torque will be produced (as torque is directly proportional to the armature current), and this high starting torque may cause huge centrifugal force which may throw off the armature winding.
To limit the starting current and torque, starter is used.

S2. Ans.(c)
Sol. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.

S3. Ans.(d)
Sol. Given that: slip(s)=3%=0.03
And P_out=10 kW
∵P_out=(1-s) P_g
Where; P_g=air gap power
∴P_g=P_out/((1-s))=10/((1-0.03))=10/0.97 kW

S4. Ans.(c)
Sol. Maximum value of slip at maximum torque is given as: s=R_2/X_2
And maximum torque: T_(max )=(KE_2^2)/(2X_2 )
From above expression it is clear that the maximum value of slip depends on rotor resistance but the maximum torque is independent of rotor resistance.

S5. Ans.(c)
Sol. Corona loss: P_c ⍺1/√d
So, by using hollow conductor d increases and P_c decreases.

S6. Ans.(c)
Sol. Tellegen theorem can be applied for linear, non-linear, active and passive, time variant and time invariant network.

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