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NHPC-JE’21 EE: Daily Practices Quiz 14-Sep-2021
Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.
Q1. For harnessing low variable water heads, the suitable hydraulic turbine with reaction and adjustable vanes runners is
(a) Impeller
(b) Pelton
(c) Kaplan
(d) Francis
Q2. The term ‘surge Tank’ is associated with which type of power plant?
(a) Low head hydro
(b) High head hydro
(c) Thermal
(d) Medium head hydro
Q3. In which of the following conditions will a 3-phase synchronous machine works as a capacitor?
(a) Over excited
(b) Under excited
(c) Critically excited
(d) None of the above
Q4. A 4-pole, 50 Hz induction motor operates at 5 % slip. The frequency of emf induced in the rotor will be
(a) 50 Hz
(b) 25 Hz
(c) 2 Hz
(d) 2.5 Hz
Q5. Slip of an induction motor ……………. when the rotor starts gaining speed.
(a) Increases
(b) Decreases
(c) Remains same
(d) Increase or decrease
Q6. Current chopping usually occurs in
(a) Oil circuit breaker
(b) Minimum OCB
(c) MCB
(d) Air blast circuit breaker
SOLUTIONS
S1. Ans.(c)
Sol. Kaplan turbine is a low head axial turbine with adjustable vane.
NOTE:
Kaplan & Francis: Reaction turbine
Pelton: Impulse turbine
S2. Ans.(b)
Sol. Surge tanks are applied in hydropower plants with long water conduits to reduce pressure forces during the acceleration of the large water masses.
S3. Ans.(a)
Sol. Over excited synchronous machine behaves as capacitor and provide reactive power.
S4. Ans.(d)
Sol. The frequency of emf induced in the rotor: f_r=sf=0.05×50=2.5 Hz
S5. Ans.(b)
Sol. Slip(s)=(N_s-N_r)/N_s
So, when the rotor starts gaining speed, due to increase of rotor speed, slip of an induction
motor decreases.
S6. Ans.(d)
Sol. Current chopping usually occurs in Air blast circuit breaker.
Current Chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero.
