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DFCCIL’21 EE: Daily Practices Quiz 15-July-2021

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

Q1. The capacitor used in single-phase capacitor motors have no
(a) Voltage rating
(b) Dielectric medium
(c) Polarity marking
(d) Definite value

Q2. The approximate efficiency of a 3-phase, 50 Hz, 4-pole induction motor running at 1350 rpm is….
(a) 90 %
(b) 65 %
(c) 40 %
(d) None of the above

Q3. In the depletion region of a PN junction, there is a shortage of
(a) Donor ions
(b) Acceptor’s ions
(c) Holes and electrons
(d) None of the above

Q4. A PN junction acts as a……….
(a) Unidirectional switch
(b) Bidirectional switch
(c) Controlled switch
(d) None of the above

Q5. The rotor of which motor does not have winding on it?
(a) Repulsion motor
(b) Universal motor
(c) Reluctance motor
(d) Hysteresis motor

Q6. The torque-slip characteristics of a polyphase induction motor becomes almost linear at small value of slip, because in this region of slips:
(a) The effective rotor resistance is very large compared to the rotor reactance.
(b) The effective rotor resistance is very small compared to the rotor reactance.
(c) The effective rotor resistance is equal to the rotor reactance.
(d) The effective rotor resistance is equal to the stator reactance.

SOLUTIONS
S1. Ans.(c)
Sol. The capacitor used in single-phase capacitor motors have no polarity marking.
The capacitor used is electrolytic and in series with an auxiliary winding for starting.
An electrolytic capacitor has no polarity marking.

S2. Ans.(a)
Sol. For 3-phase induction motor: efficiency=(1-s)=N/N_s
Where, s=slip
N= rotor speed
And N_s=synchronous speed
ACQ: N_s=(120 f)/P=(120×50)/4=1500 rpm
∴ efficiency=(1-s)=N_r/N_s =1350/1500×100=90 %

S3. Ans.(c)
Sol. Due to diffusion across the PN junction during its formation, the charge carries (free electrons and holes) are absent in the depletion region.
Depletion region of a p-n junction is formed due to the shortage of holes and electrons.

S4. Ans.(a)
Sol. A PN junction conducts current only when forward biased while no current conduction takes place when reversed biased. Therefore, a PN junction behaves as UNIDIRECTIONAL switch.

S5. Ans.(d)
Sol. A Hysteresis Motor is a synchronous motor with a uniform air gap and without DC excitation.
The rotor of the motor is made up of smooth chrome steel cylinder and it has no winding.

S6. Ans.(a)
Sol. The torque-slip characteristics of a polyphase induction motor becomes almost linear at small value of slip, i.e., X_2/R_2 ≪1.

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