Quiz: Electrical Engineering 17 May 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Single phase induction motor

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. The starting torque of a single-phase induction motor is
(a) Low
(b) High
(c) Uniform
(d) Zero

Q2. A single-phase induction motor, rotating in the direction of forward field, the slip of the rotor with respect to forward and backward rotating fields are respectively:
(a) s and s
(b) s and (1-s)
(c) s and (2-s)
(d) (1-s) and s

Q3. Capacitor in a single-phase induction motor is used for
(a) Improving the power factor
(b) Improving the starting torque
(c) Reducing the harmonics
(d) Starting the motor

Q4. In a single-phase induction motor at start, the two revolving fields produce:
(a) Equal and opposite torque in the rotor conductors
(b) No torque in the rotor conductors
(c) Unequal torque in the rotor conductors
(d) Equal torque in same direction in the rotor conductors

Q5. In a shaded-pole motor, shading coil is used to
(a) reduce winding losses
(b) produce rotating magnetic field
(c) reduce friction losses
(d) protect against sparking

Q6. A capacitor-start, capacitor-run motor has …………
(a) low power factor
(b) high power factor
(c) low efficiency
(d) high starting torque

Q7. The direction of rotation of shaded-pole motor depends upon ………….
(a) supply frequency
(b) number of poles on the stator
(c) which half of the pole is shaded
(d) supply voltage

Q8. In case of split-phase motor, the phase shift between currents in the two winding is around
(a) 30⁰
(b) 90⁰
(c) 70⁰
(d) 120⁰

Q9. An 8-pole, 50 Hz, 1-phase induction motor runs at a speed of 690 r.p.m. Its slip for forward rotating flux is
(a) – 0.08
(b) + 0.08
(c) +1.92
(d) 1.00

Q10. A 6-pole, 50 Hz, 1-phase induction motor runs at a speed of 900 r.p.m. The frequency of currents in the cage rotor will be
(a) 5 Hz, 50 Hz
(b) 5 Hz, 55 Hz
(c) 5 Hz, 95 Hz
(d) 55 Hz, 95 Hz

SOLUTIONS

S2. Ans.(c)
Sol. For 1-phase Induction motor: s_f=s & s_b=(2-s).

S3. Ans.(d)
Sol. 1-𝟇 induction motor is not self-started.
The capacitor is used to introduce a phase difference of 90⁰ between auxiliary and main winding to start the motor.

S4. Ans.(a)
Sol. In a single-phase induction motor at start, the two revolving fields produces equal and opposite torque in the rotor conductors.

S5. Ans.(b)
Sol. The effect of shading coil is to produce a shift in the flux from the unshaded portion to the shaded portion of the pole. This shift in flux produces a weak rotating magnetic field and is sufficient to develop a small starting torque.

S6. Ans.(b)
Sol. The capacitor-start, capacitor-run motor is similar to the capacitor-start type except that the starting winding remains in the circuit after the motor is started. Consequently, such a motor operates at a higher power factor

S7. Ans.(c)
Sol. The direction of rotation of shaded-pole motor depends upon which half of the pole is shaded. The rotor runs from unshaded portion to the shaded portion of the pole.

S8. Ans.(a)
Sol. In case of split-phase motor, the phase shift between currents in the two winding is around 30⁰.

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