Quiz: Electrical Engineering 24 April 2020

Quiz: Electrical Engineering
Exam: UPPSC AE
Topic: Single phase induction motor

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

Q1. Which of the following motor can work satisfactorily of both AC and DC?
a) Induction motor
b) Synchronous motor
c) Series motor
d) Shunt motor

Q2. A vacuum cleaner employ ______ motors
a) Resistance split phase
b) Capacitor start
c) Shaded pole
d) Single phase series.

Q3. The Capacitor start induction run motor is used because of
a) High starting torque
b) Good power factor
c) High efficiency
d) Minimum cost

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

Q5. In case of a split phase motor, the phase shift between currents in the two windings is around
a) 30°
b) 70°
c) 90°
d) 120°

Q6. Which of the following types of motor is not a self-starting?
a) Synchronous motor
b) DC series motor
c) Induction motor with medium slip
d) Induction motor with help slip

Q7. When a single-phase induction motor is excited with single phase ac voltage, the magnetic field set up is equivalent to
a) Two field, rotating in opposite directions with different speeds.
b) Two fields, rotating at synchronous speed in opposite directions.
c) Two fields, rotating at synchronous speed.
d) Two fields, rotating in the same direction but at different speeds.

Q8. A single-phase induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is s, what is the slip with respect to the backward field?
a) s
b) – s
c) (1 – s)
d) (2 – s)

Q9. An 8-pole single phase induction motor is running at 690 rpm. What is its slip with respect to forward and backward fields, respectively?
a) 0.08, 2.0
b) 0.08, 1.92
c) 1.92, 0.08
d) 2.0, 0.08

Q10. Why is a centrifugal switch used in a single-phase induction motor?
a) To protect the motor from overloading.
b) To improve the starting performance of the motor
c) To cut off the starting winding at an appropriate instant.
d) To cut in the capacitor during running conditions

 

Solution

Sol 1.(c)
Series motor as the AC series motors are also known as the modified DC series motor.
Sol 2.(d)
A vacuum cleaner employs Single phase series motor.
Sol 3.(a)
Because of its High starting torque
Sol 4.(c)
Equal and opposite torque in the rotor conductors
Sol 5.(a)
It is 30°.
Sol 6.(a)
Synchronous motor is not self-starting.
Sol 7.(b)
Two fields, rotating at synchronous speed in opposite directions
Sol 8.(d)
Sf= s
Sb=(Ns+N)/Ns
= 1+1-s
=2-s ( As N/Ns = 1-s )
Sol 9.(b)
P = 8
Ns = (120×50)/8 = 750 rpm
Sf = (Ns-N)/Ns
Sf = (750-690)/750
= .08
Sb = 2-Sf = 1.92
Sol 10.(c)

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