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Quiz: Electrical Engineering 10 July 2021

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Miscellaneous

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

Q1. What is the value of current I_4 (in A) for the given junction?

dfccil electrical executive, dfccil electrical quiz |_30.1
(a) 4
(b) -4
(c) 6
(d) -6

Q2. Select the incorrect statement.
A single-phase induction-motor
1. Requires only one winding
2. Can rotate in one direction only
3. Is not self-starting
(a) Only 1 and 2
(b) Only 2 and 3
(c) 0nly 3 only
(d) Only 1 and 3

Q3. In a single-phase induction motor, how are the windings placed?
(a) Both main and auxiliary windings on stator
(b) Both windings are on rotor
(c) Auxiliary on stator and main on rotor
(d) Main winding on rotor

Q4. Which of the following statements is correct for a single-phase hysteresis motor?
(a) It can run at synchronous and sub-synchronous speed
(b) It can run at sub synchronous speed only
(c) It can run at synchronous and super synchronous speed
(d) It can run at synchronous speed only

Q5. If a 500 KVA, 200HZ transformer is operated at 50HZ, its KVA(s) rating will be–
(a) 250 KVA
(b) 125 KVA
(c) 1000 KVA
(d) 2000 KVA

Q6. The SI unit of magnetic flux is……
(a) Weber
(b) Maxwell
(c) Tesla
(d) Gauss

Q7. Which of the following is a diamagnetic material?
(a) Aluminium
(b) Oxygen
(c) Lead
(d) Nickel

Q8. Which one of the following statements is NOT TRUE about the MI type instruments? (a) MI type Instruments are suitable for both AC and DC circuits.
(b) Frictional error in MI type instruments is very less.
(c) The torque weight ratio of MI type instruments is high.
(d) The instrument cost is much higher as compared to PMMC type instruments.

Q9. In a shaded pole single-phase motor, the revolving field is produced by the use of
(a) inductor
(b) capacitor
(c) Resistor
(d) shading coils

Q10. A pole pitch in an electrical machine is
(a) Equal to 180° Electrical
(b) Equal to 180° Mechanical
(c) Less than 180° electrical
(d) Greater than 180° electrical

SOLUTIONS

S1. Ans.(c)
Sol. According to Kirchhoff’s 1st law (KCL):
Σ_i=0
ie,7+2+I_4+(-8)+(-3)+(-4)=0
⇒I_4 =15-9
=6 A

S2. Ans.(a)
Sol. A single-phase induction motor is not self-starting and has two types of windings: main and auxiliary winding and direction of rotation can be changed by changing polarity of either main or auxiliary winding.

S3. Ans.(a)
Sol. The main parts of a single -phase induction motor is the Stator, Rotor, Windings. The stator is the fixed part of the motor to which A.C. is supplied. The stator contains two types of windings. One is the main winding and the other is the Auxiliary winding. These windings are placed perpendicular to each other.

S4. Ans.(d)
Sol. Hysteresis Motor is a synchronous motor with a cylindrical rotor and does not require any dc excitation to the rotor and it uses non-projected poles. It is a single-phase motor with rotor made up of ferromagnetic material.
initially, when hysteresis motor is started it behaves as a single-phase induction motor and while running it behaves as a synchronous motor.

S5. Ans.(b)
Sol. For constant load;
KVA i.e., S is directly proportional to induced emf and emf is directly proportional to frequency.
⇒ S α f
So, S_1/f_1 =S_2/f_2
⇒ 500/200=S_2/50
⇒ S₂ = 125 KVA

S6. Ans.(a)
Sol. Φ = BA = Weber*m2/m2 = Weber

S7. Ans.(c)
Sol. Example of diamagnetic material are: – Silver, lead, copper, water, mercury, carbon.
Aluminium & oxygen: paramagnetic
Nickel: ferromagnetic
Note:
If a value of μ_r is slightly less than unity then it is a diamagnetic material (μ_r< 1).
If the value of μ_ris slightly greater than unity, then it is paramagnetic material (μ_r≥ 1).
If the value of μ_ris very large than unity 〖(μ〗_r>>1), then it is the ferromagnetic material.

S8. Ans.(d)
Sol. The following are the advantages of the moving iron instruments.
Universal use – The MI instrument is independent of the direction of current and hence used for both AC and DC.
Less Friction Error – The friction error is very less in the moving iron instrument because their torque weight ratio is high. The torque weight ratio is high because their current carrying part is stationary and the moving parts are lighter in weight.
Cheapness – The MI instruments require a smaller number of turns as compared to PMMC instrument. Thus, it is cheaper.

S9. Ans.(d)
Sol. In a shaded pole single-phase motor, the revolving field is produced by the use of
Shading coils.
The shaded pole induction motor is simply a self-starting single-phase induction motor whose one of the poles is shaded by the copper ring. The copper ring is also called the shaded ring. This copper ring act as a secondary winding for the motor.

S10. Ans.(a)
Sol. 1 pole pitch=180⁰ electrical

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