Quiz: Electrical Engineering 24 Sep 2020

Quiz: Electrical Engineering
Exam: RSMSSB-JEn
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/3 mark
Time: 10 Minute

Q1. The relation between μ and H for a specimen of iron is as follows-
μ = [0.4/H + 12 × 10^(-4)] Henry / meter
The value of H which produces flux density of 1 Tesla will be-
(a) 250 A/m
(b) 500 A/m
(c) 750 A/m
(d) 103 A/m

Q2. The SI unit of magnetic flux is……
(a) Weber
(b) Maxwell
(c) Tesla
(d) Gauss

Q3. Which of the following is a diamagnetic material?
(a) Aluminium
(b) Oxygen
(c) Lead
(d) Nickel

Q4. 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.

Q5. 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

Q6. 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

Q7. Demand factor of a power plant is:
(a) (maximum demand)/(connected load)

(b) (average demand)/(plant capacity)

(c) (station energy output)/(plant capacity×hrs of use)

(d) (average demand)/(maximum demand)

Q8. In a power plant if the plant capacity is equal to the maximum demand then
(a) Plant reserve capacity will be 50%
(b) Plant reserve capacity will be zero
(c) Load factor will be unity
(d) Load factor will be nearly 70%

Q9. The maximum demand i.e. sum of connected load to the simultaneous maximum demand is expressed through
(a) Derivative factor
(b) Demand factor
(c) Diversity factor
(d) Dividend factor

Q10. If a resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct line starting:
(a) Both the starting current and torque reduce
(b) Both the starting current and torque increase
(c) The starting current reduces but starting torque increases
(d) The starting current reduces but starting torque

SOLUTIONS
S1. Ans.(b)
Sol. B = μH
or B = [0.4/H + 12 × 10^(-4)] H
or 1 = 0.4 + 12×10^(-4) H
H = 500 A/m

S2. Ans.(a)
Sol. Φ = BA = Weber*m2/m2 = Weber

S3. Ans.(c)
Sol. Example of diamagnetic material are: – Silver, lead, copper, water, mercury, carbon.
Aluminium & oxygen: paramagnetic
Nickel: ferromagnetic
Note:
1.If a value of μ_r is slightly less than unity then it is a diamagnetic material (μ_r< 1).
2. If the value of μ_ris slightly greater than unity, then it is paramagnetic material (μ_r≥ 1).
3. If the value of μ_ris very large than unity 〖(μ〗_r>>1), then it is the ferromagnetic material.

S4. 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.

S5. 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.

S6. Ans.(a)
Sol. 1 pole pitch=180⁰ electrical

S7. Ans.(a)
Sol. Demand factor = (Maximum demand)/(connected load)
Demand factor is less than 1.

S8. Ans.(b)
Sol. Plant capacity factor = (Peak load)/(Plant capacity)×load factor
If maximum demand = Plant capacity then
Load factor = capacity factor
And reserve capacity = Plant capacity – max. demand
= 0

S9. Ans.(c)
Sol. Diversity factor = (Sum of indivdula max.demand)/(Max demadnd of plant)
It is always greater than 1.

S10. Ans.(c)
Sol. Starting torque is directly proportional to the rotor resistance of an induction motor.
〖i.e.Tst α R2
In slip ring induction motor, the ends of the rotor windings are externally connected by a variable rheostat. So more the resistance, more the torque. When we add resistance to the rotor, the torque is high, the slip is high and the current is reduced. Therefore, slip ring induction motor is used for providing high starting torque.

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