Quiz: Electrical Engineering 22 Oct 2020

Quiz: Electrical Engineering
Exam: SSC JE
Topic: Miscellaneous

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

Q1. Owing to skin effect
(a) current flows through the half cross-section of the conductor
(b) portion of the conductor near the surface carries more current and core of the
conductor carries less current
(c) portion of the conductor near the surface carries less current and core of the
conductor carries more current
(d) any of the above

Q2. The speed regulation of a synchronous motor is always
(a) positive
(b) zero
(c) negative
(d) 100%

Q3. An induction motor and a synchronous motor are connected to a common feeder line;
To operate the feeder line at unity p.f., the synchronous motor should be
(a) normally excited
(b) under-excited
(c) over-excited
(d) none of the above

Q4. A synchronous condenser (or capacitor) means
(a) synchronous motor operating at no-load
(b) over-excited synchronous motor with capacitor
(c) under-excited synchronous motor
(d) none of the above

Q5. A synchronous capacitor is used to
(a) improve p.f. of the system
(b) reduce losses in the system
(c) improve voltage regulation
(d) achieve all of above

Q6. n similar resistors each of resistance r when connected in parallel have the total
resistance R. When these resistances are connected in series, the total resistance is
(a) nR
(b) R/n²
(c) n²R
(d) R/n

Q7. Fig. shows part of a closed circuit. What is the value of VA–VB?

(a) 12 V
(b) 9 V
(c) 18 V
(d) 6 V

Q8. A wire of resistance 0.1 Ω /cm is bent to form a square ABCD of side 10 cm. A similar
wire is connected between B and D to form the diagonal BD. If a 2 V battery of negligible
internal resistance is connected between A and C, then total power dissipated is
(a) 2 W
(b) 3 W
(c) 4 W
(d) 6 W

Q9. In a power station,
(a) Reserve capacity = Plant capacity + Max. demand
(b) Reserve capacity = Plant capacity – Average demand
(c) Reserve capacity = Plant capacity – Max. demand
(d) Reserve capacity = Plant capacity + Average demand

Q10. In a 3-phase system, the line losses are
(a) directly proportional to cos ϕ
(b) inversely proportional to cos ϕ
(c) inversely proportional to cos² ϕ
(d) none of the above

SOLUTIONS
S1. Ans.(b)
Sol. The distribution of current over the cross section of the conductor is uniform in DC
only. In alternating current flow through a conductor does not distribute uniformly but it
will concentrate near the surface of the conductor. In fact, in AC system no current flow
through the core because the whole current will flow through the surface region. Due to
this reason the effective area of conductor is reduced causing an increase in AC resistance
. The effective AC resistance is usually referred as the effective resistance of the conductor
.This phenomenon is called the Skin effect as it will cause concentration of current at the
skin of conductor.

S2. Ans.(b)
Sol. The speed of a synchronous motor is constant (= synchronous speed, N_s) from no
load to full-load. Therefore, speed regulation of a synchronous motor is always zero

S3. Ans.(c)
Sol. An over-excited synchronous motor (i.e.E_b>V) takes power with a leading power
factor and behaves like a capacitor,

S4. Ans.(a)
Sol. A synchronous motor operating at no-load is called a synchronous capacitor or
condenser. It is specially designed for power factor control and has no external shaft.

S5. Ans.(d)
Sol.

S6. Ans.(c)
Sol. In parallel,
R=r/n→r=nR
In series,
R_eq=nr=n²R

S7. Ans.(b)
Sol.

V_AB=3+3×2=9V

S8. Ans.(c)
Sol.

This is a wheat stone bridge in balanced form.
∴ R_eq between A and C
R_eq=(2‖2)=1Ω
∴ Total power =V^2/R= 4/1=4 W

S9. Ans.(c)
Sol. Reserve capacity = Plant capacity – Max. demand

S10. Ans.(c)
Sol. Line current, IL=P/(√3 〖 V〗L cos⁡ϕ )
Since line losses are proportional to the square of I_L, it follows that line losses are proportional to 1/cos² ϕ

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