Quiz: Electrical Engineering 4 April 2020

Quiz: Electrical Engineering
Date: 04/04/2020
Exam: UPPSC-AE
Topic: Miscellaneous

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

Electrical Misc. Quiz: UPPSC-AE

Q1. In a synchronous machine, in case the axis of field flux is in line with the armature flux then.
(a) the machine is working as synchronous motor
(b) the machine is working as synchronous generator
(c) the machine is said to floating
(d) the machine will Vibrate violently

L1Difficulty 3
QTags Synchronous motor
QCreator Devbrat Kumar

Q2. For 100% excitation in synchronous motor, the relationship between back emf E and applied voltage V is-
(a) E>V
(b) E<V
(c) E=V
(d) none of these

L1Difficulty 3
QTags Synchronous motor
QCreator Devbrat Kumar

Q3. Short-circuit ratio of a synchronous machine yields
(a) 1/(Xs(unsaturated)(PU) )
(b) 1/(Xs (unsaturated) )
(c) 1/(Xs (adjusted)(PU) )
(d) 1/(Xs (adjusted) )

L1Difficulty 3
QTags Alternators
QCreator Devbrat Kumar

Q4. A synchronous generator is feeding power to infinite bus bars at unity power factor. Its excitation is now increased. It will feed
(a) the same power but at a leading power factor
(b) the same power but at a lagging power factor
(c) more power at unity power factor
(d) Less power at unity power factor

L1Difficulty 3
QTags Alternators
QCreator Devbrat Kumar

Q5. The minimum phase-neutral voltage at which corona starts occur in power transmission lines is called
(a)Knee voltage
(b)Visual critical voltage
(c)Critical disruptive voltage
(d)Flash voltage

L1Difficulty 3
QTags Power generation, transmission and distribution
QCreator Devbrat Kumar

Q6. For equilateral spacing of conductors of an untransposed 3-phase line, we have
(a)Balanced receiving end voltage and no communication interference
(b)Unbalanced receiving end voltage and no communication interference
(c)Balanced receiving end voltage and communication interference
(d)Unbalanced receiving end voltage and communication interference

L1Difficulty 3
QTags Power generation, transmission and distribution
QCreator Devbrat Kumar

Q7. In …… instruments the deflecting torque depends on the frequency
(a) Induction type instruments
(b) Hot wire instruments
(c) Moving iron instruments
(d) Moving coil instruments

L1Difficulty 3
QTags Measurement and measuring instrument
QCreator Devbrat Kumar

Q8. Commutation conditions at full load for large DC machines can be efficiently checked by the
(a) Brake test
(b) Swinburne’s test
(c) Hopkinson’s test
(d) Field test

L1Difficulty 3
QTags DC Motor
QCreator Devbrat Kumar

Q9. Susceptibilities of diamagnetic material is-
(a)unity
(b)zero
(c)negative
(d)positive

L1Difficulty 3
QTags Magnetic circuit
QCreator Devbrat Kumar

Q10. A series or parallel RLC circuit is said to be over damped if:
(a)α=ωn
(b)α>ωn
(c)α<ωn
(d)2α=ωn

L1Difficulty 3
QTags Network & Circuit Theory
QCreator Devbrat Kumar

SOLUTIONS

S1. Ans.(c)
Sol. when the axis of field flux is in line with the armature flux then the condition is called “FLOATING” in case of synchronous motor.
In case of floating –

Er = V ie armature current = 0
Losses = zero
Er = 0

S2. Ans.(c)
Sol. 100% excitation means full or normal excitation:
Excitation Relation b/w Eb&V
Normal Eb=V
Under Eb<V
Over Eb>V

S3. Ans.(c)
Sol. SCR=1/(Xs (adjusted)(PU) )

S4. Ans.(b)
Sol.

S5. Ans.(c)
Sol. Critical disruptive voltage: it is the minimum phase neutral voltage at which the corona occurs in a power transmission line.
go=Vc/(rlog⁡(d/r) )
Where, go= potential gradient at conductor surface
Vc= critical disruptive voltage
go= 30KV/cm(max.) …… at 76 cm of Hg and 25⁰ C.

S6. Ans.(a)
Sol. If conductors are equally spaced then per phase inductance will be same and receiving end voltage will be balanced and communication interference will not take place. So here no need of transposition.

S7. Ans.(a)
Sol. for induction type instruments –
T_d=kwϕ1mϕ2m sin α
ϕ1m,ϕ2m = maximum fluxes produced by the currents
α = phase difference between the two flukes
So,

So, in induction type instrument deflecting torque depends on the frequency.

S8. Ans.(c)
Sol. Hopkinson’s test is to check the commutation condition at Full-load for large DC machines.
Hopkinson’s: – Also known as Regenerative, back to back test and heat run test

S9. Ans.(c)
Sol. χv = μv − 1

S10. Ans.(b)
Sol. Natural frequency =1/√LC
And damping coefficient(α)=R/2L
Damping factor=α/ω

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