Quiz: Electrical Engineering 27 July 2020

Quiz: Electrical Engineering
Topic: Miscellaneous

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

Q1. In a four-pole D.C. machine:
(a) all the four poles are north poles
(b) alternate poles are north and south
(c) all the four poles are south poles
(d) two north poles follow two south poles

Q2. The field coils of DC generator are generally made up of:
(a) Mica
(b) Copper
(c) Cast Iron
(d) Carbon

Q3. The basic requirement of a d.c. armature winding is that it must be………
(a) a lap winding
(b) a closed one
(c) a wave winding
(d) either ‘a’ or ‘c’

Q4. The brushes of a dc machine are made of:
(a) iron
(b) brass
(c) mica
(d) carbon

Q5. To convert a galvanometer into voltmeter, the value and type of connection of the resistance to be connected with should be
(a) Low and parallel
(b) High and parallel
(c) Low and series
(d) High and series

Q6. A voltmeter must have a very high internal resistance so that the:
(a) accuracy is high
(b) resolution is high
(c) meter draws minimum current
(d) loading is maximum

Q7. Which starting method is not used in squirrel cage induction motors?
(a) Resistance in rotor circuit
(b) Resistance in stator circuit
(c) Auto-transformer starting
(d) Star delta starting

Q8. What is condition for maximum starting torque of three phase induction motor if R₂ and X₂ are rotor resistance and reactance respectively?
(a) R₂ = 0, X₂ = 1Ω
(b) R₂ = 2 X₂
(c) R₂ = 0.5 X₂
(d) R₂ = X₂

Q9. The supply voltage to an induction motor is reduced by 10% By what percentage, approximately, will the maximum torque decrease?
(a) 40
(b) 20
(c) 10
(d) None of the above

Q10. Insulation resistance of a cable of length 20 km is 2 MΩ. What will be the insulation resistance of the same cable but for a length of 200 km?
(a)2 MΩ
(b)20 MΩ
(c)0.2 MΩ
(d)200 MΩ


S1. Ans.(b)
Sol. alternate poles are North-south or South-north

S2. Ans.(b)
Sol. because copper is an excellent conductor and becomes electromagnet when current passing through it.

S3. Ans.(b)
Sol. a closed one

S4. Ans.(d)
Sol. The brushes can be made of carbon or copper, but the resistance of the carbon brushes allows less current inrush during commutation between adjacent commutator segment.

S5. Ans.(d)
Sol. A galvanometer is very sensitive instruments so it can’t measure low value of current A Galvanometer can be converted into voltmeter by connecting a high resistance in series with it.

S6. Ans.(c)
Sol. A voltmeter having very high resistance so that if will draw minimum current and drop will be negligible and will correctly measure the CKT voltage.
▭(Rseries=Rm (m-1) ) when(m=I/Im or V/Vm )

S7. Ans.(a)
Sol. Rotor resistance starting is used only for slip-ring induction motor because rotor of squirrel cage induction motor is short circuited.

S8. Ans.(d)
Sol. Starting torque (Ts)= (KR₂)/(〖R₂²〗^ +x₂²) (Here S=1)
For maximum value (d(Ts))/(dR₂)= 0
ie, K[(R₂²+x₂²-r^2×2R₂)/((R₂²+x₂)²)]=0
⇒ R2=x₂ → condition for maximum starting torque.

S9. Ans.(b)
Sol. For Induction motor: –
If V is reduced by 10% then
T1∝(0.9v) ²
So, % change = (V^2-(0.9v)²)/V²×100
= 0.19×100
= 19%

S10. Ans.(c)
Sol. insulation resistance of cable ⍺ 1/ length
∴R2/R1 =l1/l2
∴R2=20/200×2=0.2 MΩ

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