DFCCIL ELECTRICAL QUIZ

Quiz: Electrical Engineering
Exam: DFCCIL
Topic: Miscellaneous

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

Q1. In a synchronous motor, the magnitude of back e.m.f depends on……….
(a) D.C excitation only
(b) Speed of the motor
(c) Load on the motor
(d) Both the speed and rotor flux

Q2. To obtain greater efficiency, the slip of an induction motor should be………………
(a) 1
(b) Low
(c) High
(d) Very high

Q3. The synchronous speed for the 7^th space harmonic mmf wave of a 3-phase, 8 pole, 50 Hz induction machine is
(a) 5250 rpm in forward direction
(b) 5250 rpm in reverse direction
(c) 107.14 rpm in forward direction
(d) 107.14 rpm in reverse direction

Q4. Reduction in the capacitance of a capacitor start motor results in reduced…………
(a) Speed
(b) Noise
(c) Armature reaction
(d) Starting torque

Q5. The law that explains about the economic size of a conductor is:
(a) Lambert’s law
(b) Newton’s law
(c) Kelvin’s law
(d) Faraday’s law

Q6. Power distribution by cable is generally adopted for line length
(a) Less than 10 Km
(b) Above 10 Km
(c) Less than 50 Km
(d) Above 50 Km

Q7. If a voltmeter is connected, like an ammeter, in series with the load,
(a) Almost no current will flow in the circuit
(b) The measurement reading will be too high
(c) The meter will burn out
(d) An inadmissibly high current will flow

Q8. With regard to filtering property, lead compensator is
(a) Low pass filter
(b) High pass filter
(c) Band pass filter
(d) Band reject filter

Q9. The peak value of the output voltage of a half-wave rectifier is 100 V. the rms value of the half-wave rectifier output voltage will be
(a) 100 V
(b) 50 V
(c) 70.7 V
(d) 35.35 V

Q10. To operate properly a transistor’s base-emitter junction must be forward biased with reverse bias applied to which junction?
(a) Base-emitter
(b) Collector-emitter
(c) Base-collector
(d) Collector-base

SOLUTIONS

S1. Ans.(a)
Sol. Eb=4.44fϕ×Nph
As speed is always constant. So, back emf depends on the excitation given to the field winding.
I.e., Eb α ϕ.

S2. Ans.(b)
Sol. Approximate value of efficiency=(rotor gross output(Pm))/(rotor input(P2))=(1-s)=Nr/Ns
So, lower the value of slip, higher will be efficiency.

S3. Ans.(c)
Sol. Ns=120f/P=(120×50)/8=750 rpm
Speed of 7^th harmonic=Ns/7=750/7=107.14 rpm
And, for7^th harmonic=7×120^0=840^0=840-360-360=+120⁰(i.e.forward direction)

S4. Ans.(d)
Sol. The value of the capacitor in a capacitor starts motor controls the starting torque.
Reduction in capacitance of a capacitor start motor will result in reduction of starting voltage and hence starting torque will reduce.

S5. Ans.(c)
Sol. Kelvin’s law states that most economic area of conductor is that for which the variable part of annual charge is equal to the cost of energy losses.

S6. Ans.(a)
Sol. Power cables are mainly used for power transmission and distribution up to line length of 10 Km.

S7. Ans.(a)
Sol. Voltmeter has very high resistance to ensure that its connection do not alter flow of current in the circuit. Now if it is connected in series then no current will be there in the circuit due to its high resistance. Hence it is connected in parallel to the load across which potential difference is to be measured.

S8. Ans.(b)
Sol. In terms of filtering property, lead compensator acts as high pass filter.

S9. Ans.(b)
Sol. For half wave rectification, RMS value will be Vmaximum/2=50 V

S10. Ans.(c)
Sol. if base -emitter junction (BEJ) is forward biased, then in order to have proper operation of a transistor as an amplifier, base collector junction (BCJ) must be reverse biased.