Quiz: Electrical Engineering
Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute
Q1. In a synchronous machine, if the filed flux is ahead of the armature field axis, in the direction of rotation, the machine works as-
(a) asynchronous motor
(b) asynchronous generator
(c) synchronous motor
(d) synchronous generator
Q2. Synchronous motor can be made self-starting by providing
(a) Damper winding on rotor poles
(b) Damper winding on stator
(c) Damper winding on stator as well as rotor poles
(d) None of these
Q3. The coil of a moving coil meter has 100 turns, is 40 mm long and 30 mm wide. The control torque is 240×10-6 N-m on full scale. If magnetic flux density is 1Wb/m2, range of meter is
(a) 1 mA
(b) 2 mA
(c) 3 mA
(d) 4 mA
Q4. If the cathode of the thyristor is made positive with respect to the anode & no gate current is applied then: –
(a) Only the middle junction is forward biased
(b) Only the middle junction is reversed biased
(c) All the junctions are forward biased
(d) All the junctions are reversed biased
Q5. In a dynamometer type wattmeter, the pressure coil connected across the load terminal is
(a) Highly inductive
(b) Highly capacitive
(c) Highly resistive
(d) Non inductive
Q6. Due to the inductance in the pressure coil of dynamometer type wattmeter, the reading will be
(a) High for both leading and lagging power factors
(b) Low for both leading and lagging power factors
(c) High for lagging power factor and low for leading power factor
(d) Low for lagging power factor and high for leading power factor
Q7. If the number of turns of a coil is increased, its inductance
(a) Is decreased
(b) Remains the same
(c) Is increased
(d) None of the above
Q8. Mho relay is used for protection in:
(a) Long transmission lines
(b) Short transmission lines
(c) Medium transmission lines
(d) Both long transmission lines and short transmission lines
Q9. Which of the following type of instrument is used only for D.C?
(a) Moving iron-attraction type
(b) Moving iron-repulsion type
(c) Permanent magnet type
(d) Hot wire type
Q10. At a frequency less than the resonant frequency_____:
(a) series circuit is capacitive and parallel circuit is inductive
(b) series circuit is inductive and parallel circuit is capacitive
(c) both circuits are inductive
(d) both circuits are capacitive
In and alternator, field flux is ahead of armature field flux in the direction of rotation.
Sol. Synchronous motor is not self-starting. It can be made self-starting by providing “Damper winding on ROTOR pole faces and around the pole shoes.
Following is the primary function of damper winding: –
(i) Provides starting torque
(ii) Reduce hunting
(iii) Used for salient pole rotors
(iv) Under normal running condition, damper winding in synchronous machine does not carry and current because the relative velocity between the rotating magnetic field (RMF) of stator and the rotor is zero.
Sol. In case of moving coil meter, Controlling torque Tc = NBIA.
Where, N = no. of turns of the coil, B – magnetic flux density, = A = area of coil.
So, I = 240×10-6 / (100 × 0.04 × 0.03) = 2mA.
Sol. Anode must be positive with respect to the cathode so that SCR will be forward biased. To turn on an SCR, a current is to be passed through the gate terminal. Hence it is a current operated device.
With the gate open, if the cathode is made positive with respect to the anode, the junctions J1 and J3 become reverse biased and J2 forward biased. Still the current flowing is leakage current, which can be neglected as it is very small. The SCR is said to be in reverse blocking state and voltage applied is called reverse voltage.
Sol. In a dynamometer type wattmeter, the pressure coil connected across the load terminal is highly Resistive.
Sol. Because of the inductance, the current of the pressure coils lags behind the voltage. Thus, the power factor of the wattmeter becomes lagging, and the meter reads high reading and vice-versa.
Sol. inductance of a coil is given by: L=(N^2µA)/l
∴L α N^2
Hence, L increases if number of turns(N) increases.
Sol. Relay used for different configuration of transmission lines……
Short transmission line: reactance relay
Medium transmission line: impedance relay
Long transmission line: mho relay
→ only for DC
→ Scale is uniform
→ Eddy current damping is used
→ F = BIL
→ Torque/Weight ratio is high to reduce frictional errors.
Sol. For series Resonant RLC circuit
Parallel RLC circuit