Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 12
Time: 12 min.
Q1. Two 100 W, 200 V lamps are connected in series across a 200V supply. The total power consumed by each lamp will be watts.
Q2. An ammeter
(a) Is inserted in series in a circuit and current to be measured flows through it
(b) Is inserted in series in a circuit and part of the current to be measured flows through it
(c) Is connected in parallel in a circuit and current to be measured flows through it
(d) Is connected in parallel in a circuit and only part of the current to the measured flows through it.
Q3. A 220 V single phase meter has a constant load current of 5.0 A passing through it for 2 hours, at unity power factor. If the meter disc makes 1056 revolution during this period. What is the meter constant in revolutions/kWh?
Q4. A mho relay is used for protection of:
(a) Protection of a transformer against external fault
(b) Long Transmission Line
(c) Protection of a transformer against all the internal faults and external fault
(d) Medium Length lines
Q5. Slip ring of an induction motor is usually made up of
(d) Phosphor bronze
Q6. The capacitance of an overhead transmission line increases with?
1. increase in mutual geometrical mean distance
2. increase in height of conductors above ground
Select the correct answer from the following?
(a) Both 1 and 2 are true
(b) Both 1 and 2 are false
(c) Only 1 is true
(d) Only 2 is true
Q7. Which one of the following statements is correct?
Corona loss increase with
(a) decreases in conductor size and increase in supply frequency
(b) increase in both conductor size and supply frequency
(c) decrease in both conductor size and supply frequency
(d) increase in conductor size and decrease in supply frequency
Q8. In a three-phase system, the relation V_L=V_ph is applicable to a ________.
(a) star-connected without neutral point
(b) star-connected load
(c) single-phase system also
(d) delta-connected load
Q9. The power developed by salient pole synchronous machine is/are
(a) Excitation power only
(b) Power due to saliency or reluctance only
(c) Both a & b
(d) None of the above
Q10. In salient pole synchronous machine, if direct axis reactance and quadrature axis reactance is equal, then power developed due to reluctance is
(c) Infinite value
(d) Any of the above
Q11. A 50 Hz transformer having equal hysteresis and eddy current losses at rated excitation is operated at 45 Hz at 90% of its rated voltage. Compared to rated operating point, the core losses under this condition;
(a) Reduce by 10 %
(b) Reduce by 19 %
(c) Reduce by 14.5 %
(d) Remains unchanged
Q12. Which of the following circuit is known as the emitter follower?
(a) CC amplifier
(b) CB amplifier
(c) CE amplifier
(d) CS amplifier
Sol. for given series connection,
Voltage rating of bulb = Source voltage
The total power consumed by each lamp will be 50/2=25 W.
Note: – if Voltage rating of bulb ≠ Source voltage, then above calculation cannot be performed.
Sol. An ammeter is connected in series with the load and most of the current to pass through it. Hence ammeter is connected in series with the low resistance.
it has very low resistance to ensure the correct measurement of current in the circuit. If it is connected in parallel across any load then all current in the circuit will choose the lower resistive path (i.e., ammeter) to cause its circuit to be damaged. Hence it is used in
Sol. Voltage V = 220 V
Current I = 5 A
Power factor cosφ = 1
Time (t) = 2 hours
Number of revolutions = 1056
Energy consumed = 220 × 5 × 1 × 2 = 2.2 kWh
Meter constant K = Number of revolution/Kwh i.e. (rev/kWh)
K = 1056/2.2 = 480 rev/kWh
Sol. Mho relay is used for protection of long transmission line.
Important point of mho relay……
Mho relay is a high-speed relay and is also known as admittance relay.
Mho relay comes in the category of the distance relay protection scheme.
Mho Relay is less affected by the power swing.
Sol. Phosphor bronze is an alloy of copper, tin, and phosphorus. Phosphorous bronze provides good electrical conductivity and low thermal conductivity.
A slip ring is an electromechanical device that allows the transmission of power from a stationary to a rotating structure. Slip ring in an induction motor is made up of phosphor bronze.
Sol. C_AN=(2πϵo )/ln(Dm/Ds)
Where Dm = Mutual GMD
Ds = self GMD or GMR
So, by increasing mutual GMD, the capacitance of overhead transmission line decreases.
And C_S=(ϵ_o A)/d
So, by increasing height of lower, mutual capacitance decreases.
Sol. corona loss = 242.2/δ×(f+25) √(x/D) (V-V_c )^2×10^(-5) kw\/ km\/phase
f = System frequency
r = radius of conductor
D = distance between conductor
V = phase voltage (kv,rms)
V_c= Critical disruptive phase voltage
Sol. for delta connection –
Line voltage (V_L )= Phase voltage (V_ph)
Line current (I_L) = √3× Phase current (I_ph )
Sol. Power due to excitation = EV/Xd sinδ .
Power due to saliency or reluctance = (V^2/2 (Xd-Xq))/(Xd×Xq) sin2δ .
Therefore, the power developed by salient pole synchronous machine contains two components.
Sol. Power due to saliency or reluctance = (V^2/2 (Xd-Xq))/(Xd×Xq) sin2δ .
If X_d=X_q then P_reluctance=0.
Sol. Total core loss initially =W_e+W_h=W_i+W_i=2W_i
Because initially both eddy and hysteresis loss are equal.
When frequency=45HZ and applied voltage = 90 % of rated voltage
B_max=(0.9 V_rated)/(0.9 f_rated )=constant.
∴hysteresis loss W_h ⍺ f
And eddy loss W_e ⍺ f^2
∴ W_h2=0.9 W_h & W_e2=(0.9)^2 W_e=0.81 W_e
∴core loss=0.9W_i+0.81 W_i=1.71 W_i
Reduction in core loss =(2W_i-1.71W_i)/(2 W_i )×100=14.5 %.
Sol. The common collector circuit configuration is more widely known as the emitter follower and it provides a high input impedance and a low output impedance.
The major characteristics of the emitter follower are as follows −
No voltage gains. In fact, the voltage gain is nearly 1.
Relatively high current gain and power gain.
High input impedance and low output impedance.
Input and output ac voltages are in phase.
Used for impedance matching.