Quiz: Electrical Engineering 6 May 2020

Quiz: Electrical Engineering
Exam: UPPSC AE
Topic: Miscellaneous

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

Q1. A Wheatstone bridge in balanced. If the galvanometer is short-circuited, the currents in the various resistors will
(a) increase
(b) decrease
(c) not change
(d) cannot say

Q2. If WC is the constant loss and Ra is the armature resistance of a d.c. generator, then load current IL corresponding to maximum efficiency is
(a) IL=√(Ra/Wc )
(b) IL=Wc/√(Ra )
(c) IL=Ra/√(Wc )
(d) IL=√(Wc/Ra )

Q3. A 4-pole, lap-wound d.c. shunt generator has an armature winding consisting of 440 turns each of 0.002 Ω. The armature resistance is
(a) 0.5 Ω
(b) 1 Ω
(c) 0.25 Ω
(d) 0.055 Ω

Q4. The current drawn by a 220 V d.c. motor of armature resistance 0.4 Ω and back e.m.f. 210 V is
(a) 250 A
(b) 25 A
(c) 2.5 A
(d) 255 A

Q5. With the increase in speed of a d.c. motor,
(a) both back e.m.f. and line current fall
(b) both back e.m.f. and line current increase
(c) back e.m.f. falls and line current increases
(d) back e.m.f. increases and line current decreases

Q6. In a d.c. series motor, if armature current is reduced to 85% of its original value the torque of the motor will be reduced by
(a) 48%
(b) 28%
(c) 72%
(d) 52%

Q7. A d.c. series motor is running at rated speed without any additional resistance in series. If an additional resistance is placed in series, the speed of motor
(a) increases
(b) decreases
(c) remains same
(d) none of the above

Q8. A main reason of being transformer an efficient device is ………………..
(a) is a static device
(b) uses inductive coupling
(c) uses capacitive coupling
(d) uses electric coupling

Q9. A transformer does not possess ……………… changing property.
(a) impedance
(b) voltage
(c) current
(d) power

Q10. How are transformers generally cooled for installations rated at less than 5 kVA?
(a) oil cooled
(b) air cooled
(c) water cooled
(d) none of the above

Solution

S1. Ans.(c)
Sol.
Since the bridge is balanced, there is no current in the branch containing galvanometer.
Therefore, whether the branch containing galvanometer is shorted on open-circuited, there will be no change in currents or voltages anywhere in the circuit.

S2. Ans (d)
Sol. The efficiency of a d.c. generator is maximum when
Variable loss = Constant loss
Or IL^2 Ra=Wc or IL=√(Wc/Ra )

S3. Ans (d)
Sol. Total resistance of 440 turns = 440 ×0.002=0.88 Ω
Since there are 4 parallel paths in armature,
Resistance of each path=0.88/4=0.22 Ω
Now, there are four such resistances in parallel each of value 0.22 Ω.
∴ Armature resistance, ra=0.22/4=0.055 Ω

S4. Ans (b)
Sol. Current drawn = (V-Eb)/Ra =(220-210)/0.4=25 A

S5. Ans (d)
Sol. For a d.c. motor, line current = (V-Eb)/Ra .

S6. Ans (b)
Sol. In a d.c. series motor, the torque developed T∝Ia^2. For the first case, T1∝Ia^2; For the second case, T2∝(0.8 Ia )^2.
∴ T2/T1 =((0.85 Ia)²)/(Ia^2 )=(0.72 Ia^2)/(Ia^2 )=0.72
∴ 1-T2/T1 =1-0.72=0.28 or (T1-T2)/T1 =0.28 or 28%

S7. Ans (b)
Sol. When additional resistance is placed in series, the voltage available across the armature decreases. Hence, the speed of the motor falls.

S8. Ans (a)
Sol. Since transformer is a static device, the friction and windage losses are absent.

S9. Ans (d)
Sol. A transformer is not a source of energy; it merely transfers power from primary to secondary with a change of voltage/current.

S10. Ans (b)
Sol. Transformers rated at less than 5 k VA are generally air cooled i.e. the heat produced is carried away by the surrounding air.

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