Quiz: Electrical Engineering

Exam: UPPSC-AE

Topic: Miscellaneous

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

Q1. The starting torque of a wound rotor induction motor can be increased by adding:

(a) external capacitance to the rotor circuit

(b) external inductance to the rotor circuit

(c) a combination of inductance and capacitance to the rotor circuit

(d) external resistance to the rotor circuit

Q2. Integral of unit impulse is a:

(a) unit ramp

(b) unit step

(c) constant

(d) unit impulse

Q3. For a bipolar junction transistor, common base current gain is 0.98 and the base is 120μA. Its common emitter current gain will be:

(a) 98

(b) 56

(c) 49

(d) 118

Q4. A sinusoidal voltage V=50 sinωt is applied to a series RL circuit. The current in the circuit is given by I=25 sin(ωt-53^0 ). The apparent power consumed by the load is

(a)375 VA

(b)625 VA

(c)2500 VA

(d)750 VA

Q5. If three amplifiers having the same bandwidth are cascaded, the bandwidth of the resulting amplifier will be

(a)Better than that of each stage

(b)Worse than that of each stage

(c)Same as that of each stage

(d)None of the above

Q6. Determine the current through the ideal diode (D).

(a)1 A

(b)2 A

(c)3 A

(d)4 A

Q7. A GTO can be turned on by applying

(a)Positive gate signal

(b)Positive drain signal

(c)Positive source signal

(d)None of the above

Q8. A single-phase half wave rectifier is feeding a resistive load. When α = 0, dc output voltage is 127 V. When α = 30°, the dc output voltage will be about

(a)127

(b)119

(c)110

(d)100

Q9. Schering bridge is used to measure

(a)Dielectric loss

(b)The inductance

(c)Low resistance

(d)Mutual Inductance

Q10. If a resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct line starting:

(a)Both the starting current and torque reduce

(b)Both the starting current and torque increase

(c)The starting current reduces but starting torque increases

(d)The starting current reduces but starting torque

SOLUTIONS

S1. Ans.(d)

Sol. In an induction motor:

(A)Generally, the initiation of these motors occurs when it draws full line voltage which is 6 to 7 times higher than the full load current. This high current can be controlled by external resistance connected in series with the rotor circuit.

(B)the external resistance reduces high resistance as soon as the motor obtains normal speed and increases the starting torque of a motor. The tweaking of external resistance also aids in decreasing of rotor and stator current but improves the power factor of a motor.

S2. Ans.(b)

Sol. The unit step function, u(t) is defined as: u(t) = 1 for t ≥ 0; = 0 for t < 0. and is related to the delta function as follows: u(t)=∫δ(t)dt :integration range: minus infinity to time t

S3. Ans.(c)

Sol. In a common base transistor, current gain is given by,

β=(△IC)/(△IB )

And In a common base transistor, current gain is given by:

α=(△IC)/(△IE )

∴α=β/(1-β)=0.98/(1-0.98)=49

S4. Ans.(b)

Sol. The apparent power consumed by the load is: S=Vrms Irms=50/√2×25/√2=625 VA

S5. Ans.(b)

Sol. If N amplifiers having the same bandwidth(B) are cascaded, the bandwidth of the resulting amplifier will be: BW=B×√(2^(1/N)-1 )

So, the bandwidth of the resulting amplifier will decrease as the number of identical amplifier stages increases.

S6. Ans.(a)

Sol. The diode is a non-linear element. So, here only source transformation is applicable and superposition theorem is not applicable.

On applying source transformation,

KVL: 5-2id-id-2=0

⇒id=1 A

S7. Ans.(a)

Sol. Gate turn off (GTO), like an SCR is a device four-layer, three junction semiconductor devices with three external terminals (anode, cathode and gate). GTO can be turned on and off by positive pulse or signal and negative pulse or signal respectively to the gate terminal.

S8. Ans.(b)

Sol. using equation:

= 0.933.

Hence Vdc for 30° firing angle = 0.933 x 127 = ≈ 119 V.

S9. Ans.(a)

Sol. Schering bridge is used to measure dielectric loss. It is an AC bridge circuit that is used for measuring the insulating properties of electrical cables and equipment. It has the advantage that the balance equation is independent of frequency.

Maxwell bridge is used to measure the value of unknown inductance.

Heaviside bridge is used to measured mutual inductance in terms of a known self-inductance.

Kelvin bridge is used to measure the value of low resistance.

S10. Ans.(c)

Sol. Starting torque is directly proportional to the rotor resistance of an induction motor.

In slip ring induction motor the ends of the rotor windings are externally connected by a variable rheostat. So more the resistance, more the torque. When we add resistance to the rotor, the torque is high, the slip is high and the current is reduced. Therefore, slip ring induction motor is used for providing high starting torque.