Quiz Electrical Engineering

Exam: GATE

Topic: Miscellaneous

Date: 01/06/2021

Each Question carries 2 Mark

Negative Marking: 1/3

Time: 20 Minutes

Q1. A shunt generator has a critical field resistance of 200 Ω at a speed of 800 rpm. If the speed of the generator is increased to 1,000 rpm, what is the change in the critical field resistance of the generator?

(a) Remains the same at 200 Ω

(b) Decreases to 160 Ω

(c) Increases to 250 Ω

(d) Increases to 312.50 Ω

Q2. In the given electric circuit, voltage V_0 and current I_0 are related as follows

The power absorbed by the load when I_0=3 A and I_0=4 A respectively

(a) 48 W, 36 W

(b) 16 W, 9W

(c) 64 W, 27 W

(d) 66 W, 84 W

Q3. The phase voltage and impedance of an ac network is given as

The average power and reactive power absorbed by the network are equal to:

(a) Zero, 80 VAR

(b) 69.3 W, 40 VAR

(c) 34.6 W, 80 VAR

(d) 80 W, 40 VAR

Q4. For the given system

Which of the following statements is true?

(a) The system is controllable

(b) The system is uncontrollable

(c) Can’t say

(d) None of the above

Q5. A 6-pole, 50 Hz, 1-phase induction motor runs at a speed of 900 r.p.m. The frequency of currents in the cage rotor will be

(a) 5 Hz, 50 Hz

(b) 5 Hz, 55 Hz

(c) 5 Hz, 95 Hz

(d) 55 Hz, 95 Hz

Q6. The voltage applied to a transformer primary is increased keeping V/f fixed. How will core loss and magnetizing current change?

(a) Core loss will increase and magnetizing current remain same

(b) Core loss will remain same and magnetizing current will remain same

(c) Core loss will decrease and magnetizing current will increase

(d) Core loss will remain same and magnetizing current will decrease

Q7. A dc shunt motor is running at 1000 rpm at the rated load torque. If few of the field windings get shot circuited then

(a) motor speed will increase and more armature current is drawn

(b) motor speed will decrease and more armature current is drawn

(c) motor speed will increase and lesser armature current is drawn

(d) motor speed will decrease and lesser armature current is drawn

Q8. For the equation, s^3-4s^2+s+6=0 the number of roots in the left half of s-plane will be

(a) Zero

(b) One

(c) Two

(d) Three

Q9. The de-ionization of the medium in current zero method of an arc extinction is NOT achieved by

(a) Splitting the arc

(b) Lengthening of the gap between the contacts

(c) Increasing the pressure of the vicinity of the arc

(d) Cooling the arc

Q10. A series resonant circuit is tuned to 10 MHz and provides 3-dB bandwidth of 100 kHz. The quality factor Q of the circuit is

(a) 1

(b) 10

(c) 30

(d) 100

SOLUTIONS

S5. Ans.(c)

Sol. Synchronous speed for forward rotating flux, Ns = 120 f/P = 120 × 50/6 = 1000 r.p.m.

Therefore, slip for forward rotating flux is

s_f=s=(N_s-N)/N_s =(1000-900)/1000=0.1

∴ Rotor current frequency for sf is f’ = sf × f = 0.1 × 50 = 5 Hz.

Slip for backward rotating flux s_b =(2 – s) =(2 – 0.1) = 1.9. Therefore, rotor current frequency, f” = (2 – s) × f = 1.9 × 50 = 95 Hz.

S6. Ans.(a)

Sol. In transformer, the magnetizing current I_m α V/f. If V/f is kept constant then magnetizing current will remain same and core loss depends on frequency. So, it will increase with increase in frequency.

S7. Ans.(a)

Sol. If field winding gets short circuited, field excitation reduces causing rise in speed and more armature current.

S9. Ans.(a)

Sol. There are two methods of extinguishing the arc in CB viz.

High Resistance Method

Low resistance Method or current zero method

Low resistance Method or current zero method: This method is employed for arc extinction in AC circuits only. In this method, the rapid increase of dielectric strength of the medium near current zero can be achieved by:

Lengthening of the gap

High pressure in the vicinity of the arc

Blast effect

Cooling

NOTE: Splitting the arc method is related with arc extinction by High resistance method.

S10. Ans.(d)

Sol. for resonant circuit, Q=f_0/BW=(10×10^6)/(100×10^3 )=100