Quiz: Electrical Engineering 03 June 2021

Quiz: Electrical Engineering
Exam: AE/JE
Topic: Miscellaneous

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

Q1. Under heavy loads, transformer efficiency is comparatively low because
(a) Secondary output is much less as compared to primary input
(b) Voltage drops both in primary and secondary become large
(c) Iron loss is increased considerably
(d) Copper loss becomes high in proportion to the output

Q2. The short circuit test in a transformer is used to determine……
(a) Eddy current
(b) Hysteresis loss
(c) Copper loss at any load
(d) Iron loss at any load

Q3. Two coils with self-inductances 1 H and 2 H having a mutual inductance of 1 H between them carry currents of 2 A and √2 A respectively. The energy stored in the field, in joules, is
(a) 2(1+√2)
(b) 3(1+√2)
(c) 2(2+√2)
(d) 3(2+√2)

Q4. Ferranti effect happens in a transmission line when line is
(a) short and unloaded
(b) long and unloaded
(c) short and loaded
(d) none of the above

Q5. What is the ratio of starting torque and maximum torque of a 3-phase, 50 Hz, 4 pole induction motor for a maximum torque at 1200 rpm?
(a) 0.5
(b) 0.6
(c) 0.421
(d) 0.384

Q6. The system matrix of a linear time invariant continuous time system is given by

What are the roots of the characteristic’s equation?
(a) -1, -4
(b) -1, -5
(c) -4, -5
(d) 0, -1

Q7. The tendency of charge carriers to move from a region of heavily concentrated charges to region of less concentrated charge is known as.
(a) Depletion current
(b) Drain current
(c) Diffusion current
(d) Saturation current

Q8. The type number of the control system with G(s) H(s) = (K(s+2))/(s(s^2+2s+3)) is
(a) One
(b) Two
(c) Three
(d) Four

Q9. A spring-controlled moving iron voltmeter draws a current of 1 mA for full scale value of 100V. If it draws a current of 0.5 mA, the meter reading is
(a) 200 V
(b) 100 V
(c) 50 V
(d) 25 V

Q10. A wattmeter reads 30.34 W. The absolute error in the measurement is – 2.22 W. What is the true value of power?
(a) 25.45 W
(b) 28.12 W
(c) 34.56 W
(d) 30.34 W

SOLUTIONS
S1. Ans.(d)
Sol.
At high load, copper loss increases as Pc α I^2 (Where; I=load current)
Due to increased copper loss, efficiency decreases.

S2. Ans.(c)
Sol. SC test of transformer:
Used to determine copper loss or ohmic loss
LV side short circuited
Instruments kept on HV side
Also determines equivalent resistance and leakage reactance

S3. Ans.(c)
Sol.
Energy stored=[1/2 L1 i1^2+1/2 L2 i2^2+Mi1 i2 ]J
∴E=[1/2×1×4+1/2×2×2+1×2×√2]=4+2√2=2(2+√2) joules

S4. Ans.(b)
Sol.
Ferranti effect occurs for medium and long transmission line.
Due to Ferranti effect, receiving end voltage is more than sending end voltage.
It occurs at light loaded or no-load condition.

S5. Ans.(d)
Sol. N_s=120f/P=(120×50)/4=1500 rpm
∴s=(Ns-Nr)/Ns =(1500-1200)/1500=0.2
∴Tst/Tm =2/(s+1/s)=2/(0.2+1/0.2)=2/5.2=0.384

S6. Ans.(a)
Sol. characteristic’s equation=|sI-A|=0

∴s(s+5)+4=0
∴(s+1)(s+4)=0
⇒s=-1, -4

S7. Ans.(c)
Sol. In a semiconductor the charge will always have a tendency to move from higher concentrated area to less concentrated area to maintain equilibrium this movement of charges will result in diffusion current.

S8. Ans. (a)
Sol. Pole at origin is one. Type of the system is determined by open loop poles at origin.

S9. Ans.(d)
Sol. For moving iron instruments, deflection ⍺ I^2.
∴Reading1/(Reading 2 ) =(I1^2)/(I_2^2 )
∴100/V_2 =1^2/(0.5)^2 ⇒V2=25 V.

S10. Ans.(c)
Sol. True value is equal to difference between measured value and absolute error.
∴AT=30.34-(-2.22)=34.56 W.

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