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DFCCIL’21 EE: Daily Practices Quiz 21-Aug-2021

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               DFCCIL’21 EE: Daily Practices Quiz 21-Aug-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 06 min.

Q1. Two heaters, rated at 500 W, 250 V each are connected in series across a 250V, 50 Hz mains. The total power drawn from the supply would be _____ watt.
(a) 500
(b) 250
(c) 1000
(d) 125

Q2. The RMS value of the voltage V(t) = 3 + 4cos (3t) is:
(a) 5 V
(b) 7 V
(c) 4.12 V
(d) 3 + 2 (1.41) V

Q3. Two identical heaters, each marked 1000 W,250 V are placed in parallel with each other and connected to a 250V supply. Their combined rate of heating will be:
(a) 250 W
(b) 500 W
(c) 1000 W
(d) 2000 W

Q4. An open loop transfer function is given by G(s)=K(s + 1)/(s + 4)(s^2+3s+ 2) . It has:
(a) One zero at infinity
(b) Three zeroes at infinity
(c) Two zeroes at infinity
(d) None of the above

Q5. Cogging of induction motor occurs at
(a) High voltage and when stator and rotor poles are equal
(b) Low voltage and when stator and rotor poles are equal
(c) High voltage and when stator and rotor poles are not equal
(d) Low voltage and when stator and rotor poles are not equal

Q6. Hot wire ammeters are used for measuring:
(a) only AC
(b) only DC
(c) neither AC nor DC
(d) both AC and DC

SOLUTIONS

S1. Ans.(b)
Sol. Power of each bulb = 500W
Voltage= 250V (same as voltage rating of source)
Both are connected in series.
∴P_T=(P_1 P_2)/(P_1+P_2 )=(500×500)/(500+500)=250 W

S2. Ans.(c)
Sol. V_rms=√(3^2+(4/√2)^2 )=√17=4.12 V

S3. Ans.(d)
Sol. Power of each bulb = 1000W
Voltage= 250V (same as voltage rating of source)
Both are connected in Parallel.
∴P_T=P_1+P_2=1000+1000=2000 W.

S4. Ans.(c)
Sol. The formula to calculate a number of zeroes at infinity is P – Z. Here, P and Z are the number of poles and zeroes in a given transfer function.
(s)=K(s + 1)/(s + 4)(s^2+3s+ 2) =(K(s+1 ))/((s+4)(s+1)(s+2))
Here, P=3 & Z=1
∴ So, the number of zeroes at infinity = P – Z= 3 – 1 = 2.

S5. Ans.(b)
Sol. Starting Torque ∝ V². When the supply voltage is low, the starting torque is low and when the poles on stator and rotor are equal, due to locking tendency motor caused by the slot harmonics reduced/failed to start.

S6. Ans.(d)
Sol. Hot wire instruments works on the heating effect of the current. The length of the wire increases because of heating effect of the current flowing through it. It is used for measurement of both ac and dc.

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