Q1-Q2 Carry 1 mark each
Negative marking: 0.33 mark
Q3-Q5 carry 2 marks each
Negative marking: 0.66 mark
Total Questions: 05
Total marks: 8
Time: 12 min.
Q1. In a communication circuit employed to turn-off an SCR, satisfactory turn-off is obtained when
(a) Circuit time constant > device turn-off time
(b) Circuit time constant < device turn-off time
(c) Circuit turn-off time > device turn-off time
(d) Circuit turn-off time < device turn-off time
Q2. If the pu impedance of a single-phase transformer is 0.01+j0.05, then its regulation at 0.8 lagging will be:
(a) 3.8 %
(b) -3.8 %
(c) 2.2 %
(d) -2.2 %
Q3. For a tachometer, if θ (t) is the rotor displace in radians, e(t) is the output voltage and K_t is the tachometer constant in V/rad/sec, then the transfer function E(s)/Ө(s) will be
(a) K_t s^2
(b) K_t \/s
(c) K_t s
Q4. For the equation, s³ – 4s² + s+6 = 0 the number of roots in the left half of s – plane will be
Q5. Match the items in List-I (to) with the items in List-II (use) and select the correct answer using the codes given below the lists.
Improve power factor Shunt reactor
Reduces the current ripples Shunt capacitor
Increase the power flow in line Series capacitor
Reduce the Ferranti effect Series reactor
A B C D
(a) 2 3 4 1
(b) 2 4 3 1
(c) 4 3 1 2
(d) 4 1 3 2
Sol. The circuit should not turn-off before making off the thyristor. i.e., Circuit turn-off time > device turn-off time.
Sol. At lagging pf: V.R=(R_pu cos〖ϕ+X_pu sin〖ϕ)×100〗 〗
∴ V.R=(0.01×0.8+0.05×0.6)×100=3.8 %
Sol. In A.C. Tachometer output voltage is directly proportional to differentiation of rotor displacement.
e(t)α d/dt [θ(t)]
Taking Laplace transformation on both sides of above equation
So, transfer function
T.F.=E(s)/θ(s) =(K_t )_s
Given a characteristic equation,
Applying Routh’s method,
There are two sign changes in the first column, so no. of right half poles is 2.
No. of roots in left half of S-plane = (3-2) = 1
• Shunt reactors are connected at receiving end to improve voltage profile.
• Shunt capacitors are connected at the load end to improve pf of the system.
• Series reactors are connected in series with line and reduces current ripples.
• Series capacitor increases power flow in the line by reducing the net transfer reactance.