Previous Year Questions- Root Locus Techniques | Control Systems - Electrical Engineering (EE) PDF Download

Q1: Consider the closed-loop system shown in the figure with
 The root locus for the closed-loop system is to be drawn for 0 ≤ K 
lt ∞. The angle of departure (between 0° and 360°) of the root locus branch drawn from the pole (−1 + j2), in degree, is ______ (rounded off to the nearest integer).  (2024)
(a) 2
(b) 9
(c) 7
(d) 11
Ans: (c)
Sol: 
Q2: The open loop transfer function of a unity gain negative feedback system is given by
The range of k for which the system is stable, is  (2022)
(a) k > 3
(b) k < 3
(c) k > 5
(d) k < 5
Ans: (c)
Sol: Characteristic equation:
R-H criteria:
Hence, for stable system,
k−5>0 ⇒ k>5

Q3: The root locus of the feedback control system having the characteristic equation s² + 6Ks + 2s + 5 = 0 where K > 0, enters into the real axis at  (SET-2 (2017))
(a) s = -1
(b) s = −√5
(c) s = -5
(d) s = √5
Ans: (b)
Sol: The point at which root locus enters real axis (break away point) is given by

Q4: The gain at the breakaway point of the root locus of a unity feedback system with open loop transfer function  is  (SET-2(2016))
(a) 1
(b) 2
(c) 5
(d) 9
Ans: (a)
Sol: Now, characteristic equation,
For break away point: dK/ds = 0
Therefore valid break away point is s = 2
Now gain at s = 2 is
K = (Product of distances from all the poles to break away point)/(Product of distances from all the zero to break away point)
Gain,

Q5: An open loop transfer function G(s) of a system is  For a unity feedback system, the breakaway point of the root loci on the real axis occurs at,   (SET-2 (2015))
(a) -0.42
(b) -1.58
(c) -0.42 and -1.58
(d) none of the above
Ans: (a)
Sol: Only s = -0.422 lie on root locus. Therefore brakaway point is s = -0.42.

Q6: The open loop poles of a third order unity feedback system are at 0, -1, -2. Let the frequency corresponding to the point where the root locus of the system transits to unstable region be K. Now suppose we introduce a zero in the open loop transfer function at -3, while keeping all the earlier open loop poles intact. Which one of the following is TRUE about the point where the root locus of the modified system transits to unstable region?  (SET-1(2015))
(a) It corresponds to a frequency greater than K
(b) It corresponds to a frequency less than K
(c) It corresponds to a frequency K
(d) Root locus of modified system never transits to unstable region
Ans: (d)
Sol: as root locus never cross asymptotes so, will remain in left sideof x-axis.

Q7: The root locus of a unity feedback system is shown in the figure.
The closed loop transfer function of the system is  (SET-1(2014))
(a)
(b)
(c)
(d)
Ans: (c)
Sol: This is converse root locus having no zero.
As, its G(s) H(s)
Its converse root locus only valid when K < 0
So, C.L.T.F.

$=\frac{G}{\left(}=\frac{\frac{K}{(}}{1}=\frac{K}{(}$Q8: The open loop transfer function G(s) of a unity feedback control system is given as
 From the root locus, at can be inferred that when k tends to positive infinity,  (2011)
(a) Three roots with nearly equal real parts exist on the left half of the s-plane
(b) One real root is found on the right half of the s-plane
(c) The root loci cross the jωjω axis for a finite value of k; k ≠ 0
(d) Three real roots are found on the right half of the s-plane
Ans: (a)
Sol: Characteristic equation, 1 + G(s)H(s) = 0
Routh array:
As k > 0, there is no sign change in the 1st column of routh array. So the system is stable and all the three roots lie on LHS of s-plane.
For k > 0 (k ≠ 0), none of the row of Routh array becomes zero. So root loci does not cross the jω-axis.
Number of Zero = Z = 1
Number of poles = P =3
Number of branches terminating at infinity = P - Z = 3 - 1 =2
Angle of asymptotes  = Since, all the three branches terminates at  So, all the three roots have nearly equal real parts.

Q9: The characteristic equation of a closed-loop system is s(s+1)(s+3)k(s+2) = 0, k > 0. Which of the following statements is true?  (2010)
(a) Its root are always real
(b) It cannot have a breakaway point in the range −1< Re[s] < 0
(c) Two of its roots tend to infinity along the asymptotes Re[s] = -1
(d) It may have complex roots in the right half plane.
Ans: (c)
Sol: Characteristic equation,
G(s)H(s) = open-loop transfer function (OLTF) = k(s+2)/s(s+1)(s+2)
Number of zeros = Z = 1 zero at -2
Number of poles = P = 3 poles at 0, -1 and -3
Number of branches terminating at infinity
= P − Z = 3 − 1 = 2
Angle of asymptotes  
Breakaway point lies in the range −1 < Re[s] < 0  and two branches terminates at infinityalong the asymptotes Re(s) = −1.

Q10: A closed-loop system has the characteristic function (s² − 4)(s + 1) + K(s − 1) = 0. Its root locus plot against K is  (2006)
(a) (b) (c) (d) Ans: (b)
Sol: Characteristic function
 Zero of OLTF s = 1; z = 1
Poles of OLTS s = -1, -2, +2, P = 3
The root locus starts from open-loop poles and terminates either on open-loop zero or infinity.
Root locus exist on section of real axis it the sum of the open-loop poles and zeros to the right of the section is odd.
Number of branches teminating on infinity.
= P - Z = 3 - 1 = 2
Angle of asymptotes
Intersection of asymptotes on real axis (centroid)
Option (B) is correct on the basic of above analysis.

Q11: Figure shows the root locus plot (location of poles not given) of a third order system whose open loop transfer function is (2005)
(a) K/s³
(b)
(c)
(d)
Ans: (a)
Sol: These are three asymptotes with angle 60°, 180° and 300°
Angle of asymptotes
Where, k = 0, 1, 2 up to (P-Z)-1 as angle are 60°, 180° and 300°
it means P - Z = 3
Intersection of asymptotes on real axis
Since, system does not have zeroes
As asymptotes intersect at origin, it means all the three poles are on right
Hence, Option (A) is correct.

Q12: A unity feedback system has an open loop transfer function, G(s) = K/s². The root locus plot is  (2002)
(a) (b) (c) (d) Ans: (b)
Sol: Angle of asymptotes = 90°, 270°
∴ Option (B) is correct.

Q13: In case of an armature controlled separately excited dc motor drive with closedloop speed control, an inner current loop is useful because it  (2001)
(a) limits the speed of the motor to a safe value
(b) helps in improving the drive energy efficiency
(c) limits the peak current of the motor to the permissible value
(d) reduces the steady state speed error
Ans: (c)
Sol: Closed loop system limits the peak value.