Question

The pole zero map and the Nyquist plot of the open-loop transfer function G(s) H(s) of a feed back system are shown below for this,

• A. Both open-loop and closed-loop systems are stable.
• B. Open-loop system is stable but closed-loop system is unstable.
• C. Open-loop system is unstable but closed-loop system is stable.
• D. Both open-loop and closed-loop systems are unstable.

Answer 1

The correct option is B Open-loop system is stable but closed-loop system is unstable.

From plot -1
Number of poles on the right side of s-plane = 0
P = 0
Open-loop system is stable.
From plot -2
Number of encirclement about (-1, 0) is = 2 in clockwise
N = -2
N = P - Z
-2 = 0 - Z
Z =2
Two closed loop poles on the right side of s-plane. Close-loop system is unstable.