Question

The polar plot of a type-1, 3-pole, open-loop system is shown in figure below. The closed loop system is

• A. always stable.
• B. marginally stable.
• C. unstable with one pole on the right half s-plane.
• D. unstable with two pole on the right half s-plane.

Answer 1

The correct option is D unstable with two pole on the right half s-plane.

Nyjquist plot will be:


$\therefore$ Number of encirclement of $(-1,j0)=-2$ and number of right sided poles in open loop system=0
$N=P-Z$
$\therefore -2=0-Z$
$\Rightarrow Z=2$
$\therefore$ Closed loop system is unstabe with two poles on the right half of s-plane.