Question

The presence of transportation lag in the forward path of a closed-loop control system _______.

• A. decreases margin of stability
• B. increases margin of stability
• C. is zero
• D. maintains margin of stability

Hint:

Time delay is dangerous in control systems—it may not reduce gain, but always reduces phase margin.

Answer 1

The Correct Option is A

Step 1: What is transportation lag?
Also called time delay, it is represented as $e^{-sT}$ in Laplace domain.
Step 2: Effect on system response.
This causes a phase lag without affecting the gain magnitude, pushing the Nyquist plot closer to the critical point.
Step 3: Consequence on stability.
Time delay:
- Does not affect gain margin
- Reduces phase margin
- Hence reduces the margin of stability and may cause oscillations
Conclusion: Transportation lag $\boxed{\text{decreases margin of stability}}$