With reference to Variable Frequency Drives (VFDs) in pumping systems:1.VFDs save energy by varying the motor speed instead of using a throttle valve.2.According to affinity laws, reducing the pump speed by 20% reduces power consumption by nearly 50%.3.VFDs are most effective in systems with high static head.Which of the statements given above is/are correct?

Correct Answer: A - 1 and 2 only

Quick Concept Explanation

For centrifugal pumps in systems dominated by friction head, affinity laws show flow approximately proportional to speed, head to speed squared and power to speed cubed. VFD speed control therefore saves much more energy than throttling when variable flow is required. Key relation: Q∝N; H∝N²; P∝N³.

Exam focus: 20% speed reduction: power≈0.8³=0.512, about 49% saving. Common trap: High static-head systems reduce the benefit predicted by pure affinity laws.

Statement-wise Verification

• Statement 1 - Correct.
  VFDs save energy by varying the motor speed instead of using a throttle valve.
  VFD speed control therefore saves much more energy than throttling when variable flow is required.
• Statement 2 - Correct.
  According to affinity laws, reducing the pump speed by 20% reduces power consumption by nearly 50%.
  20% speed reduction: power≈0.8³=0.512, about 49% saving.
• Statement 3 - Incorrect.
  VFDs are most effective in systems with high static head.
  VFD savings are greatest where friction/variable head dominates; high static head reduces the cubic-law savings

Core Concept

For centrifugal pumps in systems dominated by friction head, affinity laws show flow approximately proportional to speed, head to speed squared and power to speed cubed. VFD speed control therefore saves much more energy than throttling when variable flow is required. Savings are less dramatic when static head dominates.

Formula / Key Relationship

Q∝N; H∝N²; P∝N³.

Why the Other Options Are Wrong

Option B is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 1. Option C is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 2. Option D is incorrect because it includes incorrect statement(s) 3.

Exam Shortcut / Approach

20% speed reduction: power≈0.8³=0.512, about 49% saving.

Common Mistake

High static-head systems reduce the benefit predicted by pure affinity laws.

Quick Revision

Pump Affinity Laws and VFD Savings is linked with Variable Frequency Drives, Pumping Systems and Energy Management. 20% speed reduction: power≈0.8³=0.512, about 49% saving.