Practical Power Plant Engineering

15Variable Frequency Drives (VFDs) and Harmonics

CHAPTER MENU

• 15.1 Why Are Variable Frequency Drives (VFDs) Needed?
  • 15.1.1 Introduction
  • 15.1.2 Principles of Operation
  • 15.1.3 Power and Torque
• 15.2 Vector VFDs for Low‐Speed Operation
• 15.3 VFDs: Variable or Constant Torque?
• 15.4 Regenerative VFDs
• 15.5 Motor and Cable Harmonics Issues
• 15.6 How to Mitigate the Harmonics?
  • 15.6.1 What are Harmonics?
  • 15.6.2 Mitigation of Harmonics
• 15.7 Harmonic Order Limits
• References

15.1 Why Are Variable Frequency Drives (VFDs) Needed?

15.1.1 Introduction

Electric motor systems are responsible for more than 65% of power consumption in industry today. Optimizing motor control systems by upgrading to variable frequency drive VFDs can reduce energy consumption for some applications by as much as 70%. Combining energy efficiency tax incentives and utility rebates, returns on investment for VFD installations can be as little as six months.

Since 1985, motor control technology has changed, and a new method of speed control using variable frequency has been accepted for the industry automation. VFDs come in different sizes to match the motor kW ratings (see Figure 15.1).

Figure 15.1 VFD cabinets.

Before the advent of VFDs, a common method of varying motor speed was in steps by means of changing the resistance of the rotors of induction motors through external resistors. Another option was to use ...