Harmonic distortion is the presence of electrical frequencies at multiples of the fundamental 60 Hz frequency caused by nonlinear loads, degrading power quality and reducing equipment efficiency. Variable-frequency drives, switched-mode power supplies, and solid-state controls are primary sources of harmonic currents in HVAC systems.
Technical Characteristics
Harmonics occur at 120 Hz (2nd), 180 Hz (3rd), 240 Hz (4th), and higher multiples of 60 Hz. Total harmonic distortion (THD) expresses harmonic magnitude as percentage of fundamental frequency. IEEE 519 limits current THD to 20% for general industrial facilities and 5% for utilities. HVAC variable-frequency drive loads typically generate 30-50% THD without filtering.
HVAC System Impacts
Harmonic currents cause motor winding overheating, reducing bearing life by 10-30% and increasing failure rates. Neutral conductors overheat when third harmonic currents from multiple phases sum in the neutral. Electromagnetic interference disrupts building automation system communication. Motor efficiency losses reach 5-15% in high-distortion environments.
Mitigation Solutions
Line reactors reduce VFD-generated harmonics by 40-50%. 18-pulse or 24-pulse converters reduce THD to 5-10%. Active harmonic filters eliminate 90%+ of distortion. Costs range from $200-2,000 per drive, justified when motor life extension and efficiency gains exceed mitigation expenses.