Understanding Harmonics and Their Impact on Variable Frequency Drives
Harmonics are a common phenomenon in electrical systems, particularly in alternating current (AC) circuits. They are waveform distortions that can significantly affect the performance of electrical equipment, including variable frequency drives (VFDs). This article explores what harmonics are, how they are generated, and their effects on VFDs, along with methods to mitigate them.
What Are Harmonics?
Harmonics are currents that flow at multiples of the fundamental frequency. The fundamental frequency is the natural frequency of the AC system. Harmonic currents do not exist in a pure AC waveform; instead, they are distortions that occur due to non-linear loads or equipment. These non-linear loads include rectifiers, switch-mode power supplies, and variable frequency drives.
How Harmonics Affect Variable Frequency Drives
AC to DC Conversion: One of the key components of a VFD is the rectifier section, which transforms the incoming AC power into DC power. This process is crucial because DC is easier to control and manage. The DC link acts as a buffer, storing energy and helping to smooth out the waveform. However, the initial conversion does not inherently generate harmonics, as the DC output doesn't contain these distortions.
DC to AC Inversion: After the rectification, the next step is the inversion process, where the VFD converts the DC power back into AC. This inversion is achieved using power electronics, which switch transistors rapidly to recreate an AC waveform. The rapid switching action is what generates the harmonic content. These harmonics can be quite significant and are characterized by higher frequencies that can cause various issues in the electrical system.
Reflective Harmonics: The harmonics generated by the VFD are not confined within the drive itself. Instead, they can be reflected back into the power supply and even back into the connected power system. This can create a ripple effect, where these harmonics can impact other electrical equipment, causing additional stress and reducing overall system efficiency.
Managing Harmonics with Reactors and Proper Front Ends
To minimize the impact of harmonics and prevent them from causing significant damage, reactive components are often employed. Here are some effective solutions:
Supply Reactors: Power supply reactors are installed to provide inductive reactance, which limits the flow of harmonic currents. By adding a reactor, the inductance of the circuit increases, making it more difficult for the harmonics to pass through. This reduces the distortion and helps maintain a cleaner, more stable power supply. VFD Output Reactors: Similarly, output reactors are used at the VFD’s output to absorb and dampen the harmonics. They act as filters, reducing the reflective nature of the harmonics and mitigating their impact on the connected system. Front End Configuration: The choice of front end topology also plays a critical role in reducing harmonics. Systems with 12-pulse and 18-pulse converters are commonly used because they further divide the input waveform into smaller, more manageable segments. This method effectively reduces the harmonic content by spreading out the distortion across a wider frequency range.Conclusion
Harmonics are a natural byproduct of AC systems and can have significant impacts on electrical equipment, particularly variable frequency drives. However, through proper design and implementation, these impacts can be minimized. By using reactors and adopting efficient front end configurations, harmonics can be managed effectively, leading to a more stable and reliable power system.