A proven tool to reduce energy consumption, variable frequency drives (VFDs) became increasingly used during the 1970s energy crisis. Today, VFDs remain a critical component to control motor speed—improving efficiency, reducing wear and tear on valuable mechanical components, and improving system performance. However, modern drives are more effective and easier to use.
Fundamentally, VFDs control the frequency and voltage supply so the motor closely matches the speed requirements for an application. However, the latest generation of VFDs is more efficient, intelligent and user friendly—ultimately leading to increased energy savings for industrial applications.
Technology advancements in capacitors, direct current (DC) link reactors, insulated-gate bipolar transistors (IGBTs), heat management, processing power, graphical interfaces, communications and monitoring technology enable the development of solutions to problems that were not recognized in the past. Additionally, new and advanced performance algorithms enhance energy efficiency.
Drives can deliver energy savings while optimizing process reliability and protecting valuable assets by controlling the motor or pump and themselves.
Advanced Performance Algorithms
New algorithms can deliver significant energy savings and stability—establishing a new level of efficiency. A novel control algorithm for VFDs reduces the input power of an induction motor that is used to drive a variable torque load, such as a fan or pump. The algorithm dynamically adjusts the volts per hertz (V/Hz) relationship supplied to the motor while maintaining optimum magnetization current to provide improved energy efficiency and reduced electrical costs. This algorithm can also be applied to constant torque applications that would not typically realize energy savings by using a VFD.
The energy-optimizing algorithm begins when the drive commands the motor to follow a reference frequency or when the user enters a new reference frequency. To assure motor stability, the algorithm initially sets the drive output voltage at the same level as the voltage based on the linear V/Hz method for the same reference frequency. It then reduces the voltage incrementally to optimize energy use.
Meanwhile, the algorithm monitors several real-time motor parameters to prevent the motor from experiencing conditions that may lead to instability. When the motor enters the optimal zone of operation, the drive output voltage remains at the same level until a change triggered by commands to the drive occurs—such as a change in the reference frequency or a change in real-time parameters. After the output voltage stabilizes, the drive monitors the motor’s real-time parameters to prevent instability conditions and motor current rise.
Experimental results demonstrate that these algorithms are capable of assuring motor stability and achieving superior energy savings compared with other static or dynamic V/Hz control methods. By using VFDs with this technology, users can go beyond drive efficiency and ultimately improve the efficiencies of the motor and the application for superior energy savings.
From Line Reactors to DC Chokes
In the latest generation of VFDs, the internal line reactor is replaced with a built-in DC link choke with input surge protection. The internal line reactor provided protection for the alternate current (AC) diode and harmonic mitigation to the line, preventing harmonic distortion from returning to the distribution channel. The line reactor is most effective at the highest order of the harmonic spectrum.
The DC link choke performs the same task as the line reactor—preventing harmonic distortion from returning on the line and acting as a filter to smooth the ripple on a DC bus. While it reduces harmonics similarly to the AC line reactor, the DC link choke reduces harmonics across the entire harmonic spectrum.
In turn, this allows end users to meet the more rigid requirements of International Electrotechnical Commission (IEC) 61800, which looks at individual harmonics, not just the total harmonics of the system. The built-in DC link choke complies with the C2 category for both radiated and conducted emissions specified in European Norm (EN)/IEC 61800-3.2 for commercial and industry environments. In other words, all drive manufacturers will need to use a DC link choke to meet this emissions standard in the next generation of drives.
Further, the DC link choke has the added benefit of a lower voltage drop than the equivalent AC line reactor, translating into overall increased efficiency for the drive. Specifically, the AC line reactor typically results in a 2 to 4 percent voltage drop, whereas the DC link choke has a voltage drop of less than 1 percent. This translates into an overall increased drive efficiency.
Enhanced Graphical Displays & Monitoring
The next generation of drives also has enhanced graphical displays and monitoring capabilities that support a shift to user-friendly interfaces. The days of referencing keypad-displayed codes in user manuals are ending with scrolling text descriptions of parameters, monitoring values, fault codes and menu structures.
In addition, users are shifting to threshold functionality, which provide the ability to customize text or units displayed for their specific application. Drive keypads are also starting to provide the ability to define user configurable soft keys on the keypad to create specific functions that enhance diagnostics or convert a process from complex to simple with the press of a button.