New Advances in Variable Frequency Drives


Written by:
Steven B. Weston, Eaton Corporation

Variable frequency drive technology is more efficient and accurate, which leads to increased energy savings. Advancements in capacitors and direct current (DC) link reactors, IGBTs, heat management, processing power and measuring technology are all enabling the development of solutions to problems that were not even recognized earlier. Additionally, new and advanced algorithms affect energy efficiency.

High Efficiency Capacitors

In the most advanced drives, the state-of-the-art capacitor uses a new type of plastic film capacitor . This device stores energy from the rectifier, and is part of a LC filter that reduces ripple on the DC bus and minimizes current surges. The advancements in capacitor technology are yielding net energy savings while limiting toxins.

Previous capacitor technology relied on electrolyte capacitors. Today, new capacitors use thin, metalized film capacitors, which do not contain toxic electrolytes. Capacitors are non-toxic and do not leak electrolytes into landfills; they are fully Restriction of Hazardous Substances (RoHS) compliant. In addition, metalized film capacitors are self healing-preventing the occurrence of hot spots and extending the life of the product, while decreasing heat loss.

State-of-the-art capacitors do not require re-charging, even after years of sitting on the shelf.

Ultimately, these advances in technology allow the newest capacitors to offer 80 percent less power loss than older capacitor technology. As a result, there are less power losses, increased efficiency and lower heat output within a drive package. Since heat is the greatest threat to the drive, these drives will have a longer life span.

DC Choke

In the latest generation of adjustable frequency drives, the internal line reactor is replaced with a built-in DC link choke. In earlier drives the internal line reactor provided protection for the AC diode and some amount of harmonic mitigation; the line reactor is most effective at the highest order of the harmonic spectrum.

The DC link choke performs a similar function as the AC line reactor; that is, it prevents harmonic distortion from getting back on the line and acts 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 users to meet the more rigid requirements of 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 both for radiated and conducted emissions specified in 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 resulted 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 overall increased efficiency of the drive.

Heat Management

In HVAC applications, drives are typically used to improve or increase energy savings while providing system efficiency. VFD manufacturers are now taking their own advice and using the same technology on the drive.

All drives generate heat, and use fans to cool themselves. Typically, cooling fans used to run at full speed whenever the drive was energized. Newer drive technology monitors the heat-sink temperature, and the fans provide only as much cooling as is necessary (0 to 100 percent), based on the ambient temperature and the drive load. This makes the fans inside the drives another variable speed application. Essentially, drives in HVAC applications are used to make fans more efficient; now, drive manufacturers are making the internal fans in the drives more efficient.

Closely matching cooling requirements to fans speed can net a 20 to 60 percent reduction of energy used to control these fans. Less losses means increased energy savings for the drive users.

Less Power Loss from Semiconductors

New generation power semiconductors-insulated gate bipolar transistors (IGBTs)-are able to operate at higher switching frequencies, resulting in less power losses and reduced heat within the drive. As this is a core component of the drive technology, it has a huge impact on overall drive efficiency.

Advanced Power Management Control (Sleep Functionality)

In run mode, a drive consumes a tremendous amount of energy even if a zero speed reference is given. This is due to the fact that most systems require a minimum speed (12 Hz for example) to keep the TEFC motor cooled.

Some drive manufacturers now offer an enhanced sleep function, which allows the drive to shut down (remove run command) when it is not needed. Typically this is used in a fan or pump application with the drive's internal PID loop controlling speed. A PID loop uses a set point and feedback from the system to determine the correct speed. When the feedback from a remote sensor indicates a low or zero speed is required the drive is able to turn itself off, or remove the run command but return to run mode when the system indicates a higher speed is required. This same function can be performed using a costly PLC with extensive programming.

Drives can now be better tuned to actual energy needs, tapering system requirements during off hours, weekends and holidays-allowing customers to see significant cost savings in their energy bill.

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