Drive actively eliminates harmonic disturbances
New VLT Low Harmonic drives from Danfoss address current distortion by performing real-time analysis and actively imposing currents, as needed, to ensure the highest quality sine waves possible from the power supply grid. VLT Low Harmonic drives are appropriate for installations that are generator-powered, or that have generator back-up power, soft power grids, or grids with limited excess power capacity.
"In essence, the filter portion of VLT Low Harmonic drives has the same working principle as a set of noise cancelling headphones, where the noise or distortion is measured and a computer phase signal is imposed to compensate for that noise," explains Gregers Geilager, product manager. "As a matter of scale, noise cancelling headphones can inject 100 mW at 50 – 1000 Hz, where VLT Low Harmonic drives can inject several hundred watts at 250 – 2000 Hz,"Geilager says that although only one out of eight motors currently is operated by a frequency converter, the increased use of ever more powerful frequency converters means the need to prevent harmonic disturbances is growing significantly.
"It’s expected that the huge potential for energy savings by employing frequency converters will increase the number of drives in use-and hence the potential harmonic disturbance on the supply grid, and the need for smarter harmonic mitigation solutions," Geilager says.
VLT Low Harmonic drives cause no increased winding stress and have no impact on bearing life, says Geilager. "They provide the user with a full readout of the unit performance towards the grid, including a graphical overview of grid behavior. Where the performance of other low harmonic technologies depend on the stability of the grid and load or affect the controlled motor, these drives continuously regulate the network and load conditions without affecting the connected motor. In fact, VLT Low Harmonic drives are motor friendly, with output impulse and shaft voltages compatible with motors conforming to IEC 60034-17/25 & NEMA-MG1-1998 part 18.104.22.168, as per standard VLT drives," he says.
A unique design uses a ducted back channel to pass cooling air over heat sinks with minimal air passing through the electronics area. This allows 85% of the heat losses to be exhausted directly outside of the enclosure, says Geilager. There is an IP 54 seal between the back-channel cooling duct and the electronics area of the low harmonic drive.
VLT Low Harmonic drives share the same modular technology platform as Danfoss’ VLT High Power drives and share many of the same features, such as high energy efficiency, backchannel cooling, and user-friendly operation. The modular design means most elements are cost-effectively produced in large scale, enabling individually configured units to be highly customized but available at the price of a mass produced unit, says Geilager.