Regenerative Drives and EMC Filters
Mitigating the effects of inverter use and overcoming the technological issues they present, by Product Manager, Tom Downing.
Here at Roxburgh EMC a large part of what we do is to mitigate the downsides of inverter use so that industry, and more widely humanity, can benefit from their many virtues. However, as drives develop, our products must keep pace to ensure that they remain effective. Modern drive design can produce more demanding environments so we must innovate and upgrade in response.
Inverters, also called variable-frequency drives (VFD) or variable speed drives (VSD) or adjustable-frequency drives (AFD), variable-voltage/variable-frequency (VVVF) drives or AC drives, were described as early as 1925, have been in common use since the 60s and continue to be used in ever increasing numbers. This is generally a good news story as inverters extend motor life, improve quality control and reduce energy consumption by reducing load demand and improving power factor.
As an estimated 25% of the world’s electrical energy is consumed by electric motors in industrial applications, VFDs are providing a useful service in reducing wasted energy.
There have traditionally been some accompanying downsides; VFDs generate harmonics (distortions of the power supply sine wave) which can cause power loss, over-heating and unreliability. VFDs also create powerful radio frequency interference which can create potentially dangerous problems in electronic equipment and cause users to fall foul of local regulations. At Roxburgh EMC we provide comprehensive ranges of EMC filters, reactors and passive harmonic filters to mitigate these effects and allow inverters to be used to their full potential.
However, a new generation of inverters are gaining popularity. These are the active front end (AFE) drives, also called regenerative drives. AFE drives tend to incorporate harmonic filtering to mitigate harmonic distortions from entering and affecting the broader electrical network. By continuously counteracting harmonics created by non-linear current generated within the drive, active filtration brings harmonics within the required range. However, the regenerated power waveform is not always entirely free from distortion, hence a need for RFI filters that can cope with harsher waveforms whilst providing the necessary attenuation. Typical RFI filters then are more likely to fail their performance curves under regen mode.
AFE drives also further improve energy efficiency by taking waste energy that previously would have been dissipated as heat in braking resistors, and returning it to the power supply.
This means that AFE drives have two modes of operation; normal, when power is being taken from the supply, and regenerative, when power is being returned to the supply. In regenerative mode the power produced is not a clean sine wave such that would typically be provided by the power supply company.
Both normal and regenerative modes create RFI and an EMC filter is needed but now there are two problems. Firstly the noise profiles for the two modes are different. Perhaps more importantly though, most EMC filters are designed to be used with the undistorted sine waves produced by power generation companies. They behave very differently when connected to a drive running in regenerative mode. Typical effects are overheating, lost performance and, in some cases, filter failure.
At Roxburgh EMC we have been working with our customers to produce EMC filter solutions that provide the necessary attenuation performance but do not suffer the detrimental effects of regenerative wave forms. We have standard products for 3 phase applications up to 820A and a wealth of experience to design bespoke solutions.
Chat to us if you require advice with filter selection.