VSDs Radiate H-Field Noise in LF Range

Posted on Dec 14, 2010

Problem: Client has sensitive imaging equipment located in a newly-designed “green” building that does not operate properly in its designated location, but operates fine in other areas of the room.

Seems you can’t do anything lately without a reference to Green technology.  That was the driving force behind a recent client’s crisis. Their customer had built a brand-new, state-of-the-art, energy-efficient facility and was experiencing repeat failures of an imaging sensor in the room built solely for that purpose.  We received a call from the imaging sensor equipment supplier who was baffled by the failures. Through a months-long effort by their field engineers, they had determined that the equipment would work in certain areas, but not inside a large circle in the center of the room. They were at a loss to understand exactly what the issue was, so we agreed to look at the problem on-site.

We did all the usual measurements, looking at low frequency AC fields, nearby transmitters, power wiring problems, but nothing showed up.  They did mention that the circle was smaller when certain pieces of building equipment were shut down, which gave us a vital clue. Turns out the HVAC blowers were on the “hot” circuit, which was routed close by the imager. Normally this isn’t an issue; induction machines don’t cause problems like this. But, this building had variable speed drives (VSD) on every big motor to allow for energy conservation by allowing the motors to operate at varying speeds. We had recently seen some MAJOR conducted noise coming from another client’s VSD, and thought that may be the culprit. After additional diagnostics, both with us on-site and via phone consults, the VSD was indeed found to be the problem.

It turns out that many major VSD vendors don’t include adequate filters on the inputs and outputs, so they radiate a tremendous amount of H-field noise in the LF range due to their switching characteristics. This can wreak havoc on many sensitive electronic devices that aren’t specifically designed to reject that type of noise.  This experience is a good example of what happens when a new technology is rolled out: there’s usually some neglected electromagnetic compatibility issue that causes all sorts of trouble later down the line (pun intended).

Just like Kermit said, “..it ain’t easy, bein’ green”.

Solution: If VSD’s are in operation, they must be adequately filtered to avoid interfering with sensing or imaging equipment