What Have I Learned? – Wiring and VFD Problems

While wiring problems may not be pervasive, when they do exist, the upset is significant and tends to persistently reoccur for months to years. The intermittent transient occurrence is difficult to diagnose. The short term steps and spikes cause kicks in the controller output from gain and rate action. In my experience, the spikes in pH transmitter outputs often go unresolved. The article by Fred Sanders titled “Watch Out for Instrument Errors” (Chemical Engineering, July 1995) gives examples of the insidious and disruptive nature of wiring problems.

The problems seemed to get worse in the 1980s and 1990s when new low cost invertors were installed for variable frequency drives (VFD). The VFD is also known as a “variable speed drive” (VSD) because the speed of the motor is regulated to be proportional to the controller output. In a recent conversation with Owen Campney, I got an idea of what happened to make the VFD inverter noise a bigger problem.

The switching in the new invertors was faster creating sharper edges. This reduced the heat, size, and cost of inverters making them more abundant. Instead of being in dedicated inverter rooms in the motor control center, they started to appear in instrument rooms among the interface panels. The faster switching created higher frequencies with higher energy. These invertors had an output choke to prevent damage to motor insulation but the input choke was optional and was often missing or insufficient. Eventually, the noise in instrument signals became bad enough that chokes were offered to meet the International Electrochemical Commission (IEC) standards. Alternatively, isolation transformers were located close to the inverter with the power wiring between the inverter and transformer in hard pipe conduit to minimize the noise from this section of wiring. You hear VFD war stories to this day.

One such war story was related to me by Owen. A large intermediate plant installed smart HART input cards on some critical HART transmitters to take advantage of the diagnostic information digitally superimposed on the analog signal. Unfortunately, 3 to 6 times each day the digital signal would be momentarily lost. It was suspected but never confirmed to be triggered by a VSD on coolant in a temperature loop when demand changed. The problem persisted for several years until an electrical engineer patiently tracked down the problem to a second ground hidden from view in the wall.

Whether the steps, spikes, and noise in an instrument signal is due to wiring or not, the wiring is always suspect particularly if the user has been burned by VFD incidents. Consequently more hours are wasted than is generally recognized on trying to track down real or imagined wiring problems. The WirelessHART network should be immune to the VFD interference and eliminate the wiring questions. The potential savings from WirelessHART in maintenance cost may be currently underestimated.

For more information on WirelessHART checkout the article “Is Wireless Process Control Ready for Prime Time?” in the May 2009 issue of Control magazine.