The ability to precisely control process rates and achieve energy savings has made the application of variable frequency drives (VFDs) widespread, and they are increasingly applied to loads in difficult environments. As with many electronic devices, environmental conditions can be a key factor in life span and reliability; temperature, humidity, shock and vibration, sun load, air cleanliness and quality are all factors that can affect the expected life span of VFDs.
There are a variety of factors to consider to ensure VFDs are meeting site requirements. In addition, there are third-party testing agencies, including Underwriters Laboratories (UL), that can assure both VFD components and engineered assemblies are applied appropriately in given circumstances.
If VFDs were applied in conjunction with an uninterruptible power supply (UPS) system, the technological peers of VFDs, then the number of premature failures and maintenance requirements would be reduced. However, the reality of application conditions is that VFDs are installed in mechanical rooms, outside on rooftops and other areas that would invariably cause computer servers to fail.
Common Environmental Concerns in VFD Applications
Poor air quality
The most pervasive problem is air quality. Poor air quality exists in many installations. For example, caustic chemicals are often airborne in water and wastewater applications; these chemicals rapidly break down the dielectrics and circuit boards in VFDs. Hydrogen sulfide and airborne chlorine gases are prime culprits. The only way to guard against these chemicals is to have all the drive boards conformal coated. The coating is able to reduce the rate of breakdown; however, it will not completely eliminate it. Conformal coating should be required on nearly all VFDs in wastewater and water treatment facilities.
Airborne particles
Airborne oil, site-specific debris—including feathers, cotton or lint—and other airborne particles can affect the life span of VFDs. Specifically, oily debris and large matter can build up and clog the narrow fins of a VFD heat sink over time, limiting airflow and causing overheating conditions in the VFD power unit.
Further impeding performance, most medium or larger horsepower (10 hp and up) VFDs have fans where the debris can get caught or build up. Even when installed in a National Electrical Manufacturers Association (NEMA) 12 cabinet, the VFD heat sink typically extends from the back of the unit and is cooled by ambient air rather than the air from within the cabinet.
In situations that require the VFD to be placed in an oily or excessively contaminant-filled environment, it is advised that the cabinet be sealed, the heat sink be inside the cabinet, and forced air cooling with filters be used. Proper filter selection and maintenance is required to provide adequate cooled air to the drive.
UL508C*, the UL standard for solid-state power conversion devices, requires the drive assembly to be loaded and heat tested with half of the filter covered, to simulate a clogged filter. Selecting UL508C-rated VFD assemblies can mitigate risk of improper fan or filter selection.
Coal dust and other small debris cannot be effectively filtered due to particulate size, but they do not pose a grave danger to clogging the heat sink unless other contaminants, like oil, are present. If a filtered solution on a higher power unit is not practical due to airflow requirements, then a scheduled maintenance program may be required to facilitate cleaning of the heat sink, fans and other components. In some applications, sealed NEMA 12 cabinets with air conditioning are used to address these issues. Although this can be effective, it uses large amounts of energy, negating much of the savings from employing the VFD.
Thermal considerations
When placing VFDs in an air-conditioned equipment room is not practical, ambient heat can pose a problem. Nearly all VFDs are rated at either 40 C or 50 C, with some at 45 C. Most enclosed VFD assemblies are rated at 40 C. This is adequate for many installation sites when adequate cooling airflow is available.
The VFDs are rated at these temperatures at full-load current with either a high (150% of full load for
1 minute) or low (110% of full load for
1 minute) overload rating. This current
rating translates to a horsepower rating, typically based on National Electrical
Code (NEC) values. Often, VFDs are
de-rated by application, approximately 1% per degree Celsius for higher than nameplate ambient temperatures.
This approach can bring negative
repercussions with NEC compliance. For this reason, it can be preferable to apply VFD assemblies that have UL508C listings at the required temperature, thereby satisfying NEC requirements.
A misconception of UL applicability to VFDs is that UL508A and UL508C can be used interchangeably. UL508A was written for industrial control panels, including relay panels or other electromechanical devices; it can only be applied at 40 C as a base rule. A further concern, UL508A does not require a heat-run test, as the standard was not written around or intended to be used with power conversion devices, which induce significant thermal considerations.
A UL508C-rated drive component can be installed into an assembly and rated with UL508A without any actual or design thermal testing performed. In fact, in a UL508A panel, the only requirements are that the wire and short circuit devices be of appropriate size and dielectric spacing. The UL508A standard is far laxer than the UL508C standard with regards to testing and certification requirements, and use of UL508A assemblies can be risky in harsh environments. Therefore, from a quality control perspective—especially in harsh (greater than 40 C) environments—it is imperative to apply VFD assemblies that have a UL508C label.
Solar heat gain
Further complicating matters, solar heat gain must be accounted for when considering thermal selection in outdoor applications. The American Society of Heating, Refrigerating & Air-Conditioning Engineers (ASHRAE) has standards for calculating solar heat gain based on surface area, absorption coefficient and the angle of the sun to the enclosure. These calculations are only suitable for sealed VFD cabinets.
One way to impact this gain is by enclosure paint selection. The coefficient of gain varies greatly from 0.15 for white, 0.30 to 0.50 for gray and upwards of 0.97 for black. Selecting a color that minimizes solar loading, using sun shields and orienting the enclosure to reduce direct time in the sun can all serve to minimize the amount of thermal absorption by the VFD enclosure.
However, a far more effective approach than sizing VFDs into sealed boxes is to mount the VFD using enclosure cooling or mounting the VFD heat sink outside the enclosure. Although using cooling fans to exhaust hot air can be effective, cooling the VFD heat sink with ambient air effectively eliminates solar gain concerns by removing the heat and cooling air from the enclosure itself.
In conclusion, there are numerous environmental conditions that need to be considered in VFD applications. It is imperative to neutralize the effects of adverse conditions—whether dirt, heat, chemicals or sun load—to achieve long equipment life and energy savings. Finally, using UL508C-listed assemblies gives users and consultants knowledge that a configuration has been rigorously tested and is suitable for use at its rated temperature.
*UL508C standard is replaced by UL61800-5-1. However, VFDs with either of these standards are available in the market. Users should ensure that the appropriate guidelines are met when installing a VFD.