Consideration of environmental factors in the selection of the VFD

The ability to precisely control processing rates and achieve energy savings has generalized the application of frequency converters (VFDs) and they are increasingly applied to loads in harsh environments. As with many electronic devices, environmental conditions can be a key factor in durability and reliability; temperature, humidity, shock and vibration, solar charge, cleanliness and air quality are all factors that can affect the expected life of VFDs.

There are a variety of factors to consider to ensure that VFDs meet site requirements. In addition, there are third party testing agencies, including Underwriters Laboratories (UL), which can ensure that VFD components and engineering assemblies are applied appropriately under given circumstances.

If VFDs were applied in conjunction with an uninterruptible power supply (UPS) technology peers of VFDs, then the number of premature failures and maintenance requirements would be reduced. However, the reality of the application conditions is that VFDs are installed in mechanical rooms, outdoors on rooftops and other areas, which would invariably result in computer servers failing.

Common environmental concerns in VFD applications

Poor air quality

The most common problem is air quality. Poor air quality exists in many facilities. For example, caustic chemicals are often airborne in water and wastewater applications; these chemicals quickly break down the dielectrics and circuit boards of VFDs. Hydrogen sulfide and airborne chlorine gases are the main culprits. The only way to guard against these chemicals is to have a compliant coating on all drive boards. The coating is able to reduce the failure rate; however, it will not eliminate it completely. Compliant coating should be required on almost 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 of VFDs. Specifically, oily debris and large material can build up and clog the narrow fins of a VFD heat sink over time, restricting airflow and causing overheating conditions in the VFD power unit. .

Further hampering performance, most mid or higher horsepower (10 hp and above) VFDs have fans where debris can get stuck or accumulate. Even when installed in a National Electrical Manufacturers Association (NEMA) 12 cabinet, the VFD heat sink typically extends out the back of the unit and is cooled by ambient air rather than air. from inside the cabinet.

In situations that require the VFD to be placed in an oily environment or excessively filled with contaminants, it is advisable to seal the cabinet, place the heat sink inside the cabinet, and use heat sink cooling. forced air with filters. Proper filter selection and maintenance is necessary to provide sufficiently cooled air to the drive.

UL508C *, the UL standard for solid-state power conversion devices, requires the drive assembly to be charged and heat tested with half of the filter covered, to simulate a clogged filter. Selecting UL508C rated VFD assemblies can mitigate the risk of incorrect fan or filter selection.

Coal dust and other small debris cannot be filtered effectively due to the particle size, but it does not pose a serious danger of heat sink clogging unless other contaminants, such as oil, do not. are present. If a filtered solution on a higher wattage unit is not practical due to airflow requirements, a scheduled maintenance program may be required to facilitate cleaning of the heatsink, fans, and other components. . In some applications, NEMA 12 sealed cabinets with air conditioning are used to resolve these issues. While this can be efficient, it uses large amounts of energy, thus negating much of the savings made through the use of VFD.

Thermal considerations

When it is not practical to place VFDs in an air-conditioned equipment room, ambient heat can be a problem. Almost all VFDs are rated at 40 C or 50 C, with some at 45 C. Most closed VFD assemblies are rated at 40 C. This is adequate for many installation sites when a flow of cooling air occurs. adequate is available.

Frequency converters are rated at these temperatures at full load current with either high current (150% of full load for
1 minute) or low (110% of full load for
1 minute) of overload. This current
the rating results in a wattage rating, usually based on National Electrical
Code values ​​(NEC). Often VFDs are
derated per application, approximately 1% per degree Celsius for ambient temperatures above those stated on the nameplate.
This approach can provide
implications for NEC compliance. For this reason, it may be preferable to apply VFD assemblies that have UL508C listings at the required temperature, thus meeting 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 basic rule. Another concern, the UL508A standard does not require a hot test, because the standard was not written or intended for use with power conversion devices, which induces important thermal considerations.

A UL508C rated drive component can be installed in an assembly and rated UL508A without any actual thermal or design testing being performed. In fact, in a UL508A panel, the only requirements are that the wire and shorting devices are of the correct size and dielectric spacing. UL508A is much more lax than UL508C when it comes to testing and certification requirements, and using UL508A assemblies can be risky in harsh environments. Therefore, from a quality control point of view, especially in harsh environments (above 40 ° C), it is imperative to apply VFD assemblies bearing a UL508C label.

Solar heat gain

To further complicate matters, solar heat gain must be considered 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 area, absorption coefficient, and angle of the sun to the enclosure. . These calculations are only suitable for sealed VFD cabinets.

One way to have an impact on this gain is to select the cabinet paint. The gain coefficient varies greatly from 0.15 for white, 0.30 to 0.50 for gray and up to 0.97 for black. Selecting a color that minimizes solar charging, using a lens hood, and orienting the housing to reduce direct sunlight time can all serve to minimize the amount of heat absorption by the VFD housing.

However, a much more efficient approach than sizing VFDs in sealed enclosures is to mount the VFD using enclosure cooling or by mounting the VFD heat sink outside the enclosure. While using cooling fans to exhaust hot air can be effective, cooling the VFD heat sink with ambient air effectively eliminates solar gain issues by removing heat and cooling air from the case. -same.

In conclusion, there are many environmental conditions that must be taken into account in VFD applications. Neutralizing the effects of adverse conditions, whether it be dirt, heat, chemicals or solar charging, is imperative to achieve long equipment life and energy savings. Finally, using UL508C listed assemblies lets users and consultants know that a configuration has been rigorously tested and is suitable for use at its rated temperature.

* The UL508C standard is replaced by UL61800-5-1. However, VFDs with either of these standards are commercially available. Users should ensure that the proper guidelines are followed when installing a VFD.