What to consider when selecting ac drives

Several considerations, industry guidelines, and third-party certifications can help ensure safe variable-speed drive operation.

By Edward Wilke, Eaton Corp., Milwaukee December 1, 2009


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Environmental considerations in the application of variable-speed drives are important to ensure safe operation and trouble-free service. Among these are humidity, ambient temperature, elevation, and other factors including solar heating. There are a number of ways to ensure that the product will meet the site needs.

Variable-speed drives’ ability to precisely control process rates and achieve significant energy savings has made their application widespread, and they increasingly are applied to loads in difficult environments. As with many electronic devices, environmental conditions can be key factors in the drives’ lifespan and reliability. Temperature, humidity, shock and vibration, solar heating, air cleanliness, and air quality all can affect the expected lifespan of ac drives.

There are a variety of factors to consider to ensure ac drives are meeting site requirements. In addition, third-party testing agencies, including Underwriters Laboratories (UL), can ensure both drive components and engineered assemblies are applied appropriately in given circumstances.

If drives were applied in conditions experienced by a UPS system, the technological peers of drives, then the number of premature failures and maintenance requirements would be reduced dramatically. However, the reality of application conditions is such that drives are installed in mechanical rooms, on rooftops, and in other areas that are not environmentally friendly to computer servers.

Poor air quality

The most pervasive problem for drives 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 drives. Hydrogen sulfide and airborne chlorine gases are prime culprits.

The only way to guard against these chemicals is to have all the drive boards conformally coated. The coating can reduce the rate of breakdown; however, it will not completely eliminate it. Conformal coating should be required on all drives 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 lifespan of variable-speed drives. Specifically, oily debris and large matter can build up and clog the narrow fins of a drive heatsink over time, limiting airflow and causing overheating conditions in the drive power unit. Further impeding performance, most medium or larger horsepower (10 hp and above) drives have fans where the debris can get caught or build up. Even when installed in a NEMA 12 cabinet, the drive’s heatsink 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 drive to be placed in an oily or excessively contaminant-filled environment, the cabinet should be sealed, the drive heatsink should be inside the cabinet, and forced air cooling with filters should be used. Proper filter selection and maintenance is required to provide adequate cooled air to the drive. UL508C, the UL file for solid-state power conversion devices, requires the drive assembly to be load- and heat-tested with half of the filter covered to simulate a clogged filter. Selecting UL508C-rated drive 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 risk of clogging the heatsink 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 heatsink, 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 variable-speed drive.

Thermal considerations

When placing drives in an air-conditioned equipment room is not practical, ambient heat can pose a significant problem. Nearly all drives are rated at either 40 C or 50 C, with some at 45 C, while most enclosed drive assemblies are rated at 40 C. This is adequate for many installation sites when adequate cooling airflow is available. The drives themselves are rated at these temperatures at full-load current with either a high (150% of full load for 1 min) or low (110% of full load for 1 min) overload rating. This current rating translates to a horsepower rating, typically based on NEC values. Drives often 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 drive assemblies that have UL508C listings at the required temperature, thereby satisfying NEC requirements.

A misconception of UL applicability to drives is that UL508A and UL508C can be used interchangeability. UL508A was written for industrial control panels, including relay panels or other electromechanical devices; it can be applied only at 40 C as a base rule. Another concern is that 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 in 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 code is far more lax than UL508C with regard 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 drive assemblies that have a UL508C label.

Solar heat gain must be accounted for when considering thermal selection in outdoor applications. Methods for calculating solar heat gain coefficient and solar gain through frame and other opaque elements can be found in the 2009 ASHRAE Fundamentals Handbook , section 15.17 .

A way to affect this gain is by enclosure paint selection. The coefficient of gain varies greatly from 0.15 for white, 0.30 to 0.50 for grey, and upward 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 drive enclosure. However, a more effective approach than sizing drives into sealed boxes is to mount the drive using enclosure cooling or, even better, mount the drive heatsink outside the enclosure. Although using cooling fans to exhaust hot air can be effective, cooling the drive heatsink with ambient air effectively eliminates solar gain concerns by removing the heat and cooling air from the enclosure itself.

Author Information
Wilke is senior product line manager for solid-state power control with Eaton’s industrial control business. He holds a bachelor’s degree in mechanical engineering from University of Wisconsin—Madison.