By Jeff Raefield, power technical consultant, Rockwell Automation
Despite the many design and material advances that have occurred in power electronics over the decades, one thing has remained unchanged: Heat still is their biggest enemy.
This certainly holds true for low-voltage AC variable-frequency drives (VFDs). If not properly managed, heat can build up in VFD junction layers and cause them to fuse or melt.
Heat also can affect the intelligent power modules (IPMs) that are used in modern drive systems. If heat compromises just one IPM component, the entire IPM no longer is functional. And the same holds true for hundreds of other smaller, discrete components and subassemblies in a VFD. They all work in concert, and overheating one can affect them all.
For these reasons, think carefully about how you can manage heat in your drives when deciding where and how to mount them.
Three Mounting Options
Three basic packaging options exist for mounting VFDs. Each has its own inherent risks, benefits and limitations regarding heat management.
1. The first option is a motor control center (MCC). This puts heat management responsibility in the MCC manufacturer’s hands, rather than the end user’s. Specifically, the manufacturer must meet UL 845 assembly requirements and test procedures that address heat management for the entire MCC lineup.
If you go this route, remember that only the MCC manufacturer can do proper thermal management and UL 845 listing of the assembly. Panel builders can’t add VFDs into an MCC and maintain a UL 845 listing, even if they’re certified under UL 508A.
2. The second mounting option is an industrial control panel (ICP). If the ICP must be sealed, it likely will require an air-conditioning unit to maintain the internal temperature within the design limits of the VFD or any other component in the ICP.
If the ICP is ventilated, the total volume of air exchanged at the maximum ambient temperature must be able to maintain the internal temperature within the design limits of the VFD or any other component in the ICP. Filters also must be used if the ambient air contains dust or moisture.
VFDs mounted in ICPs also will have specific requirements for maintaining clear space areas to support proper airflow. Failure to meet these requirements will impact the cooling of internal boards and components. For example, some panel fabricators will mount a slotted wire duct too close to the VFD because they assume incorrectly it is not an obstruction. This can lead to a drive system prematurely failing.
3. The third packaging option is a wall-mounted or cabinet-mounted VFD. Wall-mounted drives typically use fans to push and pull air through the drive housing for cooling. Give careful attention to what might be in this air. Elements such as dust, moisture, chemicals, gases and machine oil can get into the drive and damage it or affect its cooling efficiency.
Some wall-mounted drives also must maintain a minimum relative humidity because if humidity is too low, static electricity becomes a problem.
Cabinet-mounted VFDs, used when VFDs are too large to be wall-mounted, require high-volume blower assemblies. And with a high volume of air can come contaminants that can build up quickly, even if they’re present only in small amounts. Using a separate air channel for cooling helps prevent this buildup.
Long-Term Savings
Cost often is the top factor that drives where and how to install VFDs. However, putting cost ahead of key considerations such as heat management — not to mention accessibility and maintenance — can increase your cost of ownership.
By understanding the full range of risks and benefits for different installation options, you can help optimize your VFD’s performance and uptime across its life cycle. For a full breakdown on these considerations, check out this white paper.
Learn about Rockwell Automation drives.
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