Beat the Heat When Mounting Low-Voltage AC Drives

Beat the Heat When Mounting AC Drives

Despite the many design and material advances that have occurred in power electronics over the decades, one thing has remained unchanged: Heat is still 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 just one IPM component is compromised by heat, the entire IPM is no longer 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

There are three basic packaging options for mounting VFDs. Each has its own inherent risks, benefits and limitations regarding heat management.

The first option is a motor control center (MCC).

This approach puts the responsibility of heat management in the hands of the MCC manufacturer, rather than the end user. 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 its UL-845 listing, even if they’re certified under UL-508a.

The second mounting option is an industrial control panel (ICP).

If the ICP must be sealed, it will likely 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 incorrectly assume is not an obstruction. This can lead to a drive system prematurely failing.

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. Careful attention must be given 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 impact 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 quickly build up, even if they’re only present in small amounts. Using a separate air channel for cooling helps prevent this buildup.

Long-Term Savings

Cost is often the top factor that drives where and how to install VFDs. However, putting cost ahead of key considerations like heat management – not to mention accessibility and maintenance – can actually increase your cost of ownership. 

By understanding the full range of risks and benefits for different installation options, you can optimize your VFD’s performance and uptime across its lifecycle. For a full breakdown on these considerations, check out this white paper (PDF).

Jeff Raefield
Posted August 4, 2017 By Jeff Raefield, Power Technical Consultant, Rockwell Automation
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