Installing VFDs: environmental and safety considerations
Cost is often the deciding factor when choosing where and how to install low-voltage ac variable frequency drives (VFDs). However, putting cost ahead of key decisions regarding installing a drive, can lead to a higher cost of ownership. It also can increase the likelihood of unscheduled shutdowns and create potential safety issues.
Whether users are planning to install VFDs in a new or existing facility, several environmental and safety issues should be considered first.
Environmental issues for VFDs
Heat is the greatest enemy of VFD reliability. If it is not effectively managed, heat can build up in the "junction layers" of the drive’s power transistors. This can cause the layers to fuse or melt.
Overheating also can compromise a VFD’s intelligent power module. And it can impact the hundreds of smaller discrete components and subassemblies that all work together in the drive.
Installing a VFD in a motor control center (MCC) assembly is a desirable option from an environmental perspective. UL-845: Motor Control Centers assembly requirements and test procedures address heat management for the entire MCC lineup. This means the MCC manufacturer is required to certify that the VFD will not be harmed by being in the MCC, nor will the heat produced by the VFD compromise other equipment in the MCC.
However, it is important to remember that proper thermal management and UL-845 listing of the assembly can only be done by MCC manufacturers. Panel builders, even those certified under UL-508a, cannot add VFDs into an MCC and maintain its UL-845 listing. If one unit in an MCC is not UL-845 listed, the listing of the entire MCC lineup is void.
Housing a VFD in an industrial control panel (ICP) rather than in an MCC assembly puts the heat-management burden on the end user.
If the ICP must be sealed, an air-conditioning unit often is required to maintain the internal temperature within the VFD’s design limits (or to the limits of other ICP components). A general rule of thumb is to estimate that the VFDs will emit approximately 3% of the total power going through them as heat into their immediate surroundings.
When ventilating the ICP, the total volume of air exchanged at the maximum ambient temperature must be sufficient to maintain the internal temperature within the VFD’s design limits. Also, filters must be used to mitigate contamination if the ambient air being circulated contains dust or moisture. Failure to maintain and periodically replace filters will result in components overheating.
Another critical, heat-related issue with VFDs mounted in ICPs is maintaining the clear space areas around the VFDs for proper airflow. Each VFD design will have specific, minimum clearance requirements—above, below, and side to side—that are critical to the cooling of internal boards and components. Often, inexperienced panel fabricators falsely assume that slotted wire duct is not an obstruction and mount it too close to the VFD. However, it is an obstruction to proper airflow, and failure to heed the clearances that often results in premature failure of the drives.
Wall-mounted VFDs typically have fans that push and pull air through the drive housing for cooling. Also consider what else may be present in this ambient air, including moisture, machine oil, dust, chemicals, and gases. These elements can get into the VFD and cause damage, or build up debris that lowers the cooling efficiency. Clearances from airflow obstruction are equally important for wall-mount drives.
Some gases, such as hydrogen sulfide, should be avoided altogether because they can corrode the printed circuit boards and connectors. Also, a minimum relative humidity must be maintained on some drives because, if too low, static electricity becomes a problem when the air is moved across the components. This is especially a concern in low-voltage drives that do not use conformal coating on their boards.
VFDs with motor sizes above 400 hp become too large to install on the wall and are built into free-standing structures that bolt to the floor. These options, referred to as cabinet-mounted VFDs, require a separate air channel for cooling the heat sinks.
Proper VFD safety
Arc flash safety is a serious concern when deciding how and where to install VFDs.
The most persuasive argument for installing VFDs in MCCs is that the safety is inherent in the overall MCC design. When VFDs are installed in MCCs, all personnel-safety issues become relevant to the entire MCC decision-making process. If users want an MCC to be arc-resistant, the VFD cubicles must be arc-resistant as well.
Beyond arc flash protection, there are other personnel-safety issues with MCC installation:
- In a UL-845 MCC unit, a VFD must be in a tested and listed series combination, performed by the MCC manufacturer, at a level that meets or exceeds the MCC short-circuit rating. As long as the overall MCC specification meets the site conditions, this provides assurance every unit within it will be certified to be connected to that system.
- User access to a VFD via the human-machine interface (HMI) is always brought out to the unit door in an MCC format unless otherwise specified. This means operators will not need to open the unit door and expose themselves to the safety hazards inside when they want to read, adjust, program, or troubleshoot a VFD from its display.
When housing VFDs inside an ICP, multiple safety aspects must be considered as well.
If users do not require a short-circuit current rating (SCCR) in purchase specifications, some ICP builders will deliver an ICP with a 5 kA "courtesy" rating. This means users cannot connect the ICP to a power system with more than 5 kA of available fault current (AFC). But 5 kA AFC is virtually impossible to attain in an industrial application, especially where 480 V service is used.
Also, arc flash safety and lockout/tagout requirements usually mean an ICP’s main disconnect will need to be opened, locked, and tagged before working on anything inside or connected to the ICP. Having multiple, through-the-door disconnect devices is extremely difficult to manage. But an ICP may make sense compared to an MCC or separate VFDs in the event one part of the system is shut down, the entire system must be shut down anyway.
Again, for wall-mounted and cabinet-style VFDs, SCCR is critical. If possible, the VFDs should be purchased as combination units, where the main disconnect and over-current protective devices are included as integral to the VFD package. This solves the SCCR issue and other electrical safety issues.
Another issue with large VFDs is that they are often very heavy. Maintenance technicians, for example, often use tools, jacks, or even forklifts in ways that may put the VFD or the workers at risk. A rollout chassis design, using a specially designed "truck" assembly that matches up to internal guide rails at the bottom of the VFD cabinet, can provide an easy way to safely remove heavy components.
The accessibility, safety, maintenance, and suitability of a VFD’s installation can have long-term impacts that will not be immediately obvious during the design and planning stages. By understanding the inherent risks and benefits of different installation options, users can optimize a VFD’s performance across its lifecycle while potentially reducing downtime and safety risks.
Jeff Raefield is a power technical consultant at Rockwell Automation. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, firstname.lastname@example.org.
- Heating and cooling requirements for VFDs.
- The risks and benefits of different installation methods.
- How a VFD’s installation impacts its performance.
Are certain installation methods ideal for particular facilities?