Consider these environmental and safety factors to help you improve safety and cut downtime and costs.
By Jeff Raefield, power technical consultant, Rockwell Automation
Cost often is the deciding factor when choosing where and how to install low-voltage AC variable-frequency drives (VFDs).
Ironically, putting cost ahead of key decisions about 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 new or existing facilities, several environmental and safety issues should be the first considerations.
Environmental: Beat the Heat
Heat is the greatest enemy of VFD reliability. If not managed effectively, 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. Also, it can impact the hundreds of smaller discrete components and subassemblies that all work together in the drive.
Motor Control Center Assembly. Installing a VFD in a motor control center (MCC) assembly is a desirable option from an environmental perspective. UL 845 assembly requirements and test procedures address heat management for the entire MCC lineup. This means the MCC manufacturer is required to certify 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.
Proper thermal management and UL 845 listing of the assembly can be done only by the MCC manufacturers themselves. Panel builders, even those certified under UL 508a, can’t add VFDs into an MCC and maintain its UL 845 listing. If one unit in an MCC isn’t UL 845-listed, the entire MCC lineup listing is void.
Industrial Control Panels. 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 to within the design limits of the VFD (or any other component in the ICP). 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.
If 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. This adds a maintenance item because failure to replace filters periodically will result in components overheating.
Proper Airflow. 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 cooling internal boards and components. Often, inexperienced panel fabricators assume falsely 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 often results in premature drive failure.
Wall-mounted VFDs typically have fans that push and pull air through the drive housing for cooling. Careful consideration must be given to 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 can build up debris that lowers the cooling efficiency. Clearances from airflow obstruction are equally important for wall-mount drives.
Other Concerns. 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 don’t 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.
Safety: Arc-Flash Protection and More
Arc-flash safety is a serious concern when deciding how and where to install VFDs.
Perhaps 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 common 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 afforded by MCC installation.
MCC Specifications. For example, 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.
Also, user access to a VFD via the human-machine interface (HMI) always is 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.
ICP Safety. When housing VFDs inside an ICP, multiple safety aspects must be considered.
First is the short-circuit current rating (SCCR). If users don’t require an SCCR in their purchase specifications, some ICP builders will deliver an ICP with a 5-kA “courtesy” rating. This means users can’t connect the ICP to a power system with more than 5 kA of available fault current (AFC). But 5-kA AFC is almost impossible to attain in an industrial application, especially where 480V service is used.
Also, arc-flash safety and lockout/tagout (LOTO) requirements usually mean an ICP’s main disconnect will need to be opened, locked and tagged to work on anything inside or connected to the ICP. Having multiple, through-the-door disconnect devices is extremely difficult to manage. However, an ICP may make sense compared to an MCC or separate VFDs in the case that, if one part of the system is shut down, the entire system must be shut down anyway.
SSCRs and VFDs. In wall-mounted and cabinet-style VFDs, SCCR again is critical. If possible, the VFDs should be purchased as combination units in which 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 they often are heavy. Maintenance technicians, for example, frequently 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 remove heavy components safely.
Finding the Best Option
The accessibility, safety, maintenance and suitability of a VFD’s installation can have long-term impacts that will not be immediately obvious during design and planning stages. By understanding the inherent risks and benefits of different installation options, users can optimize a VFD’s performance across its life cycle while potentially reducing downtime and safety risks.
Learn about Allen-Bradley® drives and motors from Rockwell Automation.
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