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Understanding inverter installation options can help users save costs, reduce downtime, and improve the safety of motion control systems.
Cost is often the deciding factor when choosing the location and method for installing low-voltage frequency converters . However, prioritizing cost over crucial decisions regarding converter installation can lead to higher total cost of ownership. It can also increase the likelihood of unexpected downtime and create potential safety issues.
Regardless of whether users plan to install the frequency converter in new or existing facilities, they should first consider the following environmental and safety issues.
Figure 1: Inverter performance can only be optimized when users understand the inherent risks and benefits of installation options. Image source: Rockwell Automation
Environmental issues of frequency converters
High temperatures are the biggest enemy of inverter reliability. If not properly managed, heat can accumulate on the junction of the power transistors in the drive. This can lead to the junction melting or fusing.
Overheating can also damage the inverter's intelligent power module. That can affect the hundreds of small discrete components and assemblies that work together inside the inverter.
From an environmental perspective, installing frequency converters within the Motor Control Center (MCC) is an ideal option. UL-845: Requirements and Test Procedures for Motor Control Centers addresses overheat management issues for the entire MCC arrangement. This means that MCC manufacturers need to demonstrate that the frequency converters installed within the MCC will not be damaged, or that the heat generated by the frequency converters will not damage other equipment within the MCC.
However, it is important to remember that proper thermal management and assembly of equipment listed in the UL-845 list can only be performed by the MCC manufacturer. Even cabinet manufacturers certified under UL-508a cannot add inverters to an MCC, nor can they maintain their UL-845 list. If one unit within an MCC is not on the UL-845 list, the entire MCC's listing is invalid.
Installing a frequency converter in an Industrial Control Panel (ICP) instead of an Industrial Control Cabinet (MCC) places the burden of thermal management on the end user.
If the ICP must be sealed, an air conditioning system is typically required to maintain the internal temperature within the inverter's design limits (or the limits of other ICP components). A general rule of thumb is that the inverter will release approximately 3% of its total power as heat to the surrounding environment.
When ventilating the ICP, the total air exchange rate must be sufficient to maintain the internal temperature within the inverter's design limits when the outdoor temperature is highest. Furthermore, if the recirculated outside air contains dust or moisture, a filter must be used to remove the contaminants. Maintenance malfunctions and regular filter replacements can lead to component overheating.
For inverters installed in ICPs, another key heat-related issue is ensuring sufficient clearance around the inverter for proper airflow. Each inverter design has minimum clearance requirements, including overhead, under, and side-to-side clearances, which are crucial for cooling internal boards and components.
It is common to see inexperienced switchboard manufacturers mistakenly assume that cable ducts with grooves will not be an obstacle, and therefore place them too close to the frequency converter. However, this obstructs normal airflow and fails to provide sufficient clearance, often leading to premature frequency converter failure.
Wall-mounted frequency inverters are typically equipped with fans that draw air through the inverter's casing for cooling. However, other substances in the surrounding air must also be considered, including moisture, engine oil, dust, chemicals, and gas. These substances can enter the inverter and cause damage, or cause residue buildup that reduces cooling efficiency. Preventing obstructions to airflow is equally important for wall-mounted frequency inverters.
Certain gases, such as hydrogen sulfide, should be avoided as they can corrode printed circuit boards and connecting components. Furthermore, relative humidity must be maintained above the minimum required level when using certain drives, because if it is too low, static electricity can become a problem as air flows through the components. This is especially important for low-voltage frequency inverters that do not use conformal coatings on their circuit boards.
For inverters with motors of 400 horsepower or higher, they are too large to be wall-mounted and must be installed in a freestanding structure that can be fixed to the floor. These inverters, known as cabinet-mounted inverters, require a separate air duct to cool the heat sink.
Figure 2: Users should understand the inherent risks and benefits of different installation options in order to optimize the performance of the frequency converter.
Appropriate inverter safety
When deciding how and where to install a frequency converter, the safety of electric arcs needs to be given special attention.
The most compelling reason to install frequency converters within an MCC is that its safety is consistent with the overall design of the MCC. When installing frequency converters within an MCC, all personnel safety concerns are intertwined with the entire MCC decision-making process. If the MCC is to be arc-resistant, the frequency converter's cabinet must also be arc-resistant.
In addition to protection against arc flash, there are other personnel safety issues related to MCC installation:
Within a UL-845 MCC unit, the inverter must be part of a tested, listed series combination (performed by the MCC manufacturer) with a level that meets or exceeds the MCC's short-circuit rating. This ensures that each unit within the MCC is certified for connection to the system, provided the overall MCC specifications meet the field conditions.
The human-machine interface (HMI) required for user access to the frequency converter is typically located on the outside of the equipment unit cabinet door in the form of an MCC (Management Control Center), unless otherwise specified. This means that when operators want to read, adjust, program, or diagnose faults in the frequency converter on its display screen, they do not need to open the equipment unit cabinet door and expose it to the safety hazards inside the cabinet.
If a frequency converter is installed inside an ICP, several safety issues need to be considered. If the user does not specify a short-circuit current rating (SCCR) in the purchase instructions, some ICP manufacturers will kindly provide ICPs with a 5kA rating. This means that the user cannot connect the ICP to a power system with a potential fault current (AFC) exceeding 5kA. However, in reality, a 5kA AFC is unlikely to be achieved in industrial applications, especially when using 480V power.
Furthermore, arc flash safety and lockout/tagout requirements typically mean that the main circuit breaker of the ICP must be disconnected, and lockout/tagout procedures must be performed before any operation or connection to the ICP is made inside. Managing multiple circuit breaker devices that run through cabinet doors is extremely difficult. In situations where shutting down part of the system necessitates shutting down the entire system, an ICP is a more sensible choice than an MCC or a separate frequency converter.
Meanwhile, SCCR (Supervisory Control Reduction) is also crucial for both wall-mounted and cabinet-type frequency converters. If possible, purchase frequency converters as modular units, as the main circuit breaker and overcurrent protection devices are integrated into the complete unit. This resolves SCCR issues and other electrical safety concerns.
Another issue associated with large frequency converters is their typically heavy weight. For example, maintenance technicians often use tools, cranes, or even forklifts, which can put both the converter and workers at risk. One rollout chassis design uses a special truck-like assembly that matches internal rails located at the bottom of the converter cabinet, providing a simple and safe method for moving heavy equipment components.
The accessibility, safety, maintainability, and suitability of inverter installations have long-term impacts that are not immediately apparent during the design and planning phases. By understanding the inherent risks and benefits of different installation options, users can optimize the inverter's performance throughout its entire lifecycle, potentially reducing downtime and safety risks.
