• What is UL 508A certification?
• What is UL 508A Listing?
• 2 types of UL 508A industrial control panel listing labels
• Addition to the UL 508A standard – Manufacturer’s Technical Representative (MTR) program

The UL 508A Standard

What is UL 508A?

UL 508A is the Underwriters Laboratories (UL) reference standard for the construction of industrial control panels in the United States. Industrial control panels are defined by the National Electric Code (NEC) section 409.2 as “an assembly of two or more power circuit components, control circuit components, or any combination of power and control circuit components.”

Based on the safety rules recommended by the NEC, control panel manufacturers follow UL 508A for guidance in various areas of design and assembly. The standard also includes specifications to ensure that industrial control panels can endure environmental factors such as dust, moisture, temperature fluctuations, and corrosion.

Key Areas of UL 508A

The UL 508A standard is organized into sections; each addresses specific aspects of industrial control panel construction, marking, and performance. Key sections include:

• Electrical Safety: Control panel design guidelines to prevent various hazards, such as electrical shock, short circuits, overheating, arc flash, and fire hazards.
• Component Selection: Specific requirements for properly selecting and using electrical components within industrial control panels to ensure their safety and reliability. Addresses compatibility, listing and certification, suitability for intended use, and documentation. 
• Wiring Methods: Specifications for control panel wiring installation and routing to avoid electrical hazards, mechanical damage, interference, and crosstalk. Includes wire sizing and selection criteria, routing and bundling, termination, grounding and bonding, and identification and marking.
• Environmental Considerations: Guidelines for ensuring that industrial control panels can tolerate environmental factors and operate reliably in their intended application. Addresses areas such as enclosure type, gasketing and sealing, ventilation and cooling, temperature variations, and corrosion resistance.
• Marking and Labeling: Instructions for labeling control panels with appropriate information about the manufacturer, standards compliance, ratings, warnings, and instructions.
• Construction and Assembly: Specifications for control panel construction and assembly, including mounting of components, enclosure design, accessibility, and serviceability.
• Documentation: Criteria for required documentation related to the design, construction, and installation of industrial control panels. Drawings, schematics, component lists, and testing records may be included.
• Special Applications: Specific requirements and considerations for industrial control panels used in specialized or unusual applications that may pose unique challenges or hazards.

UL 508A Certification

What is UL 508A Certification?

UL 508A certification refers to the process industrial control panel manufacturers go through to obtain a UL Listing Mark on their panels. While the term UL 508A certification is used frequently, the official UL term is UL Listing.

UL 508A Listing

The UL 508A Listing Mark on an industrial control panel indicates that the panel has been independently evaluated and tested by a UL inspector and certified to comply with the standard. UL 508A Listing is important to the municipal inspection authority and the purchaser of the panel, indicating that the control panel meets recognized safety standards for electrical equipment used in industrial settings.

The UL 508A Listing covers panels operating from a voltage of 1000 volts or less.

Why Your Industrial Control Panel Should Be UL 508A Listed

There are many reasons to obtain a UL 508A Listing for your industrial control panels. Some include: ensuring compliance with globally recognized safety standards, increasing customer confidence in your products, maximizing market opportunities, facilitating inspection, and preventing panel rejection.

Types of UL 508A Control Panel Listing Labels

There are two types of UL Listing labels for industrial control panels:

1. Enclosed Industrial Control Panel
2. Open Industrial Control Panel

The two types differ primarily in their design and level of protection for electrical components.

Enclosed Industrial Control Panel

Industrial control panels can be installed in two ways: In an enclosure as “enclosed type” equipment, or arranged onto a mounting panel as “open type” equipment. The enclosed type listing label refers to a complete control panel assembly housed within a protective enclosure. The enclosure provides a barrier between the electrical components and the outside environment, protecting against hazards such as dust, moisture, and accidental contact. Enclosed panels commonly have doors or covers that permit access to the internal components while ensuring safety and adhering to applicable standards.

Open Industrial Control Panel

The Open Industrial Control Panel listing label refers to a control panel assembly where the electrical components are not housed within a protective enclosure. Instead, the components are exposed and accessible without any barrier. Open panels must be installed in an enclosure that is built into the machine or is available at the installation site.

UL 50 Electrical Enclosure Ratings

Type 1 – Indoor Use

Provides basic protection against contact with the enclosed equipment and against falling dirt

A UL 508A panel shop is able to build an industrial control enclosure but is limited to rating the enclosure at Type 1. 

Type 2 – Indoor Use

Provides protection against contact with the enclosed equipment, falling water or dirt, and dripping and light splashing of non-corrosive liquids

Type 3 – Indoor or Outdoor Use

Provides protection against contact with the enclosed equipment, falling dirt, windblown dust, rain, sleet, and snow, and will be undamaged by the formation of ice on the enclosure

Type 3R – Indoor or Outdoor Use

Provides protection against contact with the enclosed equipment, falling dirt, rain, sleet, and snow, and will be undamaged by the formation of ice on the enclosure

Type 3S – Indoor or Outdoor Use

Provides protection against contact with the enclosed equipment, falling dirt, windblown dust, rain, sleet, and snow; external mechanisms remain operable while ice-laden

Type 4 – Indoor or Outdoor Use

Provides protection against contact with the enclosed equipment, falling dirt, splashing water, windblown dust, rain, sleet and snow, hose-directed water, and corrosion; will be undamaged by the formation of ice on the enclosure

Type 4X – Indoor or Outdoor Use

Provides protection against contact with enclosed equipment, falling dirt, splashing water, windblown dust, rain, sleet and snow, hose-directed water, and corrosion; will be undamaged by the formation of ice on the enclosure

Type 5 – Indoor Use

Provides protection against contact with enclosed equipment, falling dirt, settling airborne dust, lint, fibers, and flyings, dripping and light splashing of non-corrosive liquids

Type 6 – Indoor or Outdoor Use

Provides protection against contact with enclosed equipment, falling dirt, rain, sleet, snow, hose-directed water, corrosion, and the entry of water during occasional temporary submersion at a limited depth; will be undamaged by the formation of ice on the enclosure

Type 6P – Indoor or Outdoor Use

Provides protection against contact with enclosed equipment, falling dirt, rain, sleet, snow, hose-directed water, corrosion, and the entry of water during prolonged submersion at a limited depth; will be undamaged by the formation of ice on the enclosure

Type 12 – Indoor Use (for enclosures without knockouts)

Provides protection against circulating dust, falling dirt, lint, fibers, flyings, dripping and light splashing of non-corrosive liquids, light splashing and seepage of oil and non-corrosive coolants

Type 12K – Indoor Use

Provides protection against contact with the enclosed equipment, dust, dirt, fiber flyings, dripping water, and external condensation of non-corrosive liquids

Type 13 – Indoor Use

Provides protection against contact with the enclosed equipment, lint, dust seepage, external condensation, and spraying of water, oil, and non-corrosive liquids

Addition to UL 508A – MTR Program

What is the Manufacturer’s Technical Representative (MTR) Program?

The MTR program relates to a provision added to the UL 508A Industrial Control Panel (ICP) Shop Program requiring UL 508A Listed panel manufacturers to maintain a certified manufacturer’s technical representative on staff. This requirement is designed to ensure there is at least one employee at each manufacturing location who can effectively interpret and accurately apply the requirements of UL 508A to control panel design and manufacturing.

MTR Certification Requirements

MTRs must complete a certification course, pass a certification exam, and recertify every three years.

How Does the MTR Program Help Me?

In order to receive authorization to apply the UL Listing Mark, panel manufacturers must complete mandatory training on the UL 508A Standard when they enter the UL Industrial Control Panel (ICP) Shop Program. However, after the initial training is completed, there are no ongoing requirements for training. Over time this means that as staff turnover and other events occur, panel manufacturers could be left with no staff formally trained on the UL 508A Standard. Methods for addressing this issue vary across manufacturers.

The MTR program is designed to ensure panel manufacturers maintain UL 508A-qualified staff.

If a customer’s engineering team is completing the company’s control panel design, customers may find the MTR course helpful as it can help eliminate mistakes that can delay manufacturing and drive unforeseen costs.

Conclusion

Based on the safety rules recommended by the NEC, industrial control panel manufacturers follow UL 508A for guidance in various areas of design and assembly, including proper component selection, calculation of short circuit current ratings, and wiring methods. The safety guidelines prevent various hazards such as electrical shock, short circuits, overheating, arc flash, and fire hazards. In addition, the standard includes specifications to ensure that industrial control panels can endure environmental factors such as dust, moisture, temperature fluctuations, and corrosion.

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There are key differences between the two categories related to the type of control panel orders the business is set up to service. This blog post examines these important distinctions.

The Key Differences Between a Control Panel Manufacturer and a Panel Shop

Industrial control panel manufacturers and panel shops primarily differ in the volume of control panels the company produces and in the supporting infrastructure in place to ensure quality for every panel. A control panel manufacturer typically produces high-volume orders of OEM panels regularly and has the required facilities, software, and processes to scale up to those volumes. A panel shop typically focuses on building smaller orders of unique custom control panels so the processes, facilities, and resources are set up accordingly. The specific differences between control panel manufacturers and panel shops can be broken down into the following 8 areas:

1. Software
2. Quality and consistency
3. Company culture
4. Efficiency
5. Processes
6. Facility
7. Delivery
8. Training

The sections below describe the differences in these 8 areas.

Software

Most control panel shops use generic off-the-shelf manufacturing software designed for custom panel builders. The software is effective for managing low-volume panel orders, but is not an integrated part of the panel-building process and is not designed to be scalable for high-volume production.

Manufacturing panels in high volume requires a control panel manufacturer to implement top-tier, customized manufacturing ERP software.

This software is an integrated part of the manufacturing process and ensures best practices are followed. It is designed to automate and streamline back-office tasks, increase productivity, and optimize job functions to enable scalability for large panel orders.

Quality/Consistency

Most panel shops depend on employees with many years of experience to ensure panel quality. Staff expertise in custom panel building enables quality to be consistent across low-volume panel orders. Any quality issues are usually addressed as they occur, with no formal quality management program in place. Because employee knowledge is the basis for quality and consistency, documentation on the panel building process is usually not formally maintained.

While this method of quality management often works well for panel shops, it is not feasible for control panel manufacturers. With typical order sizes of 500 or more panels, to meet customer quality standards manufacturers must have a proactive approach to quality control, a formal quality management system (QMS), and proven consistency across high-volume orders.

Control panel manufacturers ensure consistency by using workflows documented in the QMS. The same workflow is used to build each panel. Documentation on workflows and quality management is revision-controlled and maintained in the QMS.

Company Culture

Since most panel shops are small businesses with every aspect developed through the hard work of the owner, the company culture can reflect an attitude of “this is the way it has always been done” and a resistance to change.

Surviving and succeeding as a control panel manufacturer requires a company culture of continuous improvement to meet the requirements of the (typically larger) companies that outsource panel manufacturing work. The entire staff must be willing to proactively make changes that will optimize the manufacturing process by maximizing efficiency, reducing costs, and ensuring quality and consistency.

Processes

Panel shops typically build a relatively low volume of custom control panels monthly. Accordingly, the processes are designed for a small number of units. If the shop handles both custom and low-volume OEM orders, the same process is often used to build both types of panels.

In contrast, control panel manufacturers produce a large number of panels per month. For this reason, processes are very important and must be scalable for manufacturing in volume. If the manufacturer builds both custom and OEM control panels, the processes are typically separate for custom and OEM panels to allow maximum efficiency for high-volume builds while enabling attention to the unique details of custom panels.

Efficiency

Most panel shops are small businesses that developed their panel-building process over many years and made improvements based on experience. For this reason, there is often hesitation about changing the process — a “we’ve always done it this way” mindset found in many small businesses. Because the focus is typically on small orders of custom panels (and likely also due to the mindset), Lean manufacturing principles to increase efficiency have not been implemented in most panel shops.   

On the other hand, control panel manufacturers must focus on maximizing efficiency to meet customer performance metrics, often implementing Lean manufacturing principles and hiring or training dedicated Lean resources. These staff members are responsible for making changes that increase efficiency.

Facility

The three main differences between a control panel manufacturer and a panel shop relative to the facility are:

1. The flexibility and scalability of the facility
2. The organization level of the work stations and inventory
3. The investment level in machinery and software

Most panel shops got their start building custom control panels; therefore, the facilities typically have set work stations designed for that purpose. The stations can be used for small orders of OEM panels, but cannot be reconfigured or expanded to increase production for high-volume OEM orders.

Because each custom panel job is different and the volume is low, each work station is arranged as it works best for that employee. Uniform organization and efficiency are not the focus. Inventory is usually arranged by the job. Due to the high variability of custom panel jobs, panel shops do not tend to invest heavily in machinery and software designed to increase efficiency and optimize production.

Control panel manufacturing facilities are designed to be reconfigurable as well as scalable to meet changing production needs. They typically have cell-based assembly, with stations on wheels for easy reconfiguration.

Because the facilities are designed for volume manufacturing, organization and efficiency are priorities in the production area as well as in inventory storage and retrieval. Control panel manufacturers often invest in high-end software and machinery that increases efficiency, reduces cost, and improves quality and consistency.

Delivery

Panel shops frequently track orders through a manual process, with order status updates provided through manual BOM lookup. On-time delivery can often be guaranteed for low-volume panel orders, but on-time performance history is usually not available for large panel orders.

If any parts listed on the BOM are unavailable, panel shops typically require customers to select alternate parts.

For control panel manufacturers, order tracking is usually handled by the manufacturing ERP software through automated BOM tracking. On-time delivery performance is tracked as one of the Key Performance Indicators (KPI) in the quality control process.

In the case of unavailable parts, manufacturers with engineering expertise on staff can often propose part substitutions by researching parts that match the original part in form, fit, and function.

Training

Since panel shops normally focus on building custom control panels that are unique, each employee may perform a task differently in the way that works best for them. Knowledge of the panel-building process is maintained within the veteran staff members and passed along to new employees via informal on-the-job training. The pace of training must be based on the availability of experienced employees; therefore, getting new employees up to speed can be challenging and slow when the shop is busy.

For control panel manufacturers, maintaining efficiency across orders of 500 or more identical panels requires that each employee perform each task in the process in the same way. Documented workflows for each task are contained in the quality management system, and all employees are trained to perform the task according to the workflows. Employee training is completed through an online Learning Management System (housed on the QMS) which is accessible to all staff members. Onboarding can be completed quickly since the training is not dependent on the availability of other employees.

Panel Manufacturer or Panel Shop Chart

Check out this summary chart to review the crucial differences between a control panel manufacturer and a panel shop.

Conclusion

When evaluating fabricators as possible control panel manufacturing partners, it is important to identify whether the company is a control panel manufacturer or a panel shop. The two types of fabricators differ in eight crucial areas stemming from the type of panel orders the business is set up to service. Understanding these differences can help determine which category of fabricator best fits your manufacturing needs.

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1. Risk Management
2. Document Control
3. Change Management
4. Internal Audits
5. Nonconformance Handling

These 5 areas should be part of an established Quality Management System. This blog post describes what to look for in each area to assess an industrial control panel manufacturer’s process of quality.

Quality Doesn’t “Just Happen” in Control Panel Manufacturing

For customers assessing control panel manufacturers, all suppliers may seem similar. They are all control panel experts who build panels every day – so quality just happens, right?

Although it may seem that way, manufacturing large orders of 1,000 or more industrial control panels is very different from building orders of ten to twenty custom panels. For small custom orders, control panel shops can rely on veteran employees who review every detail of each panel to ensure quality. However, for orders of more than 30 panels, this type of review is no longer feasible.

So, if the experts can’t review every panel in large orders, how can quality be ensured for each panel? Achieving a process of quality for control panel manufacturing requires the development of a Quality Management System.

A Quality Management System Specifies the Entire Process

According to the American Society for Quality, a Quality Management System is “a formalized system that documents processes, procedures, and responsibilities for achieving quality policies and objectives. A QMS helps coordinate and direct an organization’s activities to meet customer and regulatory requirements and improve its effectiveness and efficiency on a continuous basis.”

For a QMS to be effective, the manufacturer must develop written procedures and store them in an accessible location. In addition, manufacturers must complete regular internal audits to assess the effectiveness of each procedure. Records of each audit must be maintained.

For failures in any area of an audit, the responsible department must generate a Corrective Action Report describing the actions required to fix the problem. Employees must complete the Corrective Actions and update the original procedure, or create a new procedure. The manufacturer must test the new procedure to ensure it resolves the problem.

The Process of Quality for Control Panel Manufacturing: 5 Key Areas

To ensure consistent quality across large and highly variable orders, as part of their QMS suppliers must have a well-established internal process of quality in risk management, document control, change management, internal audits, and nonconformance handling.

These areas are especially important since many control panel manufacturers are smaller than the companies that outsource to them. To meet the manufacturing quality standards of large companies, control panel manufacturers must have documented and continuously improved processes in the form of a QMS. All employees must consistently follow the system.

Risk Management

In industrial control panel manufacturing, risk mitigation strategies become even more important as the volume of customer panels increases. The following sections describe five areas in which manufacturers can reduce risk in the control panel manufacturing process.

Work Standardization for Control Panel Manufacturing

Manufacturers can minimize risk by defining a standard panel manufacturing process and work, which allows them to easily identify and highlight exceptions in the process.

Elimination of Tribal Knowledge

Eliminating tribal knowledge, and ensuring that all employees are trained to perform a task in the same way improves consistency in industrial control panel manufacturing. It can also reduce risk by developing a team of employees with standard knowledge of the process.

Panel Manufacturing Quoting Method Consistency

Manufacturers can significantly reduce risk on the front end of the industrial control panel manufacturing process by using a consistent method for providing estimates. For example, the manufacturer can use software that allows reuse and produces very consistent output.

UL 508A/UL 698A Knowledge

A manufacturer with extensive knowledge of industrial control panel manufacturing standards reduces risk for customers by providing the ability to identify possible design or safety risks in the customer’s panels before manufacturing begins. Possible issues can be clarified with the appropriate application of standards.

Post-Production Job Review

Suppliers can reduce risk in panel manufacturing by using post-production processes and production management software. Post-production job review as part of the process can reduce risk by providing ongoing opportunities for improvement.

Consider the following when assessing a supplier’s level of risk management:

• What risk mitigation strategies does the supplier employ when manufacturing customer panels?
• Are the strategies part of a consistent process the supplier follows for every order?
• Does the supplier possess a level of technical knowledge about industry standards that allows them to help customers by identifying possible areas of risk in the customer’s design?

Document Control

Since a Quality Management System requires written documents and records, how should manufacturers create, organize, and manage these documents?

The supplier should have a clear, written procedure guiding the creation, organization, and storage of documents. The procedure should make it easy for all employees to find the information they need by facilitating consistent document numbering and storage as well as a consistent way of organizing information.

The following questions can help you assess a supplier’s Document Control system:

• Does the supplier have a written procedure outlining how to number, organize, and store documents?
• Do employees consistently follow the procedure for every document?
• Can any employee access the document storage system and easily find what they need?

Change Management

To manage and track customer requests to change a control panel order, it is important that the panel manufacturer has a controlled and scalable process to manage changes while still ensuring quality.

The supplier’s process becomes even more important if a customer needs to change an order that is already in process.

Since situations may arise that require changes to in-process orders, it is important to evaluate potential suppliers in this area. In-process changes may be beyond the scope of the manufacturing contract in place, so a supplier may not be obligated to make changes.

Therefore, it is important to understand the supplier’s customer service culture. Suppliers with a strong focus on customer service are more likely to go above and beyond to truly help a customer find a solution even in situations that are outside the scope of the contract.

Evaluate the following to assess a supplier’s process and customer service focus:

• Does the supplier use a well-defined, scalable process for every control panel order?
• Is customer service a core part of the culture at this company?

Internal Audits

Maintaining a process of quality in industrial control panel manufacturing requires a commitment to continuous improvement. Conducting regular internal audits is an important part of this commitment.

Consider the following questions to help you evaluate a supplier’s level of commitment:

• Is the supplier internally committed to continuous improvement as a company, or are improvements driven only by customer requests?
• Does the manufacturer evaluate their internal procedures and then take corrective action to make improvements?
• How often does the supplier perform these internal audits?
• How does the company make sure the improvements happen?

Nonconformance Handling

Nonconformances for control panel manufacturers are typically defined as any failure to meet customer specifications or documented internal processes.

Corrective Action Reports are investigations carried out internally to determine the source of a nonconformance.

Corrective Actions are reactions to a nonconformance, dictated by a corrective action report, and intended to fix the root cause and/or modify a process or processes so that the cause is monitored to prevent a reoccurrence.  

The questions below can help you evaluate a supplier’s commitment to addressing nonconformances:

• How does the supplier handle nonconformance?
• Is there a written policy, or does the supplier just “wing it” when nonconformances occur?
• Does everyone involved in panel manufacturing consistently follow the policy?
• Is there a nonconformance system or does the “system” consist of one person with a spreadsheet?
• Does the company follow up to make sure corrective actions happen?
• Does the company complete and log documentation showing that the department responsible corrected the problem and installed proper controls to prevent a reoccurrence?

Panel Manufacturing Quality Process: Conclusion

To ensure quality and consistency in the manufacturing of large volumes of industrial control panels, manufacturers must develop a quality process that the company regularly audits and improves. The presence of this process is a key area that differentiates control panel manufacturers from panel shops.

Consider assessing potential manufacturing partners based on their process of quality in risk management, document control, change management, internal audits, and nonconformance handling. Decide whether suppliers fit your outsourcing needs based on their assessment score.

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On a servo drive system it is relatively easy to set torque limits in the parameter programming of the machine. When doing so, one must remember that the electronic torque limit is at the motor only. This means that, from a safety standpoint, the motor’s electronics do not account for the masses of gears, couplings, shafts, etc. further along down the drive line, while in many cases a manufacturing process is many mechanical power transmission components away from the motor.

As a result, the servo drive and/or PLC monitoring the torque of the motor may not pick up an over-torque condition quickly enough to prevent damage from occurring. In the case of rotating equipment, there are often gearboxes and shafts which have a lot of rotating inertia not accounted for by electronic means. Additionally, linear applications impart their inertia back into the rotating components driving them when they stop or crash. Precision mechanical torque limiters and safety couplings are often a good option where blockage or malfunction is a possibility.

When we examine what happens in a machine crash, it is often useful to look at an impact force equation. F=(.5m*v^2)/s where F=force in Newtons, m=mass in kilograms, v=velocity in meters per second, and, s= stopping distance in meters. Examining this equation tells us that the force imparted by an impact is directly proportional to the mass and or velocity, while being inversely proportional to the stopping distance. That being said, the more massive any component is and the faster it’s moving, the more impact force exhorted during a crash. Because the stopping distance is in inverse proportion to the impact force, the smaller it gets, the more force is imparted by the crash.

Consider also the same principle in terms of rotating inertia and torque. T= (ω/t)*J where T=torque in Nm, ω=angular velocity in radians per second, t=acceleration/deceleration time in seconds, and J=rotational moment of inertia in kg*m^2. Here we see that likewise, stopping time is inversely proportional to torque. When this equation is applied to common industrial drive applications, the torque resulting from stopping times in the 1-3 millisecond range can be astronomically high.

Once pointed out this is fairly straightforward, although many engineers forget to account for this principle while limiting currents and or torque values, forgetting that in many instances the mechanical energy plays a much greater role than any additional current supplied to the motor after the impact. It can be relatively simple to find the mass moment of inertia of mechanical power transmission components and know how fast they will be moving. What is difficult to gauge is how the machine will likely crash and what will cause this to happen. Because many people from different backgrounds are often involved in machine design, aspects of how the machine will operate holistically can be unintentionally neglected when engineers concentrate on one area alone.

Mechanical drive experts may focus on the process and drive components without thinking much about the full capabilities of the motor and electronics. Conversely, electrical programmers and designers do not always remember to consider how the total mechanical inertia of drive systems can impact their overload settings. 

It is common practice in electrical design to implement multiple levels of overload protection into circuits. Most industrial control boxes normally have main breakers and or fuses and protection on each branch circuit. Many individual devices also have their own overload protection. This concept has not taken as deep of a root in the mechanical design side of machine building, often making drive components such as belts, chains, and couplings into mechanical fuses. In other cases machines are built with a series of shear pins to serve as the fuses to protect the drive line.

Meanwhile, the well established technology of the ball-detent torque limiter has been refined in recent years to be much better suited to dynamic drive applications. A precision ball-detent torque limiter is essentially a mechanical circuit breaker instead of a fuse. Rather than having a component that breaks and needs to be replaced, a torque limiter can trip and be reset many times during its life. In addition to eliminating maintenance, spare parts, and downtime after overload, another key advantage of using mechanical torque limiters over shear pins and or relying on a belt or coupling to break apart in an over torque condition is that they are much more accurate. A precision ball-detent torque limiter is typically able to maintain an accuracy of +/-5% while a shear pin is often in the range of +/-20% depending on the design and materials of construction. Most precision mechanical torque limiters are also adjustable, allowing users to fine tune disengagement torque values after installation.

The best advantage of using them in conjunction with electronic torque limiting is that they can usually be installed very close to the device a where crash could occur as well as at multiple points in the system. Mechanical torque limiters are designed to instantaneously detect an over-torque condition and disengage very quickly. In many cases mechanical torque limiters are capable of disengaging an over-torque drive line before an electronic device such as servo motor even begins to pick up the condition. Most mechanical torque limiters allow for an integration of mechanical and electrical design. An electronic proximity switch can be positioned near an actuation mechanism which moves in the event of an overload. This system works well because the overload is detected and disengaged followed by an electronic signal to a PLC or process controller to shut down the part of the system with the over-torque condition. Because torque limiters can be placed in multiple parts of a machine, the source of the jam can be detected very quickly using proximity sensors.

To summarize, the peak process torque value should always be calculated through the driveline back to the servomotor, and the associated current limit programmed into the servo drive parameters. Bear in mind that this will only truly protect the motor from the over-torque condition. The best option to limit torque at the process is to install a mechanical torque limiter as close to the area which jams as practical (both for maintenance of the process and possibly resetting the limiter). Essentially, machine builders should be aware not to put all of their eggs in one basket when mitigating machine crashes. Circuit breakers, fuses, and electronic limiting should be used at multiple levels on the electrical design. Torque limiters, guarding and bumpers should be employed on the mechanical design. As always, consult the manufacture’s of each component if there are any questions or concerns.

For more information, call 1-800-882-1881 or email sales@cates.com

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Effects of Power System Harmonics Includes:

• Excessive heating in rotating machinery
• Torque pulsations on AC motor applications
• Control equipment, instrumentation, signal conditioning equipment, and computers
• Overheating of power factor correction capacitors / Blown capacitor fuses
• Excessive current in neutral conductors of 3-phase 4-wire systems
• Interference with Communication Equipment
• kWh meter errors
• Overheating of transformers
• Premature tripping or failure to trip of solid-state protective relays
• Reduced power factor (increased utility rates or penalties?)
• Overloading of UPS or emergency generators
• Emergency generator instability due to voltage distortion effects on the voltage regulator

IEEE 519-1992 defines harmonics as, “A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency.” This body is working to assist customers avoid the downside of VFD application.

IEEE 519,  “Recommended Practices and Requirements for Harmonic Control in Electric Power Systems” was first published in 1981 to establish levels of voltage distortion acceptable to the distribution system.  This document has been widely applied in establishing needed harmonic correction throughout the electrical power industry.  The newer IEEE-519-1992 sets forth limits for both harmonic voltages on the utility transmission and distribution system and harmonic currents within the industrial distribution systems, based on the stiffness of the bus which can be measured by short circuit current which is measured at the Point of Common Coupling (PCC).  Following is a table from IEEE519-1992 describing harmonic current limits, and following the table are terms which are defined for clarifying their meaning.

For a harmonic analysis of your system with the help of dedicated software, answers to the following questions are needed. The software allows convenient estimation of harmonics, which allows proper decisions to be made about installing variable frequency drives.

• What is the input voltage to the VFD?
• What is the input transformer kVA rating and % impedance?
• What is your input transformer available short circuit current?
• What is the impedance ratio of all transformers (X/R) in the analysis?
• Are there AC line reactor, phase shift transformer, DCL, etc,
• What size and type of ASD is going to be used?
• What is the total linear load connected to the above power system?
• Where is the Point of Common Coupling (PCC)?
• How many other drives are to be running on the same bus and what is the total capacity?

With respect to analysis, the preceding paragraph mentions Point of Common Coupling. It is instructive to carefully define that point.

PCC is probably the most important and most controversial term in the entire IEEE document.  It is defined as the electrical connecting point or interface between the utility distribution system and the customer’s (user) electrical distribution system.  While simple in concept, identification of this point is sometimes misunderstood, which leads to confusion and mis-application of the specifications in the table.

In conclusion, the care and attention that is given to any VFD controlled system will dictate the success of the application. Most manufacturers of squirrel cage induction motors will recommend that motors be applied in conjunction with proper cabling and installation techniques, dv/dt filters, and line reactors to protect the circuit.

Article Provided by:

Lafert North America,  MOTR: Motor Operation & Technical Review

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• “Motor Temperature Ratings”,
• “Understanding Torque”
• “Electric Motors and Voltage”
• “Operating Motors in Wet and Damp Environments”

There are many more subjects thoroughly covered in this outstanding reference. All of the subjects are useful in the design of an application that will wring the most life out of the motor that is selected.

This collection is now a classic reference that is still highly relevant today. In fact, now more than ever, today’s users of electric motors are energy and quality conscious due to the cost of power as well as stiff competition. This document will help you work out the details that will allow you to select the right motor for your application. The bonus is, this information is independent of the brand of motors that you select. So even if you don’t buy Baldor motors, this document can help you.

You can download the Cowern Papers here: The Cowern Papers PR2525

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Bearing re-greasing interval time for industrial motors is an issue which tends to be as much of an art as it is a science. This post explains why it is challenging to make a blanket recommendation for bearing greasing intervals.

From the scientific end of the spectrum, greasing intervals are dependent on bearing type, bearing size, grease type and rotational speed. If ideal conditions were to exist in all motor/ bearing applications, re-greasing intervals would be easy to calculate.

The challenge comes into play when we apply the motor/ bearing combination into the real world and then try to make a responsible re-greasing interval recommendation that will give rated bearing life in 99% of the cases. Required re-greasing intervals might be shortened by a multitude of external influences such as vibration, moisture, dust and other contaminants, the type of grease used, high ambient temperatures and high radial load applied to the shaft of the motor. Every motor in an industrial application tends to be subject to a unique combination of varying degrees of the previously listed factors. This makes it virtually impossible to recommend a blanket re-greasing interval that satisfies the bearing’s needs and satisfies the customer’s wishes of having the longest greasing interval possible and still allow the bearings to attain rated life expectancy in 99% of all applications.

Most motor manufacturers are conservative in their recommendations for greasing intervals. They mimic the recommendations that are published by such bearing manufacturers as SFK, NTN and NSK. The lubrication manufacturers themselves publish almost identical recommended greasing intervals as the bearing manufacturers do. (Most manufacturers follow the guidelines set forth by such organizations as the National Lubricating and Greasing Institute.) The one factor that the recommended greasing intervals have in common besides being very similar in time frames is that they are published on the conservative side.

This is where the “art” aspect of recommending bearing greasing intervals comes into play. Bearing manufacturers point out that greasing intervals can be significantly stretched out if ideal conditions exist.

This increase in re-greasing intervals can be as much as 2.5 times for roller bearings and as much as 3 times for ball bearings. Somewhere in between the “worst case” scenario and the “ideal” situation lies a re-greasing interval which is appropriate for each specific motor/ bearing application. We feel that it would be inappropriate to make a blanket statement that extends the recommended greasing interval beyond the time frame recommended by the bearing and grease manufacturers unless very specific external parameters are known. Basic guidelines and types of lubricant used from the factory are usually included in the manufacturer’s instruction or maintenance material shipped with each motor.

The single most significant thing that increases greasing intervals, that is under the control of the manufacturer, is to supply ball bearings in lieu of roller bearings. The greasing interval for a ball bearing is almost exactly double that of a roller bearing if both bearings are the same physical size. Ball bearings have the capacity of handling a limited amount of radial thrust as compared to roller bearings, but are still suitable for many applications and therefore should be seriously considered when making motor/bearing selections. Please note that higher HP motors, will require roller bearings for belt drive applications but the ultimate decision in this instance rests with the user.

Chart #1

Common Bearing Greasing Intervals

(in all cases deference to manufacturers specific recommendations is preferred)

Chart #2

SERVICE CONDITIONS DEFINED

For the purposes of this article, Service Conditions are outlined so as to tie into Intervals chart shown above.

STANDARD DUTY      

Eight hours per day, Light to normal loading, Clean condition, free from dust

SEVERE DUTY             

24 hours per day, Light to normal shock loading, vibration, Exposure to dirt or dusty conditions

VERY SEVERE DUTY

24 hours per day, High ambient, Normal to high shock loading, vibration, Dusty conditions, Confined mounting conditions

Article Provided by:

Lafert North America,  MOTR: Motor Operation & Technical Review

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