The infographic explores the various areas that can lead to electric motor failure, including bearings, stator windings, external conditions, rotor bar, shaft coupling, and external conditions.

You can reduce the risk of failure by ensuring your electric DC motors are maintained properly. Be sure to read our General Guide to DC Motor Maintenance for tips and solutions to ensuring your electric motors run a top performance.

Source: L&S Electric

The term “quality” has many meanings. Most of the time, it is defined in subjective terms such as, exceeding expectations, or, in more casual terms, like “doing the job right the first time.” While it’s true that a quality product should exceed a customer’s expectations and be manufactured correctly, these definitions of quality are generic and subjective, especially for a motor manufacturer who has long-term relationships with its customers. At a DC motor manufacturer such as Ohio Electric Motors, quality is defined more aptly in objective and measurable terms. Specifically, quality is defined through the processes used to produce DC motors. Since manufacturing processes can be defined, controlled and measured to an objective standard, the “quality” of a DC motor is one that is produced as close as possible to that standard.

These processes are not limited to the manufacturing operations performed on the shop floor. Quality processes of a DC motor manufacturer extend throughout the supply chain and include the design processes as well as the performance of documented production tasks. Ohio Electric Motors has integrated several quality systems that provide a checks-and-balances “gauge” to verify that its custom designed DC motors are produced by standardized processes that stamp “quality” on each an every motor that leaves the plant. These systems include:

• The ISO 9000 Quality Management System
• The Lean Six Sigma Quality System
• Independent Product Safety Testing

The ISO 9000 Series Quality Management System

The ISO 9000 is a series of standards to manage the quality control processes of an organization. Published by the International Organization for Standardization, the standards are based on eight management principles that include, customer focus, process management, decision-making, supplier relationships, among others. The ISO 9000 standards have evolved over the years. Presently, Ohio Electric Motors is certified to the most current iteration of the series: ISO-9001:2008. OEM has taken advantage of the ISO 9000 quality management system to standardize the documentation of production procedures, the monitoring of supplier quality, and the implementation of a comprehensive testing program which includes 100% end of line testing and first piece inspections. Every motor that is produced by OEM is tested for peak performance and reliable operation.

The Lean Six Sigma Quality System

Lean Six Sigma is a business process management system that combines “Lean” management concepts and “Six Sigma” quality concepts to improve quality through process control and the reduction of process errors, defects and waste. The system emphasizes the continuous improvement of processes. Ohio Electric Motors has been a Lean Six Sigma company for many years. It uses Lean Six Sigma to achieve bottom line results such as, improving operational processes, identifying problems and solutions quickly and systematically, and reducing waste & cycle time.

Independent Product Safety Testing

Quality products are safe products that can measure up to independent testing. To verify the safety of its products, Ohio Electric Motors uses independent testing organizations such as, Underwriters’ Laboratories (UL), to independently test the entire line of its custom design, made-to-order DC motors. All of OEM’s motors are listed by UL and the CSA.

Most industry experts would agree that successful motor manufacturing requires significant competencies in design, manufacturing and quality assurance. No doubt these are crucial to the business success of a custom DC motor producer like Ohio Electric Motors. But OEM would add one other business competency into the success formula of a motor manufacturer: product support.

The real sign of a mutually beneficial, business partnership between a electric motor manufacturer and a customer is how the product is supported after it leaves the plant. While the goal of product support is total customer satisfaction, this term encompasses many different activities of the manufacturer-to-customer relationship that culminate in total product support. At OEM, product support is implemented in three separate but related support channels: Applications, Sales and Service.

Applications Support

As a critical, before-market support function, applications support is vital to matching as well as sizing a DC motor to an application. Motors are usually part of a complex electro-mechanical, motion control system, so knowing how the motor is going to be used and under what conditions is essential to promising a long service life. Product application issues addressed here are as varied as audible noise, bearing choices, vibration issues, torque-speed requirements, type of motor (brushed/brushless), EMI/EMC compliance and the operating environment. Since OEM specializes in custom designed, built-to-order, DC electric motors, our engineering staff is highly proficient with applications support.

Sales Support

If there were a motto that OEM would use to describe our philosophy regarding sales support, it would be “Sell and Support Locally.” Nowadays, business relationships can be started or facilitated by technology, but are only sustained by local, sales and support professionals. OEM has built a local network of sales engineers who meet with customers in their offices or at their facilities every day of the week. We have sales offices in all 50 U.S. states plus Canada, with a few states having more than one sales office, such as CA, IL, IN, NV, NY, OH and PA. These sales representatives provide before- and after-market sales support to our customers. Local contacts and relationships are integral to OEM’s core business: custom DC motor design and manufacturing.

Service Support

In any business, customer service is the first line of support to a customer. But OEM has structured its customer service department as much more than a call center or a place to order spare parts. Staffed with factory-trained professionals, our representatives are service support experts. Many of them have held different roles in motor manufacturing so they can speak from the viewpoint of a manufacturer, a service provider and an end-user. Personable, flexible and knowledgeable, they can take a customer through the steps of resolving a problem in short period of time. We know that many of our motors are installed in production applications so resolving their problems promptly is critical for eliminating lengthy equipment downtimes. In addition, we offer in-house testing and repair services for our line of motors with fast turnaround times at a reasonable price for non-warranty repairs.

Most businesses describe their devotion to customer satisfaction as a prime benefit of their products and services. Yet, in the business press, surveys of customer satisfaction often reveal this devotion is not always a reality. Too often customer service is compartmentalized to the call center as an after-market business process. We at Ohio Electric Motors approach customer service in a much different way. For OEM, customer service is synonymous to motor quality and is integrated into engineering, production, and order fulfillment using a four step system:

• Analysis of Customer Needs
• Planning for the Accomplishment of Goals
• Implementing the Project
• Evaluating our Customer’s Requirements

Analysis of Customer Needs

The production of quality DC motors is a complex task that requires a cross-functional team of professionals. The first task our team takes on is first analyzing the customer and determining what he needs. This task ensures that what is produced by OEM really answers our customers needs, which is the first step in the long road of integrating customer service into our motors. After determining what our customers need, our team analyzes what we need to do, as far as our organization, our suppliers and our processes, in order to move the project forward. We then address what our performance processes, such as, benchmarks and metrics. This period of analysis is an on-going process that is revisited throughout the development and production project and forms the first step of integrating customer satisfaction into our products.

Planning for the Accomplishment of Goals

It’s one thing to say you are a customer-focused organization and another to make it a reality that a customer perceives as such. Ohio Electric Motors uses an in-depth planning phase to establish the priorities, mission objectives and roles & responsibilities of each member of our organization. This is perhaps the most involved step of our approach to customer service. Our planning process is much more than an extended management strategy session. Rather, it establishes the project’s organizational structure, resources, training needs (if applicable), project management systems, risk assessments, budgets & cost accounting and supply chain management. Finally, we conclude the planning phase with ways to kick-off the project and build team cohesion. We want our employees to believe in the job they are about to do. And OEM management wants to empower them do the best job possible. When our workers believe they “own” the job, what happens is they improve the quality of the electric motors OEM produces, which is customer service by any other name.

Implementing the Project

A successful planning phase leads to a rapid and efficient implementation of the electric motor manufacturing project. This where we take all the things we learned while analyzing our customers needs and planning for the project and make them a reality. Throughout the implementation phase we are comparing what we thought was necessary for a successful project to what is happening in real-time on the engineer’s CAD station and all the way to the activities on the shop floor. We monitor how our resources are being used. We address problems, conflicts and changes, so we can take immediate corrective action. This is where engineering and production quality meets customer service even before our motors are shipped.

Evaluating Our Customer’s Requirements

The conclusion of our successful project analysis, planning and implementation phases is the final step: evaluating our results relative to customer satisfaction and requirements with quantitative methodologies. We develop reports to document costs, performance and the testing of our motors. We conduct process reviews and meet to see how we can improve them. This type of checks-and-balances evaluation tries to close any gaps, so to speak, and verify that what we are producing is what our customers want. This is the endgame for Ohio Electric Motors: DC electric motors built with quality and answer the requirements of our customers.

If you are an original equipment manufacturer who needs the services of a DC motors manufacturer who can design, build and test custom DC motors, what factors should guide your decision on whom to select? A basic starting point is investigating the manufacturer’s capabilities in design, manufacturing and its ability to deliver on schedule. As a successful, custom DC motor manufacturer nearly 100 years, Ohio Electric Motors has found three important requirements for a custom motor manufacturer to possess to ensure a profitable business partnership: (1) a professionally trained design and manufacturing team, (2) comprehensive manufacturing facilities and (3) standardized work processes verified by quality control systems.

Professionally Trained Engineering and Manufacturing Team

Producing custom-made, DC motors is a complex process. Even before the raw materials are trucked to the manufacturing plant and brought to the production line, both the client and electric motor manufacturer have started to solidify their working relationship by laying the ground work that’s required prior to the commencement of production. Successful working relationships require trained professionals, including engineering, manufacturing, supply chain, logistics, quality control, among others, to create not only the motor design and prototypes, but also the manufacturing processes and delivery channels. Ohio Electric Motors has built a design and manufacturing team of highly trained and customer-focused professionals. It leverages both the technical expertise and customer service skills of its staff to ensure production goals occur within budget.

Comprehensive Manufacturing Facilities

Manufacturing any product requires both the equipment and facilities to handle projected production volume, capacity or throughput. Beyond capacity considerations, what’s key for a custom electric motors manufacturer is to have all the machinery, equipment and testing capabilities in house and on-site so it can produce motors from start to finish in a timely fashion. Ohio Electric Motors has built a state-of-the-art comprehensive production facility that’s fully equipped to manufacture even high volume runs. It’s onsite process equipment includes, CNC machining, shaft splining, automated magnetizing, heat treating, epoxy coating insulation, automated coiling, winding, commutator slotting and balancing, varnishing and welding/painting/special fabrication. It also has a testing laboratory, which includes a 30 HP dynamometer and 500 Amp testing capability, to conduct both standard and special tests.

Standardized Work Processes Verified by Quality Control Systems

One of the motivations for using a custom motor manufacturer is the belief that a specialist can manufacture a motor, quicker, less expensively and with a greater level of quality. And it is the ability to manufacture a quality product that reaps both cost and time benefits. To obtain a high level of quality, a successful DC motor manufacturer should implement standardized work processes to enhance consistency and reliability. In addition, the motors should be tested throughout the production chain to verify this quality. Ohio Electric Motors understands the vital importance of manufacturing process control to produce consistent and reliable product. That’s why it has invested in ISO-9001: 2008 registration and implemented Lean Six Sigma initiatives. All of its machining operations undergo first piece inspections and all of its motors go through end-of-line testing. It uses documented manufacturing procedures and its production work-stations are linked to the engineering department to ensure all parts are manufactured to the latest revisions.

As of year 2010, the market size of the motor and generator manufacturing market is $11,340 million. On average, the industry has declined 9.5% since 2005. Despite this decay in market size, long term forecasts for the industry is expected to grow by 16.5% by year 2015.

Check out the infographic below and gather a real sense of the United States Motor and Generator Manufacturing Industry.

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At face value, the purpose of compliance is to ensure a product meets basic safety requirements and has been designed with good engineering practices and built with quality manufacturing processes so it poses little threat to the consuming public. On a much larger scale, compliance affects national and international trade and, in some cases, may reduce product liability litigation for the manufacturer. Compliance engineering is no doubt a very broad topic that covers many activities from initial product design through manufacturing and workplace safety. This article discusses the basic elements, organizations and testing involved with compliance engineering, including information specific to the compliance of electric motors. In general, compliance is broken down into the following categories:

Product Compliance:

• Health and safety designs and construction
• Environmental
• Energy efficiency
• Electromagnetic compatibility (EMC)

Workplace Safety Compliance:

• Worker safety
• Ergonomics
• Hygiene

Agencies and Organizations

Compliance standards are developed by governmental agencies and independent (third party) organizations. There are numerous agencies involved with standards writing; ¹they write both mandatory and advisory (consensus) standards. Mandatory standards can be developed by governmental agencies or independent organizations and are made into law. Advisory (consensus) standards are developed by independent organizations as recommendations for safe practices; many times, all or part of these recommendations become mandatory standards.

The main agencies involved electric motor compliance and electrical safety standards are:

• Underwriters’ Laboratories (UL): an independent, third party, testing organization that publishes over 250 safety standards and certifies the safety of products.
• Canadian Standards Association (CSA): an independent testing organization that certifies motors to its standards.
• Institute of Electrical and Electronic Engineers (IEEE): a technical organization that recommends safe practices, which often become mandatory standards. Regarding motors, the IEEE publishes standards on temperature rise, rating methods, classification of insulating materials.
• American National Standards Institute (ANSI): a national organization that writes consensus standards that represent manufacturers, distributors and consumers. Its standards cover topics such as dimensions, specifications of materials, methods of test, performance, definition of terms, etc. Its primary motor standard is C-50: Rotating Machinery.
• National Electrical Manufacturers Association (NEMA): an employer association that participates in writing voluntary standards in conjunction with the IEC and represent general practices in the industry. “It participates extensively in the IEC at both technical and management levels and provides support for six IEC Technical Committees (TCs).” For motors, its standards cover frame sizes, torque classifications, and basis of rating.
• International Electrotechnical Commission (IEC), a global non-governmental organization that prepares and publishes international standards for electrical, electronic and related technologies. Several of its standards are highly relevant for motor manufacturing. IEC 60034 is referred to by regulators for the classification of electrical motor efficiency.
• International Organization for Standardization² (ISO): a non-governmental organization that promotes the development of standardization around the world. For motor manufacturing, the two major ISO standards are ISO9000, which focuses on quality assurance in manufacturing; and ISO14001, which focuses on environmental management systems.
• The European Committee for Electrotechnical Standardization (CENELEC): Based in the European Union (EU), it writes standards to develop harmonized electrical standards for the European community. To sell motors in the EU, a manufacturer must comply with EU standards for the CE mark.

Workplace and Worker Safety Compliance

While much of compliance engineering is accomplished in the design and manufacturing stages of a product’s life cycle, it does not end when the product is shipped from the factory. Compliance extends into the workplace with standards for workplace safety, installation (construction) and maintenance. The regulatory standards bodies³ involved with workplace safety compliance are the American National Standards Institute (ANSI), Institute of Electrical and Electronic Engineers (IEEE), the National Fire Protection Association (NFPA), American Society for Testing and Materials (ASTM), American Society of Safety Engineers (ASSE), Occupational Health and Safety Administration (OSHA), National Electrical Safety Code (NESC), and the National Electrical Code (NEC) (sponsored by the NFPA).

The regulatory bodies cited above typically produce voluntary (advisory) standards developed by industry professionals from public/private organizations with two exceptions: OSHA and the NEC. OSHA’s requirements are legal, hence, mandatory, so “failure to follow these standards could result in a citation, a work shutdown, fines or other sanctions.”⁴ The NEC, sponsored by the NFPA,⁵ has been “adopted by local law in many states, municipalities, cities and other such areas.” ⁶ In fact, many countries outside of the U.S. have adopted the NEC in all or part of their national installation code. OSHA’s health and safety regulations are detailed in its 29 CFR 1910 Subpart S on Electrical Safety. This standard covers a broad range of electrical safety issues⁷ that are relevant to the installation and maintenance of electric motors. ⁸ To assist in the compliance of its regulations, OSHA provides technical consultations, voluntary protection programs and training & education. ⁹

Independent Testing: Third Party Certifications

To verify conformity to compliance standards, a manufacturer must submit its product to an independent or third party for testing and certification. ¹⁰ In the U.S., there are a number of independent testing laboratories with the most common being UL and CSA. For motor testing and compliance, tests will be conducted to ensure “compliance with the norms of design, material inputs and manufacturing accuracy. It determines the mechanical soundness and electrical fitness of the machine for its electrical and mechanical performance.” ¹¹

Depending on the application, UL tests electric motors according to the following standards: UL 1004-1, the Standard for Safety of Rotating Electric Machines;
UL 1004-2, the Standard for Safety of Impedance Protected Motors; UL 1004-3, the Standard for Safety of Thermally Protected Motors; and UL 2111, Standard for Overheating Protection of Motors. ¹²

During the manufacturing process, electric motors endure a variety of tests to ensure proper operation and integrity. But for compliance testing, the UL thermal tests are the primary interest. For example, UL 2111 ¹³ stipulates the following acceptable temperature rises for NEMA B electric motors:

• Locked Rotor: 225^oC in the first hour; 200^oC in the second hour.
• Full-load, heat run: 165^oC
• Running overload: 165^oC if protector opens; 175^oC if protector doesn’t open.

Energy Efficiency Compliance

Since the passage of the 1992 Energy Policy Act (EPACT)¹⁴ that mandated full-load motor efficiencies, energy efficiency compliance has increased and influenced the introduction of energy efficient motors. For compliance, motor efficiency must be confirmed by a “test procedure acceptable to the U.S. Department of Energy” ¹⁵ and accomplished in an approved testing facility, such as UL. ¹⁶

(Note: D.C. motors are excluded from the EPACT legislation because “unlike an induction motor, a d-c machine inherently offers a range of operating speed, determined by field control settings, throughout which efficiency is a variable.”¹⁷

EMC Compliance

Electromagnetic compatibility (EMC) compliance has become a greater concern due to the increased use of power converters (D.C. drives) and inverters (A.C./D.C. drives) for industrial motor control, causing electromagnetic interference (EMI) with other equipment. It’s been known for a long time that switches, relays and brushed-commutator motors are sources of EMI¹⁸. But the high-switching frequencies of power converters and switched-mode power supplies has exacerbated the EMI problem.

EMI results from “electromagnetic radiation, [which] is a form of energy at a particular frequency that can propagate through a medium. This intentionally or unintentionally generated EM energy is considered electromagnetic interference (EMI) if it degrades the performance of electronic systems. Due to the increasing amount of man-made generated EMI around the globe, allowable limits as well as measurement techniques on RF noise/interference have been set at national and international levels. The Federal Communications Commission (FCC) and the Military are the two governing bodies in the U.S. setting standards on EMI.” ¹⁹ Globally, the IEC is setting the standards on electromagnetic compatibility. It publishes the IEC 61000 series of standards that deal with the EM environment and describe electromagnetic phenomena, including measurement and testing techniques, as well as guidelines on installation and mitigation.

“EMC is defined as the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable disturbances to anything in that environment.” ²⁰ For electric motors and industrial contro1, the standards typically used for EMC compliance²¹ are:

• UL508, Industrial control equipment for the USA
• IEC 61010, Industrial and Lab equipment
• EN 50178, Power Equipment
• IEC 61000, Generic EMC standards

The CE Mark: Global Compliance

Globalization, world trade, free and open markets–all these words are synonymous with the nature and context of business today. American manufacturers (be it D.C. motors or washing machines) need to prepare their products so they can be sold in other parts of the world. Global certification and compliance is part of this preparation. For U.S. companies, the most common type of global safety compliance is the CE mark issued by the European Union. ²²

In 1985, when it was first introduced, the CE mark was meant to encourage trade among the European Union’s member nations. But, in 1986, many imports into the EU experienced trade barriers due to “inconsistent national product requirements.” ²³ This provided an additional impetus for the adoption of the CE mark.

The development of the CE marking system decreased these inconsistencies and created “a set of product safety standards and a series of conformity assessment procedures that are used to prove that these standards have been properly implemented.” Both China and Japan are developing compliance programs similar to the CE mark. For the U.S., gaining the CE mark compliance is critical since the EU is the “largest foreign market for the U.S.”²⁴

The CE mark is not a quality certification; rather, it’s certification that indicates the product is in compliance with “the health and safety requirements of the directive that applies to the product.”²⁵

Three EU directives related to the compliance of electric motors. They are:

• Machine Directive
• Electromagnetic Compatibility (EMC) Directive
• Low Voltage Directive

The Machine Directive focuses on fundamental health and safety requirements concerning the design and construction of machinery. The EMC Directive focuses on the requirements for EMI emissions and immunity. The Low Voltage Directive focuses on electrical, physical and mechanical product safety, including motor efficiency.

The purpose of D.C. Motor protection is to extend a motor’s lifespan by protecting it from conditions that can damage that the motor’s windings, both electrically and mechanically. Motor winding damage can result from any of the following conditions¹:

• Mechanical damage
• Excessive moisture
• High dielectric stress
• High temperature

While each of the above conditions can lead to winding damage, the apparent failure is “thermal degradation of the insulation or burnouts. Insulation life is reduced by about half for each 10^oC increase in winding temperature.” ² To avoid thermal degradation of the insulation, there are a number of methods, devices and circuits used to monitor potential motor hazards and fault conditions and deeneergize the motor when these conditions are met.

Potential Motor Hazards and Fault Conditions

A review of motor hazards and common fault conditions is useful in understanding the different approaches taken to protect motors. These fault conditions are divided into the following categories³:

• Motor-induced faults
• Load-induced faults
• Environment-induced faults
• Power source-induced faults
• Application-induced faults

Motor-induced faults⁴ are directly related to the motor and its associated wiring. Common motor-induced faults include burnt out insulation, bad bearings, loss of field, and other mechanical failures. Wiring problems, chafed or exposed wiring, cabling faults or abrased insulation can cause “short circuits between power phases or between a power phase and earth ground in the motor winding or its connections.”⁵ (Note: Even though wiring or cabling faults are related to power-source induced faults, they are categorized as a motor-induced fault.)

Load-induced faults⁶ are “the prolonged overloading as a result of the application of excessive mechanical load”⁷ Jamming (locked rotor) is a common load-induced fault that causes an apparent overload or high inertia (Wk²d). In pump applications, for instance, oil that is cold or highly viscous may cause a fault; oil heaters are possible solution to correct this fault condition.

Environment-induced faults⁸ include high ambient temperature, cold/damp environment, high contaminant level, blocked ventilation, among others. These conditions can increase the temperature of the windings by collecting moisture, degrading by corrosion or insulating the windings from contaminants. Loss of ventilation especially at low speeds also increases winding temperature.

Power source-induced faults⁹ typically will cause high motor currents that can thermally degrade the motor windings from I²R heating. These fault conditions are numerous and include, overvoltage, undervoltage, phase reversal, open phase failures¹⁰, unbalances, ground-faults, power transients, harmonics, and loss of field.

Application induced-faults¹¹ are caused by operating conditions that typically cause overcurrent or overload conditions. These conditions include high duty cycle, jogging, rapid plugging (or plug reversing), overspeeding¹², and synchronization problems.

Motor Protection Methods

Motor protection methods include devices and circuits that are used within the motor or used with the motor’s control circuitry to monitor fault conditions. They include:

• Thermal overload relays
• Transient voltage protectors
• Ground fault relays
• Distance relays
• Fuses, contactors and circuit breakers
• Undervoltage protection
• Locked rotor protection
• Loss of field relays
• Reversed current protection
• Isolation transformers
• Harmonic filters
• Power conditioners

Thermal overload relays¹³ protect motors from overload conditions. There are two main types: inherent and external. Inherent thermal overloads¹⁴ are bi-metal devices embedded in the motor’s windings. They are essentially thermostats with two dissimilar metals bound together that will bend to open (in some cases, close) a trip switch¹⁵ at a temperature setpoint, which is proportional to motor current in an overloaded condition. The switch is connected to the motor’s control circuitry to alarm and/or deenergize the motor. External thermal overload protection ¹⁶ use heaters that are connected in series with the motor’s windings and mounted on the motor contactor or circuit breaker. There are two types of heaters: solder pot and bimetal strip. Solder pot overloads will melt when the heat generated by the motor current in an overload condition occurs; this action opens the motor control circuit and trips the motor off the power line. Bi-metal strip¹⁷ thermal overloads operate similar to inherent overload protection. While thermal overload protection are most commonly used, electronic and magnetic overload protection are also used for overload protection.¹⁸ Electronic overloads are current sensors. They sense actual motor current and when the motor current reaches a predetermined level, a relay will trip and open the motor control circuit. Magnetic overloads use electromagnetism to sense an overload. When an overload condition is sensed, a relay coil will pull in (close) and trip the motor off the power line.

Fuses and circuit protectors are not overload protectors; rather, they are overcurrent protectors designed to “protect the motor from a direct ground or short circuit condition” ¹⁹ in the motor or its associated wiring and cabling. Short circuit protection is incorporated into a motor contactor with “high breaking capacity fuses” or a circuit breaker with “instantaneous attracted armature type relays.” Ground fault relays or interrupters are another type of overcurrent protection. They monitor “unintentional current paths between a current-carrying conductor and a grounded surface”²⁰. For motors, ground fault current paths are typically found through dust, water, or worn insulation. Ground faults pose worker safety hazards. ²¹ Reverse current relays are a protective feature used in motor-generator applications where a standby battery is being charged by the generator. The reverse current relay prevents the battery from discharging and motorizing the generator. ²²

For D.C. motors, the loss of field can potentially cause a dangerous, overspeed condition, ²³ Hence, loss of field relays are used to monitor the motor’s field. They are connected in parallel with the field and monitor the D.C. motor’s field current. In the event that the field current decreases below a certain limit²⁴, the loss of field relay will drop out and deenergize the motor’s armature.

When a motor fails to start or accelerate after it’s been energized, it is exhibiting a locked-rotor condition. In this condition, the “motor is subject to extreme heating, much more so than in an overload condition since the heat has very little time to be dissipated in the conductors and the other parts of the motor.” ²⁵ Locked-rotor conditions can be protected by an overcurrent relay set for permissible I²t times and currents. But for large D.C. motors, another solution is to build a zero-speed switch into the motor. ²⁶ If the motor does not accelerate to open the zero speed switch, the motor’s power supply is deenergized. However, there’s a disadvantage to the zero-speed switch. In situations where the motor starts but locks up at less than full speed, the zero-speed switch can close and deenergize the motor’s power supply. Locked-rotor protection can also be accomplished by a distance relay²⁷.

Power source-induced faults include undervoltage, overvoltage, open phasing, phase rotation and phase imbalances. (Note: Generally speaking, phase imbalances, phase rotation faults and open phasing are associated with A.C. motors and will not be covered in this article. But it should be noted that if a D.C. motor is powered by a D.C. converter, this controller protects the motor from these conditions.) ²⁸) Undervoltage faults can cause either high motor currents or a failure to start. Hence, most undervoltage protection is part of the motor starter. However, for prolonged undervoltage conditions, an inverse time undervoltage relay can be used to proect from this condition. ²⁹

Rather than using discrete components to protect a D.C. motor from overvoltages or surges, D.C. drives, isolation transformers³⁰ and power conditioning equipment³¹ are typically used to provide this type of protection. However, MOVs³², arrestors³³, harmonic filters³⁴ and power factor correction capacitors³⁵ can also provide overvoltage protection.

Interlocks: Indirect Motor Protection

Interlocking is used to “prevent [motor] contactors from being energized simultaneously or closing together and causing a short circuit.” ³⁶ In this respect, interlocking is an indirect type of motor protection and generally is used with motor starters for reversing and/or auxiliary control. There are three types of interlocks:

• Mechanical
• Electrical
• Auxiliary Contact

Mechanical interlocks will physically prevent two motor contactors (Forward and Reverse) from closing simultaneously. “This interlock locks out one contactor at the beginning of the stroke of either contactor”. On the other hand, electrical interlocks use a pushbutton control or auxiliary contact to electrically isolate one contactor while energizing the other contactor. ³⁷ Auxiliary contact interlocking is a wiring modification of pushbutton interlocking. There are two types of auxiliary contacts: Normally Closed (NC) and Normally Open (NO). For interlocking protection in a reversing circuit, a normally closed (NC) auxiliary contact is wired in series with the opposing motor contactor coil. Thus, when a motor is running in the forward direction, the forward contact coil is energized through the NC auxiliary contact. When the reverse direction is selected, the NC contact will open and deenergize the forward contact coil while the reverse coil will energize through its NC auxiliary contact.

Environmental Protection

Environmental contamination can adversely affect normal motor operation. Dust, air particulates, explosive vapors, water, humidity, high ambient temperatures can all shorten the lifespan of a motor. To protect a motor from these environmental conditions, the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) have classified motor enclosures based upon the level of protection they provide. ³⁸ The two major classification of motor enclosures are open and totally enclosed. Open motors are further classified as dripproof, splashproof, weather protected, semi-guarded and guarded. Totally enclosed motors are classified as totally enclosed non-ventilated, fan-cooled, explosionproof, dust ignition proof, air-to-water cooled and air-to-air cooled.

The cost of electrical energy in the U.S. is increasing¹ and since electric motors account for over two thirds of industrial energy (60-70% in a manufacturing plant²) and about 23% of all electricity sales in the U.S., according to the U.S. Department of Energy³, motor energy management and efficiency has generated immense concern. Motor manufacturers have responded by introducing energy efficient motors that reduce energy consumption. In addition, the replacement of bulky wound field motors with compact, brushless D.C. and permanent magnet motors that have high efficiency/power density is a purchasing trend that will continue for many years to come. But the issue of motor energy management and efficiency is more complex than merely selecting a new motor replacement. This complexity merits a thorough examination because successful energy management is really a systems approach.

An historical aside may be useful at this point to illustrate where the industry was a few decades ago and where it is now. Energy management was not always a concern. From post-WWII to the early 1970s, motor design was inefficient and emphasized low initial costs without much concern about lifetime or operating costs. But, in 1973, the first oil crisis⁴ occurred and completely changed the industry as electricity costs skyrocketed. A new imperative was born in motor design: limiting operating costs (Table 1). In response, most motor manufacturers added higher efficiency motors, featuring “optimized designs, more generous electrical and magnetic circuits and higher quality of materials.” ⁵

In 1997, the U.S. Energy Policy Act, which created mandatory efficiency standards, was passed. At about the same time, the Consortium for Energy Efficiency (CEE) developed a voluntary premium efficiency standard, which evolved into the NEMA Premium designation. Premium efficiency motors offer an efficiency improvement in the range of 4% for 1 hp motor to 2% for a 150 hp motors. ⁶

Table 1: Lifecycle Costs of An Electric Motor ⁷

Motor Losses

Electric motors experience a variety of losses⁸ that reduce their efficiency. These losses⁹ include:

• I²R (power) losses
• Core losses
• Mechanical losses

Power losses (I²R losses) can account for 20-30% of a motor’s total losses. ¹⁰ Power losses depend on the winding resistance and motor current and result in wasted power in the form of heat. Since winding resistance increases with temperature, controlling motor temperature can improve efficiency. Typically, winding resistance reductions are achieved by using more copper. High efficiency motors can have about 20% more copper in the windings than standard motors.

Magnetic core losses are the results of hysteresis and eddy currents.¹¹ Hysteresis losses can be reduced by using high quality steel in the core, such as silicon steel instead of carbon steel. Eddy currents are caused by circulating currents in the motor core. They can be reduced by using thinner and longer laminations and ensuring adequate insulation between the laminations. ¹²

Stray load losses are the “difference between total motor losses and the sum of the other four losses, including I²R, iron/magnetic, friction and windage.” ¹³ Some causes of stray load losses include, imperfections in the slotting of the stator and rotor and saturation effects. They are eliminated by design and careful manufacturing processes. High efficiency motors have much less stray load losses than standard motors. ¹⁴

Mechanical losses are the result of windage and friction. Fan blades, brush-commutator contact and bearings are the typical sources of mechanical losses. High quality bearings, improved ventilation fan designs and brush-commuator improvements are ways to alleviate these losses. ¹⁵

Other Factors that Affect Motor Efficiency

Beyond motor losses, there are other factors that can impact electric motor efficiency. They include¹⁶:

• Proper sizing
• Electrical power quality
• Type of control
• Distribution Losses
• Type of transmission
• Maintenance
• Operating Temperature¹⁷
• Application (Mechanical efficiency of driven equipment)

The oversizing of motors (lightly loaded) is a common cause of electric motor inefficiency. An oversized motor, defined as less than 50% loaded, not only lowers the efficiency also but also the power factor. “This decrease in performance is especially noticeable in small motors and standard efficiency motors.”¹⁸ In addition, at light loads, iron core losses are high. To increase efficiency, some motor controls will “automatically lower the motor voltage when the load drops.” This will lower energy costs.¹⁹

Poor power quality can also decrease the efficiency of an electric motor.²⁰ Power quality problems can include phase voltage imbalances (i.e., undervoltage, overvoltage), harmonics, interference, frequency imbalances, etc. Phase voltage imbalances can cause high input currents (I²R losses). Voltage flicker (brownouts or sags) are short-term voltage variations. They add to motor losses and cause motor current to increase. Powerline harmonic distortions increase motor losses and cause motor reactive heating, and decrease efficiency.²¹

Motor temperature affects efficiency. Cooler running motors will have a lower winding resistance and result in lower motor losses. For example, a “10^oC reduction in motor temperature will reduce the D.C. resistance losses of the conductors by 3-4%.” ²²

Design Features That Improve Motor Efficiency

Motor efficiency is improved by reducing motor losses: power, core and mechanical. This is accomplished in any of the following ways²³:

• Build with closer tolerances
• Reduce vibrations
• Increase the amount of copper in the stator windings
• Use higher-grade electrical steel
• Improve the power factor to reduce reactive current heating
• Use high efficiency mechanical loads (pumps, fans, etc.)
• Use electronic controllers instead of across-the-line start/stop controls
• Use energy efficient belts and/or gear reducers
• Use power conditioning equipment.

Closer tolerances, vibration control, low-friction bearings, and energy efficient belts are some of the ways to reduce mechanical losses. Improved mechanical load designs help improve motor efficiency by “reducing internal friction through smoother and more carefully contoured internal surfaces, tighter tolerances, higher quality bearings.” ²⁴ “Additional savings are possible by replacing V-belts with energy-efficient cogged belts. This can raise system efficiency by up to 2 percent. Switching a right-angle worm speed reducer to an inline helical or right-angle bevel gear reducer can raise efficiency by 20 to 50 percent. This means that a lower-horsepower motor that consumes less electricity [ ] can be used to drive the load.” ²⁵ Core losses are reduced with high-grade steel and better lamination designs. Power conditioning equipment can improve the electrical mains’ power factor. This reduces motor running temperature (I²R losses) by eliminating the reactive component of motor current. In manufacturing, winding resistance can be reduced by maximizing slot fill²⁶ and minimizing the end-turn radius²⁷.

D.C. Motors: High Efficiency Designs

Improvements in the efficiency of D.C. motors break down into three broad categories:

• High efficiency designs
• Advances in the brush-commutator area
• New D.C. motor types: Permanent Magnet and Brushless types.

Much of the effort in producing high efficiency D.C. motors revolves around ways to understand thermal pathways through the motor as well as limiting I²R losses (heat). In addition, “software modeling tools [are] used get a better understanding of both the magnetic flux and thermal flows in the motor laminations … Slight changes in lamination geometries, metallurgy, and insulating materials allow for increased power density and smaller motors.” ²⁸

Since the brush-commutator area of a D.C. motor is a source of mechanical losses (as well as high maintenance), there has been research conducted on how to improve these motor parts. One of the ways is to pursue designs that increase the power density of the motor by decreasing the size of the commutator. “As the circumference of the commutator shrinks, there’s less brush wear with every turn of the rotor. Reduced brush wear results in extended intervals between brush changes. Engineers also have redesigned brush blocks, pressure fingers and springs to allow for longer brushes. With longer brushes, the interval between brush changes extends further, providing for longer periods of operation without a maintenance shutdown. DC motors can be purchased with brush wear sensors, which warn that a brush is worn down to its lowest level and requires changing. Brush wear sensors often prevent commutator damage from a worn brush being left in too long and resulting in costly repairs.” ²⁹

Improvements in D.C. motor power density have led to the introduction of brushless D.C. (BLDC) motors and permanent magnet D.C. (PMDC) motors. Both of these motors types are inherently efficient. PMDC³⁰ motors use permanent magnets in lieu of power-consuming electromagnets to generate the motor’s magnetic field. These high performance, permanent magnets are made from neodymium-iron-boron alloys which have a large energy density and “offer the possibility of achieving high efficiency and compact motors.” ³¹ The permanent magnets are mounted in the stationary field (stator) which allows or better heat dissipation through the motor housing and into the atmosphere. This arrangement allows the motor to run cooler and more efficiently.

A PMDC motor can be a brushed type (commonly called a D.C. commutator motor³²) or a brushless type. The BLDC³³ motor has increased its market share because it has high efficiency, good power density and much less maintenance than the D.C. commutator motor. It does not use brushes or a mechanical commutator; rather, a controller electronically commutates the motor. Similar to a D.C. commutator motor, the BLDC motor uses permanent magnets to develop its field; the magnets are mounted on the rotor and the armature windings are mounted on the stator, where heat can be dissipated easily to the atmosphere, maintaining a cooler motor and promoting greater efficiency.

The selection of a D.C. motor for an initial installation or replacement can be an involved process requiring information¹ about the load and torque requirements, operating environment, efficiency², frame size, mounting configuration, enclosure type³, duty type⁴, among others. This guide will overview the typical D.C. motor selection and replacement process⁵. For specific or unique applications, this guide should be considered as a general guideline only.

Overview

In general, the selection of a D.C. motor consists of the following:

• Determine whether the load will be driven at motor speed (direct drive) or at some other speed that will require gearing, coupling, pulley, belts, etc. based upon the maximum load speed.
• Calculate load torque requirements.
• Calculate motor horsepower, speed and full load torque requirements.
• Based on the speed-torque and load requirements, select the motor type.
• Determine the operational requirements: start/stopping, accel/decel open/closed speed loop, braking (and holding), type of controller, etc.
• Select the motor enclosure based on environmental considerations and cooling requirements.
• Select mounting configuration: floor- or flange-mounted.

Selecting a Replacement D.C. Motor: Motor Nameplate Identification

Selecting a D.C. motor is very much an investigative process. The time it takes to gather all the information depends on whether the motor selection is for an initial installation, (e.g., new machine design), or for a simple replacement. ⁶ For direct replacements, the motor nameplate identification⁷ can provide much of the information needed. It can also tell a lot about the motor application. If no other changes have been made to the application, the motor nameplate⁸ can very well be the only thing needed to properly select a replacement. The motor nameplate identification is located on the motor enclosure. The information contained on the nameplate varies with motor manufacturer, but it can contain the following information:

• Motor Manufacturer
• Model, Type or Catalog No.
• Serial Number: this provides specific information about a model or general type of motor. For example, it may include a data code to indicate the manufacturing date, revision code or applicable drawings.
• HP (Horsepower) or KW (kilowatts)
• RPM (Revolutions per Minute): motor speed at full load
• ARM. (Armature Voltage): maximum D.C. voltage applied to the armature. Generally, 24, 48, 90 or 180 VDC.
• FLD. (Field Voltage) D.C. voltage applied to motor field winding. Generally, 100, 150 or 200 VDC.
• A (Amps): full load current.
• Fr (Frame): physical dimensions of the motor based upon NEMA standards⁹
• Enc. (Enclosure): type of enclosure based upon NEMA/IEC standards¹⁰
• CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)
• Maximum Ambient Temperature: Related to Insulation Class, this specifies the maximum operating temperature of the motor.¹¹
• Insulation Class: classifies motor winding insulation by maximum allowable temperature. Typical classes are B, F and H. ¹²
• Duty: type of rated duty cycle (e.g., continuous, short-time, periodic, intermittent, etc.)¹³
• Bearing Type: ¹⁴
• Form Factor (Power Supply Code): NEMA classification of quality of D.C. drive power quality based on the ratio of ripple D.C. current to average RMS D.C. output. ¹⁵ Typical codes are A, C, D, E and K.
• Winding Type: shunt, series or compound connections.
• Connection Diagram

Basic Calculations: Sizing a D.C. Motor

Sizing a D.C. motor is the process of ensuring the motor is properly matched to the load requirements so the motor will perform normally. ¹⁶ This entails first calculating the mechanical characteristics of the application, (i.e., load torque.). The general case for calculating torque is:

Τ = Fd (F=Force; d= rsin θ = moment arm) ¹⁷

Calculating load torque for a specific application depends on a variety of factors, including type of drive mechanism (e.g., ball screw, pulley, belt, direct, etc.), motor speed, length of shaft (moment arm) and horsepower. ¹⁸ While all the permutations of load torque calculations are beyond the scope of this article, the general case for calculating motor full load torque is:

Motor Full Load Torque = Horsepower (HP) x 5252 / Speed (RPM) ¹⁹

Since torque is related to speed, to size a motor that can deliver the necessary torque for the application, the load curve of the application must be matched to the motor’s torque-speed characteristics. The load demand in terms of torque must match or overlap the torque-speed operating region of the motor. ²⁰

Another useful value in sizing a motor is the K_m constant. ²¹. It can be useful in sizing a motor because of its relationship to torque, power and line-to-line resistance. Other considerations in motor sizing are thermal losses. ²² The motor current and power dissipation of a motor is important because they are related to the motor’s temperature rise and affect motor efficiency. Motor losses in the form of heat dissipation (called I²R losses) are calculated:

Power _dissipation = I² x Winding Resistance.

Winding resistance varies with temperature based on the type of wire and its temperature coefficient, which defines how its resistance will vary with temperature. ²³

Some Other Considerations in D.C. Motor Selection

Once a D.C. motor is properly sized in terms of torque, power and speed, there are several other factors to consider prior to making the final selection. Some of them include:

Wound Field vs. Permanent Magnet (PM) : PM motors have linear speed-torque characteristics while wound field motors are non-linear. ²⁴ PM motors have good acceleration torque, run cooler and have a smaller frame size than wound field motors. ²⁵

Brushed vs. Brushless: Brushless D.C. motors require less maintenance and can be operated at much higher speeds than brushed D.C. motors. However, they require a complex controller for electronic commutation; brushed D.C. motors can be operated with a simple control system. ²⁶

Insulation Class: An insulation class gives temperature ratings above which would damage the insulation and, at some point, the motor. The most common classes are B, F and H, with Class H having the highest temperature rating. For D.C. motors being powered by variable speed SCR converters or PWM inverters, the higher insulation class ratings (F or H) are recommended. ²⁷

Open vs. Closed Loop Control: Closed loop control of speed or torque provides better regulation than open loop control. However, closed loop systems require a feedback signal that is produced by a tachometer or optical encoder and adds to the cost of the motor.²⁸

Selecting the Motor Enclosure: Environmental and Cooling Requirements

D.C. motors are operated in a variety of environments some of which can adversely affect normal motor operation. Some of these environments include corrosive, wet/humid, high ambient temperature, dust (air particulates) and explosive. Selecting a motor enclosure to prevent environmental contamination is a critical factor in the motor selection process. Conveniently, the National Electrical Manufacturer’s Association (NEMA) and the International Electrotechnical Commission (IEC) have classified motor enclosure types according the environmental protection. ²⁹

Motor enclosures consist of two main categories: open and totally enclosed. Open enclosures consist of drip-proof, splash-proof, semi guarded, guarded, and weather protected. The totally-enclosed types include Totally Enclosed Non Ventilated (TENV), Totally Enclosed Fan Cooled (TEFC), Explosion Proof, Dust Ignition Proof, among others. In general, open enclosures have pathways in and around the motor for cooling air to flow by convection. These enclosures are selected for normal environments but are modified to adapt to minor environmental factors, such as raindrops, snow and airborne particles. ³⁰

Totally enclosed types are selected for severe environments, such as corrosive, explosive, submerged and dust-ignition. They are force cooled by an internal or external fan/blower; however, there are water-to-air cooled types where air cools the motor and water cools the air via a heat exchanger. All totally enclosed enclosures are sealed from the outside environment to prevent contamination. ³¹

Explosion proof enclosures are totally enclosed with extra features that prevent the expulsion of explosive gases or vapors from the motor prior to them being cooled and no longer able to ignite. ³²

Selecting a Motor Mounting Configuration

Selecting a mounting configuration is an essential part of the motor selection process because motors can be mounted in a variety of ways. The two basic mounting configurations are flange-mounted and foot-mounted. ³³ Foot-mounting is designed for horizontal mounting on the floor, wall or ceiling. Flange-mounting also can be mounted in various orientations; however, they are designed to be mounted on the machine or driven load. ³⁴. The mounting configuration is combined with the motor dimensions (shaft height) in the frame size code. The frame size is a numerical code with a letter suffix. In general, the number refers to motor shaft height; frame numbers increase with “increasing horsepower or decreasing speeds.” ³⁵. The frame size suffix is a letter code that refers to the mounting configuration. ³⁶.