Pumps and Systems, April 2009
The industrial motor world has a new specification available for defining the requirements of general purpose, severe duty motors in the 250- to 3,000-hp range.
The American Petroleum Institute (API) published the API 547 specification to fill the gap created when the API 541 specification was revised to focus only on critical service machines. The new API 547 specification can be used to compliment the HP range defined by the Institute of Electrical and Electronic Engineers (IEEE®†) 841 severe duty motor specification for smaller HP machines. A voluntary API 547 monogram program has been implemented that pre-qualifies a motor manufacturer's ability to consistently produce motors to meet API specifications, minimizing the user's need to audit each unit.
General Purpose Motors
Industrial motors of less than 500-hp are typically designed to meet the basic requirements found in NEMA MG 1 standard to allow for ease of application, interchangeability and quick delivery. This specification has evolved to create standardized products that operate successfully in many industrial environments and can be applied to general equipment applications. Some users have found that installations contain unusual factors that lead to unacceptable product life from standard NEMA®† (National Electrical Manufacturers Association for Motors & Generators) MG 1 motors. Based upon these experiences, users have worked with motor manufacturers to create several derivative product specifications marketed under the term Severe Duty motors.
Severe Duty Motors
Severe Duty does not have a common definition among the different motor manufacturers. As large users have worked to improve their processes and equipment, they have developed motor specifications that include key features they feel provide longer motor operating life and reduced cost of ownership. In some instances, severe duty may only indicate motors with cast iron construction and oversized bearings. Other users may require severe duty motor features go beyond the basic failure mechanisms and include features that reduce installation time.
As motor manufacturers have become suppliers to these large users, the manufacturers have included these products as their severe duty offering for other customers. Pulp and paper, chemical processors and petroleum refiners are some of the primary drivers for current severe duty motor specifications.
IEEE 841 Severe Duty Motors
Experienced users in the petroleum and chemical industries have developed motor specifications to address the most common causes of failures in their installations. The IEEE 841 specification has evolved to define the "premier" severe duty product for NEMA frame motors less than 500-hp. Due to the standardization of features and implementation support from key users, IEEE 841 severe duty motors have become an off-the-shelf product. As volume has increased on the IEEE 841 product line, the cost premium has become reasonable for other users that value reliability and total ownership costs.
API 541 Severe Duty Motors
The petroleum and chemical industries use custom in-house and/or the API® 541 specification to purchase severe duty motors for above NEMA frame sizes. General purpose or critical duty motors could both be defined by the API 541 standard (3rd edition). Specifying engineers had the option to select key features on the data sheets that were common for critical duty applications, or they could skip these high-end features to define a general purpose motor.
Regardless of the type of motor the engineer specified, the API 541 specification required the motor manufacturer to receive six completed data sheets before proceeding to quote and manufacture the custom motor. This approach in specifying above NEMA frame motors required every unit to be a production unit with longer lead times and higher costs.
In the 4th edition of the API 541 specification, the data sheets have been changed to include critical duty features as standard on every motor. Given the extra time and costs the API 541 4th edition specification added to general purpose motors, API created the new 547 specification.
API 547 Severe Duty Motors
The new API 547 specification allows any user to easily define and purchase general purpose, severe duty motors that will provide reliable service in many installations. This new specification limits HP levels and base operating speeds, but many of the centrifugal pump, fan and compressor applications should fall within its limits. Motors for critical duty applications would still be purchased referencing the API 541 4th edition document.
In addition to creating a dedicated general purpose specification for above NEMA frame motors, the API 547 document streamlines the specification and purchasing processes. In its simplest form, this specification removes the need for engineers to complete any data sheets and creates the opportunity for an off-the-shelf product offering. Just as the IEEE 841 specification has improved the reliability and availability levels of NEMA size severe duty motors, the new API 547 specification has the potential to have the same influence on motors from 250- to 3,000-hp.
API 547 Monogram Program
Educated users of the API 541 specification realize the six page data sheet can be useful in allowing the customer to specify important requirements to help ensure a motor's suitability and reliability for the application before it is delivered to the jobsite. By carefully selecting specific documentation and witness tests, specifying engineers can minimize the chance of having problems once the motor is commissioned.
The new API 547 specification does not require the customer to submit data sheets to purchase a motor built to this standard. As an alternative to over specifying and adding costs to general purpose motors by referencing the API 541 4th edition document, API created a 547 Monogram accreditation program. Manufacturers can voluntarily participate in this program in which a factory audit process certifies their ability to consistently meet the API 547 requirements. Once a manufacturer completes the certification process, they are allowed to install an API 547 Monogram Logo plate on each of its API 547 motors.
Advantages of Using the API 547 Specification
API 547 motors are designed to provide the same high level of reliability as the non-critical API 541 3rd edition units, but without the extreme costs and lead times associated with a full custom product. Initially defined by the petroleum and chemical industries, any user that is concerned with the total costs of owning a motor can benefit from using an API 547 product. Users that adopt the default features of an API 547 motor have the potential to purchase high quality motors up to 3,000-hp off-the-shelf from their local motor supplier.
The potential savings associated with purchasing a stock API 547 product are in addition to the benefits realized from simplified installation, improved reliability and improved operating efficiency. Refer to the API 547 quick facts chart and start identifying opportunities today.
†All non-Emerson Electric Co. marks shown within this document are properties of their respective owners.