Motor 'Musts' for Demanding Environments

End-users must understand the operation and maintenance requirements associated with hazardous-location or severe-duty motors to ensure a safe and productive environment

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Severe-duty motors

Two widely accepted petroleum and chemical industry standards are available for severe-duty application motors ," American Petroleum Institute (API) 541, Form-Wound Squirrel Cage Induction Motors-250 Horsepower and Larger, and Institute of Electrical and Electronics Engineers Inc. (IEEE) 841, IEEE Standard for Petroleum and Chemical Industry-Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel-Cage Induction Motors,"up to and Including 370 kW (500 hp).

A key difference between the standards is that API 541 deals with sleeve-bearing motors, whereas IEEE 841 applies to ball-bearing motors. The focus here is on horizontal IEEE 841 severe-duty motors. Table 2 identifies key features and benefits that distinguish these motors from standard TEFC motors.

 

A number of features set the IEEE 841 severe-duty motor apart from standard TEFC motors. Among them are corrosion-resistant coatings, stainless steel nameplates, seals to prevent water spray from entering the bearing chamber, cast-iron terminal boxes for ratings up to 600 volts (V) and oversized terminal boxes.

The enhanced corrosion resistance of the exterior substantially increases the time between recoatings. Shaft seals can withstand water projected from a nozzle, thereby extending bearing and, possibly, winding life, while reducing maintenance and repair requirements. Cast-iron terminal boxes provide durability in severe-duty applications.

Motors rated above 600 V could have terminal boxes fabricated from cast iron, cast steel or thick steel plate. The terminal box volume is at least twice that required for standard motors. Corrosion protection of the terminal box virtually eliminates maintenance. 

 

 

Built-in benefits

The electrical design of severe-duty motors is geared for reliability, employing techniques that extend winding thermal life, voltage endurance and contamination resistance. The insulation system is at least Class F (155 Degrees C), and a contamination- and moisture-resistant sealed system vacuum-pressure impregnation winding is required for most form-coil windings.

Thermal life is enhanced by a winding temperature rise limit of 80 Degrees C, considerably lower than the 115 Degrees C rise for standard Class F TEFC motors. The increased thermal reserve of the winding improves reliability, even at the higher temperatures associated with operation on nonsinusoidal sources such as variable-frequency drives. Winding design features that provide greater resistance to contamination and moisture also increase voltage endurance, primarily by reducing or eliminating voids in the insulation system. Lower winding temperatures also mean less reflected heat to the bearings and a lower bearing temperature.

The standard TEFC motor protects against the ingress of small solid objects and the harmful effects of water splashed against it from any direction. In comparison, the protection afforded severe-duty motors prevents dust from entering in sufficient quantity to interfere with satisfactory motor operation. It also protects the bearings from the harmful effects of water projected in any direction by a nozzle. The replaceable shaft seals that often are used to fulfill this requirement are noncontact or noncontacting-while-rotating types, with a minimum expected life of five years under normal conditions. This degree of protection enhances reliability and reduces maintenance requirements.

Dynamic balance levels are about half those of standard TEFC motors, providing smoother operation and longer bearing life. Bearing temperature limits are likewise reduced to extend bearing and lubricant life. Shaft runout limits are about half those of NEMA-standard motors. Extended bearing life equates to less-frequent bearing change maintenance tasks and reduced downtime.

Corrosion protection extends beyond the frame of a severe-duty motor. Exposed interior parts of the stator, rotor and shaft must have a corrosion-resistant coating, with the rotor typically epoxy coated.

For durability, the frames, end brackets and fan covers of severe-duty motors must be of cast-iron construction. All unplated threaded surfaces are lubricated to facilitate removal. The joints between stator and end brackets are coated with a corrosion-resistant compound such as silicone rubber. These last two features facilitate disassembly when maintenance or repair is required.

In addition to stricter requirements for dynamic balance and shaft runout, mechanical integrity of the severe-duty motor is enhanced by a foot flatness tolerance. The motor feet must be coplanar within .005 in. and have a maximum draft angle of 1.5 degrees on the top surface of the area surrounding the feet mounting holes. These features reduce or eliminate "soft foot" conditions and shifting of the motor position during the alignment.

Tables 3 and 4 provide checklists of key items applicable to hazardous location and severe duty motors.

Nyberg and Bishop are technical support specialists at the Electrical Apparatus Service Association (EASA), St. Louis. EASA is an international trade association of more than 2,200 firms in 56 countries that sell and service electrical, electronic and mechanical apparatus. They can be reached at (314) 993-2220; fax: (314) 993-1269; Web site: www.easa.com.

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