Current Affairs: Is The Spec Tough Enough?
But what do you do when faced with specifying motors for wet, corrosive, or other harsh environments? Is your standard motor specification adequate, or should you include more stringent requirements?
Wet LocationsGenerally, most fully enclosed motors are acceptable for use in wet environments, but special features should be considered for critical motors, or where normal life expectancy in the presence of moisture is required.
Small motors are available in "washdown duty" - fully sealed construction with stainless steel hardware and special paint systems to accommodate the constant moisture and washdown duty found in food processing and other manufacturing applications. these designs are ideal for critical wet location motors.
In larger sizes, standard TEFC motors may be specified with one or more options that increase their life expectancy, including: tropical or fungus-proof winding treatments to provide additional moisture protection of the insulation; heaters to prevent internal condensation; bearing seals to prevent ingress of water; and sealed or encapsulated windings, which provide added protection against the effects of moisture on the insulation.
Corrosive LocationsMotors exposed to corrosive liquids, gases, or solids require protection of not only the electrical insulation but the mechanical parts of the motor as well. This construction traditionally has been termed "mill and chemical duty" due to widespread use in paper mills and petrochemical plants, may include: corrosion-resistant epoxy enamel paint system; coating of the rotor and other internal surfaces; stainless steel hardware; bearing seals; and sealed windings.
Features of motors marketed as corrosion-resistant may vary between manufacturers, as there is no industry standard specifying strict requirements for use of this terminology.
Where a standard is required, Institute of Electrical and Electronic Engineers (IEEE) 841, Standard for Petroleum and Chemical Industry - Severe Duty Totally Enclosed Fan Cooled (TEFC) Squirrel Cage Induction Motors Up to and Including 350 kW (500 HP), may be applied. To the basic features described above, this will add requirements for:
- Efficiencies one NEMA band above EPACT levels;
- Increased enclosure and bearing environmental protection;
- Increased terminal box size and environmental protection requirements;
- Limits on the product of bearing diameter and rotational speed;
- Reduced tolerances on motor shaft runout;
- Reduced permissible vibration levels; and
- More stringent production testing.
Much of this standard is aimed at reducing bearing failures, which are often cited as the cause of 50% or more of all motor failures in industrial applications. These requirements won't come cheap (one source indicates that the premium for IEEE 841 compliance can range from 35% to 50% over the cost of a standard motor) so a careful evaluation should be performed before including this standard in your motor specification.
Hazardous LocationsHazardous locations are those areas with risk of fire or explosion from ignition of flammable vapors, liquids, or gases or combustible dusts or fibers. The basic rule is that motors must be approved by an independent agency such as Underwriter's Laboratories (UL) for the specific conditions. Testing focuses on maintaining the temperature of all motor surfaces below the ignition temperature of the hazardous substance.
The traditional North American system of specifying the hazard by class, division, and group has now been supplemented by the European Zone system, making this subject even more complicated for those who don't work with it frequently. When specifying hazardous location motors, consult with the electrical engineer for the current code requirements. ES