Motor Enclosures: What You Need to Know

All electric motors (motors) have a housing that contains the working components of the motor. In the U.S., the enclosure describes this housing. The enclosure should meet specific environmental requirements for restricting foreign objects, such as water, dust, and tools, and safety requirements for personal protection. Depending upon the extent of containment, cooling considerations add to the design of the enclosure.

General Motor Enclosure Considerations

When selecting the correct motor enclosure, numerous considerations must be made for determining the overall requirements of such enclosures. Fundamentally, these are governed by three major influences, which must then be analyzed further based on specific industry and application variables. All resulting requirements are driven by the common need to safeguard the functionality of the equipment and to protect both personnel and environment. These considerations are summarized in Table 1.

NEMA Standards MG 1-2003

The National Electrical Manufacturers Association (NEMA) provides a minimum standard for general-purpose industrial AC alternating current squirrel-cage induction motors. This NEMA Standard is designated as MG 1-2003. Within this standard, descriptions are provided for various classifications of protection for motor enclosures in Section 1 - Classification According to Environmental Protection and Methods of Cooling. NEMA provides definitions for various motor enclosures. In general, there are two primary categories - open and totally enclosed. An open motor has openings that allow external air to pass over and around the motor windings that provides required cooling. Although it is not airtight, the enclosure of a totally enclosed motor limits cooling of the windings from the external atmosphere. Motor cooling for totally enclosed motors is typically done by some external means such as a fan or water cooling. Table 2 provides a summary of the NEMA motor enclosure definitions. The enclosure is selected depending upon the environment and cooling method in which the motor will be operated. The application environment will determine the degree of protection for personal safety, water, or vapors. It is the responsibility of the purchaser to specify the motor enclosure.

IEC Designations

The national standards of Europe and developing countries are, in general, based on the International Electrotechnical Commission (IEC). Many of the motor requirements in their applicable standards are similar to those of NEMA. The IEC standard has provided a more detailed description of motor protection and how to conduct tests to determine the enclosure designation. These classifications of degrees of protection have been included in the 2003 version of MG-1.

Classification of Degrees of Protection Provided by Enclosures (IP Designations)

The IEC designation for degrees of protection consist of the letters "I" and "P" followed by two numerals. The first represents the enclosure's level of protection against incidental contact with internal components. The second defines the amount of water ingress that the enclosure must protect. This may be followed by a letter indicating whether the protection was tested dynamic (S) or static (M). No letter indicates the motor will be operational under normal conditions to the degree of protection designated. Tables 3 and 4 define the IP designation system. For example, a motor with a Degree of Protection of IP13 would not allow accidental contact with moving parts exceeding 1.968-in. (50-mm) and would not be adversely affected by a spray of water up to 60-deg from vertical. IP designations with first numerals 4 or higher are typically used when describing totally enclosed machines. Guards must also protect external fans to the degree of the motor enclosure and are tested in a similar manner. For motors with an IP3x or IP4x enclosure that will be operated with open drain holes, the drain hole may comply with the IP2x protection requirements. For motors with an IP5x enclosure that will be operated with open drain holes, the drain hole may comply with the IP4x protection requirements.

Methods of Cooling (IC Designations)

Electric motors must dissipate the heat generated within their windings in order to operate. If a unit fails to adequately cool itself, it can overheat and cause damage to itself and the driven equipment. To guard against this damage, thermal protection devices are available that will trigger the safe shutdown of a motor if the temperature exceeds a predetermined maximum. There are varieties of cooling methods used in motor design. When the cooling air is drawn from the surrounding environment, circulated around the internal components, and expelled back into the surroundings, the cooling method is called an open circuit. This type of cooling is only possible in open enclosure motors. Closed circuit cooling involves internal coolant in a closed loop that passes heat to another coolant either through the surface of the machine or a heat exchanger. This type of cooling is by definition associated with totally enclosed machines since the primary coolant remains contained within the motor. Most motors use shaft mounted fans to circulate air as the primary coolant. One drawback of this approach is that the velocity at which the cooling air is circulated decreases if the speed of the motor decreases. This is one limitation of utilizing an adjustable speed drive with a standard motor not specifically designed for use with these drives. In some applications, a constant velocity of air is necessary. In these cases, separately powered fans are often employed to deliver a regular velocity of air regardless of the motor's rotational speed. Although air is the most common fluid used as primary and/or secondary coolant in electric motor design, units can be built using other cooling media such as refrigerant, hydrogen, nitrogen, carbon dioxide, water, and oil. Although the IEC classifications are included in the NEMA MG-1 standard, industry is accustomed to the descriptive definitions for protection and cooling, not the more defined degrees of classification provided by IEC. Tables 5 and 6 provide a comparative guideline for protection and cooling between the two standards. Within the IEC, a short and complete code exists for designating the cooling method. It is typically preferred to use the short code for the cooling designation, and the complete code is intended for use when the short code is not applicable for the equipment or application.

Enclosures for Hazardous Applications

Some motors are designed and approved to meet Underwriters Laboratories or Canadian Standards Association (CSA) standards for use in the hazardous (explosive) locations, shown by a designating label on the motor. The motor purchaser or user must specify the explosion proof motor classification required prior to purchase. There are two divisions. Division 1 is a location in which hazardous materials are present in the atmosphere under normal operating conditions. Division 2 is a location in which the atmosphere may become hazardous as the result of some abnormal condition. The locations are considered hazardous because the atmosphere contains or may contain gas, vapor, or dust in quantities that may cause an explosion. Once the location is defined as hazardous, the location is further defined by the class and group of the hazard. The National Electrical Code (NEC) divides these locations into classes and groups according to the type of hazardous agent. The following list has some of the agents in each classification. See Article 500 of the National Electrical Code for a complete list. Class I (Gases, Vapors)

• Group A: Acetylene
• Group B: Butadiene, ethylene oxide, hydrogen, propylene oxide
• Group C: Acetaldehyde, cyclopropane, diethlether, ethylene, isoprene
• Group D: Acetone, acrylonitrile, ammonia, benzene, butane, ethylene dichloride, gasoline, hexane, methane, methanol, naphtha, propane, propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylene

Class II (Combustible Dusts)

• Group E: Aluminum, magnesium and other metal dusts with similar characteristics
• Group F: Carbon black, coke or coal dust
• Group G: Flour, starch or grain dust

A new European directive, called the ATEX (ATmospheres EXplosibles) directive, became effective July 2003. The directive, (94/9/EC), deals with electrical, mechanical, hydraulic or pneumatic equipment in areas exposed to explosive atmosphere and is only valid in the EU area. For a common and increased awareness of safety in these risk areas, manufacturers of this type of equipment have to comply with the basic safety requirements stated in the new directive. The safety requirements in the ATEX directive imply that pumps and motors have to carry a clear indication of the equipment group and category in which they belong and in what areas they can be used. The ATEX directive affects a wide range of industries, dealing with the handling of combustible dust - such as cereals, animal feed, paper and wood - and industries that generate explosive gases, such as chemicals, plastics and petroleum.

Other Industry Enclosure Designations

The applications in which motors are applied may require more physical design features than the NEMA standards provide. The motor industry has provided other advanced enclosure and motor descriptions that meet the needs of the market. Some of these are described in general below. Most motor manufacturers have branded descriptions of these general descriptions.

Corrosion duty

Industries with aggressive environments, such as high humidity or corrosive; additional enclosure features are required for extended protection. These motors are typically TEFC and have a degree of protection of IP54 via the use of a rotating shaft slinger. A higher degree of protection via the use of bearing isolator(s) is also available. Rotating slingers are provided to minimize entrance of moisture and contaminants into the bearing chamber. Condensation drain holes are provided at the low points in the end brackets and are supplied with corrosion resistant breather drain plugs. All fastening hardware is grade 5, zinc or cadmium plated. Motor cast iron components are typically oxide primed and painted with vinyl-phenolic paint or other chemical resistant paint. This coating is chemical solvent, salt water and acid resistant. Motor nameplate is stainless steel.

Automotive duty

The major motor manufacturers have developed expanded motor specifications that meet the requirements of the manufacturing environment. The frame size ("U" frame) is a previously used NEMA designation indicating frame size and dimension (prior to 1965 the standard frame sizes per horsepower rating).

• Ford Spec EM1-1996 - TEFC motors, which meets IEEE 841 frame, conduit box, paint and nameplate requirements.
• GM Spec 7E-1970 - TEFC motors, cast iron frame and end brackets, steel or iron t-box with lead separator and gasket, shaft slinger.

Marine duty

For use on ships, a standard exists (IEEE-45) for motor drivers.

• Above Deck (waterproof): A motor with corrosion duty construction with a shaft slinger on the opposite pulley end. Frame surface under conduit box base must be flat to ensure full gasket fit and prevent water entry.
• Below Deck: Corrosion treatment - consisting of anti-rust compounds on metal to metal fits, plated hardware, epoxy painted aluminum parts and air deflectors, stainless steel nameplate, resin and hardener or equivalent on rotor.

IEEE-841 standard - 1994

These motors are a cast iron, heavy duty, industrial design motor, intended for the chemical and petroleum industries. Other industries, such as mining, food processing, pulp and paper, marine and automotive industries also consider this construction because of the heavy-duty, reliable, energy efficient design. The motors are TEFC and have a degree of protection of IP55 on 143 to 5811 frames. Motor bearings have a degree of protection of IP55 via the use of a non-contact bearing isolator for motors with a 324 frame or larger. Corrosion resistant hardware is also used. ASTM B117-90, Test Method of Salt Spray (Fog) Testing, is completed to confirm protection. The enclosure is all-cast iron construction with epoxy paint. It must also be noted that the efficiency of this design exceeds Energy Policy Act of 1992 (EPAct) requirements but is below the NEMA Premium levels.

Food and beverage duty

Depending upon the specific food or beverage industry, specific enclosures and motor designs may be required because of food contamination concerns or cleaning procedures.

• U.S.D.A. Specifications - The requirements for motors involves the paints, primers and sealants; must be U.S.D.A. approved.
• Wash down duty - Because of the cleaning procedures required in most food and beverage plants, all equipment could be washed down with high pressure, cleaning water. Motors with enclosure features beyond TEFC are typically required.
• Basic features - TEFC motor with USDA-approved, white epoxy paint.
• Medium features - Stainless steel frame; specially processed endbells.
• Advanced features - All exterior surfaces stainless steel, including shaft, with IEEE-841 severe-duty features; o-ring endbell seals. (May also be described as "dirty duty.")

Aggregate industry/quarry duty motors

Motors used in the aggregate or quarry industry are in a very dirty, abrasive environment. The motors are typically all-cast iron construction with larger frames with roller bearings.

Cooling tower motors

Motors in an environment near a cooling tower can see much moisture, in the form of a spray or mist. These motors are all-cast iron construction, salt-spray tested, with corrosion resistant nameplate and hardware, and slinger seals. ANSI/API 661 Air-Cooled Heat Exchangers for General Refinery Services provides some motor design requirements for this difficult service.

American Petroleum Institute

The American Petroleum Institute (API) has developed two standards for induction motors for use in general-purpose petroleum, chemical and other industrial severe duty applications: 1. API 541 - Form-Wound Squirrel Cage Induction Motors This standard provides minimum requirements for large, all form-wound squirrel cage induction motors, 500-hp and larger. Typically this standard is used in refinery services where:

• The service is critical.
• The motor is larger than 3000-hp (2250-kW) for speeds 1800-rpm and below.
• The motor is rated 800-hp (600-kW) or greater for two-pole (3000-gpm or 3600-rpm) machines of totally-enclosed construction, or rated 1250-hp (930-kW) or greater for two-pole machines of open or guarded construction (including machines with WP-I or WP-II type enclosures).
• The motor drives a high-inertia load (in excess of the load Wk² listed in NEMA MG-1 Part 20).
• The motor uses an adjustable speed drive as a source of power.
• The machine is an induction generator.
• The motor is a vertical machine rated 500-hp (375-kW) or greater.
• The machine operates in abnormally hostile environments.

2. API 547 - General-purpose Form-wound Squirrel Cage Induction Motors, 250-hp and larger This standard provides minimum requirements for form-wound squirrel cage induction motors that are used in general-purpose petroleum, chemical and other industrial severe duty applications. For motors larger than that described and motors in other applications, they should be specified in accordance with API Standard 541. It is recommended that API Standard 547 is applied to motors that have the following characteristics:

• Rated 250-hp (185-kW) through 3000-hp (2250-kW) for 4, 6 and 8 pole speeds.
• Rated less than 800-hp (600-kW) for two-pole (3000-rpm or 3600-rpm) motors of totally-enclosed construction.
• Rated less than 1250-hp (930-kW) for two-pole motors of WP-II type enclosures.
• Drive centrifugal loads.
• Drive loads having inertia values within those listed in NEMA MG 1 Part 20.
• Are not induction generators.

Other Issues

Motor noise coming from motors is caused by a number of variables, including the type of enclosure and cooling, motor power size, speed, and load conditions. For totally enclosed motors utilizing fans for cooling, the air turbulence produced by the cooling fan can create the greatest amount of noise, especially at 2-pole speeds. As larger motors may require higher cooling, larger cooling fans are required, developing greater air flow and more noise. As motor speeds are decreased, less air turbulence is created, which can reduce the noise developed. Other external methods of noise reductions can be used. External enclosures with noise insulating material are used to reduce the noise. Although these can be effective in noise reduction, they can impact the effectiveness of the enclosure (especially with cooling) or make the enclosure substantially larger. Other external factors can increase the noise that the motor produces:

• On undamped baseplate mountings, motor noise can be transmitted, amplified, and radiated by non-motor structures. A motor suspension system or cushioned mounting can be added to the installation to reduced noise and vibration.
• The physical design of internal motor components, such as the rotor and laminations, can also affect the amount of noise and vibration produced by the motor. The motor manufacturer is responsible for their design and to minimize the noise that may be designed in their product.
• Motor noise will also be affected by the carrier frequency when controlled by a variable frequency drive. Isolated gate bipolar transistors, designated as IGBTs, can minimize motor noise with variable frequency drives due to their fast switching speed and higher pulse or carrier frequency.

References

• American Petroleum Institute, Washington, D.C., http://www.api.org
• Joe Hillhouse, Reliance Electric Motors, "HI Drivers Spec.doc"
• Leeson Electric, "Basic Training - Industrial-Duty & Commercial-Duty", 1999, Grafton, Wisconsin
• National Electrical Manufacturers Association, "NEMA Standards Publication MG 1-2003 - Motors and Generators", Rosslyn, Virginia
• Andy Easton, Comparison of IEC and NEMA / IEEE Motor Standards, Hydraulic Institute 2001 Annual Meeting, Las Palmas Resort, Palm Springs, CA