Motor Enclosures: What You Need to Know

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Pumps and Systems, February

All electric motors have a housing that contains the motor's working components. In the U.S., this housing is referred to as the enclosure. The enclosure must adhere to specific environmental standards to restrict the entry of foreign objects like water, dust, and tools, as well as safety requirements for personnel protection. Depending on containment levels, cooling considerations also play a role in the enclosure's design.

General Motor Enclosure Considerations

When choosing the right motor enclosure, several factors must be considered to determine the overall requirements. Fundamentally, these are influenced by three major factors, which should be further analyzed based on specific industry and application variables. The ultimate requirements aim to ensure the equipment's functionality while protecting both personnel and the environment. Table 1 summarizes these considerations.

Table 1. Influences on the selection of type and design of motor enclosures.

NEMA Standards MG 1-

The National Electrical Manufacturers Association (NEMA) sets minimum standards for general-purpose industrial AC squirrel-cage induction motors, referred to as NEMA Standard MG 1-. Within this standard, various classifications of motor enclosure protection are described in Section 1 - Classification According to Environmental Protection and Methods of Cooling.

NEMA defines various motor enclosure types. Generally, there are two primary categories: open and totally enclosed. An open motor has openings that allow external air to flow over and around the motor windings for cooling. While not airtight, a totally enclosed motor restricts cooling from the outside atmosphere. Cooling for totally enclosed motors is typically achieved through external means, such as fans or water cooling. Table 2 summarizes the NEMA motor enclosure definitions.

Table 2. Common NEMA Motor Enclosures.

The selection of an enclosure depends on the operating environment and cooling method. The application environment dictates the necessary degree of protection for personal safety, water, or vapors. It is the purchaser's responsibility to specify the motor enclosure.

IEC Designations

European and developing countries' national standards are generally based on the International Electrotechnical Commission (IEC). Many motor requirements in these standards are similar to NEMA's. The IEC standard offers a more detailed description of motor protection and testing procedures for determining enclosure designations. These classifications for degrees of protection have been incorporated into the MG-1 version.

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

The IEC designation for degrees of protection consists of the letters "I" and "P," followed by two numerals. The first numeral indicates the enclosure's protection against incidental contact with internal components; the second defines the extent of water ingress protection. A letter may follow, indicating whether the protection was tested dynamically (S) or statically (M). Absence of a letter indicates that the motor will operate normally under the designated degree of protection.

Tables 3 and 4 define the IP designation system. For example, a motor rated IP13 protects against accidental contact with moving parts larger than 1.968 inches (50 mm) and can withstand a spray of water up to 60 degrees from vertical. IP designations with a first numeral of 4 or higher usually describe totally enclosed machines.

Table 3. Summary of IEC Code for Degree of Protection.

Table 4. Summary of IEC Code for Methods of Cooling.

Guards must also protect external fans to the same degree as the motor enclosure and undergo similar testing. For motors with IP3x or IP4x enclosures operating with open drain holes, the drain holes may meet IP2x protection requirements. For motors with IP5x enclosures and open drain holes, the drain holes may meet IP4x protection standards.

Methods of Cooling (IC Designations)

Electric motors must dissipate heat generated within their windings to function properly. Failure to cool adequately can lead to overheating and potential damage to both the motor and the equipment it drives. To mitigate this risk, thermal protection devices are available to safely shut down a motor when it exceeds a predetermined maximum temperature.

Various cooling methods are utilized in motor design. Open circuit cooling refers to cooling air drawn from the environment, circulated through internal components, and expelled back into the surroundings. This method only works with open enclosure motors.

Closed circuit cooling involves internal coolant in a loop that transfers heat to an external coolant via the machine surface or a heat exchanger. This cooling method is typically associated with totally enclosed machines, as the primary coolant remains within the motor.

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Most motors employ shaft-mounted fans to circulate air as the primary cooling method. However, a drawback is that the cooling air's velocity decreases if the motor speed decreases. This limitation is particularly relevant when using an adjustable speed drive with a standard motor not specifically designed for such applications. In scenarios requiring a constant airflow, separately powered fans are often utilized to maintain a consistent air velocity, regardless of the motor's rotational speed.

While air is the common primary and secondary coolant in electric motor design, other cooling media may include refrigerants, hydrogen, nitrogen, carbon dioxide, water, and oil.

Although IEC classifications are incorporated in the NEMA MG-1 standard, the industry tends to favor the descriptive definitions for protection and cooling rather than the more detailed classifications from IEC. Tables 5 and 6 provide a comparative guideline between the two standards.

Table 5. Comparison of NEMA and IEC Protection Designations.

Table 6. Comparison of IEC and NEMA Cooling Designations.

Within the IEC framework, a short and complete code specifies the cooling method. It is generally preferred to use the short code for the cooling designation; the complete code is used when the short code does not apply to 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 hazardous (explosive) locations, indicated by a label on the motor. The motor purchaser or user must specify the required explosion-proof motor classification prior to purchase. There are two divisions: Division 1, where hazardous materials are present under normal operating conditions, and Division 2, where the atmosphere could become hazardous due to abnormal conditions.

Such locations are considered hazardous due to the presence of gas, vapor, or dust in quantities that could cause an explosion. Once identified as hazardous, the location is further classified by the class and group of the hazard. According to the National Electrical Code (NEC), the locations are divided into classes and groups based on the type of hazardous agent. Refer to Article 500 of the NEC for the complete list.

Class I (Gases, Vapors)

• Group A: Acetylene
• Group B: Butadiene, ethylene oxide, hydrogen, propylene oxide
• Group C: Acetaldehyde, cyclopropane, diethyl ether, 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, known as the ATEX (ATmospheres EXplosibles) directive, became effective in July. This directive addresses equipment in areas subjected to explosive atmospheres and is valid only within the EU. To enhance safety awareness in these risk areas, manufacturers of such equipment must comply with basic safety requirements outlined in the ATEX directive.

The ATEX directive mandates that pumps and motors clearly indicate the equipment group and category they belong to and their permissible usage areas. The directive encompasses various industries dealing with combustible dust handling—such as cereals, animal feed, paper, and wood—and industries that generate explosive gases, including chemicals, plastics, and petroleum.

Other Industry Enclosure Designations

Motor applications may necessitate more design features than those specified by NEMA standards. The motor industry has introduced advanced enclosure and motor descriptions to accommodate market needs, of which several are generally outlined below. Most motor manufacturers have branded specifications for these general descriptions.

Corrosion Duty

Industries facing aggressive environments, such as high humidity or corrosive conditions, require additional enclosure features for extended protection. These motors are typically TEFC (Totally Enclosed Fan Cooled) and possess an IP54 protection rating through a rotating shaft slinger. A higher level of protection from bearing isolators is also an option. Rotating slingers minimize moisture and contaminant ingress into the bearing chamber. Condensation drain holes located at low points in end brackets are fitted with corrosion-resistant breather drain plugs. All fastening hardware is grade 5 and zinc or cadmium plated. Cast iron components are generally oxide primed and coated with vinyl-phenolic paint or another chemical-resistant paint, while the motor nameplate is stainless steel.

Automotive Duty

Major motor manufacturers have developed expanded specifications tailored to manufacturing environments. The frame size ("U" frame) denotes a previously used NEMA designation indicating frame size and dimension.

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

Marine Duty

Standards exist for motor drivers used on ships (IEEE-45), specifying:

• Above Deck (waterproof): Motors with corrosion-duty construction and a shaft slinger opposite the pulley end. The frame surface under the conduit box base must be flat to ensure full gasket fit and prevent water entry.
• Below Deck: Anti-rust treatment on metal-to-metal fittings, plated hardware, epoxy-painted aluminum parts, air deflectors, stainless steel nameplate, resin, and hardener or equivalent applied to the rotor.

IEEE-841 Standard

This standard pertains to heavy-duty, industrial design motors intended primarily for chemical and petroleum industries. Other industries, including mining, food processing, pulp and paper, marine, and automotive, also utilize this robust and energy-efficient construction.

These motors are TEFC and possess an IP55 degree of protection across frames 143 to 324. Motor bearings achieve IP55 protection through non-contact bearing isolators for motors with frame sizes of 324 and larger. Corrosion-resistant hardware is also applied. The ASTM B117-90, Test Method of Salt Spray (Fog) Testing, verifies protection. The all-cast iron enclosure features an epoxy coating, achieving efficiency that surpasses the Energy Policy Act of 2005 (EPAct) requirements but does not meet NEMA Premium levels.

Food and Beverage Duty

Depending on the specific food or beverage industry, certain enclosures and motor designs may be required to prevent contamination or facilitate cleaning procedures.

• U.S.D.A. Specifications: The requirements dictate that motors must use U.S.D.A.-approved paints, primers, and sealants.
• Wash Down Duty: Due to prevalent cleaning procedures in food and beverage facilities, equipment frequently undergoes high-pressure washing. Motors with enclosure features beyond TEFC are typically needed.
  • Basic features: TEFC motors coated in U.S.D.A.-approved white epoxy paint.
  • Medium features: Stainless steel frame with specially processed end bells.
  • Advanced features: All exterior surfaces, including the shaft, are stainless steel, featuring IEEE-841 severe-duty characteristics and O-ring end bell seals ("dirty duty" may also be referenced).

Aggregate Industry/Quarry Duty Motors

Motors used in the aggregate or quarry industry operate in extremely dirty and abrasive environments. Generally, they feature all-cast iron construction with larger frames and roller bearings.

Cooling Tower Motors

Motors situated near cooling towers are exposed to considerable moisture from sprays or mist. These motors typically consist of all-cast iron construction, salt-spray tested with corrosion-resistant nameplates, hardware, and slinger seals. ANSI/API 661 Air-Cooled Heat Exchangers for General Refinery Services outlines design requirements for these challenging service conditions.

American Petroleum Institute

API has established two standards for induction motors used in general-purpose petroleum, chemical, and severe industrial applications:

1. API 541 - Form-Wound Squirrel Cage Induction Motors

This standard sets minimum requirements for large, all form-wound squirrel cage induction motors of 500 hp and higher. Typically applied in refinery services where:

• The service is critical.
• The motor exceeds 100 hp (75 kW) for speeds of 1800 rpm and below.
• The motor's rating ranges from 800 hp (600 kW) or greater for two-pole (3600 rpm or 1800 rpm) totally-enclosed machines to over 1000 hp (930 kW) for two-pole open or guarded machines, including those with WP-I or WP-II type enclosures.
• The motor drives a high-inertia load (exceeding the load Wk2 outlined in NEMA MG-1 Part 20).
• The motor is driven by an adjustable speed drive.
• The machine operates as an induction generator.
• Vertical machines must be rated 500 hp (375 kW) or more.
• The motor operates in severely challenging environments.

2. API 547 - General-Purpose Form-Wound Squirrel Cage Induction Motors, 250 hp and larger

This standard outlines minimum requirements for form-wound squirrel cage induction motors used in general-purpose petroleum, chemical, and other severe industrial applications. For motors exceeding the specified sizes or in supplementary applications, they should comply with API Standard 541. It is recommended that API Standard 547 applies to motors with these characteristics:

• Rated 250 hp (185 kW) through 500 hp (373 kW) for 4, 6, and 8 pole speeds.
• Rated below 800 hp (600 kW) for two-pole totally-enclosed construction motors.
• Rated under 1000 hp (930 kW) for two-pole motors with WP-II type enclosures.
• Drive centrifugal loads.
• Drive loads with inertia values within those listed in NEMA MG 1 Part 20.
• Not induction generators.

Other Issues

Motor noise originates from various factors, including enclosure type, cooling, power size, speed, and load conditions.

In totally enclosed motors using fans for cooling, the air turbulence created by the cooling fan can generate significant noise, especially at 2-pole speeds. Larger motors typically require larger cooling fans, resulting in more airflow and increased noise. However, operating at reduced speeds generates less air turbulence, which can minimize noise levels.

Additional methods for noise reduction are available. External enclosures utilizing noise-insulating materials can help, although these can impact the enclosure's effectiveness (particularly regarding cooling) or increase its overall size.

Furthermore, external factors may amplify motor noise:

• On undamped baseplate mountings, motor noise can be transmitted, amplified, and radiated by surrounding structures. Adding a motor suspension system or cushioned mounting can reduce noise and vibration.
• The design of internal components, such as the rotor and laminations, influences noise and vibration levels. Motor manufacturers are responsible for optimizing these designs to minimize noise.
• Carrier frequency affects motor noise when controlled by a variable frequency drive. Isolated gate bipolar transistors (IGBTs) can reduce noise with variable frequency drives due to their fast switching speeds 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", Grafton, Wisconsin
• National Electrical Manufacturers Association, "NEMA Standards Publication MG 1 - Motors and Generators", Rosslyn, Virginia
• Andy Easton, Comparison of IEC and NEMA / IEEE Motor Standards, Hydraulic Institute Annual Meeting, Las Palmas Resort, Palm Springs, CA

Selecting the right electric motor for a specific vehicle isn't straightforward. With numerous variables to weigh, pinpointing where to begin can be challenging. Given the cost of batteries and electric motors, finding the most economical solution requires an examination of powertrains that closely match the necessary vehicle performance.

This article will review 10 basic questions that need answering before selecting the right motor for your project. Identifying the vehicle's most demanding requirements and evaluating how various road conditions will affect the powertrain's performance are essential:

1. Vehicle characteristics

Key vehicle characteristics, such as size, weight, overload capacity, and aerodynamics, are critical in determining the speed, torque, and power requirements of the electric motor. Understanding these aspects of vehicle operating conditions is essential for choosing the appropriate powertrain. Keep these considerations at hand for the subsequent steps.

2. Driving cycles

Understanding the vehicle's usage is vital. What will its typical driving cycles entail? Will it navigate an urban environment with frequent stops, or cover long distances with few stops? These details are crucial in determining the vehicle configuration (series hybrid, parallel hybrid, all-electric) and battery pack size, ultimately influencing the choice of powertrain.

3. Vehicle configuration (electric, hybrid)

Is the vehicle hybrid or fully electric? If hybrid, is it parallel or series hybrid? Generally, hybrid architectures are favored for unpredictable routes or extended travel distance.

A fully electric configuration suits city driving, where distances between charging stations are manageable, speeds are lower, and stops are frequent.

4. Maximal speed

What is the target maximum speed of the vehicle? How long must it be maintained? Is it primarily used for passing?

Consider the available gearbox ratios (if applicable) and the differential ratio. What is the wheel's rolling radius? Answering these questions is essential in calculating the maximum speed required of the electric motor for your application.

5. Maximal torque

Maximum torque is vital for the vehicle's ability to ascend slopes. Determine the steepest grade the vehicle will encounter, as this information allows for the calculation of the highest torque needed from the electric motor, considering the differential and gearbox (if equipped). Maximum weight must also be factored.

6. Maximal power

Some grades necessitate climbing at minimum speed, while others do not. Often, maximum power is required when reaching top speeds, particularly in vehicles with large frontal areas or ones that operate at high velocities. Thus, the motor must be sufficiently powerful to handle all condition variations.

Maximum power enables the vehicle to achieve and sustain constant speeds under challenging slope and speed scenarios. A simulator that considers drag and friction coefficients, along with forces needed for climbs, is essential for calculating maximum power.

The duration of peak conditions influences calculations: unlike combustion engines, electric motor peak power cannot be sustained continuously, and selecting an electric motor to handle the most strenuous hill-climb scenarios without time constraints would lead to overengineering.

7. Battery Capacity

Battery capacity is typically computed through a simulator that models a reference cycle that mirrors the vehicle's usage. The simulator outputs vehicle consumption in kWh/km, which can then be multiplied by the desired range to calculate the battery's capacity.

8. Battery Voltage

Battery voltage depends on vehicle size. Higher voltage decreases the current output. In larger vehicles where continuous power is elevated, higher battery voltage accommodates more manageable conductor sizes.

Two primary voltage ranges typically exist: 300-450Vdc and 500-750Vdc, corresponding to the IGBT voltage limitations in motor controllers and standard available voltages of 600Vdc and other common standards.

9. Gearbox or direct-drive?

Is a gearbox required for the powertrain configuration? Would it be more cost-effective to eliminate the transmission or simplify the system?

TM4's SUMO electric powertrain offers a direct-drive solution; the motor's high torque/low speed allows it to connect directly with standard axle differentials without needing an additional gearbox. This approach enhances system reliability, reduces overall maintenance costs, and improves the powertrain's efficiency, maximizing energy use from the battery pack.

10. Cost

Finally, consider your budget. In a previous blog post, we examined various electric motor technologies available in the market, along with their pros, cons, and relative usage in electric vehicles.

To sum up

Once you've collected all relevant information, the right tools will enable you to calculate your vehicle components' requirements based on performance. TM4 can assist you in making an informed choice regarding motor selection. Contact us with the information gathered.

Contact us to discuss your requirements for cast iron Electric Motor Housing. Our experienced sales team can help you identify the best options for your needs.