Pumps and Systems , February 2007
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.
Table 1. Influences on the selection of type and design of motor enclosures.
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.
Table 2. Common NEMA Motor Enclosures.
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.
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 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
