An electric motor frame refers to the structural part of the motor that provides physical support and protection for the internal components, such as the stator, rotor, and bearings. The frame also serves as the mounting interface for the motor, allowing it to be securely attached to equipment or machinery.

The frame’s design, size, and material can influence the motor’s cooling, alignment, and overall durability.

Frame Materials

Rolled Steel Frames

A rolled steel electric motor frame is a durable, lightweight, and cost-effective option for housing electric motors. Made from rolled steel sheets that are formed and welded into shape, these frames are suitable for a wide range of applications, from industrial machinery to household appliances.

While they offer several advantages, such as cost savings and ease of handling, they may have some limitations in terms of rigidity, thermal conductivity, and vibration dampening compared to other materials like cast iron.

Stainless Steel Frames

A stainless steel electric motor frame is a robust and corrosion-resistant housing made from stainless steel, offering high durability, excellent hygiene, and an attractive appearance. These frames are particularly suited for applications in the food and beverage industry, pharmaceuticals, marine environments, chemical processing, and wastewater treatment, where resistance to corrosion and easy cleaning are essential.

While more costly and heavier than some alternatives, stainless steel frames provide significant advantages in terms of durability, thermal stability, and overall performance in harsh and demanding conditions.

Aluminum Frames

An aluminum electric motor frame is a motor housing made from aluminum, offering advantages such as reduced weight, excellent thermal conductivity, and corrosion resistance. These frames are used in various applications, including automotive, HVAC systems, industrial equipment, aerospace, and consumer electronics.

While they may be more costly and less rigid than some other materials, their benefits in terms of weight, cooling efficiency, and corrosion resistance make them an excellent choice for many applications.

Cast Iron Frames

A cast iron electric motor frame is a robust and durable housing made from cast iron, known for its strength, vibration dampening properties, heat dissipation capabilities, and longevity. These frames are widely used in industrial machinery, mining equipment, marine applications, energy generation, and rail transportation due to their reliability and ability to withstand harsh operating conditions.

While heavier and potentially more costly than some other materials, cast iron frames offer significant advantages in terms of durability, thermal stability, and mechanical strength, making them a preferred choice for demanding applications requiring robust motor construction.

Fabricated Steel Frames

A fabricated steel electric motor frame is a robust and versatile motor housing constructed from steel sheets or plates that are cut, shaped, and welded together. These frames offer high strength, durability, and cost-effectiveness, making them suitable for a wide range of industrial and commercial applications.

While they are prone to corrosion if not properly treated and are heavier than some alternative materials, their customizable design and scalability in production make them a popular choice for many industries.

Frame Sizes

Motor frame dimensions have been standardized with a uniform frame size numbering system developed by NEMA. The current standardized frames for integral horsepower induction motors range from 143T to 449T. These standards cover most motors in the range of 1 through 250 Horsepower.

The numbers used to designate frame sizes have specific meanings based on the physical size of the motor. The first two digits are related to the motor shaft height and the remaining digit or digits relate to the length of the motor.

The shaft height of horizontal motors can be calculated by dividing the first two digits of frame size by 4.

For example, a 256T frame motor would have a shaft height (“D”) dimension of 25/4 = 6.25 inches. Similarly, a 444T frame motor would have a shaft height of 44/4 = 11 inches.

The third digit of the frame size is related to the length of the motor. There is no general rule of thumb that can be easily applied to determine the length of the motor.

It is important to note that when standard foot mounted motors have frame sizes that differ only in the third digit, the shaft diameters, shaft lengths and distance from the end of the shaft to the bolt holes in the feet on the shaft end of the motor will be the same. It is the motor length or distance from the drive end bolt-down holes to the “front” bolt-down holes that will vary.

Frame Suffix Letters

When a motor is provided with special mounting dimensions, flanges or special shaft features, this is indicated by adding a suffix letter to its frame number.

FOR EXAMPLE: A 254TS frame motor indicates that the motor has a shaft extension length and diameter different from that of a 254T frame motor.

The standard suffix letters and their meanings are as follows:

  • C = C flange mounting
  • D = D flange mounting
  • P or PH = Vertical P or PH flange mounting
  • T = Standard shaft for belted service
  • TS = Standard short shaft for coupled service
  • Y = Special mounting dimensions
  • Z = Special shaft extension