Heed Design Letters When Replacing Motors

DC Electric motor (3D xray red and blue transparent isolated on black background)

By Mike Howell, Electrical Apparatus Service Association (EASA)

Too often, replacement specifications for three-phase squirrel-cage induction motors cover only basic nameplate data such as power, speed, voltage, and frame size, while overlooking other important performance characteristics such as the design letter. This can lead to misapplication of a motor, causing poor performance, inoperability, or failures that result in unnecessary downtime. To avoid these problems, familiarize yourself with the following speed-torque characteristics and typical applications for design letters that NEMA and IEC commonly use for small and medium machines (up to about several hundred kilowatts/horsepower).

NEMA Designs A and B, IEC Design N

• Characteristics include low starting torque, normal starting current, low slip, and relatively high efficiency. (Slip, the difference between rotor speed and synchronous speed, is necessary to produce torque. As load torque increases, slip increases.)
• NEMA Design A typically has higher starting current and lower maximum torque than NEMA Design B and IEC Design N.
• Typical applications include fans, pumps, and compressors where starting torque requirements are relatively low.

NEMA Design C, IEC Design H

•Characteristics include high starting torque, low starting current, and medium slip (achieved by using a double-cage, high-resistance rotor design).
• The high-resistance rotor results in greater losses at normal operating speed and, consequently, lower efficiency than NEMA Designs A and B and IEC Design N.
• Typical applications include conveyors, crushers, reciprocating pumps, and compressors that require starting under load.

NEMA Design D

• Characteristics include very high starting torque, low starting current, and high slip.
• The robust rotor design typically incorporates a single-cage with brass alloy or high-resistance aluminum alloy rotor bars.
• The high-resistance rotor results in lower efficiency at the operating point.
• Typical applications include high-impact loads, sometimes involving flywheels, such as punch presses and shears. These motors see significant slip increases with increased torque, which, for example, can facilitate delivery of kinetic energy from the flywheel to the impact.

Using the wrong motor design for an application is another way of spelling trouble. For example, replacing a NEMA Design D motor in a shear application with a NEMA Design B unit can result in rapid failure, even if the power rating of the machine is doubled.

When replacing motors, give your supplier as much information as possible about the existing motor and application. If you need more information about design letters, see NEMA MG-1 and IEC 60034-12. MT

Mike Howell is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis. EASA is an international trade association of more than 1,900 electromechanical sales and service firms in 62 countries that helps members keep up to date on materials, equipment, and state-of-the art technology. For more information, visit easa.com.