Heed Design Letters When Replacing Motors

Tue, 12/06/2016 - 14:31 -- Decatur Industr...

Heed Design Letters When Replacing Motors

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.


December 2016 Newsletter

Thu, 12/01/2016 - 11:48 -- Decatur Industr...

The December newsletter is out!  
Take a moment and watch our 3-minute compay video, featuring President Trent Thompson.
Read how we saved a multinational grain processing company from repeated bearing failures. 
 - Are you planning a shut-down over the winter break? Call Decatur Industrial and find out more about PdMA testing
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To read the full newsletter click here. 

Decatur Industrial Crane and Hoist Services

Mon, 11/28/2016 - 08:44 -- Decatur Industr...

Decatur Industrial Electric has been a trusted choice in crane and hoist services for customers for over 60 years.

We are a reliability company so we focus on OSHA compliance.

Some reasons customers use us include:

  • Inspections and service VS trying to sell you new hoists
  • Already a trusted partner in motor repair and other solutions
  • Inspections per OSHA 29 CFR 1910.179 requirements by factory trained technicians
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  • Non-destructive testing
  • Detailed reporting
  • Preventative and predictive maintenance
  • Training of your personnel
  • In-house and on-site hoist repair service

Click here to learn more.

Older, Wiser...and More Efficient?

Mon, 11/21/2016 - 08:54 -- Decatur Industr...


According to the Gallup Poll in January of 2015 only one-third of the baby boomers in the US were still working. Worse, the Star Tribune research finds that as older workers retire productivity slows due to the loss of senior working efficiency and influence on younger workers efficiency. If your facility is facing a similar retirement era, visit the PdMA website to read the latest article Tips for a Lean Approach to Motor Reliability.

We Are Hiring Drivers

Mon, 11/07/2016 - 09:17 -- Decatur Industr...

We are hiring drivers! All you need is a Class C drivers license. Decatur Industrial Electric provides excellent pay and benefits and is hiring NOW! What are you waiting for? Military veterans are encouraged to apply.
Commercial driving experience and an up-to-date D.O.T. medical card is preferred