Have you ever wondered how accurate your motor nameplate speed is? You might be surprised that the National Electrical Manufacturers Association (NEMA) MG-1 allows +/- 20% of rated slip rpm. Slip is the difference between synchronous speed of the stator magnetic field and the actual speed of the rotor. Given this level of tolerance it’s not surprising that finding speed in your current spectrum isn’t always that cut and dry. The MCEGold software also allows you to select an operating speed in addition to the nameplate speed to improve the auto speed acquisition. Think of it like calibrating the nameplate speed for actual operating conditions.

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.

The October newsletter is out! Meet our featured October employee, Tom Gadberry, PLUS:

• Food Processing Company saves $100K
• Plant shut-down services
• Motortronics LV and MV RVSS
• National Manufacturing Day
• And more

Click here to read the full newsletter. 

Gearboxes have a tendency to be installed and left to run until oil is leaking excessively or until the unit starts to make weird noises.

Our Reliability Team has the ability to perform Reliability Asset Review of your critical gear units. The following are a few items that will be reviewed:

•  Manufacturer’s print, laying out parts with part numbers
• Type of breather and changing intervals
• Oil analysis and type of oil used
• Greasing of seals and other greaseable bearings, if applicable
• Visual internal inspection of unit
• Vibration readings
• Motor loading and performance
• Coupling types and maintenance practices
• Alignment/soft-foot checks Storage procedures

Gearboxes with the appropriate care and maintenance can give your company many years of satisfactory performance.

Decatur Industrial Electric wants to partner with your company throughout your unit’s life through commissioning, maintenance PM, reliability PdM review and the repair/re-manufacturing process as needed.  As a Falk Rexnord Select Partner, a SEW Eurodrive Distributor, and a Siemens Flender Distributor, our team has the expertise to support your plants needs.

At D.I.E we love motors – all motors! Check out the video below showing The Genovation Extreme Electric car (GXE) making history in July 2016 by breaking the land speed record for a street legal all-electric vehicle. The GXE became the world’s first street legal electric car to exceed 200 mph on its way to reaching the final record breaking speed of 205.6 mph.