Archives for December 2016

The Potential to Failure Curve (or P-F Curve) gives the user information on how an asset behaves before a failure occurs. This example is focused on failure due to misalignment. The goal of a reliability focused plant is to be as far to the left on the curve as possible. While some companies are doing predictive maintenance work in an effort to reach the left side, many companies today are on the right domain of the curve, doing reactive work.  Being in the reactive domain—putting out fires as they say— increases maintenance costs. This forces a company to perform unplanned work, causes unscheduled downtime, and higher costs to expedite parts. Using technologies like ultrasound, thermography, and vibration analysis will catch an asset in a pre-failing state. This allows time to plan and schedule the repair to take place. However, with the right processes in place, the technician should recognize the misalignment of the machine before it causes components to fail. The ultimate goal is to be so far left on the curve, that it is off the chart, at the point where all the efforts (flat and rigid bases, accounting for thermal growth, eliminating soft foot, precision alignment, etc) are made so that the machine never runs misaligned.

Source: http://ludeca.com/blog/alignment/6720/the-importance-of-the-p-f-curve-wh…

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.