Figure out the benefits of NEMA Premium motors

Are NEMA Premium motors always better than their non-Premium predecessors?

02/19/2012


An impact study by the American Council for an Energy Efficient Economy estimated that if all motors in the U.S. were upgraded to NEMA Premium, the annual energy savings could reach nearly 10,000 GW of electricity every year. That’s equal to the power generated by three coal-burning power plants.

But are NEMA Premium motors always better than their non-Premium predecessors?

Although in most cases the mechanical package of the motor is the same for NEMA Premium designs as for a lower efficiency motor, there are definite advantages. NEMA Premium motors operate at a higher efficiency at the same load point. They often run cooler than lower efficiency motors. However, other application criteria must be considered before making a final decision to use NEMA Premium designs.

Energy savings is dependent on many variables beyond just motor efficiency—including cost per kilowatt hour, number of operating hours, peak energy charges, motor loading percent, load matching, and power factor penalties. In the long run, switching from a lower efficiency motor to a NEMA Premium significantly impacts overall energy savings.

Motor speed is one of the more critical application considerations when evaluating the use of NEMA Premium motors. The speed of an induction motor is the synchronous speed minus slip. On an induction motor, slip is the difference between motor operation speed and the synchronous speed of the motor.

The synchronous speed is determined by the electrical frequency of the power supply such as 60 Hz (standard in most of North America) and 50 Hz (standard in most of the rest of the world) and the number of poles of the motor.

60 Hz = 7,200 ÷ No. of poles

Motor speed is an issue because as a motor increases in efficiency, it is “strengthened” by adding more active material to achieve the efficiency required. “Strengthening” the motor results in less slip and a higher operating speed. The difference between the synchronous and the operating speed is smaller. In constant torque applications the difference in speed is not a big deal. In variable torque applications, it is a very big deal.

A variable torque load increases horsepower at the cube of the change in speed. So, if the slip changes by less than 1%, the resulting power consumption changes 3%. The flow of the variable torque load will also increase in varying levels, but typically at the square of the change in speed, or 2% on a 1% change in speed. This is a good thing if an increased flow is desired. But if a system is designed correctly, a change in flow is not a desired outcome and the increased flow becomes wasted energy.

To achieve the savings a new higher motor efficiency can gain for the application, a process change is required. This could be a small change such as a change to the sheave diameter or trimming a fan or impeller blade. Efficiency can be gained, but speed change must be considered, particularly if an older standard efficient motor is being replaced with a premium efficient design.

Is power factor a consideration for your facility? It is a component of many plant power bills. As electrical designs change to reach NEMA Premium, the power factor can change as well. Specifying, or at least considering, the power factor of the motor being removed to a new NEMA Premium replacement can be important to the long-term power cost. U.S. Motors brand motors incorporate very high power factors into NEMA Premium designs to allow users to maintain system power factors.

Motor efficiencies can be stated in alphanumeric designations, such as IE-1, IE-2, IE-3, and IE-4, or by descriptors like Standard, Epact, Energy, Premium, and NEMA Premium. The IE nomenclature refers to IEC efficiency designations. Descriptors are used to refer to NEMA.

Standard motor efficiencies have never been defined by NEMA and vary widely by manufacturer.

  • Energy Efficient (EPACT) motors are defined by NEMA in the MG1 Standard in Table 12-11 for 60 Hz designs 1-200 hp.

  • NEMA Premium Motors are defined by three different tables in MG1. Table 12-12 defines efficiency for 60 Hz 1-200 hp. Table 12-13 defines it for 60 Hz 250-500 hp, and Table 12-14 defines efficiency for 50 Hz 1-200 hp.

  • In the IEC designations, IE-1 is considered standard efficient. IE-2 closely matches the Energy Efficient levels but defines 50 Hz and 60 Hz efficiencies. IE-3 compares to the NEMA Premium levels for 60 Hz and is exactly the same as the 50 Hz NEMA levels. IE-4 is a table of efficiencies that currently is neither defined by NEMA, nor used as a standard for any efficiency regulation in the world. There is an ongoing effort to harmonize the two primary “world” efficiency standards.

Perhaps the most important “best practice” when it comes to motors in general would be to establish a plan for every critical motor—50 hp or larger—and identify the appropriate replacement motor.

Albers is director of marketing, Business Division, at Nidec Motor Corp.



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