Understanding System Efficiency in Motor-Driven Rotating Equipment

Energy efficiency has become a major focus for the U.S. government, municipalities, power utilities and the industrial sector, with much of the attention falling on components such as motors and pumps. For end users, understanding the difference between component efficiency and system efficiency as applied to motor-driven equipment is critical for evaluating a total system and making appropriate upgrades. The Energy Independence and Security Act (EISA) is one standard that users must understand and comply with to successfully improve system efficiency.

Efficiency Standards as Defined by EISA

For each general-purpose rating (Subtype 1) from 1 to 200 horsepower (HP) that was previously covered by EPAct, the law specifies a nominal full-load efficiency level based on National Electrical Manufacturers Association (NEMA) premium efficiency as shown in NEMA MG 1, Table 12-12. All 230- or 460-volt (and 575-volt for Canada) motors currently under EPAct that were manufactured after December 19, 2010, must meet or exceed this efficiency level. General-purpose electric motors (Subtype II) not previously covered by EPAct will be required to comply with energy efficiencies as defined by NEMA MG 1, Table 12-11. The term general-purpose electric motor (Subtype II) refers to motors that incorporate the design elements of a general-purpose electric motor (Subtype I) that are configured as one of the following:

• U-frame motor
• Design C motor
• Close-coupled pump motor
• Footless motor vertical solid shaft normal thrust motor (as in a horizontal configuration)
• An 8-pole motor (900 rpm)
• A poly-phase motor with voltage of not more than 600 volts (other than 230 or 460 volts)

Motors that are 201 to 500 HP that were not previously covered by EPAct will be required to comply with energy efficient efficiencies as defined by NEMA MG I, Table 12-11. This information and the Tables referenced above are readily available on the Department of Energy (DOE) website. So, what does the new EISA Standard have to do with system efficiency? Many end users believe that any system efficiency improvement is the result of an increase in motor efficiency; however, that is not always the case. For example, consider a centrifugal pump system operating at a fixed speed. The system requires variable flow and is controlled by a motor-operated valve. One might believe that replacing the standard-efficiency motor with the new EISA premium-efficient motor would lead to an incremental gain in efficiency and a lower operating cost. This seems reasonable, but more factors must be considered. In order to meet the EISA standard, motor original equipment manufacturers (OEMs) had to redesign their equipment to achieve the increased efficiency as mandated by government regulations. To understand what is meant by "increased efficiency," users must know the definition of a premium-efficiency motor and what affects that efficiency.

Motor Losses

Losses in a motor include stray losses, rotors, stators, core losses and fan design (windage). To make a motor more efficient, a manufacturer must add more or better material. These additions and adjustments could include more active material such as copper in the winding, a longer stator, rotor cores and improved electrical steel (silicon steel is used for the stator and rotor). A low-loss fan design could also be used to reduce friction and windage losses. To reduce the stray load losses, manufacturing processes are assured through International Organization for Standardization (ISO) 9001 procedures. Some advantages of energy efficient motors are:

• Maximum Efficiency – Energy-efficient motors operate at maximum efficiency even when they are lightly loaded because of better design.
• Longer Life – Energy-efficient motors dissipate less heat compared with standard motors. Use of energy-efficient fans keeps the motor at a lower temperature, which increases the life of the insulation and windings as well as the overall life of the motor.
• Lower Operating Cost – The total energy cost of energy-efficient motors during its life cycle is much lower when compared with conventional motors.
• Other Benefits – Energy-efficient motors have better tolerance to thermal and electrical stresses, the ability to operate at higher temperatures, and the ability to withstand abnormal operating conditions such as low voltage, high voltage or phase imbalance.

System Efficiency

Energy-efficient motors can also improve system efficiency, but end users must consider the following factors:

• Motors meeting higher efficiencies tend to run faster than their less efficient counterparts.
• Matching speeds to application need (such as pump flow) is important to consider.
• Drives may be required, which offers the opportunity to increase system efficiency in applications with variable output requirements. Variable frequency drives (VFDs) require further considerations for optimum reliability and efficiency.
• In some cases, mounting dimensions for motor into machinery may be slightly different.

Case Study

The following case study graphically illustrates the impact of a premium-efficient motor in a centrifugal pumping application. Figure 1 provides four separate scenarios for reducing energy consumption in a cooling tower pumping system. The portrayed system is a typical closed loop configuration where the discharge is being throttled over a range of operation. The system in this example operates 24/7, 365 days per year. At this particular load point, that means it operates 70 percent of the time—or 6,250 hours per year. Columns 1 and 2 in Figure 1 indicate the various components factored into the system efficiency calculation. Column A is the base condition where the system operates 50 percent of the time. The component efficiencies for the VFD and gearbox are at 100 percent because they were not used. Under the base condition, the total power required is approximately 1,777 HP; almost 356 HP is being lost (wasted) across a control valve. In addition, the pump is operating back on the curve at 65 percent efficiency. Under these conditions, the total system efficiency is 49 percent. Column B provides the new operating conditions with the addition of a VFD. The head required has been reduced to 150 feet because the loss across the valve has been eliminated by reducing the speed of the pump to meet required system demand. Motor efficiency remains the same, and a 2 percent loss has been added as a result of heat generated across the drive. Note the dramatic improvement in the overall system efficiency (81 percent) and the total operating cost reduction from $414,306 to $187,360. The total cost savings is $226,946 per year. Column C addresses the impact on the system by improving the efficiency of the pump. Nothing else in the system was changed. The minimal improvement of the overall system efficiency (53 percent) results from increasing the pump efficiency by 5 percent. The 50 feet of head loss across the control valve remains, so the total power required is 1,650 HP. This scenario does not present huge savings based on the cost of a new pump and installation and potential piping changes. Factor in the ongoing reliability issues, such as the pump operating back on the curve, and $29,593 would be difficult to justify. Column D identifies potential savings when motor efficiency is improved by 2 percent. Again, nothing has changed in the system with the exception of an additional 5 feet of friction loss across the valve as a result of the reduced slip in the premium-efficient motor (head increases to the square of the speed). In this case, the system efficiency remains the same at 49 percent. Note that the power required for the additional friction has increased to 330 HP. The total power required was reduced to 1,650.2 HP (a reduction of 127 HP) with a total savings of $518 per year. References

1. EISA Standards Department of Energy
2. WEG Electric