This piece was originally published in the June 2016 issue of ei, the magazine of the electroindustry.
By William Livoti, Power Generation Business Development Manager, WEG Electric Corporation
Energy efficiency has become a major focus for government, municipalities, power utilities, and the industrial sector. Each entity focuses its attention on specific components—e.g., motors and pumps.
This article will discuss component efficiency versus system efficiency, as applied to motor-driven equipment, and why it is important to evaluate the total system when making upgrades.
Efficiency Standards as Defined by EISA
The Energy Independence and Security Act of 2007 (EISA) originally applied to induction motors and was recently revised to include additional motors.
For each general-purpose rating (subtype 1) from 1 to 200 horsepower that was previously covered by the Energy Policy Act of 1992 (EPAct), the law specifies a nominal full-load efficiency level based on NEMA Premium efficiency as shown in NEMA MG 1, table 12-12. All 230- or 460-volt motors (and 575-volt motors for Canada) currently under EPAct, manufactured after December 19, 2010, must meet or exceed this efficiency level (addendum 2015).
General-purpose electric motors (subtype II) not previously covered by EPAct will be required to comply with energy efficiencies defined by NEMA MG 1 Motors and Generators, table 12-11. The term general-purpose electric motor (subtype II) refers to motors that incorporate the design elements of general-purpose motors (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)
- Eight-pole motor (900 rpm)
- Polyphase motor with voltage of not more than 600 volts (other than 230 or 460 volts)
- 201–500-horsepower motors not previously covered by EPAct will be required to comply with energy efficiencies defined by NEMA MG I, table 12-11
So, what does the new EISA standard have to do with system efficiency? Most believe that the system efficiency improvement is due to the increase in motor efficiency; this is not always the case. Take for example a centrifugal pump system operating at a fixed speed. The system requires variable flow and is controlled by a motor-operated valve.
One would believe that replacing the “standard” efficiency motor with the new EISA premium efficient motor would result in an incremental gain in efficiency and consequently reduced operating cost. Seems like a no-brainer, right? Not so fast; there is more to the story.
In order to meet the EISA standard, original motor equipment manufacturers had to redesign their motors to achieve the increased efficiency as mandated by government regulations.
Let’s define a premium efficient motor. What makes a motor more or, for that matter, less efficient?
There are many possible losses in a motor:
- Stray losses
- Core loss
- Fan design (windage)
To make a motor more efficient, you add more or better material: more active material (e.g., copper in the winding and longer stator and rotor cores) or improved electrical steel (silicon steel is used for the stator and rotor). A low-loss fan design is used to reduce friction and windage losses. To reduce the stray load losses, manufacturing processes are assured through ISO9001 procedures.
What are the advantages of a energy-efficient motor?
- Maximum efficiency: Energy-efficient motors operate at maximum efficiency even when they are lightly loaded because of their better design.
- Longer life: Energy-efficient motors dissipate less heat compared to standard motors. Use of energy-efficient fans keeps the motor at a lower temperature. This increases the life of the insulation and windings, in addition to increasing the overall life of the motor.
- Lower operating cost: The total energy cost of an energy-efficient motor during its life cycle is lower when compared to conventional motors.
- Other benefits: There is better tolerance for 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.
While the advantages of energy-efficient motors as defined above are accurate, let’s look at some other claims that are being made regarding premium efficient motors.
- Utility rebate
- Reduced load
- Energy savings
Reality check! Since the implementation of the EISA standard, most, if not all, utilities have eliminated the rebate for premium efficient motors. Why? Because it is now mandated by the federal government. Will the premium efficient motor reduce loading? Will it save energy?
Pumping System Considerations
- Motors meeting higher efficiencies tend to run faster than their less efficient counterparts.
- Matching speeds to application needs (pump flow and fan cubic feet per minute measurement) is important to consider.
- Drives may be required, which provides the opportunity to increase system efficiency in applications with variable output requirements. Variable-frequency drives (VFDs) require further considerations to achieve optimum reliability and efficiency.
- In some cases, mounting dimensions for motor into machinery may be slightly different.
The following is a case study that graphically illustrates the impact of a premium efficient motor in a centrifugal pumping application.
The table below presents four separate scenarios for reducing energy consumption in a cooling tower pumping system. The system outlined in the table 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 a year and, at this particular load point, 70 percent of the time, or 6250 hours per year.
Columns one and two 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. Note that the component efficiencies for the VFD and gearbox are at 100 percent since they were not used. Under the base condition, the total power required is approximately 1777 horsepower (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 in. since 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; a two-percent loss has been added due to heat generated across the drive. Observe the dramatic improvement in the overall system efficiency (81 percent) and total operating cost reduction from $414,306 to $187,360, which amounts to cost savings of $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. Note the minimal improvement of the overall system efficiency (53 percent) by increasing the pump efficiency five percent. The 50-inch head loss across the control valve remains; therefore, the total power required is 1650 hp. Not much of a saving based on the cost of a new pump, installation, and potential piping changes. When factoring in the ongoing reliability issues (the pump is still operating back on the curve), the $29,593 is very difficult to justify.
Column D identifies potential savings when motor efficiency is improved by two percent. Again, nothing has changed in the system, with the exception of an additional five feet of friction loss across the valve, as a result of the reduced slip in the premium efficiency 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 1650.2 hp (a reduction of 127 hp) with a total savings of $518.00 dollars per year.
Premium efficient motors have been mandated by the federal government. The new efficiency standard has many advantages. When replacing or retrofitting a premium efficient motor, one must take into account the impact on the total system—especially in fan and pump applications with variable load requirements under fixed speed conditions.
Mr. Livoti is the recipient of the first ever Hydraulic Institute Pump Systems Matter Leadership Award for his leadership and active involvement in promoting educational opportunities.