Efficiency and Your Bottom Line: Getting to the Meaning Behind the Nameplate

Because operational costs ride on efficiency determinations, accurate measurements of losses occurring within the motor are paramount. The reliability of efficiency data is key to any energy-savings plan, and knowing the meaning behind the rating can make or break a smart purchasing decision. Whether you are a municipality or an industrial facility, you know that operating costs are one of management's biggest concerns when considering how to power your pumping system. So, now that you have convinced management to lower costs by purchasing a premium efficient motor, what's next? It's time to ensure you're really getting that efficiency rating you purchased. The topic of efficiency has been a mainstay in industrial circles since the Energy Policy Act went into effect in 1997, requiring motors to meet minimum efficiency standards. Facilities using highly efficient systems are taking advantage of costs savings on their utility bills and positively contributing to the bottom lines - real financial returns for an industry the U.S. Department of Energy estimates consumes more than $30 billion a year in electricity to power motorized systems. While the DOE provides a wealth of information on what industrial facilities can do to maximize efficiency, the only way to obtain a reliable figure for operational-cost purposes is to measure it. In technical terms, efficiency is a ratio of energy watts out to energy watts in. Watts in are determined by measuring the losses occurring in the motor's stator winding, rotor or iron, as well as by friction and windage. The more mysterious stray losses that occur within the motor are also taken into account when determining efficiency. Because operational costs are riding on efficiency determinations, accurate measurements of losses occurring within the motor are paramount. To measure these losses, IEEE 112 helps motor manufacturers determine efficiency ratings by approving several methods of measuring motor losses. Motor manufacturers typically use two methods for a majority of motor efficiency testing today. For variable loads like fans and pumps, the most popular motor designs follow the NEMA design class B guideline for speed and torque characteristics. For these machines, IEEE 112 Method B is commonly considered the preferred method of testing. This method uses a carefully calibrated dynamometer to measure the motor's power and torque - key components of determining efficiency percentages. However, larger horsepower motors (500-hp and above) require substantial electrical power systems and load cells to run performance testing, making the dynamometer an impractical option for some manufacturers. In these instances, testers use IEEE 112 Method F, an equivalent circuit methodology based on calculations and assumptions.

Method B: By Dynamometer

This testing procedure determines the machine's efficiency by direct coupling it to a dynamometer that measures performance at the rated load on the nameplate. The input power applied to the machine is set to the nameplate values for frequency and volts. The dynamometer then loads the machine to a specific torque value and the test operator measures and records the volts, amperes and watts, and resulting speed of the machine. This step is repeated for various torque values, giving the tester the total losses under a loaded condition. Accuracy of the measured motor losses is enhanced by allowing the machine to stabilize at the operating temperature and performing the test at the rated load. A no-load test is then performed at different input voltages to measure the resulting amps and watts. This helps determine the core, friction and windage losses of the machine under the assumption that they are independent of the load. With this data, the stray loss can be calculated by subtracting the sum of the measurable losses from the total losses. The concept of "smoothing" is a statistical method that correlates the resulting stray loss, minimizing error and optimizing the reliability of the data (refer to IEEE 112 - 6.4.2.8 for details). A clear indication that the test results are valid is when the correlation factor is 0.9 or larger.

Method F: By Equivalent Circuit Methodology

IEEE 112 Method F relies on an equivalent circuit calculation to determine efficiency. Data on items such as amperes, watts and winding temperature are determined through various tests, including a Direct Current test, a locked-rotor test and a no-load test. But because a dynamometer is not involved in the process, characteristics, such as stray loss and rotor resistance, need to be obtained by completing a set of calculations. These may or may not represent the motor under actual running conditions. Under Method F, the stray losses are determined either by taking direct measurement or by using an assumed percentage of rated output that is determined in Table 2 of IEEE 112. By using the predetermined percentage, stray losses may not have a direct relationship to the design, nor do they take into consideration the machine's manufacturing variation. Similarly, the direct measurement option obtains results through a complicated process of calculations open to the chance of human error. Take, for example, the stray losses of a standard efficient motor versus a premium efficient motor. While their efficiencies are different, IEEE 112 Table 2 treats their stray loss values the same. Stray losses are typically 8 percent to 15 percent of the total losses of the machine. If the table is calculating the stray losses as 8 percent of the total losses, but the stray losses are actually 15 percent, the overall efficiency rating of the motor may be operating 0.3 percent lower in efficiency. The only way to uncover the stray-loss differences among the motors under Method F is to measure the fundamental frequency and high-frequency components of the stray-load loss and then calculate the sum of the results. In addition to the complications associated with this process, the calculations have the capability to produce erroneous results. Although Method F offers a way to measure several variables when efficiency cannot be measured at the motor's rated operating point, the opportunities for results that differ from Method B extend well beyond stray losses. Another problematic issue related to Method F is the calculation for rotor resistance, which determines stator losses, and the assumption used. Stator and rotor leakage reactance are portions of magnetizing reactance and the coefficients of these variables are typically assumed. Method F calls for measuring these resistance and reactance parameters with a locked-rotor test that produces a significant disconnect from the actual operating point. Under the locked-rotor test, alternating current frequency for rotor and stator are identical. This is another opportunity to introduce error because it is not consistent with the operating conditions. The Method F tester must choose a low frequency (IEEE 112 recommends 15-Hz or below). But to get an acceptable measurement of the stator, there is a tendency to keep the frequency higher than 15-Hz. This results in another approximation of motor performance perpetuated through the use of this data to calculate rotor resistance. The impact this has on the overall efficiency is difficult to quantify. Nevertheless, this part of Method F walks us through yet another set of variables that are approximate, resulting in data that is not exactly representative of the machine's operating characteristics. Method B requires measurement tolerances for watts, volts, amps, temperature, frequency and torque when testing the motor on the dynamometer. This test equipment must undergo regular calibrations to maintain consistent results. Measuring five of the losses with a machine produces stray-loss data that are calculated based on more reliable sources. Moreover, since a regression is made on six (or more) operating points, the opportunity is minimized for measurement error and other inaccuracies. The Method F logically measures several variables, but requires the tester to make several assumptions in the process. Understanding what the assumptions are and how they relate to your motor may be challenging. Solving all the calculations needed to evaluate motor performance using Method F may not deliver the same results as using Method B. Nevertheless, the reliability of efficiency data is key to any energy-savings plan, and knowing the meaning behind the rating on the nameplate can make or break a smart purchasing decision. Under Method B, the user can be assured more reliable data by requesting calibration documents for the dynamometer and the correlation of stray loss smoothing. Under Method F, they might want to keep their calculators nearby.