Efficiency of motor

When maldug any economic analysis, care should be taken to be certain that the efficiency ratings of all motors being considered are on the same basis. While this should not be a problem for motors rated 1 to 500 horsepower as covered by the NEMA Standards for efficiency marldug, it is common practice for several different test methods to be used when measuring the efficiency of motors rated over 500 horsepower. A particular test method may need to be selected by the test facility on the basis of available test equipment and power supply. All test methods that may be used to test any one motor will not necessarily give the same result for efficiency. 

There has been not only gi owth m the total number of electric motors (more standard appliances in use), but also a proliferation in their use for new, novel applications. Both trends will continue to increase demand for the electricity to run electric motors. In the United States, electric motors arc responsible for consuming more than half of all electricity, and for the industrial sector alone, close to two-thirds. Since the cost of the electricity to power these motors is enormous (estimated at more than 90 billion a year), research is focused on finding ways to increase the energy efficiency of motors and motor systems. 

E = efficiency of motor and pump expressed as a fraction B = a constant independent of Dt... 

E = efficiency of motor and pump expressed as a fraction f = Fanning friction factor, dimensionless, or function for dynamic programming in Eq. (91) indicating optimum return depends on that input... 

Efficiencies. Efficiencies for centrifugal pumps and over-all efficiencies of motor-driven centrifugal pumps are presented in Figs. 4-7 to 4-11 in terms of hydraulic horsepower. 

Fig. 4-11. Over-all efficiencies of motor-driven centrifugal pumps. [Courtesy of R. M. Braca and J. Happel, Ckem. Eng., 60(1) 181 (1953),]...

The efficiency of motors is a fimcdon of load and speed. The elecmic motor manufiicturers provide performance curves and tables fi>r their products that show, among other infiirmadon, the efficiency of the motor. 

Cr = coefficient of rolling resistance a = inclined angle of road (radians) e= efficiency of motor, controller, and gearing... 

FIGURE 3.7 Efficiency of motors historic inventions. NOTE F = efficiency, where 1 = 100%. SOURCE Adapted from Marchetti (1985) and Ausubel (1996). 

Energy costs ate not direcdy related to the energy efficiency of the process (6,42). Even if the thermal efficiency of a steam ejector, for example, is less than that of mechanical equipment mn by an electdc motor, the overall cost of the energy to mn the steam ejector may still be less. 

From equation 60 one can obtain a theoretical power requirement of about 900 kWh/SWU for uranium isotope separation assuming a reasonable operating temperature. A comparison of this number with the specific power requirements of the United States (2433 kWh/SWU) or Eurodif plants (2538 kWh/SWU) indicates that real gaseous diffusion plants have an efficiency of about 37%. This represents not only the barrier efficiency, the value of which has not been reported, but also electrical distribution losses, motor and compressor efficiencies, and frictional losses in the process gas flow. 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

Further incentives to use energy-efficient motors are provided by various cost rebate programs offered by utilities based on norsepower rating and efficiency level. Another factor that will have a significant impact is the Energy Policy Act of 1992, in which the U.S. Congress established limits on the lowest level of nominal efficiency that certain classes of motors of standard design can have after 1997. 

The declared efficiency and power factor of a motor are affected by its loading. Irrespective of the load, no-load losses as well as the reactive component of the motor remain constant. The useful stator current, i.e. the phase current minus the no-load current of a normal induction motor, has a power factor as high as 0.9-0.95. But because of the magnetizing current, the p.f. of the motor does not generally exceed 0.8-0.85 at full load. Thus, at loads lower than rated, the magnetizing current remaining the same, the power factor of the motor decreases sharply. The efficiency, however, remains practically constant for up to nearly 70% of load in view of the fact that maximum efficiency occurs at a load when copper losses (f R) are equal to the no-load losses. Table 1.9 shows an approximate variation in the power factor and efficiency with the load. From the various tests conducted on different types and sizes of motors, it has been established that the... 

The mechanical output of the motor for crimes iind hoists in lifting the hook load is the useful work done by it. The losses produced in the crane or hoist mechanism are taken into account by the mechanical efficiency of the hoisting mechanism. 

These motors have a lower efficiency as a result of running in liquid, causing more liquid drag and also axial thrust bearing loss, which is also a part of the motor. However, this lower efficiency of the motor is compensated by fewer mechanical and hydraulic losses in a submersible motor-pump installation, compared to a vertical turbine pump installation. 

The actual operating speed will be slightly less by the amount of slip. Slip varies with motor size, load, and application. Typically, the larger and more efficient the motor, the less full-load slip. A standard 10-hp motor may have 2 A% slip whereas, motors over 1,000 hp may have less than M75. slip. Operating slip can be approximated by multiplying % load by full-load slip. 

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