Efficiency of electric motors

Figure 14-37 gives values that are suitable for design estimates of centrifugal-pump efficiencies. Because pump and driver efficiencies must both be considered when total power costs are determined, necessary design data on the efficiency of electric motors are presented in Fig. 14-38. 

Since most pumps are driven by electric motors, the efficiency of the electric motor must be taken into account to determine the total electric power input to the motor. Typical efficiencies of electric motors are as follows 75% for-j-kW motors, 80% for... 

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. 

Efficient use of electric motors requires appropriate control systems. Typical control systems for AC and DC electric motor operations are now discussed [ 10]. 

To calculate the size of an electric motor, divide the compressor shaft power by an electric-motor efficiency. Efficiencies for electric motors are given in Table 5.9. The size of electric motors are standardized according to horsepower, as shown in Table 5.10. If less than the standard horsepower is calculated, then the next standard horsepower is selected. 

An electric motor is to be used to drive a compressor of 1,119 brake horsepower (BHp), The efficiency of the motor is 95%. Therefore, the electrical input to the motor must be l,119(0.7457)/0.95 = 878 kW. This is equivalent to 3,000,000 Btu/hr, which is the basis for the previous example. Calculate the kW-hr required per year for the motor if the plant-operating factor is 0.9, and calculate the cost of electricity per year. 

Table 5.35 compares the nominal full-load efficiencies of standard motors and energy-efficient motors for both open and TEFC 1800 r/min motors. Energy-efficient induction motors carry a premium price over standard motors. However, based on the cost of electric... 

However, this power loss is just the power delivered to the rotor of the compressor. This power comes from an electric motor, which has an efficiency of less than 1, and there are also losses in the connecting shaft and the bearings of the compressor rotor. If we express the combined efficiency of the motor and drive system as r m, then Ihe electrical power used will be greater than the compressor power by a factor of I/rjm, and so there will be a loss of electrical power given by the equation... 

At least half the electricity generated in the power systems of developed countries is consumed by just one type of electric motor - the induction motor (Walters, 1999). This type of motor requires an AC supply, three phase for preference. Such a three-phase supply can easily be generated by an inverter as described in Section 10.3.2, and so they are sometimes used with fuel cells. A brief description of this type of motor is given in Section 10.4.2. Although hugely successful, and generally highly efficient, the induction motor is not the best in terms of power density and efficiency. Two other modem... 

Thermally conductive materials can conduct heat away from devices into a heat sink or the surrounding air and extend product life. Thermal conductive ETP can increase the electrical efficiency of encapsulated motors by lowering the operating temperature, resulting in more power and torque and longer device life than hotter-running devices. 

Horizontally Mixing Aspirator Aerators. An aerator using a horizontally mixing aspirator has a marine propeller, submerged under water, attached to a soHd or a hoUow shaft. The other end of the shaft is out of the water and attached to an electric motor. When the propeller is rotated at high velocity, at either 1800 or 3600 rpm, a pressure drop develops around the propeller. Air is then aspirated under the water and mixed with the water, and moved out. This type of aerator, shown ia Figure 3g, is very efficient ia mixing wastewater. 

The compressor can be driven by electric motors, gas or steam turbiaes, or internal combustion (usually diesel) engines. The compressor can also be a steam-driven ejector (Fig. 7b), which improves plant reUabiUty because of its simplicity and absence of moving parts, but also reduces its efficiency because an ejector is less efficient than a mechanical compressor. In all of the therm ally driven devices, turbiaes, engines, and the ejector mentioned hereia, the exhaust heat can be used for process efficiency improvement, or for desalination by an additional distillation plant. Figure 8 shows a flow diagram of the vertical-tube vapor compression process. 

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. 

Recently, the regulation of impeller rotational velocity has become a popular regulation mode for volume flow. Electric-motor rotational velocity is regulated by a frequency changer, and its price has dropped lately. Changing the rotational speed also affects the circumference velocity of the impeller. The volume flow can be changed by the same ratio as rotational speed. The form of the velocity triangles and the efficiency remain the same. 

Similarly, an electric motor can use electricity that costs more than the motor during a year of continuous operation. Even if the motor is in perfect condition, it may be cost effective to replace it with a new motor that is a few percentage points more efficient at converting electricity into work. In many applications, however, an electric motor operates only a few hours per year. In such cases, the cost of the electricity is negligible relative to the cost of a new motor, so that even a large gain in energy efficiency is not worth the cost. 

Even when the time comes to make a purchasing decision, an energy-efficient motor purchase is not a certainty. Sometimes an energy-efficient motor will be the economically efficient choice at other times, not. The capital investment decision is based on the cost in relation to performance, efficiency and reliability. Moreover, the decision depends on the application and the amount of time the motor is in operation. It can be the major component of a product (drill or mixer), or a minor component (computer disk drive) it can be the major component cost of a product (fan), or it can be a minor component cost (stereo tape deck) it can run almost constantly (fan, pump, and machinery), or only a few minutes a day (vacuums and power tools). For example, contractors purchase circular saws almost solely based on performance and reliability. Time is money, and since the saw is operating only a few minutes a day and the contractor is often not responsible for the electricity costs to run the motor, energy efficiency is not a consideration performance and reliability are what matter most. On the other hand, an industrial user, who runs huge electric motors twenty-four hours a day to work pumps, machinery, and ventilation equipment, is very concerned tvitli energy efficiency as well as performance and reliability. 

Efficiency. In CRC Handbook of Energy- Efficiency, ed. F. Kreith and R. E. West. Boca Raton, FL CRC Press. Beaty, H., and Kirtley, J. (1998). Electric Motor Handbook. New York McGraw-Hill. 

The North American electric power transmission system has been described as the largest, most complex machine ever built by humanity. It is a massive network of generating stations, transmission lines, substations, distribution lines, motors, and other electrical loads all interdependently linked for the conversion, transportation, and control of electrical energy. Approximately 60 percent of all energy utilized in the United States passes through the interconnected electric power system. The major goal of the system is to most efficiently and reliably deliver electric power from generating stations to residential, commercial, and industrial consumers. 

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