Electric motors selecting size

For the designer, the use of a standardised component size allows for the easy integration of a piece of equipment into the rest of the plant. For example, if a standard range of centrifugal pumps is specified the pump dimensions will be known, and this facilitates the design of the foundations plates, pipe connections and the selection of the drive motors standard electric motors would be used. 

As turbine-driven plants generally cost 2-4 million Euro more than similar plants driven by electric motors the selection of drive will reflect the cost of the plant versus capacity as indicated on Figure 17 showing the regression line for LNG plant cost development versus time (year). Likewise, Figure 18 shows the cost development versus the development of LNG train size [10]. The investment may also reflect various requirements for the liquefaction train and necessary support facilities. 

Because electric motors are available in standard sizes from Table 5.1a, select a standard 3000 hp (2.24x10 kW) motor. This choice results in a safety factor of 9%. 

Most chemical-plant-size compressors are electrically driven [43]. Moore [25] discusses the characteristics of squirrel-cage induction and synchronous electrical motors. Wound rotor induction motors have not been used for compressor drives. For 370 to 4500 kW (500 to 6,000 hp), the induction motors are the first choice. The squirrel-cage induction motor is the most coimnonly used driver in the process industries from 1/8 to 1,5000 hp (0.0932 to 1,120 kW [25]. From 15,000 hp (149 to 11,200 kw) the synchronous motor could be used [25]. If the compressor is operated at 7,500, 11,000, and 23,000 rad/s (1,200, 1,800 and 3,600 rpm), no step-up gears are required. The least costly speed for an induction motor is 1,000 rad/s (1800 rpm) so that this speed is usually selected. Step-up gears are used to obtain higher speeds. 

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. 

From Table 5.10, the nearest standard size electric motor is 5 hp (373 kW). The safety factor for this selection will only be 2.5 %. Therefore, select the next larger-size motor, which is 7.5 hp (559 kW). The safety factor for this selection is 53.8 %. 

The efficiency is set by the pump manufacturer when the final pump selection is made. It is usually based on their shop tests for the same model and size pumps. The pump efficiency can vary between 10% and 80%. Pump electric motor specifications require mechanical and electrical requirements. Motors can vary in size depending upon power, speed (RPM), frame size, area classification, orientation, service factor and type of enclosure (e.g. totally enclosed fan closure). 

Before contact with the workpiece, the press is slowed to a pressing speed that may or may not be variable, depending on the press design. As the pressing operation requires power to be drawn directly from the electric motor, the pressing speed is often based on the selection of a motor of reasonable size. 

The start-point for electrical design is the electric motor and other consumers list. This list is typically drawn up by process and mechanical engineers involved in the selection of the driven equipment. The electrical engineer needs to probe this data and understand how it was drawn up, with special reference to the expected load factors (namely, ratio of actual kW or kVA to the value used as design basis). Rarely, drive sizes are underestimated. More usually, they are consistently overestimated as a result of the practice of putting factors on factors . There may be a factor of conservatism for increased process performance requirements, a factor for driven-machine performance tolerances and deterioration of efficiency, and then perhaps a factor to allow for uncertainty, and, then again, rounding up to the nearest standard motor size. 

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

In this arrangement the compression heat from the compressor is used to heat up the helium to the desired temperature. The required compressor power was 90 MW with 45 MW generated and regained by the expansion in the gas turbine and 45 MW introduced by the electric drive motor. The selected combination of the turbine and compressor on one shaft resulted in dimensions being comparable with a helium turbine of 300 MW capacity (the reference plant size at the time). 

Select standard size motor. A motor that require a minimum size of (79.1 hp)/0.85 = 93 motor and impeller assembly is correctly sized because of the gassed power factor, that is, P/Pq the impeller power would increase to 78/0.38 = this problem, some method (typically electrical supply. When the gas supply is off, the control two-speed motor, goes to a lower speed. 

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