International Appliance Manufacturing 2008 - (Page 31) Matching Motor/Fan Performance Motor design and fan blade interaction are also important factors in optimizing the performance of an evap system. AC single phase, 2 pole, 60 Hz, C-frame induction motors with a maximum (synchronous) speed of 3,600 revolutions per minute (RPM) are standard for use in these evaporator systems. A typical dynamometer curve for such a motor with important areas highlighted is shown in Figure 2. A sample fan load line is also included in the figure; the point of intersection between the motor and fan curves defines the system’s operating point. For stable, efficient operation, the operating motor speed must be above the “cusp” and preferably above the “breakdown torque” as shown in Figure 2. Figure 3 provides a plot of efficiency vs. speed for the same C-frame motor shown in Figure 2. It can be seen from Figure 3 that this desired operating region just above breakdown typically corresponds to the region of maximum motor efficiency which is important in minimizing power input to the motor/fan system. Both the motor and fan have design elements which can adjust operating speed and maximum efficiency ranges. Examples for the motor include wire size and number of turns and rotor end ring resistance. Hub and tip diameters and blade angles are examples of fan design elements. Evap energy minimization requires Figure 2: C-frame dyno curve with load line too low, then interior temperatures will not be maintained without high evap run times, thereby negating any power reduction benefit. Likewise, higher airflow pushes too much cold air from the freezer compartment into the refrigerator proper, potentially causing problems such as the freezing of refrigerated vegetables. Breakthrough in Fan Performance Fan design is a mature technological sector wherein performance and efficiency improvements are typically minimal. A leading source of inefficiency for axial fans lies in their tendency to throw air outward, necessitating a shroud to collect and redirect airflow along the axis as intended. The Pax Group™ develops fans which employ streamlined blades with patented geometrical shapes derived from a naturalistic design approach, providing better airflow direction and improved efficiency. Potential benefits from using a more efficient PAX fan include a greater output for the same input or reductions in noise, input power, and motor size. The primary benefit sought by using a PAX fan in an evaporator application is reduced input power. PAX Scientific develops the airmoving technologies. A. O. Smith Electrical Products Company has licensed exclusive rights from The Pax Group to use PAX fan technology in kitchen and bath applications. Figure 1 shows the PAX fan designed for evap systems that offers an efficiency improvement over other evaporator fan designs. International Appliance Manufacturing 2008 31
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