November 2011 Electrical Apparatus

November 2011 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2011 featured technical article, by Richard L. Nailen, P.E.   

Whether open or enclosed, all but the smallest a-c motor sizes require forced air circulation to dissipate heat from internal energy losses. Centrifugal fans are most often used, externally (and sometimes internally also) in totally enclosed motors, and internally for open machines.

For ratings up to about 375 kilowatts, most fans are radial with straight blades that function equally well in either direction of rotation. In larger machines, fan blades will often be inclined at an angle and sometimes curved for higher efficiency and reduced noise.

Internal fans at each end of the rotor assembly force cooling air over stator winding end turns, and in some motors over the stator core outer diameter as well.

Larger ratings include radial air ducts through both stator and rotor cores, so that the rotor itself provides most of the air circulation. This is seldom effective at 3,000 RPM and above, because electromagnetic and shaft diameter constraints allow little or no room for air to enter the rotor Inner-diameter fans in two-pole motors are therefore of the axial flow or propeller type, forcing air down the air gap from which it discharges through stator ducts.

Fan design–particularly choosing the number of fan blades–is more art than science. Although logic may seem to dictate increasing the number of blades increases air flow, tests indicate that this often has little effect. Many large motors have successfully used no more than six or eight fan blades, individually attached to spider arms.

Laws of proportionality are useful in ratioing centrifugal fan dimensions from a series of tests reported almost 90 years ago. Opinions differ, however, on the accuracy of extrapolating from those tests that could not reproduce the actual air flow patterns found in motors. Experience has proven to be the best guide. Many compromises are needed, because the cost of such refinements as air-foil fan blade construction is seldom justified. Modification of existing fans during motor maintenance or repair is unwise.

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