MIL-PRF-24751A(SH)
3.12.4.1 Face mounted. Motors shall be face mounted. Mounting shall be designed to dampen rotor vibration.
3.12.4.2 Integrally mounted. Stator mounting may be integrally cast or manufactured with fan casing. Means
shall be provided to prevent rotational and axial displacement of stator after insertion. If mechanical fasteners are
used, they shall be of corrosion-resistant material.
3.12.5 Conduit box. A conduit box shall be mounted on the outside of the fan casing in line with motor lead
entrance. The box shall permit connection from either side or from one end. Boxes shall have the same degree of
enclosure as motors with which they are used. For horizontal installation on shipboard, a fan should be installed
with a conduit box above the horizontal centerline. This will prevent drainage of motor condensate to conduit box.
Preferred installation is with conduit box 45 degrees above horizontal centerline.
3.12.6 Conduits. The lead wires from the conduit box to the motor shall be enclosed in a watertight conduit or
race. An effective electrical ground shall be provided from the motor frame to the fan casing and the conduit box.
3.12.7 Bellmouths. When specified (see 6.2), a fan bellmouth shall be provided.
3.12.8 Fan impellers. The fan impeller shall require no assembly or disassembly except to be removable as a
unit from the drive shaft. Impellers shall be of airfoil design. Impellers shall be designed to prevent retention of
water when the fan is installed vertically with the impeller up or down, or horizontally. The total impeller shall have
at least a safety factor of 8, based on the ultimate tensile strength of the material. The hub of the impeller shall not
distort or the fit loosen on the drive shaft during the life of the equipment.
3.12.8.1 Nosepiece. If a nosepiece is required, it shall be match marked with the impeller or attached to it to
ensure reinstallation in the same position relative to the impeller.
3.12.8.2 Balancing. The impeller shall be statically and dynamically balanced in accordance with
manufacturer's recommended procedures. The preferred method of balancing impellers is by removal of material.
Provision of sufficient rim thickness or of cast pads on the inside diameter of the impeller rim is recommended for
this purpose.
3.13 Electrical equipment.
3.13.1 Motors. In consideration of the cooling effect of the air stream, motors for vaneaxial fans need not
conform to referenced motor specifications in regard to horsepower rating of a standard frame size. Motors shall not
have excessive horsepower, but shall have sufficient torque to start the fans at 90 percent of rated voltage, and to drive
them continuously without overloading when fan delivery is above 50 percent of the minimum delivery as shown in
Figure 1. Motors shall be in accordance with IEEE STD 45, IEEE STD 429, and NEMA MG 1.
3.13.1.1 General requirements. Motors shall conform to the following:
a.
Motor - 60 Hz, ac, integral-horsepower.
b.
Voltage, phase, and frequency - 440V, 3-phase, 60 Hz.
c.
Ambient temperature - 65 °C
d.
Navy service A - Shock tested to MIL-S-901, type A.
e.
Duty - Continuous air over.
f.
Enclosure - Spraytight in accordance with IEEE STD 45 and NEMA MG 1.
g.
Insulation - Class F or better sealed insulation - with class B temperature rise.
h.
Structureborne noise - Motor structureborne noise levels shall not exceed the levels shown for type II
in MIL-STD-740-2 or lower when specified by auxiliary manufacturer.
i. Drains - Tapped drain holes with metal pipe plugs shall be provided in each spraytight motor to drain
condensate from the motor when the fan is installed vertically with fan impeller up or down, or
horizontally.
j. Balancing - Motor rotor shall be statically and dynamically balanced.
k. Revolutions per minute (rpm) - 3600 (synchronous) constant speed.
l. Type - Squirrel cage induction. With not less than 80 percent power factor full load.
m. Efficiency - All motors shall be of the high efficiency type.
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