MIL-PRF-24755B(SH)
v. Speed-torque characteristics: The speed-torque characteristics shall be coordinated with the combined
inertia of the rotating assembly, including fan wheel. It shall be possible to accelerate the fan unit from standstill to
high speed at 90 percent of rated voltage when the current rating of the controller overload protective device does
not exceed the motor rated full load current.
w. Protection: Motor shall be equipped with thermal protection in each phase to detect over-temperature.
Thermal protectors shall be of the automatic reset type with appropriate locked rotor temperature limit (see 6.2).
Design B: In accordance with MIL-DTL-17060.
x.
y. Drains: Drain holes with plugs shall be furnished in each spraytight motor to drain condensate from the
motor when fan is installed vertically with the impeller up or down, or horizontally. For drainage in the horizontal
position, an adequate number of holes shall be provided to allow a 360-degree rotation of the fan.
3.18.2 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.
3.18.3 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.
3.18.4 Bearings. Motors shall be equipped with sealed, grease-filled bearings. Bearings shall be in accordance
with ANSI/ABMA precision classification of ABEC-1 in accordance with ANSI/ABMA 4, 9, 13, and 20 and
ISO 3290-1. Bearings shall be Type 120 in accordance with MIL-B-17931. Means shall be provided to ensure that
grease is always available to the bearing and to prevent the leakage of oil or grease along the shaft. When specified
(see 6.2), special bearings for quiet operations shall be in accordance with ANSI/ABMA 4, 9, 13, and 20, ISO 3290-
1, and MIL-B-17931.
3.19 Fan performance.
3.19.1 Volume and pressure. The design point for each size fan shall be volume-pressure point at 14 inches of
water gauge total pressure shown for that size in table II. The total pressure shall rise continually from free delivery
to a value at least as high at the stated volume as shown in table II, and throughout this range of stable performance
the volume in ft3/min shall be within 2 percent of the volume indicated in table II. The pressure developed by a fan
is the pressure at the fan discharge, and the volume is the volume at the fan inlet.
3.19.2 Aerodynamic stall. Effective stall shall not occur at capacities greater than 85 percent of the fan design
or rated ft3/min.
3.19.3 Airborne noise. The total sound power levels for the fan-motor unit shall not exceed the levels stated in
table III when tested in accordance with procedures in 4.7.3. Exceeding the total sound power levels specified in
table III shall be cause for rejection.
3.20 Noise level. The fan-motor unit shall not exceed the sound power levels specified in table III (see 4.7.3).
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