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Q
What is a fan operating point, and why is it different from free-air CFM?
AA fan operating point is the intersection between the fan P-Q curve and the system impedance curve. Free-air CFM is measured at zero static pressure, while installed airflow depends on filters, heat sinks, ducts, guards, and enclosure openings. For defense electronics cooling, selection should be based on the required airflow at the expected pressure drop, not on free-air airflow alone.
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Q
Should I select a cooling fan by static pressure, total pressure, or airflow?
AStart with the required heat removal and installed airflow, then check whether the fan can provide that airflow at the system pressure drop. Static pressure is usually the practical value used to overcome restrictions such as fin stacks, filters, ducts, and compact VPX or avionics enclosures. Total pressure may be reviewed in formal test reports, but product selection should confirm the full operating point.
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Q
Where should axial and centrifugal fans operate on a P-Q curve?
AThe preferred operating region depends on the fan type and system impedance. Axial fans are commonly selected away from stall-prone low-flow regions and should be checked against the actual enclosure resistance. Centrifugal blowers are often used when the system needs higher static pressure through ducts, filters, or dense heat sinks. Final selection should be verified with the model-specific P-Q curve.
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Q
Can one PWM signal control multiple BLDC cooling fans?
AOne PWM command can often be shared by multiple compatible fans when the control reference is correct and the controller can support the electrical interface. The PWM reference should normally be tied to the fan negative return unless an isolated interface is specified. The design should also check pull-up behavior, signal voltage, cable length, and noise immunity before release.
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Q
Can FG or RD feedback outputs from multiple fans be connected in parallel?
AFG and RD feedback outputs should normally be routed separately to the controller. These outputs are commonly open-collector or open-drain style signals, and parallel wiring can hide an individual fan fault or distort the tachometer reading. For redundant cooling trays or multi-fan avionics modules, each fan feedback line should be reviewed as an independent status or speed signal.
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Q
Can a fan feedback wire be used as the PWM ground reference?
ANo. A feedback output such as FG or RD is not a ground return. It may be pulled low, floating, or switching depending on fan status and the external pull-up circuit. Using it as a PWM ground reference can cause unstable speed control, false fault detection, and poor noise immunity. The PWM reference should use the defined fan negative return or an approved isolated interface.
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Q
Why can PWM speed control fail even when the fan is powered correctly?
ACommon causes include an incorrect PWM reference ground, reversed signal wiring, incompatible control voltage, an unintended external voltage divider, excessive cable noise, or a missing isolation strategy between digital ground and power return. In avionics or RF equipment, PWM wiring should be checked together with EMC planning, cable routing, and controller input protection.
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Q
How should replacement fan selection start when only a competitor model is known?
ABegin with the public datasheet or measured sample data for the existing fan: frame size, mounting pattern, voltage range, current, speed, airflow, static pressure, acoustic limit, connector or lead interface, PWM logic, and FG/RD output type. The replacement should then be checked against the system operating point, environmental requirements, and qualification plan rather than by size match alone.
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Q
Which environmental tests matter for naval, outdoor, or high-salt cooling applications?
ANaval and outdoor defense systems should review salt fog, humidity, rain, dust, vibration, shock, and corrosion protection together with the enclosure design. Useful planning references include MIL-STD-810H environmental methods and project-specific protection requirements such as IP rating, coating system, drainage, connector sealing, and maintenance access.
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Q
What should be checked when fan noise increases after installation?
ANoise after installation can increase because of inlet restriction, outlet obstruction, grille turbulence, structural resonance, incorrect mounting torque, unexpected PWM duty cycle, or operation near an unstable point on the P-Q curve. The review should compare free-air data with installed airflow, pressure drop, vibration path, and the actual mounting structure.
AA fan operating point is the intersection between the fan P-Q curve and the system impedance curve. Free-air CFM is measured at zero static pressure, while installed airflow depends on filters, heat sinks, ducts, guards, and enclosure openings. For defense electronics cooling, selection should be based on the required airflow at the expected pressure drop, not on free-air airflow alone.
