Fans
Fans are electromechanical devices used to move air and remove heat from electronic and electrical equipment. They are widely used in computers, power supplies, amplifiers, industrial equipment, telecommunications equipment and other systems that require forced-air cooling.
What Is a Fan?
A fan uses a rotating motor and blades or an impeller to move air. Moving air increases heat transfer from hot components to the surrounding environment.
Electrical Power
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Motor
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Rotating Blades
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Airflow
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Heat Removed
Why Are Fans Used in Electronics?
Electronic components generate heat during operation. If the heat is not removed effectively, the temperature of the components can rise above their safe operating range.
Fans force air across heat-producing components and heatsinks, improving heat transfer.
Hot Component
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Heatsink
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Airflow ─────────►
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Heat transferred
to surrounding air
Common Fan Applications
- Computer cooling
- Power supplies
- Audio amplifiers
- Industrial controllers
- LED lighting equipment
- Battery chargers
- Telecommunications equipment
- Server systems
- Inverters
- Motor controllers
- Cabinet cooling
- Electrical enclosures
DC Fans
DC fans are powered from a direct-current supply and are very common in electronic equipment.
Typical DC fan supply voltages include:
- 5 V
- 12 V
- 24 V
- 48 V
The correct voltage depends on the particular fan.
AC Fans
AC fans are designed to operate from alternating-current supplies. They are commonly used in larger equipment and electrical enclosures.
AC fans can be designed for different supply voltages and frequencies, so the fan's rating must always be checked before connection.
Brushless DC Fans
Most modern electronic cooling fans use brushless DC motors rather than traditional brushed motors.
A brushless motor uses electronic commutation instead of mechanical brushes and a commutator.
DC Supply
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Electronic Controller
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Motor Windings
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Permanent Magnet Rotor
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Fan Blades
Why Brushless Fans Are Common
- No mechanical brushes
- Long operating life
- Low mechanical wear
- Good efficiency
- Compact construction
- Suitable for continuous operation
Axial Fans
An axial fan moves air approximately parallel to the axis of rotation.
Airflow
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┌─────────┐
│ FAN │
│ ↻ │
└─────────┘
│
Rotation
Axis
Axial fans are widely used for electronics cooling because they can move a large amount of air through relatively open systems.
Centrifugal Fans
Centrifugal fans, often called blowers, draw air into the center of an impeller and discharge it approximately perpendicular to the inlet direction.
Air Out
↑
│
┌────┴────┐
│ Impeller│
Air ───►│ ↻ │
In └─────────┘
Blowers are useful where higher static pressure is required.
Axial Fan vs Blower
| Feature | Axial Fan | Centrifugal Blower |
|---|---|---|
| Air direction | Approximately parallel to shaft | Approximately radial |
| Typical airflow | High | More pressure-oriented |
| Typical application | Open ventilation | Ducts and restricted airflow paths |
| Common form | Fan | Blower |
Fan Motor
The motor converts electrical energy into mechanical rotation.
In a brushless DC fan, an internal electronic controller switches the motor windings in the correct sequence.
Fan Motor Windings
Brushless fan motors typically contain multiple stator windings. Electronic switching energizes the windings in sequence to rotate the magnetic rotor.
Electronic Controller
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├── Winding A
├── Winding B
└── Winding C
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Magnetic Rotor
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Rotation
Fan Bearings
The bearings support the rotating shaft and allow the rotor to turn smoothly.
Common bearing arrangements include:
- Sleeve bearings
- Ball bearings
- Other specialized bearing systems
Sleeve Bearing Fans
Sleeve-bearing fans use a sleeve bearing around the rotating shaft.
They are generally economical and are widely used in applications where cost is important.
Ball Bearing Fans
Ball-bearing fans use rolling bearings to support the rotor.
They are commonly selected for applications requiring long operating life or operation at elevated temperatures, depending on the specific fan design.
Fan Blades
The blades transfer mechanical energy from the motor into airflow.
Blade geometry affects:
- Airflow
- Static pressure
- Noise
- Efficiency
- Power consumption
Fan Airflow
Airflow describes the volume of air moved by the fan over a period of time.
Fan specifications may express airflow using units such as CFM or liters per second.
A higher airflow rating does not necessarily mean a fan will perform better in every application.
Static Pressure
Static pressure indicates how effectively a fan can maintain airflow when the airflow path provides resistance.
Applications involving filters, heatsinks, ducts or restricted openings may require a fan with suitable static-pressure capability.
Airflow vs Static Pressure
A fan's airflow and pressure capability should be considered together.
Open Air Path
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High airflow requirement
Restricted Air Path
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Higher static-pressure requirement
The correct fan depends on the resistance of the cooling system.
Fan Speed
Fan speed is normally expressed in revolutions per minute, or RPM.
Increasing fan speed can increase airflow, but it can also increase noise and power consumption.
Fan Noise
Fan noise can come from several sources:
- Air turbulence
- Blade design
- Motor noise
- Bearing noise
- Mechanical vibration
- Resonance of the enclosure
A larger fan can sometimes provide the required airflow at a lower speed, reducing noise.
Fan Vibration
An unbalanced fan can produce mechanical vibration.
Possible causes include:
- Damaged blade
- Dust buildup
- Worn bearing
- Loose mounting
- Distorted fan housing
Vibration can transfer mechanical energy into the equipment enclosure and produce additional noise.
2-Wire DC Fan
A basic two-wire DC fan normally has:
Red → Positive Black → Ground
However, wire colors are not universal. Always verify the fan manufacturer's wiring information.
3-Wire Fan
A three-wire fan commonly provides power, ground and a tachometer signal.
A typical arrangement is:
VCC GND TACH
The exact pinout must be verified for the particular fan.
4-Wire PWM Fan
A four-wire fan commonly provides:
1. Ground 2. +V 3. Tachometer 4. PWM Control
The exact pin order depends on the fan and connector standard.
The PWM input allows the fan speed to be controlled electronically.
Fan Tachometer Signal
A tachometer output allows the controller to determine the fan's rotational speed.
The controller can use the signal to detect whether the fan is rotating and estimate its RPM.
Fan
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└── TACH ─────► Controller
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RPM
Fan PWM Control
PWM, or Pulse Width Modulation, can be used to control the speed of compatible fans.
The controller changes the PWM control signal to request a different fan speed.
Controller │ │ PWM ▼ Fan Controller │ ▼ Motor Speed
Fan Speed Control
Fan speed can be controlled using several methods depending on the fan design.
- PWM control
- Supply-voltage control for suitable fans
- Electronic motor control
- Temperature-based control
The control method must be compatible with the fan.
Temperature-Controlled Fan
Electronic equipment can automatically control fan speed according to temperature.
Temperature Sensor │ ▼ Controller │ ▼ Fan Speed Control │ ▼ Fan │ ▼ Temperature decreases
This allows the fan to operate more slowly when the equipment is cool and increase speed when additional cooling is required.
Thermal Fan Control
A simple thermal-control system can use a temperature sensor and a controller to determine when the fan should start.
More advanced systems can vary the fan speed continuously according to temperature.
Fan Voltage
A fan must be operated at its specified voltage.
Applying excessive voltage can damage the motor or internal electronics.
Operating below the specified voltage may cause:
- Reduced speed
- Failure to start
- Unstable operation
- Reduced airflow
Fan Current
The fan's current rating indicates the approximate current required under its specified operating conditions.
The power supply and switching circuit should be capable of supplying the required current.
Fan Power Consumption
For a DC fan, approximate electrical power can be calculated as:
P = V × I
where P is power, V is voltage and I is current.
The actual electrical and mechanical efficiency depends on the fan design.
Fan Starting Current
Some fans can draw a different current during startup compared with steady-state operation.
The power supply and control circuit should therefore be capable of handling the fan's startup requirements.
Fan Protection
A suitable protection circuit can protect the fan driver and power supply from abnormal conditions.
Protection requirements depend on the fan architecture and controller design.
Fan Filter
A filter can prevent dust from entering an electronic enclosure.
However, the filter also adds resistance to airflow.
A clogged filter can substantially reduce cooling performance.
Dust and Fans
Dust accumulation can reduce cooling performance and increase fan mechanical problems.
Dust can:
- Block airflow
- Coat heatsinks
- Increase bearing contamination
- Unbalance fan blades
- Increase operating temperature
Fan Maintenance
Maintenance can include:
- Inspecting the fan
- Removing accumulated dust
- Checking airflow
- Checking unusual noise
- Checking vibration
- Inspecting connectors
- Checking mounting screws
The appropriate cleaning procedure depends on the equipment and fan construction.
Fan Testing
A DC fan can be tested by verifying its supply voltage and observing whether the motor starts correctly.
For a basic two-wire fan:
- Verify the rated voltage.
- Disconnect the fan from the equipment if appropriate.
- Connect it to a suitable power supply.
- Observe whether the blades start rotating.
- Listen for abnormal noise.
- Check for excessive vibration.
Do not exceed the manufacturer's voltage rating.
Testing Fan Current
A multimeter or suitable current meter can be used to measure fan current.
The measurement method must be appropriate for the meter and circuit. Incorrectly connecting a current meter can short a power supply.
Testing Fan Tachometer
A tachometer output can be checked using an oscilloscope, logic analyzer or suitable controller input.
The signal frequency can be related to fan rotational speed according to the fan's tachometer specification.
Fan Troubleshooting
If a fan does not operate, check the system in the following order:
- Check the supply voltage.
- Check the connector.
- Check polarity where applicable.
- Check the fan wiring.
- Check the control signal.
- Check for mechanical obstruction.
- Check the fan current.
- Test the fan from a suitable independent supply if appropriate.
Fan Does Not Start
Possible causes include:
- No supply voltage
- Incorrect polarity
- Insufficient supply voltage
- Failed motor
- Failed internal controller
- Blocked blades
- Damaged bearing
- Incorrect PWM control
Fan Starts and Stops
Intermittent operation can be caused by:
- Unstable power supply
- Loose connector
- Thermal protection
- Damaged motor electronics
- Worn bearings
- Incorrect control signal
Fan Is Noisy
Possible causes include:
- Worn bearings
- Dust buildup
- Damaged blades
- Mechanical vibration
- Loose mounting
- Air turbulence
Fan Spins Slowly
Possible causes include:
- Low supply voltage
- Incorrect PWM control
- Worn bearings
- Mechanical obstruction
- Excessive dust
- Motor or controller fault
Fan Draws Excessive Current
An unusually high current can indicate:
- Mechanical obstruction
- Bearing failure
- Damaged motor windings
- Faulty electronic controller
- Incorrect operating voltage
Do not continue operating a fan that is overheating or drawing abnormally high current.
Fan Replacement
When replacing a fan, do not select a replacement based only on its physical dimensions.
Check:
- Voltage
- Current
- Fan size
- Mounting-hole spacing
- Thickness
- Airflow
- Static pressure
- Connector
- Number of wires
- Rotation direction
- Airflow direction
- Noise level
Fan Airflow Direction
Fans normally have a defined direction of airflow.
Many fans have arrows on the housing indicating:
- Direction of rotation
- Direction of airflow
Installing a fan in the wrong direction can reduce the cooling performance of the equipment.
Fan Size
Fan dimensions are normally specified by the physical size of the housing.
Common electronic cooling fan sizes include various compact and larger formats.
The replacement must fit the available mounting space.
Fan Thickness
Fans with the same length and width can have different thicknesses.
The thickness affects the available space, airflow characteristics and sometimes the motor and bearing design.
Fan Mounting
Fans are commonly mounted using screws or other mechanical fasteners.
The mounting system should keep the fan secure without distorting its housing.
Fan Connector
Electronic fans can use different connectors.
Before replacing a fan, verify:
- Connector type
- Number of contacts
- Pinout
- Wire arrangement
- Voltage
A connector that physically fits is not necessarily electrically compatible.
Fan Selection
- Determine the required cooling capacity.
- Determine the available mounting dimensions.
- Determine the supply voltage.
- Determine the required airflow.
- Determine the required static pressure.
- Check the electrical current.
- Choose the required bearing type.
- Check the connector and control method.
- Consider noise requirements.
- Check operating temperature.
- Verify airflow direction.
Fan Selection for Amplifiers
Power amplifiers can generate significant heat, particularly when operating at high output power.
Fan selection should consider:
- Heatsink size
- Amplifier power
- Expected heat generation
- Airflow path
- Heatsink resistance
- Enclosure size
- Noise requirements
Cool Air │ ▼ Fan │ ▼ Heatsink │ ▼ Hot Air │ ▼ Outside Enclosure
Push-Pull Cooling
Some equipment uses one fan to bring cool air into the enclosure and another to exhaust warm air.
Cool Air │ ▼ [ Intake Fan ] │ ▼ Components │ ▼ [ Exhaust Fan ] │ ▼ Hot Air
The effectiveness depends on the complete airflow path and the resistance of the enclosure.
Positive and Negative Air Pressure
Multiple fans can be arranged to create different airflow-pressure conditions inside an enclosure.
The objective should be to create a predictable airflow path through the heat-producing components.
Fan Faults
| Symptom | Possible Cause |
|---|---|
| Fan does not start | No power, failed motor, controller fault or obstruction |
| Fan spins slowly | Low voltage, bearing problem or control problem |
| Fan is noisy | Worn bearing, dust, vibration or damaged blades |
| Fan starts and stops | Power, thermal, controller or connection problem |
| Fan draws high current | Mechanical obstruction, bearing or motor fault |
| Low airflow | Wrong fan, blocked filter, dust or restricted airflow path |
| Excessive vibration | Unbalanced blades, bearing failure or loose mounting |
Advantages of Fans
- Simple cooling solution
- Relatively low cost
- Available in many sizes
- Low-voltage models are widely available
- Can provide substantial heat removal
- Speed can be controlled electronically on suitable models
- Easy to replace
Limitations of Fans
- Produce mechanical and aerodynamic noise.
- Consume electrical power.
- Moving parts eventually wear.
- Dust can reduce performance.
- Fan failure can cause equipment overheating.
- Airflow depends on enclosure design.
- High airflow does not always mean high static-pressure capability.
Common Fan Design Mistakes
- Selecting a fan based only on physical size
- Ignoring static pressure
- Ignoring airflow direction
- Using the wrong voltage
- Connecting a PWM fan incorrectly
- Ignoring startup current
- Blocking the intake or exhaust
- Ignoring dust filters
- Failing to provide a complete airflow path
- Installing the fan in the wrong direction
- Ignoring fan noise
Key Points
- Fans remove heat by forcing air through or across electronic equipment.
- Most modern electronic cooling fans use brushless DC motors.
- Axial fans move air approximately parallel to the shaft.
- Centrifugal blowers are useful where higher static pressure is required.
- Fan performance depends on airflow, static pressure, speed and the complete cooling system.
- Two-wire fans generally provide power connections only.
- Three-wire fans commonly add a tachometer output.
- Four-wire fans commonly add PWM speed control.
- Dust, worn bearings and damaged blades can reduce fan performance.
- Fan replacement requires checking voltage, size, airflow, pressure, connector and mounting dimensions.
- Temperature-controlled fan systems can automatically adjust cooling according to equipment temperature.
- Always verify the fan's electrical specifications before connecting it to a circuit.