Fan Features

This is a subsidiary page to the Fan Selection & Application Guide, which elaborates on the different values for the Features attribute used to characterize AC and DC fans at Digi-Key.

The image below illustrates the relative number of times that a given feature is listed for AC and DC fans sold by Digi-Key, at the time of writing. The lack of any specific enumerated features is the most common by far, with DC fans being roughly twice as likely as their AC counterparts to have some listed feature.

Fan feature sets and the documentation thereof are not especially well-standardized industry-wide, and as a result the feature enumerations presented are best interpreted loosely; some particularly common features or behaviors may be cited as a feature by one manufacturer, but not another.


Alarm features in a context of fans generally refer to a signal-level output that indicates presence of a fault, failure, or over/under threshold condition of some form. Most commonly amounting to a locked rotor sensor by a different name, different functions and implementations are possible; consult the manufacturer’s documentation for information on device-specific behaviors.

Auto Restart

An auto restart feature indicates that a device will attempt to resume operation following clearance of a fault such as a locked rotor or over-temperature condition, as opposed to a latching self-protection behavior requiring supply power to be removed and reapplied in order to restart the device. It is an extremely common feature, to the extent that it may be incorporated into a device whether enumerated as a feature or not.


A “capacitor” feature attaches to a device whose datasheet makes specific mention of a capacitor of some form. It most frequently refers to an AC-input fan incorporating a permanent split capacitor (A.K.A capacitor run) induction motor, but is occasionally found in other contexts.

An example of a typical fan based on a PSC motor is shown above, with the capacitor highlighted. The construction of a typical PSC motor is simpler than that of an electronically commutated type though the result is typically less energy efficient. In general, PSC motors exhibit the same higher efficiency/higher complexity tradeoff when compared to shaded pole motors, which are a third type commonly used for small AC fans.

Depending on initial product quality, temperature, and other application factors, there is a potential for the capacitor in a PSC motor to fail prior to the bearing system in a fan. Though often more readily user-replaceable than a bearing system, this failure mechanism should be taken under advisement if maintenance-free service life is a matter of interest.

Conformal Coating

A conformal coating feature in a fan indicates that the electrical/electronic portions thereof have been coated with an environmentally resistant sealant material in order to reduce the risk of failure caused by atmospheric contaminants. It should be noted that not all devices incorporating comparable protective measures will specifically list it as a feature. In particular, devices with an IP rating indicating protection against moisture ingress are likely to incorporate a conformal coating if not more extensive protective measures.

Dual Speed

A dual speed fan feature is not particularly common, but is more so among AC-input devices and refers to a device designed to operate at either of two nominal rotational speeds, often selected by switching which two among 3 input leads the AC supply voltage is connected to.

An example of such is shown above, with a diagram on the label indicating the different connections for speed selection highlighted.

Dual Voltage

A dual voltage fan feature refers to an AC-input fan that capable of operating from either of two different nominal supply voltages, such as 115VAC and 230VAC. This traditionally implies a fan based on induction motor technology with two sets of windings that can be connected either in series or parallel, to facilitate operation from either of two nominal supply voltages. Rated voltages for such units are commonly listed in the form “X/Y VAC” indicating that the applied voltage should be within some limited range of either X or Y for proper operation. Such devices are commonly equipped with four supply leads, and an example of such is shown above; note the wiring diagram on the label indicating the necessary connections for the two configurations.

More modern devices based on electronically-commutated motors may also list a dual voltage feature to highlight compatibility with different nominal input voltages. Such devices are typically equipped with only two supply leads, and since many are capable of proper operation from a supply voltage anywhere within a wide range, their rated voltage is often listed in the format “X~Y VAC”

Electronic Commutation

An electronic commutation feature indicates that a fan’s motor is based on permanent magnet AC, brushless DC, or similar architecture that relies on the use of electronic means to create rotation, rather than more traditional motor architectures that rely on the use of brushes or the oscillatory nature of AC current to achieve the same goal. Nearly all DC-input fans in current production are of this sort so the feature is rarely mentioned in that context, but for AC fans it is a substantial distinction since it enables stable fan performance despite wide variations in supply voltage and frequency, greater flexibility with regard to speed control, and higher operating efficiencies. Rated voltages for such devices are commonly listed in the form “X~Y VAC” implying operability at any point within the indicated span, rather than within a limited range of the listed values as is the case with devices based on induction motors with a dual voltage feature.

An example of an electronically commutated AC fan is shown above. Compared with the dual-voltage example shown previously, only two lead wires are provided; no changes in wiring configuration are required for operation from a supply voltage anywhere in the 115~230VAC range.

Fan Performance Sensor

A fan performance sensor feature refers to a signal level status output found on select devices manufactured by Comair Rotron, designed to provide a logic-compatible output signal that indicates whether or not fan speed is in excess of some pre-determined level.

The excerpt below from the datasheet for the FPS-equipped MC24B3NDN describes the behavior of this output, with the fan speed at which the output changes logic state highlighted.



A GR-487 fan feature indicates that a device has been tested in accordance with some portion of the Telcordia GR-487 standard, most typically in regards to corrosion resistance in a salt fog environment. It is used more typically in context of DC than AC fans.


A kit feature indicates a fan that is sold co-packaged or pre-assembled with accessory equipment commonly sold separately, such as filters, louvers, guards, power adapters, mounting brackets, etc. Examples of two products sold in kit form are shown below.

Locked Rotor Protection

A locked rotor protection feature indicates that a fan’s motor structure is protected to some degree against a condition where the rotor is physically prevented from rotating, such as by interference from a foreign object or bearing failure. The degree of protection afforded may range from minimal (e.g. “fan may be rendered inoperable by a sustained fault, but is not expected to catch fire”) to comprehensive (e.g. “locked rotor condition may be maintained indefinitely with no ill effect to the device”). It should be noted that this concept of protection is distinct from one of notification indicated by a locked rotor sensor feature. While a locked rotor sensor feature is an option, locked rotor protection of some form is a nearly universal safety feature likely to be found on any small fan, regardless of whether it is enumerated as a feature or not.

Locked Rotor Sensor

A locked rotor sensor feature indicates that a fan provides some form of status output signal that indicates whether or not the fan’s rotor is being physically prevented from moving, such as by interference from a foreign object or bearing failure. This concept of providing notification of a locked-rotor condition is distinct from protection against such a condition; a device equipped with a locked rotor sensor may conceivably be reliant on external equipment to monitor the status signal provided and disconnect the power source to prevent damage. The distinction between locked rotor and speed sensor outputs is that the former maintains a stable logic level during operation and cannot be used to determine actual fan speed.

Permanent Split Capacitor Motor

A permanent split capacitor motor is a form of single-phase AC induction motor that uses a permanently-connected capacitor (as opposed to one that is disconnected after startup) to create a phase-shifted version of the input voltage, which is used in conjunction with an auxiliary winding to improve starting torque and operation over a range of rotational speeds. When listed as a feature, it indicates a fan that is based on such a motor architecture. Note that in many cases, this is functionally equivalent to a “capacitor” feature.

PWM Control

A PWM control feature indicates a variable-speed fan controlled by means of a low-level input signal, typically a square wave, with fan speed being proportional to the duty cycle of the signal applied.

The excerpt from the 9GA0612P6S001 spec sheet below characterizes the relationship between input signal duty cycle and resulting rotational speed for that device. The dashed portion of the chart indicates a region where device behavior becomes less predictable and in which operation is not recommended.

See the section on electrical interfacing for more information on typical implementations and characteristics of PWM control inputs.

Reverse Air Flow

A reverse air flow feature indicates a fan designed to move air in the opposite direction with respect to its mechanical construction than is typical for type. Most commonly this is applied in context of tubeaxial fans that intake, rather than exhaust air across their struts. While not particularly common, it’s a feature easily confused with the concept of reversible flow

Reversible Flow

A reversible flow feature is applied to fans having an ability to change the direction of air movement through the device. Though not common, it’s easily confused with the concept of a reverse air flow feature.

The excerpts from the 9RF0924P1H001 datasheet below illustrate how this example integrates control of a reversible flow feature with a speed control feature by adjusting the customary interpretation of the applied control signal.

Salt Fog Rated

A fan described as salt fog rated has been tested or qualified to some level regarding performance in a saltwater fog environment, such as is outlined in in the Telcordia GR-487 standard. It is used more commonly in context of AC than DC fans.

Speed Sensor (Tach)

A speed sensor or tachometer feature provides a low-level output signal in some format that correlates with a fan’s rotational speed, commonly as a square wave making one full cycle for every half-rotation of the fan rotor. It differs from a locked rotor signal in that it has no stable logic level during normal operation, and thus is less convenient when simple detection of a locked-rotor condition is the objective. See the section on electrical interfacing for more information on typical implementations.

Thermal Overload Protection

Fans with thermal overload protection incorporate measures to provide a level of fail-safe behavior in the event of an over-temperature condition. More commonly listed in context of AC-input fans, a similar feature in context of DC fans is usually described as “locked rotor protection.” The basic objective for both is to mitigate risk of fire should a locked rotor condition occur with rated voltage applied, however the concept of thermal overload protection is more general and encompasses thermal overloads occurring from other causes such as high ambient temperature.

Exact implementations and behaviors can vary; some devices may incorporate a one-time fuse that renders a device inoperable once tripped, others a thermo-mechanical switch having a finite operational life, and others an electronic protection mechanism capable of nearly limitless operational cycles.

Another thermal protection technique that is often not enumerated as such is the concept of impedance protection; devices protected in this way are designed so that the combined inductance and resistance of the motor windings is sufficient to limit current flow to a value that does not produce an unsafe rise in device temperature when rated voltage is applied. Importantly, this protection mechanism only works when ambient temperatures are below some specified maximum value.

Thermistor/ Thermistor, External

A thermistor fan feature refers to the integration of resources needed for temperature-based speed control using a thermistor temperature sensor, either incorporated within the device itself enabling local temperature measurement, or connected externally to allow temperature measurement at a remote location.

Devices designed for use with external thermistors are often characterized in terms of their behavior when used with a specified thermistor assembly, as in the example shown below. Use of this suggested assembly is not strictly necessary; other thermistor devices with similar characteristics should yield similar behaviors.