AC Input Electronically Commutated (EC) Fans

Electronically Commutated (EC) fans are used to create a forced air flow in equipment to aid in thermal management which in turn extends equipment life. There are AC and DC input fan types available. This article will focus on the AC input versions, highlighting their features and benefits.

Figure 1: An example of an electronically commutated fan.

A little motor background

Every electric motor is caused to turn by the interaction of two magnetic fields, one on the rotor and one on the stator, influencing each other. Different motor types create and control these fields differently.

Brushed DC motors

Brushed DC motors use a series of coils on the rotor to generate a magnetic field when a DC input is applied via mechanical carbon contacts (brushes) and a commutation ring. The field generated by the rotor interacts with permanent magnets embedded in the stator to cause the rotor to turn. However, as the rotor turns, the brushes mechanically wear and sparking occurs. Both of these actions will eventually degrade the brushes to a point where they will need to be replaced in order for the motor to operate normally again. Also, being DC driven, an external motor driver will be necessary to run this type of motor from AC mains. This adds complexity and cost to a design.

Single-phase induction motors

Single-phase induction motors, of which shaded-pole and permanent-split capacitor (PSC) types are commonly used for fans, use a series of stator coils controlled by an AC input to create a magnetic field. This stator field electromagnetically induces a rotor field which will interact with the stator field to turn the rotor.

Since induction motors are AC driven, they are designed to operate at peak efficiency at one point on their performance curve. The operating efficiency can decrease drastically when operating on either side of this operating point which makes them limited by synchronous speed requirements and susceptible to voltage fluctuations.

Electronically commutated (EC) motors

EC motors (also referred to as brushless DC, or BLDC, motors) are electric motors with permanent magnets embedded in the rotor. To get these motors to turn, a magnetic field is generated by a series of coils in the stator and caused to commutate via electronic circuitry. This commutating magnetic field interacts with the permanent magnets on the rotor in a way that causes the rotor to turn and follow the magnetic field of the stator.

Since EC motors have no brushes, they avoid the sparking and brush wear of brushed motors, resulting in longer motor life. Also, with electronic commutation of the stator and the unnecessary requirement to waste power to induce a rotor field, they provide superior controllability and performance over induction motors. Additionally, traditional induction motors typically have a smaller operating range than EC motors which exhibit an almost flat efficiency curve that has relatively little variance across their rated speed range. This allows one EC motor to replace a number of induction motor models, decreasing the number of different fan models required by a typical customer. This also means that EC fan product lines don’t have to include as many motor models as their induction counterparts to cover the same applications.

Today, EC motors are used in many fractional-horsepower applications where high motor efficiency, reliability, and controllability are desired, such as fans.

Powering EC fans

For many years there was the need for a separate DC power supply in brushless DC motors. This introduced added complexity and cost in applications using only an AC supply. However, EC fans which could be powered directly from AC mains with integrated electronics became available in the early 2000s. The electronics in these fans converted the AC to DC, performed the commutation, and controlled the fan speed by regulating the power to the motor - typically with pulse-width modulation (PWM).

The efficiency of EC fans

DC motors, which include EC fan motors, exhibit about 30% better efficiency than AC motors since the permanent magnets generate the secondary magnetic field rather than copper windings. AC motors consume additional energy solely in creation of the magnetic field induced by a current in the rotor. Of course, fan efficiency will vary by manufacturer along with the power rating and load conditions. A general rule of thumb is that shaded-pole fans have a 15-25% efficiency range and PSC fans range from 30-50% efficiency while EC fans typically achieve efficiencies of 60-75+%.

Keep in mind that when the speed of a motor doubles, the power required increases by a factor of eight, making it very inefficient to operate a fan faster than is required by the application. Since an EC fan motor’s functionality is controlled by software, users can optimize operation of a fan to match the application needs which can potentially save a lot of energy. In addition to optimizing fan speed, features like data communications, constant volume control, variable speed, etc. can be incorporated into the EC fan application as needed.

A byproduct of having a high efficiency is that EC motors run cooler than their comparable counterparts. This can dramatically reduce the heat produced when compared to non-EC fans for similar applications. A result of this is that the fan(s) will not have to do extra work to remove the heat produced by the fan(s) itself in heat removal applications. Therefore, EC fans can aid in improving efficiency for the entire system.

Another benefit of the motors running cooler is that it improves the lifespan of the highly loaded motor parts including the bearings and windings, resulting in less lifetime maintenance.

Finally, since EC fan motor control does not require TRIACs or frequency inverters, there are no high pitched droning noises. An EC fan motor under speed control operation is practically silent.

Conclusion

Electronically commutated fans offer many benefits over the comparable induction and brushed DC motor-driven versions. The primary advantage is their high energy efficiency over their entire range of operation. This improves not only the fan’s energy performance, but the entire system’s efficiency. Other benefits include longer lifetimes, lower overall system maintenance, and quieter operation. It’s no wonder that EC fans are becoming the go-to fan type for new, energy-efficient designs.