BLDC Motor Commutation: A Better Way!

Electronically commutated motors powered by a direct current electric source through an external motor controller are known as brushless direct current electric motors or brushless DC motors. BLDC motors require an external controller to provide commutation for the process of switching current in the motor phases to produce motion, in contrast to their brushed siblings. 

In contrast to BLDC motors, which do not have actual brushes to accomplish this operation twice per spin, brushed motors can have any number of pole pairs for commutation because of the nature of their architecture.  

Basics of BLDC Motor Commutation  

A 3-phase BLDC motor design is the most typical. While the rotor poles might have any number of pairs depending on the application, the number of phases corresponds to the number of stator windings. To efficiently operate the three motor phases of brushless DC motors, the stator pole position must be tracked because the rotor is affected by the rotating stator poles.   

A motor controller is utilized to create a 6-step commutation pattern on the three motor phases. Through these six processes, also known as the commutation phases, an electromagnetic field is moved, which causes the rotor's permanent magnets to move the motor shaft.  

Since the invention of the brushless motor, hall effect sensors have been a common choice for commutation feedback. The controller must constantly know the rotor's position accurately to commutate the motor. For three-phase control, three sensors are often needed.   

To detect the rotor position and switch the transistors in the three-phase bridge to operate the motor, the Hall effect sensors are built within the motor's stator. Common names for the three sensor outputs include U, V, and W channels. Unfortunately, this approach to position input has certain shortcomings. Despite the cheap Hall effect sensor BOM cost, the expense of incorporating these sensors into the brushless DC motors can more than double the motor's overall cost.   

Additionally, the Hall effect sensors only provide the controller with a partial view of the motor's location, which can be problematic in systems that depend on accurate position feedback to function effectively.  

Encoders Offer Higher Precision  

Today, systems that use BLDC motors require a higher position measuring precision than ever before. In addition to Hall effect sensors, the BLDC motor can also be connected with incremental encoders to achieve this. This technique offers better position feedback, but the motor manufacturer now requires adding the incremental encoder and both Hall sensors after assembly. Better yet, omit the Hall effect sensors entirely and switch to a commutation encoder instead of the incremental one.  

Conclusion  

When given a tight control loop and highly accurate position-sensing feedback, brushless DC motors continue to expand in popularity and can excel in various applications. Due to their low BOM costs, hall effect sensors have long been the preferred option. However, they frequently need an incremental encoder to provide a complete image of a motor's position.