Understanding Position Sensing in Brushless DC Motors

Brushless DC (BLDC) motors are widely used in various applications because of their high efficiency and long life. Unlike brushed DC motors, BLDC motors do not have mechanical brushes and commutators. Instead, they rely on electronic commutation.

Electronic commutation requires knowing the exact position of the rotor (shaft) at all times. This information is crucial for the electronic controller to energize the correct stator windings at the right sequence and timing. Without accurate position information, the motor cannot generate continuous torque and rotate smoothly.

How is Shaft Position Determined?

Let's look at the options provided:

• Three-phase inverter: The three-phase inverter is the part of the electronic controller that switches the current to the motor's stator windings based on the control signals. It does not determine the shaft position.
• Electronic controller: The electronic controller is the brain that processes information and controls the motor. It receives position feedback and uses it to decide when and how to switch the inverter. However, it is the device *providing* the position information that determines the position, not the controller itself.
• Permanent magnets: Permanent magnets are part of the rotor in a BLDC motor. They create the magnetic field that interacts with the stator field to produce torque. While their position defines the rotor position, the magnets themselves do not actively sense or report their position.
• Hall effect sensor: Hall effect sensors are electronic components that detect the presence and strength of a magnetic field. In many BLDC motors, these sensors are placed strategically around the stator. As the permanent magnets on the rotor pass by these sensors, the sensors detect the magnetic field changes and output signals. These signals correspond to specific rotor positions or sectors.

Therefore, the Hall effect sensor is commonly used to determine the position of the shaft (rotor) in brushless DC motors. This position feedback allows the electronic controller to perform the necessary electronic commutation.