Brushless direct current electric motors, or BLDC motors for short, are electronically commutated motors powered by a DC electric source via an external motor controller. Unlike their brushed relatives, BLDC motors rely on external controllers to achieve commutation. Put simply, commutation is the process of switching the current in the motor phases to generate motion. Brushed motors have physical brushes to achieve this process twice per rotation, while BLDC motors do not, hence the name. Due to the nature of their design, they can have any number of pole pairs for commutation.
Motor commutation
Before delving too far into feedback options for BLDC motors, it is important to understand why they are necessary. BLDC motors come in single phase, 2-phase, and 3-phase configurations; the most common configuration being 3-phase. The number of phases matches the number of windings on the stator while the rotor poles can be any number of pairs depending on the application. Because the rotor of a BLDC motor is influenced by the revolving stator poles, the stator pole position must be tracked in order to effectively drive the 3 motor phases. Hence, a motor controller is used to generate a 6-step commutation pattern on the 3 motor phases. These 6-steps, or commutation phases, move an electromagnetic field which causes the permanent magnets of the rotor to move the motor shaft.
Position feedback
Since the inception of the brushless dc motor, Hall-effect sensors have been the workhorse for commutation feedback. For 3-phase control, only three sensors are required, and with a very low per-unit cost, they are easily the most economical option to achieve commutation from a pure BOM cost perspective. Hall sensors are embedded into the stator of the motor to detect rotor positon, which is used to switch the transistors in the 3-phase bridge to drive the motor. The three Hall-effect sensor outputs are commonly noted as the U, V, and W channels. While Hall sensors are an effective solution for commutating BLDC motors, they only address half the needs of a BLDC system.
How to Use BLDC Hall Sensors as Position Encoders
How to Use Hall Effect To Drive Brushless DC Motor
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