Brushless DC motors (BLDC) overcome the inherent defects of brushed DC motors by replacing the mechanical commutator with an electronic commutator. Therefore, brushless DC motors possess both the excellent speed control performance of DC motors and the advantages of AC motors, such as simple structure, no commutation sparks, reliable operation, and ease of maintenance. Essentially, a brushless DC motor is a three-phase AC permanent magnet synchronous motor with rotor position feedback, using a DC power input and an electronic inverter to convert it to AC. It offers significant performance advantages over traditional DC motors and is currently the most ideal speed-regulating motor.

  I. Introduction to Brushed DC Motors

  Before discussing brushless DC motors, let's examine brushed motors:

  DC motors are renowned for their excellent starting performance and speed regulation capabilities. Among them, brushed DC motors utilize mechanical commutators, simplifying the drive method. Their schematic model is shown below.

  An electric motor mainly consists of a stator made of permanent magnet material, a rotor (armature) with coil windings, a commutator, and brushes. As long as a certain DC current is applied to the A and B terminals of the brushes, the commutator of the motor will automatically change the direction of the magnetic field of the motor rotor, thus allowing the rotor of the DC motor to continue to rotate.

  DC motors present the following drawbacks:

  Relatively complex structure, increasing manufacturing costs;

  Susceptibility to environmental factors (e.g., dust), reducing operational reliability;

  Spark generation during commutation, limiting application scenarios;

  Prone to damage, increasing maintenance expenses.

  II. Introduction to Brushless DC Motors

  The “BL” in BLDC motors stands for “brushless,” signifying the absence of the “brushes” found in conventional DC (brushed) motors. Brushless DC motors (BLDC) replace mechanical commutators with electronic commutation. Consequently, they retain the excellent speed regulation characteristics of DC motors while also possessing the advantages of AC motors: simpler structure, absence of commutation sparks, reliable operation, and ease of maintenance.

  A brushless DC motor primarily consists of a rotor made of permanent magnet material, a stator with coil windings, and a position sensor (optional). As evident, it shares many similarities with DC motors: the stator and rotor structures are nearly identical (the original stator becomes the rotor, and vice versa), and the winding connections are largely the same.    However, a key structural difference exists: BLDC motors lack the commutator and brushes found in DC motors, replacing them with position sensors. This simplifies the motor's structure, reducing manufacturing and maintenance costs. However, brushless DC motors cannot automatically reverse polarity (phase), resulting in higher controller costs. For example, in a three-phase DC motor, a brushed DC motor's drive bridge requires 4 power transistors, while a brushless DC motor's drive bridge requires 6 power transistors.

  Brushless DC motors exhibit the following characteristics: They possess excellent external characteristics, delivering high torque at low speeds and enabling substantial starting torque.   They operate across a wide speed range, maintaining full power output at any speed. Their high efficiency and strong overload capacity ensure outstanding performance in drive systems. Excellent regenerative braking capability: The permanent magnet rotor allows the motor to function as a generator during braking. Compact size and high power density. No mechanical commutator and fully enclosed structure prevent dust ingress, ensuring high reliability. Simpler drive control compared to asynchronous motors.

  III. Working Principle of Brushless DC Motors

  The stator of a brushless DC motor consists of coil windings forming the armature, while the rotor comprises permanent magnets. If only a fixed DC current is applied, the motor generates a static magnetic field and fails to rotate. Only by continuously detecting the rotor's position and applying corresponding currents to different phases based on this position can the stator produce a uniformly rotating magnetic field. This enables the motor to rotate in sync with the magnetic field.

  The motor's stator windings are typically configured as a symmetrical three-phase star connection, closely resembling a three-phase asynchronous motor. Permanent magnets are bonded to the motor's rotor. To detect the rotor's polarity, a position sensor is installed inside the motor. The driver consists of power electronic components and integrated circuits, performing the following functions: Receiving position sensor signals and forward/reverse direction signals to control the switching of power transistors in the inverter bridge, generating continuous torque; Receiving speed command and speed feedback signals to regulate and adjust rotational speed; Providing protection and display functions, etc.

  DC motors offer rapid response, high starting torque, and the ability to deliver rated torque from zero to full speed. However, their strengths also represent limitations: to maintain constant torque under rated load, the armature field and rotor field must consistently maintain a 90° phase difference. This requires carbon brushes and a commutator. During motor rotation, carbon brushes and commutators generate sparks and carbon dust, leading to component damage and limiting application scenarios. AC motors lack these components, offering maintenance-free operation, robustness, and broad applicability. However, achieving performance comparable to DC motors requires complex control techniques. Rapid advancements in semiconductor technology have significantly increased switching frequencies in power components, enhancing motor drive performance. Microprocessors are also becoming increasingly faster, enabling the placement of AC motor control within a rotating two-axis Cartesian coordinate system. By appropriately controlling the AC motor's current components along both axes, performance comparable to DC motors can be achieved.

Furthermore, many microprocessors now integrate essential motor control functions directly onto the chip, with sizes becoming increasingly compact. such as analog-to-digital converters (ADC) and pulse width modulators (PWM). Brushless DC motors represent an application where AC motor commutation is electronically controlled, yielding characteristics similar to DC motors while avoiding the mechanical drawbacks inherent in DC motor designs.

  IV. Driving Methods of Brushless DC Motors Brushless DC motors can be driven in various ways, each with its own characteristics.

  1. By driving waveform: Square wave drive: This method is easy to implement and facilitates sensorless control of the motor.

  2. Sine wave drive: This method improves motor performance and produces uniform output torque, but its implementation is relatively complex. Furthermore, this method has two variations: SPWM and SVPWM (Space Vector PWM). SVPWM performs better than SPWM.