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What is the working principle of a BLDC motor for an indoor air-conditioning unit?
Release Date:
2025-12-15
The core principle of the BLDC motor used in the indoor unit of an air conditioner is electronic commutation replacing mechanical brushes, combined with the interaction between the permanent magnet field and the stator winding field, to achieve continuous rotor rotation and drive the cross-flow fan to complete air circulation.
The core principle of the BLDC motor used in the indoor unit of an air conditioner is electronic commutation that replaces mechanical brushes. By leveraging the interaction between the permanent magnet field and the stator winding field, the rotor rotates continuously, driving the cross-flow fan to circulate air. Specifically, this can be divided into two parts: the structural foundation and the operating process.
I. Core Structural Foundations
A BLDC motor consists mainly of four parts: stator, rotor, position sensor, and electronic commutator (driver). The structure is the prerequisite for realizing its working principle:
Stator: It is assembled by laminating silicon steel sheets and has multiple sets of symmetrically arranged windings (typically three-phase windings) wound on its surface. When DC is applied, a rotating magnetic field is generated.
Rotor: It incorporates permanent magnets—typically made of neodymium-iron-boron—to generate a stable permanent magnetic field, thereby replacing the rotor windings found in conventional brushed DC motors.
Position sensors: Mostly Hall sensors, mounted on the stator to continuously detect the magnetic pole positions of the rotor’s permanent magnets and feed the position signals back to the electronic commutator. Some high-end models employ sensorless control, estimating the rotor position by monitoring the back EMF in the windings.
Electronic commutator: Equivalent to the “brain” of the motor, it comprises power switching devices (such as IGBTs and MOSFETs) and control circuitry, responsible for receiving sensor signals and adjusting the energization sequence and current magnitude of the stator windings.
II. Complete Operational Process
Power-On Initialization of the Air-Conditioner Indoor Unit: Upon power-up, the electronic commutator first receives the speed command from the controller (such as the fan-speed signal from the thermostat), while the Hall sensor detects the initial magnetic pole position of the rotor and transmits this position data to the commutator.
When the stator windings are energized, a directional magnetic field is generated. The electronic commutator, based on the initial position of the rotor, controls the switching of the power semiconductor devices to apply DC current to a designated set of stator windings, thereby establishing a directional magnetic field in the stator that is oriented at an angle relative to the rotor’s permanent-magnet field. According to the principle that like poles repel and unlike poles attract, the rotor’s permanent magnets begin to rotate in the direction that aligns them with the stator’s magnetic field under the repulsive or attractive forces exerted by the stator field.
Real-time electronic commutation maintains continuous rotation: as the rotor reaches the next preset position, the Hall sensor instantly detects the change in magnetic polarity and sends a feedback signal to the commutator. The commutator then promptly switches the energization sequence of the stator windings—e.g., cycling from Phase A to Phase B to Phase C—thereby causing the stator magnetic field to rotate in synchrony. This process replaces the mechanical brush commutation used in conventional brushed motors; with no physical contact, it eliminates brush wear and friction-induced noise.
The air-conditioning controller precisely regulates the fan speed by sending speed-control signals—typically 0–10 V analog signals or PWM pulse signals—to the electronic commutator in response to indoor temperature demands (such as changes in cooling or heating load). The commutator adjusts the magnitude of the stator winding current, thereby altering the strength of the stator magnetic field and controlling the rotor’s rotational speed, enabling stepless speed regulation—for example, smoothly ramping from a low-speed setting of 300 rpm to a high-speed setting of 1200 rpm.
Shutdown or Standby: When the air conditioner receives a shutdown command, the electronic commutator disconnects power to the stator windings, causing the stator magnetic field to collapse and the rotor to gradually decelerate and come to a stop due to inertia. In standby mode, the motor maintains only a very low power consumption, while the sensor continuously monitors the rotor position in real time, remaining ready at all times to respond to a start-up command.
III. Key Considerations for Compatibility with the Air Conditioner Indoor Unit
The load on the through-flow fan of the indoor unit of an air conditioner is relatively stable. Based on the above principle, a BLDC motor can operate efficiently at low loads (e.g., low-speed operation during the constant-temperature stage, with energy consumption reduced by more than 30% compared with AC motors). In addition, since there is no brush friction, the operating noise can be kept below 20 dB(A), making it suitable for quiet environments such as bedrooms and studies.
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