How Does a BLDC Motor Work: A Detailed Explanation of the Principles of Brushless DC Motors

Over the past two to three decades, owing to the continuous advancement of electronic technology and permanent magnet materials, BLDC motors have transformed from niche motors to mainstream power solutions for various industries. BLDC motors have been widely used in applications, including household appliances, power tools, electric vehicles, and industrial automation equipment due to their high efficiency, high reliability, and long lifespan. Yet, the operating principles of BLDC motors are still not known to lots of individuals. This article will explain whatever you need to realize about the working principle of BLDC motors so that you can get a more detailed view of this incredible innovation in present-day electrical engineering.

The Basic Structure of a BLDC Motor

Now, Before getting into the working principle, we should familiarize ourselves with its basic structure:

Stator: Fixed inside the motor housing, it is generally made up of laminated silicon steel sheets, and coils are wound around it to create several electromagnet poles.

Rotor: The rotor is usually mounted on the shaft and contains permanent magnets. BLDC motors can be classified into several varieties based on the various configurations of the permanent magnets.

Hall sensors: For determining the position of the rotor and the feedback to the electronic control system.

Controller: An electronic control system that determines the timing of the current that flows through the stator windings based on the feedback from the Hall sensors or other feedback mechanisms.

It is worth noting that the outrunner BLDC motor structure is unique from the inner rotor structure. The rotor is outside, and the stator is inside. This type of design is especially prevalent in applications that necessitate high torque, like in the drive of drone propellers.

The Principles of Electromagnetism: The Foundation of BLDC Motor Operation

Two basic principles of electromagnetism determine the operation of a BLDC motor:

Ampere's law: When a current-carrying conductor is in a magnetic field, the conductor will be subjected to a force. The stator of a BLDC motor is constructed of coiled copper windings, and after these windings are energized, they create a magnetic field that interacts between the rotor's permanent magnets to generate a force that provides the mechanical movement for the rotor to turn.

Faraday's law of electromagnetic induction: An electromotive force will be induced in a conductor when it cuts magnetic field lines or is in a changing magnetic field. It is widely used in sensorless control, where the rotor position is detected by sensing the back electromotive forces.

These principles are applied in the design of industrial brushless motors. They accomplish this by finely regulating the current flowing through the stator windings, ensuring favorable electromagnetic interactions with the rotor's permanent magnets to attain high efficiency and stability.

The Commutation Process of a BLDC Motor

The most important working mechanism in the BLDC motor is called electronic commutation. It is the most important and key distinction factor between brushless motors and traditional brushed motors.

Principle of Commutation: Commutation is the process of changing the direction of the current flow in regards to the coils of the motor so that there can exist continuous electromagnetic interaction between the rotor and stator, which creates continuous torque.

Six-step commutation: The traditional control method of BLDC motors divides the electrical cycle into six steps. In every step, two out of the three-phase-windings are powered, while one phase is off.

Hall sensor feedback: Hall sensors detect the position of the rotor's permanent magnets, so that the controller can determine which winding should be energized and the current direction.

Sensorless control: A more advanced method is to determine the position of the rotor by monitoring the back electromotive force in the un-energized phase, no Hall sensors are needed to implement this complex strategy, resulting in greater reliability of the drive.

Big BLDC motors usually adopt more complex control strategies, such as sine wave drive or vector control, to achieve smoother torque output and higher efficiency.

The Role of the Controller in a BLDC Motor System

BLDC motor cannot work on its own and needs to be combined with a specialized electronic controller:

The motor driver: Usually a three-phase bridge inverter based on power MOSFET or IGBT that switches the current path according to the control signal

Microcontroller: Receives signals from the position sensor, performs control algorithms, and generates PWM signals to drive the power devices.

Closed-loop control: Provides accurate speed control or position control based on application needs.

Safe protection function: over-current protection, over-temperature protection, under-voltage protection, etc.

Applications for 48V BLDC motor systems are widespread in electric bicycles, small electric vehicles, and some other industrial applications. Its controller and handling circuitry need to deal with higher voltages and currents and usually have a more complex set of performance and protection functions.

The Performance Characteristics and Application Scenarios of BLDC Motors

Now let's take a look at their performance advantages after learning BLDC motor operation principles:

High efficiency: Due to the absence of friction losses from brushes and commutators, their efficiency is typically above 85%, even above 95% in some cases.

Good speed-torque characteristics: Provides a wide range of torque output.

Greater lifespan: Without mechanical wear elements, the lifespan is only ultimately constrained by the bearings.

Improved heat dissipation efficiency: Direct contact between the stator windings and the motor housing for more effective heat dissipation.

Low noise and electromagnetic interference: There are no sparks and noise generated by brush commutation.

Because of its special structure, the outrunner BLDC motor is more capable of providing higher torque at low speeds, making them especially suitable for direct drive systems, like drone propellers and fans, etc. With the advantages of stability and durability, the brushless motor makes it the choice of power supply in automation equipment and precision instruments.

The Control Algorithms of BLDC Motors

The more developed modern BLDC control technology has surpassed this simple six-step commutation method:

Trapezoidal wave control: The most basic control method is a trapezoidal current waveform. It is simple to put into practice but produces torque ripples of considerable amplitude.

Sinusoidal control: driving the motor by sinusoidal current, which can reduce the motor torque ripple and make the running smoother.

Field-oriented Control (FOC): By applying a mathematical transformation, the 3-phase current is translated to the rotating coordinate system for control, leading to optimal torque control and energy efficiency.

Sensor fusion technology: Integrates multiple feedback signals (e.g., Hall sensors, encoders, and current sampling) for better control accuracy and robustness.

Due to the challenges in high-power applications, such as thermal management, efficiency optimization and dynamic response, more sophisticated control algorithms are typically used for Big BLDC motors.

Conclusion: The Working Principles and Advantages of BLDC Motors

BLDC motors precisely control the current in the stator windings through an electronic control system and interact with the rotor's permanent magnets to achieve efficient conversion of electrical energy into mechanical energy. From small consumer electronic devices to large industrial equipment, from low-voltage applications to 48V BLDC motor systems, brushless DC motors, with their high efficiency, high reliability, and excellent control performance, are driving technological progress and energy efficiency improvement in various industries. With the continuous development of electronic control technology and permanent magnet materials, we can foresee that BLDC motors will have a broader application prospect and more outstanding performance.

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