1 Introduction

DC motors have been widely adopted in motion control systems due to their excellent torque characteristics. Nevertheless, conventional DC motors rely on mechanical commutation and brushes, resulting in poor reliability and frequent maintenance requirements. Electromagnetic interference and noise generated during commutation restrict their further application in control systems. To tackle the drawbacks brought by mechanical commutation, brushless motors utilizing electronic commutation instead of mechanical commutation have emerged.In 1955, D. Harrison et al. from the United States filed the first patent for replacing mechanical brushes with transistor commutation circuits, marking the birth of modern brushless motors. It was not until the launch of the classic MAC brushless DC motor and its driver in 1978 that electronically commutated brushless DC motors entered practical application. Since then, in-depth research on brushless DC motors has been carried out globally, leading to the development of square-wave brushless motors and sine-wave brushless DC motors.Over the past two decades, driven by advances in new permanent magnet materials, microelectronics, automatic control technology and power electronics (especially high-power switching devices), brushless motors have achieved substantial progress. The term brushless DC motor no longer exclusively refers to DC motors with electronic commutation; it generally denotes electronically commutated motors featuring the external characteristics of brushed DC motors [1].

Brushless DC motors retain the favorable static and dynamic speed regulation performance of traditional DC motors, and possess a simple structure, reliable operation and easy controllability. Their application scope has expanded rapidly from the initial military industry to aerospace, medical equipment, information technology, home appliances and industrial automation.

Structurally different from brushed DC motors, a brushless DC motor uses stator windings as armature, while excitation windings are replaced by permanent magnet materials. According to the waveform of current flowing into armature windings, brushless DC motors are classified into square-wave brushless DC motors (BLDCM) and sine-wave permanent magnet synchronous motors (PMSM).BLDCM adopts electronic commutation to replace mechanical commutation in traditional DC motors, and uses permanent magnets for the rotor to eliminate brushes. PMSM replaces the excitation windings on the rotor of synchronous motors with permanent magnets, removing excitation windings, slip rings and brushes. Under identical operating conditions, it is easier to generate square waves with simpler control via drive circuits. Therefore, BLDCM has far broader applications than PMSM [2].

A typical brushless DC motor consists of three parts: electronic commutation circuit, rotor position detection circuit and motor body. The electronic commutation circuit generally comprises a control unit and a drive unit, and rotor position detection is mostly realized by position sensors. During operation, the controller sequentially triggers each power transistor in the drive circuit according to the rotor position measured by position sensors to perform orderly current commutation and drive the DC motor [3]. This paper analyzes the development of brushless motors from the perspective of these three components.

2 Development Status of Each Component

2.1 Motor Body

Electromagnetically, brushless DC motors are fundamentally similar to brushed DC motors. However, armature windings are placed on the stator, and permanent magnets are adopted for the rotor. This reduces rotor weight, simplifies the structure, improves performance and enhances reliability.The development of brushless motors is inseparable from advances in permanent magnet materials. The evolution of magnetic materials has roughly gone through three stages: Alnico, ferrite magnetic materials, and neodymium iron boron (NdFeB). Featuring high magnetic energy product, NdFeB triggered a revolution in magnetic materials. The application of third-generation NdFeB permanent magnets further cuts copper consumption of motors, driving brushless motors toward higher efficiency, miniaturization and energy conservation [4].

At present, transverse flux permanent magnet motors have been developed to boost motor power density. The stator slots and armature coils are spatially perpendicular to each other, and the main magnetic flux inside the motor flows along the axial direction. This structure increases air-gap magnetic flux density and can deliver much higher output torque than traditional motors [5]. This type of motor is still under research and development.

2.2 Electronic Commutation Circuit

Control CircuitA brushless DC motor regulates rotating speed, rotation direction, torque and implements protection functions (overcurrent, overvoltage, overheating protection, etc.) by controlling power switching devices in the drive circuit. Early control circuits adopted analog circuits with relatively simple control functions. Digitalized circuits enable many hardware functions to be realized by software, reducing hardware circuits, improving reliability and enhancing anti-interference capability. Accordingly, control circuits have evolved from analog circuits to digital circuits.

Currently, control circuits are mainly implemented in three forms: application-specific integrated circuits (ASIC), microprocessors and digital signal processors (DSP). For scenarios with low motor control requirements, control circuits built with dedicated integrated circuits offer a simple and practical solution. Benefiting from fast computing speed, few peripheral circuits, simple and reliable system configuration, DSPs greatly simplify the overall structure of brushless DC motors and improve performance, facilitating motor miniaturization and intellectualization. Hence, DSP represents the development trend of control circuits [6].

Drive CircuitThe drive circuit outputs electric power to drive the armature windings of the motor and is governed by the control circuit. It is composed of high-power switching devices. The emergence of thyristors enabled the transition from brushed DC motors to brushless DC motors. Nevertheless, thyristors are half-controlled switching devices that can only be turned on without self-turn-off capability, featuring low switching frequency which restricts further performance improvement of brushless DC motors.With the rapid advancement of power electronics technology, fully-controlled power switching devices have emerged, including gate turn-off thyristors (GTO), power metal-oxide-semiconductor field-effect transistors (MOSFET), insulated gate bipolar transistor (IGBT) modules, integrated gate-commutated thyristors (IGCT), and the newly developed injection-enhanced gate transistors (IEGT) in recent years [7]. Continuous performance improvement of these power devices has driven rapid development of corresponding drive circuits for brushless motors.At present, fully-controlled switching devices are gradually replacing conventional thyristors with complicated circuits, large volume and low performance indicators. Drive circuits have transitioned from linear amplification mode to pulse-width modulation (PWM) switching mode. Meanwhile, discrete power transistor circuits have been replaced by modular integrated circuits, laying a foundation for intellectualization, high-frequency operation and miniaturization of drive circuits.

2.3 Rotor Position Detection Circuit

Permanent magnet brushless motors are closed-loop mechatronic systems. Rotor pole position signals serve as commutation signals for electronic switching circuits. Therefore, accurate detection of rotor position and timely switching of power devices based on rotor position are critical for stable operation of brushless DC motors.

Installing position sensors on the rotor shaft to realize real-time rotor position detection is the most direct and effective solution. The earliest magnetoelectric position sensors were bulky and complicated and have been phased out. At present, Hall-effect magnetic sensors are widely applied in brushless DC motors, together with photoelectric position sensors.The adoption of position sensors increases the weight and overall dimension of brushless DC motors, hindering miniaturization. Sensors suffer from wear during rotation and are difficult to maintain. Besides, the mounting precision and sensitivity of sensors directly affect motor operating performance. In addition, numerous signal wires tend to introduce interference, and hardware signal acquisition reduces system reliability.To accommodate further development of brushless motors, sensorless control technology has been developed. It generally detects rotor pole position indirectly by utilizing back electromotive force (back-EMF) induced in armature windings. Compared with direct detection methods using sensors, sensorless control eliminates physical position sensors, simplifies the motor structure, achieves favorable effects and has been widely applied. However, sensorless brushless DC motors relying on back-EMF detection cannot generate back-EMF at standstill, so smooth startup remains a key challenge to be solved.

In recent years, a novel sensorless brushless motor scheme has been proposed. Instead of detecting rotor position via back-EMF, non-magnetic conductive materials are attached to the rotor surface. When stator windings operate under high-frequency switching, eddy current effects occur in non-magnetic conductive materials, causing the voltage of open-phase windings to vary with rotor position. Rotor position can be judged by detecting open-phase voltage. This sensorless brushless motor overcomes the startup and low-speed operation difficulties of conventional sensorless brushless motors, yet it requires specially customized motors and imposes stringent requirements on motor manufacturing processes [8].

3 Research Challenges

3.1 Torque Ripple

The most prominent problem existing in brushless DC motors is torque ripple. Torque ripple limits their application in AC servo systems. Especially in direct-drive applications, torque ripple deteriorates motor speed control performance. For brushless DC motors used in audio-visual equipment, film machinery and computer equipment, smooth operation and low noise are highly demanded. Therefore, suppressing or eliminating torque ripple is the key to improving servo system performance.

The main causes of torque ripple include: torque ripple induced by cogging effect and magnetic flux distortion; torque ripple caused by harmonics; commutation torque ripple resulting from equivalent armature inductance.Universities and research institutions have carried out extensive research on torque ripple and proposed multiple suppression and mitigation methods targeting different causes, improving the performance of brushless motors to varying degrees. However, these solutions only weaken or compensate torque ripple based on existing structures and schemes, without fundamentally eliminating torque ripple from the mechanism. Further research on torque ripple is still required.

3.2 Sensorless Rotor Position Detection

Major sensorless rotor position detection approaches include the back-EMF method, freewheeling diode method, inductance method and state observer method. Among them, the back-EMF method is the most common and widely adopted. However, this method is established on the assumption of negligible armature reaction, which inherently introduces theoretical errors. For high-power brushless motors, armature reaction exerts a more obvious influence on air-gap flux density, leading to larger detection errors.Furthermore, back-EMF is zero or extremely weak during motor startup and low-speed operation, making rotor position detection difficult. Startup difficulty remains a major obstacle for sensorless brushless motors [9].Hence, it is urgent to find solutions to compensate rotor position signal errors generated by the back-EMF method in high-power brushless motors and resolve startup problems. For startup issues, a common strategy is to start the motor via alternative methods before switching to sensorless operation.

4 Development Trend of Brushless DC Motors

With advances in electronic and control technology, rotor position detection can be realized by microchips combined with appropriate algorithms. The emergence of high-speed microprocessors, DSPs and dedicated control chips greatly enhances operation speed and processing capacity. The inherent computing power of DSPs can be utilized to implement sensorless control on brushless motors [10].Sensorless control based on DSP has become a research hotspot. Low-cost DSP-based sensorless brushless DC motors represent the mainstream development direction of brushless DC motors.