What are the applications of brushless motors?

What are the applications of brushless motors?

DC motors have been widely used in the field of motion control due to their excellent torque characteristics. However, ordinary DC motors require mechanical commutation and brushes, resulting in poor reliability and frequent maintenance; Electromagnetic interference and high noise are generated during commutation, which affects the further application of DC motors in control systems. In order to overcome the drawbacks of mechanical commutation, brushless motors that replace mechanical commutation with electronic commutation have emerged. In 1955, D. Harrison and others in the United States first applied for a patent to replace mechanical brushes with transistor commutation circuits, marking the birth of modern brushless motors. The brushless DC motor with electronic commutation truly entered the practical stage in 1978 with the introduction of the classic MAC brushless DC motor and its driver. Afterwards, in-depth research was conducted on brushless DC motors internationally, resulting in the development of square wave brushless motors and sine wave DC brushless motors. Over the past 20 years, with the development of permanent magnet new materials, microelectronics technology, automatic control technology, and power electronics technology, especially high-power switching devices, brushless motors have made significant progress. Brushless DC motor no longer specifically refers to DC motors with electronic commutation, but generally refers to electronic commutation motors with external characteristics of brushed DC motors [1].  

Brushless DC motors not only maintain the good dynamic and static speed regulation characteristics of traditional DC motors, but also have a simple structure, easy operation, and are easy to control. Its application has rapidly expanded from the initial military industry to aerospace, medical, information, home appliances, and industrial automation fields.  

Structurally, unlike brushed DC motors, brushless DC motors use the stator winding as the armature and the excitation winding is replaced by permanent magnet materials. According to the different waveforms of the current flowing into the armature winding, brushless DC motors can be divided into square wave DC motors (BLDCMs) and sine wave DC motors (PMSMs). BLDCMs replace the mechanical commutation of the original DC motor with electronic commutation and use permanent magnet materials as rotors, eliminating the need for electric brushes; PMSM replaces the excitation winding in the synchronous motor rotor with permanent magnet materials, eliminating the need for excitation winding, slip ring, and electric brush. Under the same conditions, it is relatively easy for the driving circuit to obtain square waves and the control is simple, so the application of BLDCM is much more extensive than PMSM [2].  

A brushless DC motor is generally composed of three parts: an electronic commutation circuit, a rotor position detection circuit, and the motor body. The electronic commutation circuit is generally composed of a control part and a drive part, while the detection of the rotor position is generally completed by a position sensor. During operation, the controller triggers various power transistors in the drive circuit in an orderly manner based on the position sensor's measurement of the motor rotor position, and performs orderly commutation to drive the DC motor [3]. This article analyzes the development of brushless motors in three parts.  

Development status of each component
2.1 Motor body

Brushless DC motors have similar electromagnetic structures to brushed DC motors, but their armature windings are placed on the stator, and the weight, simplified structure, and improved performance of the rotor are used to enhance their reliability. The development of brushless motors is inseparable from the development of permanent magnet materials. The development process of magnetic materials has basically gone through the following stages: aluminum nickel cobalt, ferrite magnetic materials, and neodymium iron boron (NdFeB). Neodymium iron boron has a high magnetic energy product, and its emergence has sparked a revolution in magnetic materials. The application of third-generation neodymium iron boron permanent magnet materials has further reduced the copper consumption of motors, promoting the development of brushless motors towards high efficiency, miniaturization, and energy conservation [4].  

At present, in order to improve the power density of electric motors, transverse magnetic field permanent magnet motors have emerged. The stator slots and armature coils are perpendicular to each other in spatial position, and the main magnetic flux in the motor flows along the axial direction of the motor. This structure improves the air gap magnetic density and can provide much larger output torque than traditional motors [5]. This type of motor is currently in the research and development stage.  

2.2 Electronic commutation circuit
Control circuit: Brushless DC motors control the speed, direction, torque, and protection of the motor by controlling the power switching devices in the drive circuit, including overcurrent, overvoltage, overheating, and other protections. The control circuit initially used analog circuits, which made the control relatively simple. If circuits are digitized, many hardware tasks can be directly completed by software, which can reduce hardware circuits, improve their reliability, and enhance the anti-interference ability of control circuits. Therefore, control circuits have evolved from analog circuits to digital circuits

At present, control circuits generally come in three forms: application specific integrated circuits, microprocessors, and digital signal processors. In situations where the requirements for motor control are not high, using professional integrated circuits to form control circuits is a simple and practical method; Due to the fast computation, few peripheral circuits, simple and reliable system composition of digital signal processors, the composition of DC brushless motors is greatly simplified, and their performance is greatly improved, which is conducive to the miniaturization and intelligence of motors. Therefore, digital signal processors are the direction of control circuit development [6].  

Drive circuit: The drive circuit outputs electrical power, drives the armature winding of the motor, and is controlled by the control circuit. The driving circuit is composed of high-power switching devices. It is precisely because of the emergence of thyristors that DC motors have made a leap from brushed to brushless. However, due to the fact that thyristors are semi controlled switching devices that only have the ability to control turn-on and no self turn off capability, their switching frequency is low and cannot meet the further improvement of the performance of brushless DC motors. With the rapid development of power electronics technology, fully controlled power switching devices have emerged, including switchable transistors (GTOs), power field-effect transistors (MOSFETs), metal gate bipolar transistor (IGBT) modules, integrated gate commutated thyristors (IGCTs), and recently developed electron injection enhanced gate transistors (IEGTs) [7]. With the continuous improvement of the performance of these power devices, the corresponding brushless motor drive circuit has also achieved rapid development. At present, fully controlled switching devices are gradually replacing ordinary thyristors with complex circuits, large volumes, and low functional indicators. The driving circuit has shifted from a linear amplification state to a pulse width modulation switching state, and the corresponding circuit composition has also been transformed from power transistor discrete circuits to modular integrated circuits, creating conditions for intelligent, high-frequency, and miniaturized driving circuits.  

2.3 Rotor Position Detection Circuit

Permanent magnet brushless motor is a closed-loop electromechanical integrated system, which uses the rotor magnetic pole position signal as the commutation signal of the electronic switch circuit. Therefore, accurately detecting the rotor position and switching the power devices in a timely manner based on the rotor position is the key to the normal operation of brushless DC motor.  

Using position sensors as rotor position detection devices is the most direct and effective method. Generally, position sensors are installed on the shaft of the rotor to achieve real-time detection of the rotor position. The earliest position sensors were magneto electric, which were bulky and complex and have been phased out; At present, magnetic sensitive Hall position sensors are widely used in brushless DC motors, and there are also photoelectric position sensors. The presence of position sensors increases the weight and structural dimensions of brushless DC motors, which is not conducive to the miniaturization of the motors; During rotation, the sensor is inevitably worn and difficult to maintain; Meanwhile, the installation accuracy and sensitivity of sensors directly affect the operational performance of the motor; On the other hand, due to too many transmission lines, it is easy to introduce interference signals; Due to the hardware signal acquisition, the system's reliability is further reduced. In order to adapt to the further development of brushless motors, position sensors have emerged. They generally use the induced back electromotive force of the armature winding to indirectly obtain the position of the rotor magnetic poles. Compared with direct detection methods, position sensors are omitted, simplifying the structure of the motor body, achieving good results, and have been widely used. But for sensorless brushless motors that detect the position of the * back electromotive force, how to start smoothly is a problem that needs to be solved because no back electromotive force is generated when the motor is stationary.  

In recent years, a new sensorless brushless motor has been proposed. It does not use back electromotive force to detect the rotor position, but instead uses non-magnetic conductive materials attached to the rotor surface to utilize the eddy current effect on the non-magnetic material during the operation of the stator winding high-frequency switch. The magnitude of the open circuit phase voltage changes with the rotor position, and the rotor position can be determined by detecting the open circuit phase voltage. This sensorless brushless motor overcomes the starting and low-speed operation problems of general sensorless brushless motors, but this method requires special motors and places high demands on the manufacturing process of the motor [8].  

3 issues to be researched

3.1 Torque pulsation

At present, the main problem with brushless DC motors is the presence of torque ripple. Due to torque pulsation, the application of brushless DC motors in AC servo systems is limited, especially in direct drive applications, where torque pulsation deteriorates the speed control characteristics of the motor. Especially for brushless DC motors used in audiovisual equipment, film machinery, and computers, they require smooth operation and no noise. Therefore, suppressing or eliminating torque ripple has become the key to improving the performance of servo systems.  

The main causes of torque ripple are: cogging effect and torque ripple caused by magnetic flux distortion; Torque ripple caused by harmonics; Due to the influence of the equivalent inductance of the armature, torque ripple is caused by commutation current. At present, various universities and research institutions have conducted in-depth research on the problem of torque ripple, and proposed various methods to suppress or weaken torque ripple based on different causes, which have improved the performance of brushless motors to varying degrees. However, these studies have proposed some weakening or compensation methods based on the original structure and scheme, without fundamentally eliminating torque pulsation in principle. Therefore, the pulsation of torque still needs further research.  

3.2 Rotor position detection without position sensor

The main methods for detecting rotor position without position sensors include back electromotive force method, freewheeling diode method, inductance method, and state observation method. Among them, the back electromotive force method is the most common and widely used method. However, this method is based on neglecting the armature reaction, and there are errors in principle. For high-power brushless motors, the influence of armature reaction on air gap magnetic density is more obvious, and the error is also greater. On the other hand, during start-up and low-speed operation, the back electromotive force of the motor is zero or very small, making it difficult to detect the rotor position through the back electromotive force. Brushless motors without position sensors have start-up problems [9]. Therefore, it is urgent to solve how to compensate for the rotor position signal error caused by the back electromotive force method in high-power brushless motors, and how to overcome the starting problem of the motor in the back electromotive force method. For startup issues, it is generally recommended to start using other methods first and then switch to the operation method without position sensors.  

4 Development directions of brushless DC motors
With the development of electronic technology and control technology, position detection can be achieved through chips combined with appropriate algorithms. The emergence of high-speed microprocessors, DSP devices, and specialized control chips has greatly improved operating speed and processing capabilities. The inherent computing power of DSP can be used to achieve sensorless control on brushless motors [10]. The use of DSP to implement sensorless control has become a hot research topic, and low-cost DSP sensorless brushless motors have become the development direction of brushless DC motors.