Views: 120 Author: Site Editor Publish Time: 2024-10-17 Origin: Site
The most basic motor is the “DC motor (brush motor)”. By placing a coil in a magnetic field and passing a flowing current through it, the coil will be repelled by the magnetic poles on one side and attracted by the other side at the same time, and will keep rotating under this action. During the rotation, the current flowing through the coil is reversed, causing it to rotate continuously. There is a part of the motor called the “commutator” that is powered by “brushes”, which are positioned above the “steering gear” and move continuously as it rotates. By changing the position of the brushes, the direction of the current can be changed. The commutator and brushes are indispensable structures for the rotation of a DC motor.
The commutator switches the flow of current in the coil, reversing the direction of the poles so that they always rotate to the right. The brushes supply power to the commutator which rotates with the shaft.
We have categorized motors by type of power supply and principle of rotation (Fig. 2). Let's take a brief look at the characteristics and uses of each type of motor.
DC motors (brushed motors), which are simple and easy to control, are often used for applications such as opening and closing of optical disk trays in home appliances. They are also used in automobiles for applications such as opening and closing of electric mirrors and direction control. Although it is inexpensive and can be used in many fields, it has its drawbacks. Since the commutator comes into contact with the brushes, it has a short life span and the brushes must be replaced periodically or under warranty.
A stepper motor will rotate with the number of electrical pulses sent to it. The amount of movement depends on the number of electrical impulses sent to it, making it suitable for position adjustment.
It is often used at home for “paper feeding of fax machines and printers”, etc. Since the feeding steps of a fax machine depend on the specifications (engraving, fineness), a stepping motor that rotates with the number of electrical impulses is very easy to use. It is easy to solve the problem that the machine stops temporarily once the signal stops. Synchronous motors, whose number of rotations varies with the frequency of the power supply, are used in applications such as “rotary tables for microwave ovens.
The motor set has a gear reducer to obtain the number of rotations suitable for heating food. Induction motors are also affected by the frequency of the power supply, but the frequency and the number of revolutions do not coincide. In the past, these AC motors were used in fans or washing machines.
As you can see, a wide variety of motors are active in several fields. What are the characteristics of BLDC motors (brushless motors) that make them so versatile?
The “BL” in BLDC motors means “brushless”, which means that the “brushes” in DC motors (brush motors) are no longer present. The role of brushes in DC motors (brush motors) is to energize the coils in the rotor through the commutator. So how does a BLDC motor without brushes energize the coils in the rotor? It turns out that BLDC motors use permanent magnets for the rotor, and there is no coil in the rotor. Since there are no coils in the rotor, there is no need for commutators and brushes for energizing the motor. Instead, the coil is used as the stator (Figure 3).
The magnetic field created by the fixed permanent magnets in a DC motor (brush motor) does not move and rotates by controlling the magnetic field created by the coil (rotor) inside it. The number of rotations is changed by changing the voltage. The rotor of a BLDC motor is a permanent magnet, and the rotor is rotated by changing the direction of the magnetic field created by the coils around it. The rotation of the rotor is controlled by controlling the direction and magnitude of the current flowing through the coils.
BLDC motors have three coils on the stator, each with two wires, for a total of six lead wires in the motor. In reality, only three wires are usually needed because they are internally wired, but it is still one more than the previously described DC motor (brushed motor). It won't move purely by connecting the positive and negative battery terminals. As to how to run a BLDC motor will be explained in the second installment of this series. This time we are going to focus on the advantages of BLDC motors.
The first characteristic of a BLDC motor is “high efficiency”. It is possible to control the rotational force (torque) to maintain the maximum value at all times, whereas with DC motors (brush motors), the maximum torque can only be maintained for a single moment during rotation, and the maximum value cannot be maintained at all times. If a DC motor (brush motor) wants to get as much torque as a BLDC motor, it can only increase its magnet. This is why even a small BLDC motor can produce a lot of power.
The second feature is “good controllability”, which is related to the first one. BLDC motorscan get the torque, number of revolutions, etc., exactly as you want them to be, and BLDC motors can feed back the target number of revolutions, torque, etc., precisely. Precise control suppresses heat generation and power consumption of the motor. In the case of battery drive, it is possible to extend the drive time by careful control. In addition to this, it is characterized by durability and low electrical noise. The above two points are the advantages brought by brushless.
On the other hand, DC motors (brushed motors) are subject to wear and tear due to the contact between the brushes and the commutator over a long period of time. The contacting part also generates sparks. Especially when the gap of the commutator touches the brush, there will be a huge spark and noise. If you do not want noise to be generated during use, a BLDC motor will be considered.
Where are BLDC motors with high efficiency, versatile handling, and long life generally used? They are often used in products that can utilize their high efficiency and long life and are used continuously. For example, home appliances. People have been using washing machines and air conditioners for a long time. Recently, BLDC motors have been adopted for electric fans, and have succeeded in dramatically reducing power consumption.
It is because of the high efficiency that the power consumption has been reduced. BLDC motors are also used in vacuum cleaners. In one case, by changing the control system, a large increase in the number of revolutions was realized. This example shows the good controllability of BLDC motors.
BLDC motors are also used in the rotating part of hard disks, which are important storage media. Since it is a motor that needs to run for a long time, durability is important. Of course, it also has the purpose of extremely suppressing power consumption. The high efficiency here is also related to the low consumption of electricity.
BLDC motors are expected to be used in a wider range of fields, and they will be used in a wide range of small robots, especially “service robots” that provide services in areas other than manufacturing. “Positioning is important for robots, so shouldn't we use stepping motors that run with the number of electrical pulses?” One might think so. However, in terms of force control, BLDC motors are more suitable. In addition, if stepper motors are used, a structure such as the robot's wrist needs to be supplied with a large amount of current in order to be fixed in a certain position. With BLDC motors, only the required power can be supplied in conjunction with an external force, thus curbing power consumption.
It can also be used in transportation. Simple DC motors have long been used in electric cars or golf carts for the elderly, but recently high-efficiency BLDC motors with good controllability have been adopted. BLDC motors are also used in drones. Especially in UAVs with multi-axis racks, since it controls the flight attitude by changing the number of rotations of the propellers, BLDC motors that can precisely control the rotations are advantageous.
How about it? BLDC motors are high quality motors with high efficiency, good control and long life. However, maximizing the power of BLDC motors requires proper control. How should it be done?
The inner rotor type BLDC motor is a typical type of BLDC motor, and its exterior and interior are shown below (Fig. 1). A brush DC motor (hereinafter referred to as a DC motor) has a coil on the rotor and a permanent magnet on the outside, while a BLDC motor has a permanent magnet on the rotor and a coil on the outside, and a BLCD motor has a permanent magnet without a coil on the rotor, so there is no need to energize the rotor. This makes it possible to realize a “brushless type” without brushes for energizing.
On the other hand, compared to DC motors, control becomes more difficult. It is not just a matter of connecting the cables of the motor to the power supply. Even the number of cables is different. It is not the same as “connecting the positive (+) and negative (-) terminals to the power supply”.
One coil is placed in the BLDC motor at 120 degree intervals, for a total of three coils, to control the current in the energized phase or coil
As shown in Figure 2-A, BLDC motors use three coils. These three coils are used to generate magnetic flux when energized and are named U, V, and W. Try energizing this coil. The current path on coil U (hereafter referred to as “coil”) is recorded as phase U, V is recorded as phase V, and W is recorded as phase W. Next, look at phase U. Let's take a look at phase U. When electricity is applied to the U phase, magnetic flux is generated in the direction of the arrow as shown in Figure 2-B. However, in reality, the U, V, and W phases are not the same as the U phase.
However, in reality, the cables of U, V, and W are all connected to each other, so it is not possible to energize only the U phase. Here, energizing from the U phase to the W phase will generate magnetic flux in U and W as shown in Fig. 2-C. The two magnetic fluxes of U and W are synthesized into the larger magnetic flux shown in Fig. 2-D. The permanent magnet will be rotated so that this synthesized magnetic flux is in the same direction as the N pole of the central permanent magnet (rotor).
Flux is energized from the U-phase to the W-phase. First, by focusing only on the U part of the coil, it is found that a magnetic flux is generated as in the arrows
Figure 2-D: Principle of rotation of a BLDC motor Passing electricity from phase U to phase W can be thought of as generating two magnetic fluxes synthesized
If the direction of the synthesized magnetic flux is changed, the permanent magnet is also changed. In conjunction with the position of the permanent magnet, switch the phase energized in the U-phase, V-phase, and W-phase to change the direction of the synthesized magnetic flux. If this operation is performed continuously, the synthesized magnetic flux will rotate, thereby generating a magnetic field and rotating the rotor.
FIG. 3 shows the relationship between the energized phase and the synthetic magnetic flux. In this example, by changing the energizing mode from 1-6 in sequence, the synthetic magnetic flux will rotate clockwise. By changing the direction of the synthesized magnetic flux and controlling the speed, the rotation speed of the rotor can be controlled. The method of controlling the motor by switching between these six energization modes is called “120-degree energization control”.
Figure 3: The permanent magnets of the rotor will rotate as if they were pulled by a synthetic magnetic flux, and the motor's shaft will rotate as a result
Next, although the direction of the synthesized magnetic flux is rotated under 120-degree energized control, there are only six different directions. For example, if you change the “energized mode 1” in Fig. 3 to “energized mode 2”, the direction of the synthetic magnetic flux will change by 60 degrees. The rotor will then rotate as if attracted. Next, by changing from “Energized mode 2” to “Energized mode 3”, the direction of the synthetic magnetic flux will change again by 60 degrees. The rotor will again be attracted to this change. This phenomenon will be repeated. The movement will become stiff. Sometimes this action will also make noise.
It is the “sine wave control” that eliminates the shortcomings of the 120-degree energized control and achieves smooth rotation. In 120-degree power control, the synthesized magnetic flux is fixed in six directions. It is controlled so that it varies continuously. In the example in Fig. 2-C, the fluxes generated by U and W are of the same magnitude. However, if the U-phase, V-phase, and W-phase can be better controlled, the coils can each be made to generate magnetic flux of different sizes, and the direction of the synthesized magnetic flux can be precisely controlled. By adjusting the current size of each of the U-phase, V-phase, and W-phase, a synthesized magnetic flux is generated at the same time. By controlling the continuous generation of this flux, the motor rotates smoothly.
Current on 3 phases can be controlled to generate synthetic magnetic flux for smooth rotation. Synthetic magnetic flux can be generated in a direction that cannot be generated by 120-degree energized control
What about the currents on each phase of U, V, and W? To make it easier to understand, think back to the 120-degree energized control and take a look. Look again at Fig. 3. In energized mode 1, current flows from U to W; in energized mode 2, current flows from U to V. As you can see, whenever the combination of coils in which current flows changes, the direction of the synthetic flux arrows also changes.
Next, look at energization mode 4. In this mode, current flows from W to U, in the opposite direction of energization mode 1. In DC motors, the switching of current direction like this is done by a combination of commutator and brushes. However, BLDC motors do not use such a contact type method. An inverter circuit is used to change the direction of the current. Inverter circuits are generally used to control BLDC motors.
The inverter circuit adjusts the current value by changing the applied voltage in each phase. For voltage adjustment, PWM (PulseWidthModulation = Pulse Width Modulation) is commonly used.PWM is a method to change the voltage by adjusting the length of time of the pulse ON/OFF, and what is important is the change of the ratio (duty cycle) between the ON time and the OFF time. If the ON ratio is high, the same effect as increasing the voltage can be obtained. If the ON ratio decreases, the same effect as voltage reduction is obtained (Fig. 5).
In order to realize PWM, microcomputers equipped with dedicated hardware are now available. To perform sine wave control it is necessary to control the voltages of 3 phases, so the software is slightly more complex than the 120 degree energized control where only 2 phases are energized. The inverter is a circuit necessary to drive a BLDC motor. Inverters are also used in AC motors, but it can be assumed that almost all BLDC motors are used in what are called “inverter-type” home appliances.
Figure 5: Relationship between PWM output and output voltage
Change the ON time at a certain time to change the RMS value of the voltage.
The longer the ON time, the closer the RMS value is to the voltage when 100% voltage is applied (ON time).
BLDC motors using position sensors The above is an overview of the control of BLDC motors, which change the direction of the synthesized magnetic flux generated by the coils, causing the permanent magnets of the rotor to change accordingly.
In fact, there is another point not mentioned in the above description. That is, the presence of sensors in BLDC motors. BLDC motors are controlled in conjunction with the position (angle) of the rotor (permanent magnet). Therefore, a sensor to acquire the position of the rotor is necessary. If there is no sensor to know the direction of the permanent magnet, the rotor may turn in an unexpected direction. This is not the case when there is a sensor to provide information.
Table 1 shows the main types of sensors for position detection in BLDC motors. Depending on the control method, different sensors are needed. For 120-degree energization control, a Hall effect sensor that can input a signal every 60 degrees is equipped to determine which phase is to be energized. On the other hand, for “vector control” (described in the next section), which precisely controls the synthesized magnetic flux, high-precision sensors such as corner sensors or photoelectric encoders are more effective.
The use of these sensors makes it possible to detect position, but there are some drawbacks. The sensors are less resistant to dust and maintenance is essential. The temperature range over which they can be used is also reduced. The use of sensors or the addition of wiring for this purpose causes costs to rise, and high-precision sensors are inherently expensive. This led to the introduction of the “sensorless” method. It does not use a sensor for position detection, thus controlling costs and eliminating the need for sensor-related maintenance. However, for the purpose of illustrating the principle, it is assumed that the information has been obtained from the position sensor.
Sensor Type | Main Applications | Characteristics |
Hall effect sensor | 120 degree energized control | Acquires signal every 60 degrees. Lower price. Not heat resistant. |
Optical Encoder | Sine wave control, vector control | There are two types: incremental type (the distance traveled from the original position is known) and absolute type (the angle of the current position is known). The resolution is high, but dust resistance is weak. |
Angle Sensor | Sine wave control, vector control | High resolution. Can be used even in rugged and harsh environments. |
Table 1: Types and Characteristics of Sensors Specialized for Position Detection
Sine wave control smoothly changes the direction of the synthesized magnetic flux by energizing 3 phases, so the rotor will rotate smoothly. 120-degree energization control switches 2 of the U-phase, V-phase, and W-phase to rotate the motor, whereas sinusoidal control requires precise control of the currents in the 3 phases. Moreover, the control value is an AC value that changes all the time, making it more difficult to control.
This is where vector control comes in. Vector control simplifies control by calculating the AC values of the three phases as the DC values of the two phases through coordinate transformation. However, vector control calculations require rotor position information at high resolution. There are two methods for position detection, namely the method using position sensors such as photoelectric encoders or corner sensors, and the sensorless method that extrapolates the current values of each phase. This coordinate transformation allows direct control of the current value associated with the torque (rotational force), thereby realizing efficient control without excess current.
However, vector control requires coordinate transformation using trigonometric functions or complex calculation processing. Therefore, in most cases, microcomputers with high computational power are used as control microcomputers, such as microcomputers equipped with FPUs (floating point units).
A brushless DC motor (BLDC: BrushlessDirectCurrentMotor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a type of synchronous motor that uses a direct current (DC) power supply.
A brushless DC motor (BLDC: Brushless Direct Current Motor) is essentially a permanent magnet synchronous motor with position feedback that uses a DC power input and an inverter to convert it to a three-phase AC power supply. A brushless motor (BLDC: Brushless DirectCurrent Motor) is a self-commutated type (self-direction switching) and is therefore more complex to control.
https://www.holrymotor.com/brushless-motors.html
BLDC motor (BrushlessDirectCurrentMotor) control requires knowledge of the rotor position and mechanism by which the motor is rectified and steered. For closed-loop speed control, there are two additional requirements, a measurement of the rotor speed/ or motor current and a PWM signal to control the motor speed power.
BLDC motors (BrushlessDirectCurrentMotor) can use either side-aligned or center-aligned PWM signals depending on the application requirements. Most applications requiring only speed change operation will utilize six separate side aligned PWM signals. This provides the highest resolution. If the application requires server positioning, energy braking, or power reversal, the supplemental center-aligned PWM signals are recommended.
To sense rotor position, BLDC motors (BrushlessDirectCurrentMotor) use Hall effect sensors to provide absolute position sensing. This results in the use of more wires and higher cost. Sensorless BLDC control eliminates the need for Hall sensors and instead uses the motor's counter electromotive force (electromotive force) to predict rotor position. Sensorless control is critical for low-cost variable speed applications like fans and pumps. Sensorless control is also required for refrigerator and air conditioning compressors when BLDC motors (Brushless Direct Current Motors) are used.
There are all kinds of motors, and the BLDC motor is the most ideal speed motor available today. It combines the advantages of DC motors and AC motors, with the good adjustment performance of DC motors and the advantages of AC motors such as simple structure, no commutation spark, reliable operation and easy maintenance. Therefore, it is very popular in the market and widely used in automobile, home appliances, industrial equipment and other fields.
Brushless DC motor overcomes the inherent defects of brush DC motor and replaces the mechanical commutator with electronic commutator, so brushless DC motor has the characteristics of DC motor with good speed regulation performance, and also has the advantages of AC motor with simple structure, no commutation sparks, reliable operation and easy maintenance.
Brushless DC motor (BrushlessDirectCurrentMotor) is the most ideal speed control motor today. It combines the advantages of DC motors and AC motors, with the good adjustment performance of DC motors and the advantages of AC motors, such as simple structure, no commutation sparks, reliable operation and easy maintenance.
Brushless DC motors are developed on the basis of brush motors, and their structure is more complex than brush motors. Brushless DC motor consists of motor body and driver. Different from brushed DC motor, Brushless DC motor (BrushlessDirectCurrentMotor) does not use mechanical brush device, but adopts square-wave self-control permanent magnet synchronous motor, and replaces carbon brush commutator with Hall sensor, and uses neodymium-iron-boron as the permanent magnet material of rotor. (It should be noted that at the time of the birth of the electric motor in the last century, the practical motors that arose were of the brushless form.)
Early models of the electric motor first appeared in the 1740s through the work of Scottish scientist Andrew Gordon. Other scientists, such as Michael Faraday and Joseph Henry, continued to develop early motors, experimenting with electromagnetic fields and discovering how to convert electrical energy into mechanical energy.
1832: Invention of the first commutator DC motor
The first DC motor that could provide enough power to drive machinery was invented by British physicist William Sturgeon in 1832, but its application was severely limited due to its low power output, which was still technically flawed.
Following in Sturgeon's footsteps, Thomas Davenport of Vermont, USA, made history by inventing the first official battery-powered electric motor in 1834. It was the first electric motor with enough power to perform its task, and his invention was used to power a small printing press.In 1837, Thomas Davenport and his wife, Emily Davenport, received the first patent for a DC motor.
1886: Invention of the practical DC motor
In 1886, the first practical DC motor that could run at constant speed with variable weight was introduced. FrankJulian Sprague was its inventor.
It is worth noting that the utility motor was a brushless form of the AC squirrel-cage asynchronous motor, which not only eliminated sparks and voltage losses at the winding terminals, but also allowed power to be delivered at a constant speed. However, the asynchronous motor had many insurmountable defects, so that the development of motor technology was slow.
In 1887, Nikola Tesla invented the AC induction motor (ACinductionmotor), which he successfully patented a year later. It was not suitable for use in road vehicles, but was later adapted by Westinghouse engineers.In 1892, the first practical induction motor was designed, followed by a rotating bar-wound rotor, making the motor suitable for automotive applications.
In 1891, General Electric began development of the three-phase induction motor (Threephasemotor). In order to utilize the wound rotor design, GE and Westinghouse signed a cross-licensing agreement in 1896.
In 1955, the United States d. Harrison and others applied for the first time with a transistor commutation line instead of brush DC motor mechanical brush patent, officially marking the birth of the modern brushless DC motor (BrushlessDirectCurrentMotor). However, at that time there was no motor rotor position detection device, the motor did not have the ability to start.
1962: The first brushless DC (BLDC) motor was invented thanks to advances in solid-state technology in the early 1960s. In 1962, TGWilson and PHTrickey invented the first BLDC motor, which they called the “solid-state commutated DC motor”. The key element of the brushless motor was that it did not require a physical commutator, making it the most popular choice for computer disk drives, robots, and airplanes.
They utilized Hall elements to detect the rotor position and control the phase change of the winding current to make brushless DC motors practical, but were limited by transistor capacity and relatively low motor power.
Since the 1970s, with the emergence of new power semiconductor devices (such as GTR, MOSFET, IGBT, IPM), the rapid development of computer control technology (microcontroller, DSP, new control theories), as well as high-performance rare-earth permanent magnet materials (such as samarium cobalt, neodymium-iron-boron), the Brushless Direct Current Motor (BrushlessDCMotor) has been rapidly developed. BrushlessDirectCurrentMotor) has been developed rapidly, and the capacity is increasing. Technology-driven industrial development, with the introduction of mac classic brushless DC motor and its driver in 1978, as well as the research and development of square-wave brushless motor and sine-wave brushless DC motor in the 80's, brushless motors really began to enter the practical stage, and get rapid development.
Brushless DC motor (BrushlessDirectCurrentMotor) consists of synchronous motor and driver, which is a typical mechatronic product. The stator winding of synchronous motor is mostly made into three-phase symmetrical star connection, which is very similar to three-phase asynchronous motor.
The structure of BLDCM control system includes three main parts: motor body, driving circuit and control circuit. In the working process, the motor voltage, current and rotor position information is collected and processed by the control circuit to generate the corresponding control signals, and the drive circuit drives the motor body after receiving the control signals.
Brushless DC motor (BrushlessDirectCurrentMotor) mainly consists of a stator with coil windings, a rotor made of permanent magnet material and a position sensor. The position sensor, as required, can also be left unconfigured.
The stator structure of a BLDC motor is similar to that of an induction motor. It consists of stacked steel laminations with axial grooves for winding.The windings in BLDC are slightly different from those in conventional induction motors.
Typically, most BLDC motors consist of three stator windings connected in a star or “Y” shape (no neutral). In addition, based on coil interconnections, the stator windings are further divided into trapezoidal and sinusoidal motors.
In a trapezoidal motor, both the drive current and the counter electromotive force have a trapezoidal shape (sinusoidal in the case of a sinusoidal motor). Typically, motors rated at 48 V (or less) are used in automotive and robotics (hybrid cars and robot arms).
The rotor part of a BLDC motor consists of permanent magnets (usually rare-earth alloy magnets such as neodymium (Nd), samarium cobalt (SmCo) and neodymium iron boron (NdFeB).
Depending on the application, the number of poles can vary between two and eight, with the north pole (N) and south pole (S) placed alternately. The diagram below shows three different arrangements of magnetic poles.
Since there are no brushes in BLDC motors, commutation is electronically controlled. In order to rotate the motor, the stator windings must be energized sequentially and the position of the rotor (i.e., the north and south poles of the rotor) must be known in order to accurately energize a specific set of stator windings.
Position sensors using Hall sensors (operating on the Hall effect principle) are commonly used to detect the position of the rotor and convert it into an electrical signal. Most BLDC motors use three Hall sensors that are embedded in the stator to detect the position of the rotor.
Hall sensors are a type of sensor based on the Hall effect, which was first discovered in 1879 by the American physicist Hall in metallic materials, but was not used because the Hall effect in metallic materials was too weak. With the development of semiconductor technology, began to use semiconductor materials to produce Hall components, due to the Hall effect is significant and has been applied and developed. A Hall sensor is a sensor that generates an output voltage pulse when an alternating magnetic field passes by. The amplitude of the pulse is determined by the field strength of the excitation magnetic field. Therefore, Hall sensors do not require an external power supply.
The output of the Hall sensor will be high or low depending on whether the north pole of the rotor is the south pole or near the north pole. By combining the results of the three sensors, the exact sequence of energization can be determined.
Unlike brushed DC motors, where the stator and rotor are completely reversed, the armature windings are set on the stator side and high-quality permanent magnet material is set on the rotor side, the motor body structure of the BLDCM consists of the stator armature windings, the permanent magnet rotor, and the position sensors, and the three-phase windings are arranged uniformly in the stator space of the motor, with a difference of 120° of electrical angle between phases, respectively. This structure is different from a purely brushed DC motor, and is similar to the stator winding structure of an AC motor, but square wave AC power is supplied to the motor by the drive circuit when it is operating.
The BLDCM selects a full-bridge, three-phase, star-wired, six-state, two-by-two conduction mode, in which two MOSFETs are energized in the drive circuit at the same time, and accordingly, the two-phase stator windings in the body of the motor are energized in series. Every electronic phase change once, the stator magnetic dynamic potential Fa turned 60 ° space electrical angle, is a step magnetic dynamic potential, the interval of 60 ° time electrical angle, Fa made a jump. Although the rotor rotates continuously, but the stator magnetic momentum rotation mode is a stepping type, which is different from the real AC synchronous motor rotating magnetic momentum.BLDCM's Fa and rotor magnetic momentum Ff space angle is always in the range of 60 ° ~ 120 ° range of periodic changes, the average value of 90 °, which ensures that the stator and rotor magnetic momentum Fa, Ff interaction to get is the average maximum electromagnetic torque T, the Strong drag permanent magnet rotor continuous rotation.
The working principle of brushless DC motor is similar to that of brush DC motor. Lorentz's force law states that as long as a current-carrying conductor is placed in a magnetic field, it will be subject to a force. Due to the reaction force, the magnet will be subjected to equal and opposite forces. When a current is passed through a coil, a magnetic field is generated, which is driven by the magnetic poles of the stator, with homopolarities repelling each other and anisotropic poles attracting each other. If the direction of the current in the coil is continuously changed, then the poles of the magnetic field induced in the rotor will also be continuously changed, and then the rotor will rotate all the time under the action of the magnetic field.
In BLDC motors, the permanent magnets (rotor) are in motion, while the current-carrying conductor (stator) is fixed.
When the stator coil receives power from the power supply, it becomes an electromagnet and begins to generate a uniform magnetic field in the air gap. The switch generates an AC voltage waveform with a trapezoidal shape despite the fact that the power supply is DC. The rotor continues to rotate due to the interaction force between the electromagnetic stator and the permanent magnet rotor.
By switching the windings to high and low signals, the corresponding windings are excited as north and south poles. The permanent magnet rotor with south and north poles is aligned with the stator poles, which causes the motor to rotate.
Brushless DC motors come in three configurations: single-phase, two-phase, and three-phase. Among them, three-phase BLDC is the most common one.
The driving method of brushless DC motor can be divided into various driving methods according to different categories:
According to the drive waveform: square wave drive, this drive method is convenient to realize, easy to realize the motor without position sensor control.
Sinusoidal drive: this drive method can improve the motor running effect and make the output torque uniform, but the realization process is relatively complicated. At the same time, this method has SPWM and SVPWM (space vector PWM) two ways, SVPWM is better than SPW.
▷ High output power
▷ Small size and weight
▷ Good heat dissipation and high efficiency
▷ Wide range of operating speeds and low electrical noise.
▷ High reliability and low maintenance requirements.
▷ High dynamic response
▷ Low electromagnetic interference
▶ The electronic controller required to control this motor is expensive
▶ Complex drive circuitry is required
▶ Extra position sensors are required (FOC is not used)
Brushless DC motors are widely used in various application needs, such as industrial control (Brushless DC motors play an important role in industrial production such as textile, metallurgy, printing, automated production lines, CNC machine tools, etc.) , Automotive (motors are found in wipers, power doors, automotive air conditioning, power windows and other parts of the car.) , Aviation, automation system (in life common printers, fax machines, copy machines, hard disk drives, floppy disk drives, movie cameras, etc., in their spindle and subsidiary movement driven control, all have brushless DC motors.) In addition, healthcare equipment (the use of brushless DC motors has been more common, can be used to drive a small blood pump in the artificial heart; in the country, surgical high-speed apparatus for high-speed centrifuges, thermal imaging and thermometry of the infrared laser modulator are used brushless DC motors.) Various loads in such fields as constant load and positioning applications.
Project category | Brushless DC motor | Brush DC Motor |
Structure | Permanent magnet as rotor, electric drive as stator | Permanent magnet as rotor, electric drive as stator |
Windings and coil links | Brushed motor characteristics, long life, no interference, no maintenance, low noise, high price. | Heat dissipation |
Good | Poor | Commutation |
Electronic switching commutator with electronic circuits | Mechanical contact between brush and rectifier | Rotor position sensor |
Hall elements, optical encoders, etc. or counterpotential generators | Self-propagating by brushes | Self-propagating by brushes |
Reversal | Changing the switching sequence of the electronic steering gear | Change of terminal voltage polarity |
Comparison of advantages and disadvantages | Good mechanical and control characteristics, long life, no interference, low voice, but higher cost. | Good mechanical characteristics and control, high noise, electromagnetic interference |
Currently, the top companies in the BLDC industry include ABB, AMTEK, Nidec, Minebea Group, Textronic, United Motion Technologies, Baldor Electronics, North American Electric Company, Schneider Electric, and RegalBeloit Corporation.
A brushless DC motor (BLDC) is a type of synchronous motor in which the magnetic field generated by the stator and the magnetic field generated by the rotor have the same frequency. It is widely used due to its advantages of high output power, low electrical noise, high reliability, high dynamic response, less electromagnetic interference, and better speed-torque.
The structure of a DC brushless motor is shown below (slotted, external rotor, sensorless motor as an example):
The brushless motor is composed of front cover, middle cover, magnet, silicon steel sheet, enameled wire, bearing, rotating shaft and back cover. Among them, the magnet, bearing and rotating shaft constitute the rotor of the motor; The stator of the motor is composed of silicon steel sheet and enamelled wire. The front cover, middle cover and back cover comprise the shell of the motor. Important components are described in the following table:
Components | Description | |
Rotor | Magnet | An important component of a brushless motor. The vast majority of the performance parameters of a brushless motor are related to it; |
Axis of rotation | The directly stressed part of the rotor; | |
Bearing | Are the guarantee of smooth motor operation; currently most brushless motors use deep groove ball bearings; | |
Rotor | Silicon steel sheet | Silicon steel sheet is an important part of the slotted brushless motor, the main function is to reduce the magnetic resistance and participate in the magnetic circuit operation; |
Enameled wire | As the energized conductor of the coil winding; through the alternating frequency and waveform of the current, a magnetic field is formed around the stator to drive the rotor to rotate; |
The rotor of a brushless DC motor (BLDC) is made of permanent magnets with multiple pairs of poles arranged alternately according to N- and S-pole (involving the pole-pair parameter).
The stator of a brushless DC motor (BLDC) consists of a silicon steel sheet (figure below) with stator windings placed in slots cut axially along the internal axis (the parameter number of core poles (number of slots N) is involved). Each stator winding consists of a number of coils connected to each other. Commonly, the windings are distributed in a three-connected star pattern.
Triple-connected star-wound coils, according to the way the coils are connected, the stator windings can be divided into trapezoidal and sinusoidal windings. The difference between the two is mainly the waveform of the generated counter electromotive force. As the name suggests: trapezoidal stator winding produces a trapezoidal counter electromotive force, and sinusoidal winding produces a sinusoidal counter electromotive force. This is shown in the figure below:
PS: When the motor is supplied without load, the waveform can be measured by oscilloscope.
Brushless DC motor (BLDC) according to the rotor distribution can be divided into internal rotor motor, external rotor motor; according to the drive phase can be divided into single-phase motor, two-phase motor, three-phase motor (the most common use); according to whether or not the sensor is divided into the sensory motors and non-sensory motors, and so on; there are many classifications of motors, the space reason, not to be over here to describe the brothers interested in their own understanding.
Brushless motors can be divided into outer rotor motors and inner rotor motors according to the row structure of rotor and stator (as shown below).
Motor | Descriptive |
Outer rotor motor | The internal energized coil winding serves as the stator, and the permanent magnets are coupled to the housing as the rotor; in common parlance: the rotor is outside and the stator is inside; |
Internal rotor motor | The internal permanent magnets are linked to the shaft as the rotor, the energized coil winding and the shell as the stator. Commonly: rotor inside, stator outside; |
Difference between internal and external rotor motor
In addition to the different rotor and stator sequencing, there are also differences between internal and external rotor motors as follows:
Characteristics | Internal Rotor Motor
| Outer Rotor Motor |
Power Density | Higher
| Lower
|
Speed | Higher
| Lower |
Lower Stability
| Lower
| Higher
|
Cost
| Relatively higher Relatively | Lower
|
Heat dissipation
| Mediocre | Worse Better |
Pole Pairs
| Less
| More
|
Parameter | Description |
Rated voltage | For brushless motors, they are suitable for a very wide range of operating voltages, and this parameter is the operating voltage under specified load conditions. |
KV value | Physical significance: speed per minute under 1V working voltage, that is: speed (no load) =KV value * Working voltage for brushless motors with size specifications: 1. The number of winding turns is large, the KV value is low, the maximum output current is small, and the torque is large; 2. Fewer turns of winding, high KV value, maximum output current, small torque; |
Torque and speed | Torque (moment, torque) : the driving torque generated by the rotor in the motor can be used to drive the mechanical load; Speed: motor speed per minute; |
Maximum current | The maximum current that can withstand and work safely |
Trough structure | Number of core poles (number of slots N) : the number of slots of the stator silicon steel sheet; Number of magnetic steel poles (pole number P) : the number of magnetic steel on the rotor; |
Stator inductance | The inductance at both ends of the stator winding of a motor at rest |
Stator resistance | Dc resistance of each phase winding of motor at 20℃ |
Dc resistance of each phase winding of motor at 20℃ | Under specified conditions, when the motor winding is open, the value of linear induced electromotive force generated in the armature winding per unit speed |
Brushless motors are of the self-commutating type (self-direction switching) and are therefore more complex to control.
BLDC motor control requires knowledge of the rotor position and mechanism by which the motor undergoes rectification steering. For closed-loop speed control, there are two additional requirements, i.e., measurements for rotor speed/ or motor current and PWM signals to control the motor speed power.
BLDC motors can have either side-aligned or center-aligned PWM signals depending on the application requirements. Most applications require only speed change operation and will utilize 6 separate side aligned PWM signals.
This provides the highest resolution. If the application requires server positioning, energy braking, or power reversal, the supplemental center-aligned PWM signals are recommended. To sense rotor position, BLDC motors use Hall effect sensors to provide absolute position sensing. This results in the use of more wires and higher costs. Sensorless BLDC control eliminates the need for Hall sensors and instead uses the motor's counter electromotive force (electromotive force) to predict rotor position. Sensorless control is critical for low-cost variable speed applications like fans and pumps. Sensorless control is also required for refrigerator and air conditioning compressors when BLDC motors are used.
Most BLDC motors do not require complementary PWM, no-load time insertion or no-load time compensation. The only BLDC applications that may require these features are high performance BLDC servo motors, sine wave excited BLDC motors, brushless AC, or PC synchronous motors.
Many different control algorithms are used to provide control of BLDC motors. Typically, power transistors are used as linear regulators to control the motor voltage. This approach is not practical when driving high power motors. High-power motors must be PWM controlled and require a microcontroller to provide starting and control functions.
A PWM voltage for controlling the motor speed
A mechanism for rectifying and commutating the motor
Methods for predicting rotor position using reverse electromotive force or Hall sensors
Pulse Width Modulation is only used to apply a variable voltage to the motor windings. The effective voltage is proportional to the PWM duty cycle. When proper rectifier commutation is obtained, the torque-speed characteristics of a BLDC are the same as those of the following DC motors. Variable voltage can be used to control the speed and variable torque of the motor.
The commutation of the power transistor enables the appropriate winding in the stator to generate the best torque depending on the rotor position. In a BLDC motor, the MCU must know the position of the rotor and be able to make the commutation at the right time.
One of the simplest methods for DC brushless motors is to use what is called trapezoidal commutation.
In this schematic, the current is controlled by a pair of motor terminals at a time, while the third motor terminal is always electronically disconnected from the power supply.
Three Hall devices embedded in the large motor are used to provide digital signals which measure the rotor position in a 60 degree sector and provide this information at the motor controller. Since the current flow is equal on two windings at a time and zero on the third, this method produces a current space vector with only one of six directions in common. As the motor is steered, the current at the motor terminals is electrically switched (rectified commutation) once per 60 degrees of rotation, so the current space vector is always at the closest 90 degree phase shift of the
The current waveform in each winding is therefore trapezoidal, starting at zero and going to positive current then zero then negative current. This produces a current space vector that will approach balanced rotation as it steps up in 6 different directions as the rotor rotates.
In motor applications like air conditioners and refrigerators, the use of Hall sensors is not a constant. Reverse potential sensors induced in unlinked windings can be used to achieve the same results.
Such trapezoidal drive systems are very common because of the simplicity of their control circuits, but they suffer from torque ripple problems during rectification.
Trapezoidal rectifier commutation is not sufficient to provide balanced and accurate BLDC motor control. This is mainly because the torque generated in a three-phase brushless motor (with a sinusoidal wave counter electromotive force) is defined by the following equation:
Rotating shaft torque = Kt[IRSin(o)+ISSin(o+120)+ITSin(o+240)]
Where: o is the electrical angle of the rotating shaft Kt is the torque constant of the motor IR, IS and IT for the phase current if the phase current is sinusoidal: IR = I0Sino; IS = I0Sin (+120o); IT = I0Sin (+240o)
will get: rotating shaft torque = 1.5I0 * Kt (a constant independent of the angle of the rotating shaft)
The sinusoidal rectifier commutated brushless motor controller endeavors to drive three motor windings with three currents that smoothly vary sinusoidally as the motor rotates. The associated phases of these currents are chosen such that they will produce smooth space vectors of rotor current in directions orthogonal to the rotor with invariance. This eliminates the torque ripple and steering pulses associated with northerly steering.
In order to generate a smooth sinusoidal modulation of the motor current as the motor rotates, an accurate measurement of the rotor position is required. Hall devices only provide a rough calculation of the rotor position, which is not sufficient for this purpose. For this reason, angular feedback from an encoder or similar device is required.
Since the winding currents must be combined to produce a smooth constant rotor current space vector and since each of the stator windings are positioned at an angle of 120 degrees apart, the currents in each wire bank must be sinusoidal and have a phase shift of 120 degrees. The position information from the encoder is used to synthesize two sine waves with a phase shift of 120 degrees between the two. These signals are then multiplied by the torque command so that the amplitude of the sine wave is proportional to the required torque. As a result, the two sinusoidal current commands are properly phased, thus producing a rotating stator current space vector in the orthogonal direction.
The sinusoidal current command signals output a pair of P-I controllers that modulate the current in the two appropriate motor windings. The current in the third rotor winding is the negative sum of the controlled winding currents and therefore cannot be controlled separately. The output of each P-I controller is sent to a PWM modulator and then to the output bridge and the two motor terminals. The voltage applied to the third motor terminal is derived from the negative sum of the signals applied to the first two windings, appropriately used for three sinusoidal voltages spaced 120 degrees apart, respectively.
As a result, the actual output current waveform accurately tracks the sinusoidal current command signal, and the resulting current space vector rotates smoothly to be quantitatively stabilized and oriented in the desired direction.
The sinusoidal rectifier steering result of stabilized control cannot be achieved by trapezoidal rectifier steering in general. However, due to its high efficiency at low motor speeds, it will separate at high motor speeds. This is due to the fact that as the speed increases, the current return controllers must track a sinusoidal signal of increasing frequency. At the same time, they must overcome the counter electromotive force of the motor that increases in amplitude and frequency as the speed increases.
Since P-I controllers have finite gain and frequency response, time-invariant disturbances to the current control loop will cause phase lag and gain errors in the motor current that increase with higher speeds. This will interfere with the direction of the current space vector with respect to the rotor, thus causing a displacement from the quadrature direction.
When this occurs, less torque can be produced by a certain amount of current, so more current is required to maintain torque. Efficiency decreases.
This decrease will continue as speed increases. At some point, the phase displacement of the current exceeds 90 degrees. When this occurs, the torque is reduced to zero. Through the combination of sinusoidal, the speed at this point above results in a negative torque and therefore cannot be realized.
Scalar control (or V/Hz control) is a simple method of controlling the speed of a command motor
The steady state model of the command motor is mainly used to obtain the technology, so transient performance is not possible. The system does not have a current loop. To control the motor, the three-phase power supply varies only in amplitude and frequency.
The torque in a motor varies as a function of the stator and rotor magnetic fields and peaks when the two fields are orthogonal to each other. In scalar based control, the angle between the two magnetic fields varies significantly.
Vector control manages to create orthogonality again in AC motors. In order to control the torque, each generates a current from the generated magnetic flux to achieve the responsiveness of a DC machine. Vector control of an AC commanded motor is similar to the control of a separately excited DC motor.
In a DC motor, the magnetic field energy ΦF generated by the excitation current IF is orthogonal to the armature flux ΦA generated by the armature current IA. These magnetic fields are decoupled and stabilized with respect to each other. As a result, when the armature current is controlled to control torque, the magnetic field energy remains unaffected and a faster transient response is realized.
Field Oriented Control (FOC) of a three-phase AC motor consists of mimicking the operation of a DC motor. All controlled variables are mathematically transformed to DC instead of AC. its target independent control torque and flux.
There are two methods of field orientation control (FOC): Direct FOC: The direction of the rotor magnetic field (Rotorfluxangle) is calculated directly by a flux observer Indirect FOC: The direction of the rotor magnetic field (Rotorfluxangle) is obtained indirectly by estimation or measurement of rotor speed and slip (slip).
Vector control requires knowledge of the position of the rotor flux and can be calculated by advanced algorithms using knowledge of the terminal currents and voltages (using a dynamic model of an AC induction motor). From an implementation point of view, however, the need for computational resources is critical.
Different approaches can be used to implement vector control algorithms. Feedforward techniques, model estimation and adaptive control techniques can all be used to enhance response and stability.
At the heart of a vector control algorithm are two important conversions: the Clark conversion, the Park conversion and their inverse. The use of Clark and Park transitions allows control of the rotor current into the rotor region. This allows a rotor control system to determine the voltage that should be supplied to the rotor in order to maximize the torque under dynamically varying loads.
Clark Conversion: The Clark mathematical conversion modifies a three-phase system into a two-coordinate system:
Where Ia and Ib are components of the orthogonal datum and Io is the unimportant homoplanar component
Three-phase rotor current versus rotating reference system
Park Conversion: The Park mathematical conversion converts the bi-directional static system into a rotating system vector.
The two-phase α,β frame representation is computed by Clarke conversion and then fed into the vector rotation module where it rotates the angle θ to conform to the d,q frame attached to the rotor energy. According to the above equation, the conversion of angle θ is realized.
The Clarke transformation uses three-phase currents IA,IB as well as IC, which are in the fixed-coordinate stator phase are transformed into Isd and Isq, which become elements in the Park transformation d,q. The Clarke transformation is based on a model of the motor fluxes. The currents Isd,Isq and the instantaneous flux angle θ, which are calculated from the motor flux model, are used to calculate the electric torque of the AC induction motor.
These derived values are compared with each other and the reference values and updated by the PI controller.
Control parameter | V/Hz control | Yari control | Sensorless sagittal control |
Speed adjustment | 1% | 0 001% | 0 05% |
Torque adjustment | Poor | +/- 2% | +/-5% |
Motor model | Don't | Demand | An accurate model is required |
MCU processing power | Low | High | High +DSP |
An inherent advantage of vector-based motor control is that it is possible to use the same principle to select the appropriate mathematical model to separately control various types of AC,PM-AC or BLDC motors.
BLDC motor is the main choice for field oriented vector control. Brushless motors with FOC can achieve higher efficiency, up to 95%, and are also very efficient for motors at high speeds.
In this mode, the winding is powered in the following order, AB/CD/BA/DC(BA means that the winding AB is powered in the opposite direction). This sequence is called the single-phase full-step mode, or wave-driven mode. At any one time, there is only one additional charge.
In this mode, the two phases are charged together, so the rotor is always between the two poles. This mode is called biphase full step, this mode is the normal drive sequence of the bipolar motor, can output the maximum torque.
This mode will single-phase step and two-phase step together power: single-phase power, and then double add power, and then single-phase power... Therefore, the motor runs in half-step increments. This mode is called half-step mode, and the effective step Angle of the motor per excitation is reduced by half, and the output torque is also lower.
The above three modes can be used to rotate in the opposite direction (counterclockwise), but not if the order is reversed.
Usually, the stepper motor has multiple poles in order to reduce the step Angle, but the number of windings and the drive sequence are constant.
General motor speed control, especially the use of two circuits of the motor: phase Angle control PWM chopper control
Phase Angle control is the simplest method to control the speed of general motors. The speed is controlled by changing the point arc Angle of the TRIAC. Phase Angle control is a very economical solution, however, it is not very efficient and prone to electromagnetic interference (EMI).
The diagram shown above illustrates the mechanism of phase Angle control and is a typical application of TRIAC speed control. The phase movement of the TRIAC gate pulse produces an efficient voltage, thus producing different motor speeds, and a zero-cross detection circuit is used to establish a timing reference to delay the gate pulse.
PWM control is a more advanced solution for general motor speed control. In this solution, the power MOFSET, or IGBT, turns on the high-frequency rectified AC line voltage to generate a time-varying voltage for the motor.
The switching frequency range is generally 10-20KHz to eliminate noise. This general purpose motor control method allows for better current control and better EMI performance, and therefore, higher efficiency.