The most basic motor is "DC motor (brushed motor)". Place a coil in a magnetic field. Through the flowing current, the coil will be repelled by the magnetic pole on one side and attracted by the magnetic pole on the other side at the same time, and it will continue to rotate under this effect. During the rotation, the current to the coil flows in the opposite direction, so that it continues to rotate. There is a part of the motor called "commutator" that is powered by the "brush". The position of the "brush" is above the "diverter" and moves continuously with the rotation. By changing the position of the brush, the direction of the current can be changed. The commutator and brushes are indispensable structures for the rotation of DC motors (Figure 1).
Figure 1: DC motor (brushed motor) running
The commutator switches the flow of current in the coil and reverses the direction of the magnetic poles so that it always rotates to the right. The brushes supply electricity to the commutator rotating with the shaft.
Motors in different industry
The motor can be classified according to the type of power source and the principle of rotation. Let's take a brief look at the characteristics and application of various motors.
The DC motor (brush motor), which has a simple structure and is easy to operate, is usually used for "opening and closing of disc trays" in home appliances. Or it can be used in the "opening and closing and direction control of electric rearview mirrors" of automobiles. Although it is cheap and can be used in many fields, it also has drawbacks. Since the commutator will be in contact with the brush, its life is very short, the brush must be replaced regularly.
The stepper motor will rotate with the number of electrical pulses sent to it. Its movement depends on the number of electrical pulses sent to it, so it is suitable for position adjustment. It is usually used for "paper feeding of fax machines and printers" in the family. Since the paper feeding procedure of the fax machine depends on the specifications (engraving, fineness), the stepping motor that rotates with the number of electric pulses is very easy to use. It is easy to solve the problem that the machine will stop temporarily once the signal stops.
Synchronous motors whose number of revolutions vary with the frequency of the power supply are used for applications such as "rotating tables for microwave ovens". There is a gear reducer in the motor unit to get the number of revolutions suitable for heating food. Induction motors are also affected by the power frequency, but the frequency and the number of rotations is not consistent. Previously, this type of AC motor was used in fans or washing machines.
It can be seen that various motors are active in many fields. Among them, what are the characteristics of BLDC motors (brushless motors) that make them so versatile?
How does the BLDC motor rotate?
The "BL" in BLDC motor means "brushless", that is, the "brush" in the DC motor (brush motor) is gone. 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? The original BLDC motor uses permanent magnets as the rotor, and there is no coil in the rotor. Since there are no coils in the rotor, no commutator and brushes for energization are needed. Instead, the coil is used as the stator (Figure 3).
The magnetic field created by the fixed permanent magnet in the DC motor (brush motor) is immobile, and it rotates by controlling the magnetic field generated inside the coil (rotor). To change the number of rotations by changing the voltage. The rotor of the BLDC motor is a permanent magnet, and the rotor is rotated by changing the direction of the magnetic field generated by the surrounding coils. The rotation of the rotor is controlled by controlling the direction and magnitude of the current to the coil.
Figure 3: BLDC motor running
BLDC motors use permanent magnets as the rotor. Since there is no need to energize the rotor, there is no need for brushes and commutators. The electricity to the coil is controlled from the outside.
BLDC motor advantages
There are three coils on the stator of the BLDC motor, each coil has two wires, and there are six lead wires in the motor. In fact, due to internal wiring, usually only three wires are needed, but there is one more than the previously mentioned DC motor (brush motor). Purely by connecting the positive and negative poles of the battery will not move. As for how to run the BLDC motor, it will be explained in the second part of this series. This time we are going to focus on the advantages of BLDC motors.
The first feature of BLDC motors is "high efficiency". It can control its turning force (torque) to always maintain the maximum value. In the case of a DC motor (brush motor), the maximum torque can only be maintained for a moment during rotation, and cannot always be maintained at the maximum value. If a DC motor (brush motor) wants to get the same torque as a BLDC motor, it can only increase its magnet. This is why a small BLDC motor can also generate great power.
The second feature is "good control", which is related to the first. The BLDC motor can get the expected torque and rotation speed precisely. The BLDC motor can give feedback of the target rotation number, torque, etc. Through precise control, the heat generation and power consumption of the motor can be suppressed. If it is battery driven, the drive time can be extended through careful control.
In addition, it is durable and has low electrical noise. The above two points are the advantages brought by brushless. The DC motor (brushed motor) will be worn for a long time due to the contact between the brush and the commutator. Sparks will also be generated at the contacted part. Especially when the gap of the commutator touches the brush, there will be huge sparks and noise. If you do not want to generate noise during use, you can consider to use a BLDC motor.
BLDC motor application
What’s the application of the BLDC motors with high efficiency, diversified control and long service life? It is often applied in products that can give play to its high efficiency and long life and are working continuously. For example: home appliances. People have used washing machines and air conditioners for a long time. Recently, BLDC motors have also been adopted in electric fans, and they have successfully reduced power consumption. The power consumption reduced exactly due to the high efficiency.
BLDC motors are also used in vacuum cleaners. In one case, the rotating speed increased significantly by changing the control system. This example reflects the good controllability of the BLDC motor.
As an important storage medium, the hard disk also uses a BLDC motor in its rotating part. Since it is a motor that needs to run for a long time, durability is vital importance. Of course, it also has the purpose of suppressing power consumption. The high efficiency here is also related to the low power consumption.
There are many other applications for BLDC motors
BLDC motors are expected to be used in a wider range of fields. BLDC motors will be widely used in small robots, especially "service robots" that provide services in areas other than manufacturing. "Positioning is very important for robots. Shouldn't you use a stepper motor that runs with the number of electrical pulses?" Someone might think so. But in terms of power control, BLDC motors are more suitable. In addition, if a stepper motor is used, a structure such as a robot wrist needs to provide a considerable amount of current to be fixed in a certain position. If it is a BLDC motor, it can cooperate with external forces to provide the required power and reduce the power consumption.
It can also be used for transportation. For a long time, simple DC motors have been mostly used in electric vehicles or golf carts for the elderly, but recently they have begun to use high-efficiency BLDC motors with good controllability. The duration of the battery can be extended by fine control. BLDC motors are also suitable for drones. Especially for UAVs with multi-axis racks, since it controls the flight by changing the number of rotations of the propellers, the BLDC motor that can precisely control the rotation.
BLDC motor is a high-quality motor with high efficiency, good controllability and long life. However, in order to maximize the power of the BLDC motor, proper control is required. How to do it?
The inner rotor type BLDC motor is a kind of typical BLDC motor, and its appearance and internal structure are as follows (Figure 1). Brushed DC motors (hereafter referred to as DC motors) have coils on the rotor and permanent magnets on the outside. The rotor of the BLDC motor has permanent magnets, and the outside has coil. The rotor of the BLCD motor has no coils and is a permanent magnet, so there is no need to energize the rotor. A "brushless type" without a brush for energization is realized.
On the other hand, control becomes more difficult compared with DC motors. It's not only to make the cable on the motor connected to the power supply. Even the number of cables is different. It is different from the method of "connect the positive (+) and negative (-) to the power supply".
Figure1 BLDC motor appearance and structure
Change the direction of magnetic flux
In order to rotate the BLDC motor, the current direction and timing of the coil must be controlled. Figure 2-A is the result of modeling the stator (coil) and rotor (permanent magnet) of the BLDC motor. Think about the rotor working with reference to the following picture. Consider the case of using 3 coils. Although there are actually cases where 6 or more coils are used, based on the principle, one coil is placed every 120 degrees and three coils are used. The motor converts electricity (voltage, current) into mechanical rotation. How does the BLDC motor in Figure 2-A rotate? Let's take a look at what happens in the motor first.
Figure 2-A: BLDC motor rotate principle
A coil is placed every 120 degrees in the BLDC motor, and a total of three coils are placed to control the current of the energized phase or coil.
As shown in Figure 2-A, the BLDC motor uses 3 coils. These three coils are used to generate magnetic flux after energization, and they are named U, V, and W. Give it try to energize the coil. The current path on the coil U (hereinafter referred to as "coil") is marked as U phase, V is recorded as V phase, and W is recorded as W phase. Next, take a look at the U phase. After the U phase is energized, the magnetic flux in the direction of the arrow shown in Figure 2-B will be generated.
But in fact, the U, V, and W cables are all connected to each other, so it is impossible to energize the U phase only. Here, energizing from the U phase to the W phase will generate magnetic flux at U and W as shown in Figure 2-C. Combining the two magnetic fluxes of U and W becomes the larger magnetic flux as shown in Figure 2-D. The permanent magnet will rotate so that the resultant magnetic flux is in the same direction as the N pole of the permanent magnet (rotor) in the center.
Energize from U phase to W phase. Firstly, pay attention to the coil U, you will find the generated magnetic flux like arrow.
Figure 2-C: BLDC motor rotate principle
Energize from U phase to W phase, 2 magnetic flux with different direction will be generated.
Figure 2-D: BLDC motor rotate principle
Energize from U phase to W phase, two magnetic flux will be generated.
If the direction of the synthetic magnetic flux is changed, the permanent magnet will also change accordingly. According to the position of the permanent magnet, switch the energized phase among U-phase, V-phase, and W-phase to change the direction of the combined magnetic flux. Continuously performing this operation, the resultant magnetic flux will rotate, thereby generating a magnetic field, and the rotor will rotate.
Figure 3 shows the relationship between the energized phase and the resultant magnetic flux. In this example, if the energization mode is changed from 1-6 in order, the resultant 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 control method for switching these 6 energization modes and controlling the motor is called "120-degree energization control".
Figure 3: The permanent magnet of the rotor will rotate as if pulled by the synthetic magnetic flux, and the shaft of the motor will also rotate accordingly
Use sine wave control for smooth rotation
Next, although the direction of the combined magnetic flux will rotate under the 120-degree energization control, there are only six directions. For example, if the "energization mode 1" in Figure 3 is changed to "energization mode 2," the direction of the combined magnetic flux will change by 60 degrees. Then the rotor will rotate as if attracted. Next, change from "energization mode 2" to "energization mode 3", the direction of the resultant magnetic flux will change 60 degrees again. The rotor will be attracted by this change again. This phenomenon will repeat itself. This action will become blunt. Sometimes this action will make noise.
It is the "sine wave control" that can eliminate the shortcomings of 120-degree energization control and achieve smooth rotation. In the 120-degree energization control, the combined magnetic flux is fixed in 6 directions. In the example of Figure 2-C, U and W generate the same magnetic flux. However, if the U-phase, V-phase, and W-phase can be controlled well, the coils can generate magnetic fluxes of different sizes, and the direction of the combined magnetic flux can be precisely controlled. The currents of the U-phase, V-phase, and W-phase are adjusted to generate a composite magnetic flux. By controlling the continuous generation of this magnetic flux, the motor can rotate smoothly.
Figure 4: sine wave control
The sine wave control can control the current on the 3 phases, generate synthetic magnetic flux, and realize smooth rotation. It can generate synthetic magnetic flux in a direction that cannot be generated by 120-degree energization control.
Inverter control motor
What about the currents on the U, V, and W phases? For ease of understanding, let's recall the case of 120-degree energization control. Please see Figure 3 again. In power-on mode 1, current flows from U to W; in power-on mode 2, current flows from U to V. It can be seen that whenever the combination of the coils with current flowing changes, the direction of the synthetic magnetic flux arrow also changes.
Next, look at power-on mode 4. In this mode, the current flows from W to U, opposite to the direction of energization mode 1. In a DC motor, the current direction conversion like this is performed by a combination of a commutator and a brush. However, BLDC motors do not use such contact type methods. Use an inverter circuit to change the direction of current. When controlling a BLDC motor, an inverter circuit is generally used.
In addition, the inverter circuit can change the applied voltage in each phase and adjust the current value. In voltage adjustment, PWM (Pulse Width Modulation=Pulse Width Modulation) is commonly used. PWM is a method of changing the voltage by adjusting the pulse ON/OFF time length. What is important is the change in the ratio (duty cycle) of 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 the voltage decrease can be obtained (Figure 5).
In order to realize PWM, there are now microcomputers equipped with dedicated hardware. When performing sine wave control, it is necessary to control the voltage of three phases, so the software is slightly more complicated than the 120-degree energization control with only two phases energized. The inverter is a necessary circuit for driving the BLDC motor. Inverters are also used in AC motors, but it can be considered that the "inverter type" referred to in home appliances almost uses BLDC motors.
Change the ON time within a certain period of time to change the effective value of the voltage. The longer the ON time, the closer the effective value is to the voltage when 100% voltage is applied (when it is ON).
BLDC motor using position sensor
The above is an overview of the control of the BLDC motor. The BLDC motor changes the direction of the synthetic magnetic flux generated by the coil to change the permanent magnet of the rotor.
In fact, there is one more point not mentioned in the above description. That is, the presence of sensors in BLDC motors. The control of the BLDC motor is coordinated with the position (angle) of the rotor (permanent magnet). Therefore, a sensor to obtain the rotor position is necessary. If no sensor knows the direction of the permanent magnet, the rotor may turn to an unexpected direction. If there are sensors to provide information, this will not happen.
Table 1 shows the main types of sensors for position detection of BLDC motors. Depending on the control method, the required sensors are also different. In the 120-degree energization control, in order to determine which phase to energize, a Hall-effect sensor that can input a signal every 60 degrees is equipped. On the other hand, high-precision sensors such as angle sensors or photoelectric encoders are effective for "vector control" (explained in the next item) that precisely controls the synthesized magnetic flux.
The position can be detected by using these sensors, but it also brings some disadvantages. The sensor is weak against dust and maintenance is indispensable. The usable temperature range will also be reduced. The use of sensors or the increase in wiring for this will cause the cost to rise, and the high-precision sensors themselves are expensive. Thus, the "sensor less" approach was introduced. It does not use position detection sensors to control costs and does not require sensor-related maintenance. But for the purpose of explaining the principle this time, let's assume that information has been obtained from the position sensor.
|Sensor type||Main application||Feature|
|Hall sensor||120-degree power supply control||Acquire signal every 60 degree. Lower cost, poor heat endurance|
|Optical encoder||Sine wave control, vector control||High resolution, poor anti-dust ability.|
|Angle sensor||Sine wave control, vector control||High resolution.|
Maintain high efficiency at all times through vector control
The sine wave is controlled to be energized in three phases, which smoothly changes the direction of the synthesized magnetic flux, so the rotor will rotate smoothly. The 120-degree energization control switches 2 phases among U-phase, V-phase, and W-phase to make the motor rotate, while sine wave control requires precise control of the 3-phase current. Moreover, the controlled value is an AC value that changes all the time, so the control becomes more difficult.
Here is vector control. Vector control can use coordinate transformation to calculate the 3-phase AC value as the 2-phase DC value, so the control can be simplified. However, vector control calculation requires rotor position information at high resolution. There are two methods for position detection, that is, a method using a position sensor such as a photoelectric encoder or a rotation angle sensor, and a senseless method that estimates based on the current value of each phase. Through this coordinate transformation, the current value related to the torque (rotational force) can be directly controlled, so as to achieve efficient control without excess current.
However, vector control requires coordinate transformation using trigonometric functions or complex calculation processing. Therefore, in most cases, a microcomputer with strong computing power is used as a control microcomputer, such as a microcomputer equipped with an FPU (floating point arithmetic unit).
The above is about the brushless DC motor and the normal use method shared by the editor of AIP. However, if you want to improve the quality of the brushless DC motor and reduce the defective rate of motor production, you also need to use the motor testing machine in the motor production process. The product launched by the editor of AIP today is: BLDC motor testing machine.
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