From small vibration motors in mobile phones to more complex motors used in home washing machines and air conditioners, motors have become everyday devices in the consumer field. Motors are also an important part of the industrial field, and are widely used in many applications, such as driving fans, pumps and other mechanical equipment. The energy consumption of these motors is very huge: research shows that in China alone, the energy consumed by motors accounts for 60% to 70% of the total industrial energy consumption, of which the energy consumed by fans and pumps accounts for nearly four One in one. Although this number may not be as high in other countries, reducing motor energy consumption in electronic systems has become a priority issue globally.
For more than a century, traditional AC motors have been widely used. AC motors are the simplest induction motors designed, but they cause a lot of wasted energy. This is because the AC motor only outputs a constant speed and cannot adapt itself to changes in operating conditions. There are some simple methods for adjusting the speed of AC motors (for example, a standard household fan that can provide three speed options), but these methods have limited scope of application and are difficult to transfer to more complex systems.
But for direct current (DC) motors, the speed can be changed and controlled by changing the voltage, so as to speed up or slow down the working speed according to the needs of the application. This can save a lot of energy because the motor can be operated according to the required conditions. In general, DC motors are more efficient than AC motors.
Figure 1: Replacing traditional AC motors with smaller, more efficient BLDC motors can save energy and reduce costs, but the algorithms required for BLDC control are so complex that many designers are reluctant to convert. A dedicated IC specifically designed for BLDC motor control can make this job easier.
Advantages of BLDC motors
The DC motor can be designed as a brush motor or a brushless motor. Brushless DC (BLDC) motors are usually the best choice for most applications. Such motors are more reliable, quieter, generate less electromagnetic radiation, and are safer because they eliminate sparks caused by brushes and commutators. BLDC motors are smaller and more efficient, which means they require less energy.
The operating temperature of BLDC motor is lower than that of AC motor. The more efficient design makes the internal parts generate less heat. This can not only increase the service life of the bearing system, but also improve the reliability of the electrical system and the fan.
In addition, the power density of BLDC motors is also higher than that of AC motors. For the same energy output, the size and weight of the DC motor are smaller than the AC motor. This makes the transportation and installation of BLDC motors easier and cheaper.
However, the trouble with using BLDC motors is that the system requires more complicated electronic equipment to manage the motors. Motor control has not always been a key area for electronic engineers, and many developers cannot easily design the necessary control circuits due to lack of experience or expertise. The development of BLDC motors requires additional time and technical support, which means longer development cycles and higher system costs, which makes it more difficult for system manufacturers to transition from familiar AC motors to BLDC motors.
However, for more and more manufacturers, the complexity of using BLDC motors will not offset with the increasing demand for more energy-efficient appliances. According to a 2011 IMS survey, about 40% of air conditioners in China use inverter-controlled BLDC motors. This situation is on the rise, and, to a certain extent, thanks to a dedicated circuit designed specifically for BLDC motor control.
Sensorless magnetic field guidance control technology
The traditional method for controlling BLDC motors uses a six-step process to drive the stator, thereby generating pulsations in the generated torque. The so-called "six-step square wave" process uses Hall effect sensors to detect the permanent magnet position in the BLDC motor.
The six-step process is relatively simple, but is prone to noise, and for more advanced applications that need to quickly change the motor speed according to changes in conditions, its responsiveness is insufficient. Taking a washing machine as an example, the load varies according to the selected washing cycle, and also changes throughout the cycle. In the drum-type washing machine, this situation is more complicated. When the laundry rotates to the top of the drum, gravity will affect the motor.
In these cases, a more advanced algorithm is needed. Field-oriented control (FOC) can provide the response time required for rapid changes in speed, and has become the choice of motor control methods for today's more advanced energy-saving appliances.
There are multiple ways to achieve FOC. One method is to use a sensor (similar to the six-step square wave process method), but the sensor is more difficult to install and maintain, especially when the application involves complex wiring harnesses or motors exposed to water. The simpler and more cost-effective way to achieve FOC is to eliminate sensors. Sensorless FOC involves a constant rotor magnetic field generated on the rotor by permanent magnets and is a very effective control method.
The FOC method can make the motor run smoothly in the full speed range, generate maximum torque at zero speed, and can quickly accelerate and decelerate. In fact, due to the small size of the motor, low cost, and low power consumption, the many advantages of sensorless FOC make it a popular choice for applications with lower performance requirements.
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