01 The concept of brushless DC motor
Brushless DC motor (BLDC) is a motor without brushes and commutator, also known as commutatorless motor or synchronous DC motor. The BLDC input terminal is direct current , but essentially it achieves the effect of alternating current by cyclically switching the inverter main switch, causing the coil winding to produce a changing magnetic field, and allowing the motor rotor to be subject to continuous rotation. Moment acts and continues to rotate.
BLDC can be configured as single-phase, two-phase or three-phase according to the number of stator windings. The three-phase motor we usually come into contact with most is the three-phase motor. The above picture is a disassembly diagram of a three-phase brushless DC motor.
02 Application of brushless DC motors
It is estimated that the BLDC motor market size will reach approximately US$19.76 billion in 2022. As BLDC motor technology matures, BLDC motors have been used in military, aviation, industrial, automotive, civil control systems, and It has been widely used in home appliances and other fields. Low-voltage and low-power devices are common, such as small robots, drones, electric bicycles, vacuum cleaners , power tools, etc.
Here are a few popular applications.
Vacuum cleaner/hair dryer: When it comes to hair dryers or vacuum cleaners, the well-known brand is Dyson ( Dyson ). The "black technology of the era" promoted by Dyson hair dryers is the V9 smart digital motor, which is smaller, lighter and faster than traditional motors.
Dyson digital motor
does not use carbon brush . The motor can rotate up to 110,000 times per minute, and the motor is smaller and lighter than other motors. The motor used in Dyson's digital motor is a "single-phase brushless DC motor", which is essentially a BLDC motor.
Dyson digital motor
drone/ gimbal :
drone motor control The key to is speed and direction control. The most popular drone is none other than DJI . The picture below is a disassembly of the Spark drone, showing the BLDC motors at the four corners.
Disassembly diagram of DJI Spark series drones
Similar applications include gimbals.
Disassembly diagram of a BLDC motor for a gimbal
Power tools:
Common handheld power tools in life, such as Bosch electric wrench , electric drill, etc. The energy saving and high efficiency of brushless DC motors, coupled with the continued decline in the cost of handheld power tools, have led to the rapid development of more power tools using BLDC motors. The most famous international manufacturers such as Bosch and Germany Wei (Dewalt), Milwaukee (Milwaukee), etc.
03 The composition of brushless DC motor
briefly introduces the composition and structure of BLDC.
stator :
The stator of the BLDC motor is composed of cast steel laminations, and the winding is placed in a slot excavated axially along the inner circumference. The stator is very similar to the stator of the induction motor , but the windings are distributed differently. Most BLDC motors have three star-connected stator windings. Each of these windings is made up of many coils connected to each other. One or more coils are placed in the slots and connected to each other to form a winding. These windings are distributed around the circumference of the stator to form evenly distributed magnetic poles .
rotor: BLDC motor uses permanent magnets as the rotor. There are no coils in the rotor, and the south magnetic pole and north magnetic pole of the rotor are arranged alternately. In addition, with the improvement of soft magnetic material technology and the decline of prices, the rotor materials are increasingly using high-performance neodymium iron boron rare earth materials to make permanent magnet rotors. Its high magnetic energy product and stable characteristics make BLDC The motor has better mechanical characteristics and dynamic response, higher efficiency and speed range.Here is a schematic diagram from a principle description document of BLDC by Microchip. The following picture is a schematic diagram of the cross-section of the rotor magnet:
Hall sensor :
The most important thing in controlling the BLDC motor is the identification of the rotor position. There are two methods of position identification. One kind uses a position sensor to identify the position of the rotor, that is, a Hall sensor; the other does not have a position sensor and identifies the position of the rotor by detecting the reverse electromotive force .
For inductive BLDC motors, most BLDC motors have three Hall sensors embedded in the stator. At each commutation, one winding is connected to the positive terminal of the control power supply (current enters the winding), a second winding is connected to the negative terminal (current flows out of it), and the third winding is de-energized. Torque is generated by the interaction between the magnetic field generated by the stator coils and the permanent magnets . When the rotor magnetic pole passes near the Hall sensor, the sensor will send out a high level or low level signal, indicating that the south/north magnetic pole is passing through the area sensed by the Hall sensor. The phase offset between the signals output by the Hall sensor can be 60° or 120°.
Hall effect : The Hall effect was discovered by the American physicist Hall in 1879. When a current passes through a conductor located in a magnetic field, an electric potential difference will be generated in the conductor perpendicular to the direction of the current and the direction of the magnetic field. And the size of the potential difference is proportional to the vertical component of the magnetic induction intensity and the size of the current.
