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Operating Principle of BLDC

来源:未知  发布时间:2017-08-20 12:42

Operating Principle of brushless motor

1.Structure and basic working principles of BLDC
    BLDC is a kind of synchronous motor, which means that its stator generates magnetic field and the magnetic field generated by the rotor is the same frequency, so BLDC will not generate the frequency difference of ordinary induction motor.In the BLDC, there is the difference between single phase, 2 phase and 3 phase motor, and the different types of the stator coil winding.Here we will focus on the most widely used three-phase BLDC.
1.1 Stator
   The BLDC stator is composed of a number of silicon steel sheets that are pressed and axial to press, each of which has a certain coil winding, which can be seen in the image below.In the traditional sense, the stator of the BLDC is similar to the stator of the induction motor, but there are some differences in the distribution of the stator winding.Most of the BLDC stator three motors in star line winding, each winding by many internal combination according to certain ways of the steel composition, an even number of distribution around the stator winding of an even number of poles.

  brushless fan motor
  BLDC stator windings of motors can be divided into trapezoidal and sine winding, whose fundamental difference is the way because of the different winding connection that they produce counter electromotive force (bemf related introduction, please attend the EMF section), trapezoid and sine waveform respectively, so use this naming.
1.2 Rotor
  The stator is 2 to 8 for the permanent magnet to be arranged around the rotor around the rotor (inner rotor type) according to the N pole and S pole. If it is the outer rotor type BLDC, then it is attached to the inner wall of the rotor.
1.3 Hall Sensor
   Unlike brushless dc motor, brushless dc motor USES electronic commutation.To get the BLDC to rotate, the stator is required to be energized in a certain order, so we need to know the position of the rotor so that the corresponding stator coils can be energized in accordance with the order of power.The location of the stator is perceived by the hall sensor embedded in the stator.Three hall sensors are usually arranged around the rotor path.No matter when, as long as the rotor's magnetic pole over the hall element, according to the rotor current magnetic polarity hall element will be output corresponding to high or low level, so as long as according to the three level produced by the hall element sequence can determine the position of the rotor current, and the corresponding electricity on stator winding. 
    Hall effect: when the electric conductor in magnetic field, because the charge in magnetic field force makes the conductor may be gathered to one side of the conductor, when a thin plate electric conductor in a magnetic field, the effect is more apparent side gathered the conductor of the charge will be the influence of the magnetic field, due to the gathered on one side of the charge in the conductor, so as to make the conductor produces voltage on both sides, this phenomenon is called the hall effect, E.H hall discovered this phenomenon in 1879, so the name.
2.1 Closed-loop control
   We can through the closed loop measurement current of the motor speed and achieve the goal of control motor speed, we expect by calculating the error of the rotational speed and actual speed, and then use the PID algorithm to adjust the PWM duty cycle to achieve the goal of control motor speed.
    For low - cost and low - speed applications, speed feedback can be obtained using hall sensors.Use a timer of the PIC18FXX31 microcontroller itself to measure the output signal of two hall components, and then calculate the actual rotational speed according to this signal. 
     In high speed applications, we can put photoelectric encoder in motor and can use its output by 90 ° signal is a measure of the speed and steering.Normally, also can output the PPR photoelectric encoder signal, allows for more accurate rotor position of the encoder scale to hundreds or even thousands of coding and coding scale, the more the higher the precision.
   According to lenz's law, when the BLDC turns, its winding will produce a reverse voltage opposite to the voltage of the end of the winding, which is the BACK EMF.Remember that the voltage on the counter electromotive force and the winding is reversed.The main factors that determine the anti-electromotive force are as follows:
   The angular velocity of the rotor; 
   magnetic field strength of rotor permanent magnet; 
   number of winding winding of each stator winding
   The formula for calculating the inverse electromotive force: Back EMF = (E) ∝ NlBw
    L=Length of rotor
    B=The flux density of the rotor
    W=Angular velocity of the rotor

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