Tengfei talks about brushless DC motors

The brushless DC motor consists of a motor body and a driver, and is a typical mechatronic product. A brushless motor refers to a motor without brushes and commutators (or collector rings), also known as commutatorless motors. As early as the 19th century, when the motor was born, the practical motor produced was the brushless form, that is, the AC squirrel-cage asynchronous motor, which has been widely used. However, asynchronous motors have many insurmountable defects, so that the development of motor technology is slow. In the middle of the last century, the transistor was born, so the DC brushless motor that uses the transistor commutation circuit to replace the brush and the commutator came into being. This new brushless motor, called an electronically commutated DC motor, overcomes the shortcomings of the first generation of brushless motors.

The brushless DC brushless motor is a kind of synchronous motor, that is to say, the speed of the motor rotor is affected by the speed of the rotating magnetic field of the motor stator and the number of rotor poles (p):

n=60.f/p. When the number of rotor poles is fixed, changing the frequency of the stator rotating magnetic field can change the rotor speed. The brushless DC motor is a synchronous motor with electronic control (driver) to control the frequency of the rotating magnetic field of the stator and feed back the speed of the motor rotor to the control center for repeated correction, in order to achieve close to the characteristics of the DC motor. That is to say, the DC brushless motor can still control the motor rotor to maintain a certain speed when the load changes within the rated load range.

The DC brushless driver includes a power supply part and a control part: the power supply part provides three-phase power to the motor, and the control part converts the input power frequency according to the demand.

The power supply can be directly input by direct current (usually 24v) or by alternating current (110v/220v). If the input is alternating current, it must first be converted into direct current by a converter. Whether it is DC input or AC input, the DC voltage must be converted from the inverter to a 3-phase voltage to drive the motor before it is transferred to the motor coil. The inverter is generally divided into upper arm (q1, q3, q5)/lower arm (q2, q4, q6) by 6 power transistors (q1 ~ q6) and is connected to the motor as a switch that controls the flow through the motor coil. The control unit provides pwm (pulse width modulation) to determine the switching frequency of the power transistor and the timing of the inverter commutation. Brushless DC motors generally want to use speed control that can stabilize the speed at the set value without changing too much when the load changes, so the motor is equipped with a Hall sensor that can induce a magnetic field, as a closed-loop control of the speed. It is also used as the basis for phase sequence control. But this is only used for speed control and not for positioning control!

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