Structure characteristics of DC brushless motor

The brushless DC 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. In the case of a fixed number of rotor poles, changing the frequency of the stator rotating magnetic field can change the rotor speed. Brushless DC motor is a synchronous motor with electronic control (driver) to control the frequency of the stator's rotating magnetic field and feedback the motor rotor's speed to the control center for repeated corrections, in order to achieve a way 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 brushless DC drive includes a power supply unit and a control unit as shown in Figure (1): The power supply unit provides three-phase power to the motor, and the control unit converts the input power frequency according to demand.

The power supply unit 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 to direct current by a converter. Regardless of whether it is DC input or AC input, it is necessary to convert the DC voltage from an inverter into a 3-phase voltage to drive the motor before it is transferred to the motor coil. Inverter (inverter) is generally divided into upper arm (q1, q3, q5)/lower arm (q2, q4, q6) by 6 power transistors (q1 ~ q6) and 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 power transistors and the timing of inverter commutation. Brushless DC motors generally hope to be used for speed control where the speed can be stabilized at the set value when the load changes, and the speed will not change too much. Therefore, the hall-sensor that can sense the magnetic field is installed inside the motor as the speed. The closed loop control is also used as the basis for phase sequence control. But this is only used as speed control and cannot be used as positioning control.

Features of Brushless DC Motor:

1. It can replace DC motor speed control, inverter + frequency conversion motor speed control, asynchronous motor + reducer speed control;

2. It has the advantages of the traditional DC motor, and at the same time cancels the carbon brush and slip ring structure;

3. It can run at low speed and high power, eliminating the need for a reducer to directly drive a large load;

4. Small size, light weight, and large output;

5. Excellent torque characteristics, good middle and low speed torque performance, large starting torque and small starting current;

6. Stepless speed regulation, wide speed regulation range and strong overload capacity;

7. Soft start and stop, good braking characteristics, can save the original mechanical braking or electromagnetic braking device;

8. High efficiency, the motor itself has no excitation loss and carbon brush loss, eliminating the multi-stage deceleration loss, and the comprehensive power saving rate can reach 20%~60%.

9. High reliability, good stability, strong adaptability, simple repair and maintenance;

10. Resistance to turbulence and vibration, low noise, low vibration, smooth operation and long life;

11. No sparks, especially suitable for explosive places, with explosion-proof type;

12. Trapezoidal wave magnetic field motor and sine wave magnetic field motor can be selected as required.