1. Pulse width modulation (PWM) is an effective technology to control analog circuits through digital output, especially in the control of motor speed, which can greatly save energy. PWM has the characteristics of strong anti-noise ability, space saving and economy. The analog control circuit has the following defects: the analog circuit is easy to drift with time, which will produce some unnecessary heat loss and is sensitive to noise. After adopting PWM technology, the above defects are avoided, and the digital control of analog signals is realized, which can greatly reduce the cost and power consumption.

2.DC brushless motor

Brushless DC motor consists of motor, rotor position sensor and electronic switch circuit. The DC power supply supplies power to the stator winding of the motor through the switch circuit, and the position of the motor rotor is detected by the position sensor and provides a signal to trigger the power switch element in the switch circuit to turn it on or off, thus controlling the rotation of the motor. In the application example, the magnetic pole rotates, the armature is stationary, and the current commutation in the armature winding is realized by position sensor and electronic switch circuit. The armature winding of the motor is three-phase, the rotor is a permanent magnet, and the position sensor connected with the rotor shaft adopts Hall sensor. Within the range of 3600, the difference between two is 1200, and three are loaded. In order to improve the characteristics of the motor, the motor adopts the working mode of two-phase conduction star and three-phase six-state. The switch circuit adopts three-phase bridge connection mode.

Speed regulation and speed stability control

The timing circuit mainly adopts time base circuit LM555 and pulse width modulator SG 1525. LM555 is used to generate a square wave signal with a certain duty cycle and a fixed frequency. SG 1525 is a monolithic PWM controller chip, which has a reference regulated power supply with an output of 5. 1V, an error amplifier, a sawtooth oscillator with an oscillation frequency of100 400 kHz, a soft start circuit, a turn-off circuit, a PWM comparator, an RS register and a protection circuit. The integration problem of PWM circuit is solved, and the speed regulation of the system is realized by using this chip in an example. In a specific circuit, the position sensor signal is first shaped to form a required waveform with steep leading edge and trailing edge and a certain width. The differential pulse generated by difference channel triggers the time base circuit LM555 to form a square wave with a duty ratio of 2: 1, and the frequency of the square wave is about 200Hzo.

The formula for calculating the square wave frequency is: f= n * p/ 60, where Y 1 is the rated speed r/min of the motor, f is the frequency of the output signal of the position sensor, and p is the number of poles of the motor. After the square wave is filtered by the filter, the DC voltage is sent to the pulse width modulator and compared with the feedback voltage of the pulse width modulator. The obtained error signal is used to control the width change of the modulated square wave pulse output by the pulse width modulator, that is, the duty cycle change of the PWM output pulse. The voltage applied to the armature winding of the motor is adjusted by changing the duty ratio, and when the voltage changes, the motor current changes, and the rotation speed is changed according to the current.

Conclusion: In an application example, PWM has the following advantages for the speed control system: the response speed and stability accuracy of the system are better; Armature current pulsation is small, it is easy to be continuous, and it can work smoothly without filter reactance. The speed range of the system is wide; Few components and simple circuit.

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Application of PWM control technology in DC brushless motor speed regulation system-chicken blog (webpage)