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Principle of BLDC Motor using PWM to Control Speed
The PWM speed-regulating principle for the BLDC motor is different from the AC motor speed-regulating principle. It regulates the motor's rotating speed not via frequency modulation but through regulation of the driving-voltage pulse width. Meanwhile, it is combined with corresponding energy-storage components in the circuit, and changes the amplitude transmitted to the armature voltage. In this way, the AC motor's rotating speed can be changed.
To be specific, pulse width modulation (PWM) is an effective method which makes use of digital output to control the artificial circuit, particularly in controlling the motor's rotating speed and largely saving the energy. The PWM has a strong noise immunity, and is space-saving and relatively economic. At present, the analog control circuit has the following defects: The analog circuit can easily flow with the time, thus generating unnecessary heat loss and sensitivity to noises. After the PWM technology is adopted, the above defects can be avoided. The analog signals are controlled in the digital way, which can significantly reduce the cost and power consumption.
In the PWM-controlled BLDC motor, PWM control is realized through the following two ways:
In the speed-regulating circuit, the time-base circuit, LM555, and the pulse width modulator, SG1525, finishes the process. LM555 is used to generate one square signal with certain duty ratio and fixed frequency. SG1525 is the single PWM controller chip and consists of the benchmark voltage-stabilized source which outputs 5.1V voltage, error amplifier, sawtooth oscillator whose vibration frequency is within the scope of 100kHz and 400kHz, soft launch circuit, shut-off circuit, PWM comparator, register, and protective circuit. It can cope with integration of PWM circuit. In this case, this chip is used to realize system speed regulation. In a specific circuit, the position sensor signals are first shaped to form the waveform whose front and back edge are sharp. After differentiation of the differentiating circuit, the differentiated circuit thus generated can be used to trigger the time-base circuit, LM555, to form the square wave whose duty ratio is 2:1, and the square wave is around 200Hz.
This square wave frequency can be calculated by the formula: f= n * p/ 60. Where, Y1 denotes the rated rotating speed of the electric motor (r/min)，f denotes the signal of the position sensor's output signals，P denotes the number of poles of the electric motor. After undergoing filtering, the square wave forms the DC voltage to be sent into PWM to compare with the feedback voltage of PWM. The error signals obtained are used to control the width changes of the modulated square wave impulse output by the PWM. In other words, changes of the PWM output impulse makes use of the duty ratio changes to regulate the voltage imposed on the electric motor's armature winding at an attempt to change the voltage and the motor's current. The rotating speed changes via the current value.
To the speed-regulating system, PWM has the following advantages:
To be specific, pulse width modulation (PWM) is an effective method which makes use of digital output to control the artificial circuit, particularly in controlling the motor's rotating speed and largely saving the energy. The PWM has a strong noise immunity, and is space-saving and relatively economic. At present, the analog control circuit has the following defects: The analog circuit can easily flow with the time, thus generating unnecessary heat loss and sensitivity to noises. After the PWM technology is adopted, the above defects can be avoided. The analog signals are controlled in the digital way, which can significantly reduce the cost and power consumption.
In the PWM-controlled BLDC motor, PWM control is realized through the following two ways:
- Use PWM signals to control the breakover time of triode. The longer the breakover is, the longer the work is, the faster the motor rotates.
- Use PWM signals to control the breakover time of triode and change the value of control voltage for realization.
In the speed-regulating circuit, the time-base circuit, LM555, and the pulse width modulator, SG1525, finishes the process. LM555 is used to generate one square signal with certain duty ratio and fixed frequency. SG1525 is the single PWM controller chip and consists of the benchmark voltage-stabilized source which outputs 5.1V voltage, error amplifier, sawtooth oscillator whose vibration frequency is within the scope of 100kHz and 400kHz, soft launch circuit, shut-off circuit, PWM comparator, register, and protective circuit. It can cope with integration of PWM circuit. In this case, this chip is used to realize system speed regulation. In a specific circuit, the position sensor signals are first shaped to form the waveform whose front and back edge are sharp. After differentiation of the differentiating circuit, the differentiated circuit thus generated can be used to trigger the time-base circuit, LM555, to form the square wave whose duty ratio is 2:1, and the square wave is around 200Hz.
This square wave frequency can be calculated by the formula: f= n * p/ 60. Where, Y1 denotes the rated rotating speed of the electric motor (r/min)，f denotes the signal of the position sensor's output signals，P denotes the number of poles of the electric motor. After undergoing filtering, the square wave forms the DC voltage to be sent into PWM to compare with the feedback voltage of PWM. The error signals obtained are used to control the width changes of the modulated square wave impulse output by the PWM. In other words, changes of the PWM output impulse makes use of the duty ratio changes to regulate the voltage imposed on the electric motor's armature winding at an attempt to change the voltage and the motor's current. The rotating speed changes via the current value.
To the speed-regulating system, PWM has the following advantages:
- The system's response speed and stabilization precision are better.
- The armature current pulsating quantity is small and might easily be continuous so that there is no need to add external filtering reactance for work stability.
- The system's speed-regulating scope is wide.
- There are few elements to be used and the circuit is simple.
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