The frequency conversion technology is actually based on the principle of motor control, and the motor is controlled by a so-called frequency converter. The motor used for this type of control is called a variable frequency motor.
Common variable frequency motors include: three-phase asynchronous motor, DC brushless motor, AC brushless motor and switched reluctance motor.
Control principle of variable frequency motor
Generally, the control strategy of the variable frequency motor is: constant torque control at the base speed, constant power control above the base speed, and weak magnetic field control in the ultra high speed range.
Base speed: The back EMF is generated when the motor is running, and the back EMF is usually proportional to the speed. Therefore, when the motor runs to a certain speed, since the magnitude of the back electromotive force is the same as the magnitude of the applied voltage, the speed at this time is called the base speed.
Constant torque control: The motor performs constant torque control at the base speed. At this time, the back electromotive force E of the motor is proportional to the rotational speed of the motor. In addition, the output power of the motor is proportional to the product of the torque and speed of the motor, so the motor power is proportional to the speed.
Constant power control: When the motor exceeds the base speed, the motor's back-EMF is kept constant by adjusting the motor's excitation current to increase the motor's speed. At this point, the output power of the motor remains basically constant, but the motor torque decreases inversely proportional to the speed.
Weak magnetic control: When the motor speed exceeds a certain value, the excitation current is already quite small, and basically can not be adjusted again. At this time, the field weakening control is entered.
The speed control and control of the motor is one of the basic technologies of various types of machinery and office and industrial and electrical equipment in industry and agriculture. With the amazing development of power electronics technology and microelectronic technology, the AC speed regulation mode of “special frequency induction induction motor + frequency converter†is leading a new place in the field of speed regulation with its excellent performance and economy. The change of the speed control method is changed. It brings the gospel to all walks of life: to greatly improve the degree of mechanical automation and production efficiency, save energy, improve product qualification rate and product quality, correspondingly improve Power System capacity, miniaturize equipment, and increase comfort. The fast speed replaces the traditional mechanical speed regulation and DC speed regulation scheme.
Due to the special nature of the variable frequency power supply and the system's demand for high speed or low speed operation and dynamic speed response, the demanding motor is a demanding requirement, which brings new problems in the electromagnetic, structural and insulation aspects of the motor. .
Application of variable frequency motor
The frequency conversion speed regulation has become the mainstream speed regulation scheme, and can be widely applied to the continuously variable transmission of various industries.
Especially with the increasing application of frequency converters in the field of industrial control, the use of variable frequency motors is becoming more and more extensive. It can be said that because of the superiority of inverter motors in frequency conversion control compared with ordinary motors, wherever inverters are used, It is not difficult to see the figure of the inverter motor.
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