CN1. The frequency converter is classified according to the nature of the DC power supply
Voltage type--the energy storage element is a capacitor, and the controlled quantity is voltage, which is equivalent to providing a voltage source. Its dynamic response is slow. When braking, an anti-parallel inverter must be installed on the power supply side to achieve energy feedback. Adapt to multi-motor dragging. The AC voltage output by its inverter is a rectangular wave or a ladder wave, while the current waveform is close to a sine wave after being filtered by the motor load, but has a large harmonic component. Since it is used as a voltage source to provide AC power to the AC motor, the main advantage is that the operation is almost not affected by the power factor or commutation of the load; the disadvantage is that when the load is short-circuited or the load is put into the inverter, it is easy to appear. In case of overcurrent, protective measures must be applied within a short period of time.
Current type--The energy storage element is a reactor, and the large DC internal resistance is equivalent to providing a current source. The dynamic response is fast, and the feedback braking can be directly realized. The current type frequency conversion speed regulation system of the induction motor can realize four Quadrant operation is more suitable for stand-alone operation where one inverter supplies power to one motor.
Its advantage is that it has four-quadrant operation capability and can easily realize the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to commutate, the structure of the device is complicated, and the adjustment is difficult. In addition, since the grid side adopts thyristor phase-shift rectification, the input current harmonics are relatively large, which will have a certain impact on the grid when the capacity is large.
Second, the frequency converter is classified according to the working principle
V/f control --- V/f control frequency converter is to ensure that the output voltage is directly proportional to the frequency control, so that the magnetic flux of the motor can be kept constant, avoiding the phenomenon of magnetic field weakening and magnetic saturation, and it is mostly used for energy saving of fans and pumps , implemented with a voltage-controlled oscillator. The torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. Under the rated frequency, the voltage must reduce the frequency, the magnetic flux will become larger, and the magnetic circuit tends to be saturated. In severe cases, the motor will be burned. When the frequency and voltage should be changed proportionally, the magnetic flux of the motor should be kept constant to avoid the occurrence of magnetic field weakening and magnetic saturation.
Slip frequency control---slip speed regulation is to change the slip of the asynchronous motor to adjust the speed. The greater the slip, the slower the speed. Around the knee is the motor rotor series resistance. The system improves the static and dynamic performance of the system to a certain extent. At the same time, it is simpler than the vector control method. It has the characteristics of simple structure, easy implementation, and high control accuracy. It is widely used in the vector control speed regulation system of asynchronous motors.
There is no need for complex magnetic flux detection and cumbersome coordinate transformation. As long as the rotor flux linkage is constant, the indirect field-oriented control can be achieved through the calculation of the mathematical model by detecting the stator current and rotor angular velocity. To improve the dynamic performance of the speed control system, it mainly depends on controlling the rate of change of the rotational speed. Obviously, the purpose of control can be achieved by controlling the slip angle frequency.
Vector control--according to the characteristics of DC motor speed control, decompose the stator winding current of the asynchronous motor according to the method of vector transformation and form the magnetic field current component and torque current component similar to the DC motor, as long as the magnitude of the stator winding current of the asynchronous motor is controlled And phase, you can control the excitation current and torque current, so controlling the speed of the AC asynchronous motor is like controlling the DC motor, which has a good speed control effect.
Its main features are large low-frequency torque, fast dynamic response, and flexible control. It is generally used in harsh working environments and systems that require high-speed response and high-precision electric drag.
