1、 Frequency converter is classified according to the nature of DC power supply

Voltage type - the energy storage element is a capacitor, and the controlled quantity is voltage, that is to say, the voltage source is provided. Its dynamic response is slow. When braking, the reverse parallel inverter should be set on the power side to realize energy feedback, which can adapt to the multi motor driving. The output AC voltage of the inverter is rectangular wave or step wave, and the current waveform is close to sine wave after the motor load filtering, but has a large harmonic component. Because it is used as a voltage source to provide AC power to AC motor, the main advantage is that the operation is almost not affected by the power factor or commutation of load; the disadvantage is that when the load is short circuited or the load is put into operation in the inverter state, overcurrent is easy to occur, so protection measures must be taken in a short time.

Current type - the energy storage element is the reactor. The large DC internal resistance is equivalent to providing the current source, with fast dynamic response, which can directly realize the feedback braking. The induction motor current type frequency conversion speed regulation system can realize the four quadrant operation frequently and quickly, which is more suitable for the single operation mode of one inverter supplying power to one motor.

Its advantage is that it has four quadrant operation ability and can easily realize the brake function of the motor. The disadvantage of the device is that it is difficult to adjust the structure of the device. In addition, due to the use of thyristor phase-shifting rectifier on the grid side, the input current harmonics are large, and the large capacity will have a certain impact on the grid.

2、 Classification of frequency converter according to working principle

V / F control - V / F control inverter is to ensure that the output voltage is proportional to the frequency of the control so that the motor flux can be maintained to a certain extent, to avoid the phenomenon of weak magnetism and magnetic saturation, mostly used for fans, pumps, energy saving, using voltage controlled oscillators. The torque of the asynchronous motor is produced by the interaction between the flux of the motor and the current flowing through the rotor. At the rated frequency, the voltage must reduce the frequency, the flux becomes larger, and the magnetic circuit tends to be saturated. In serious cases, the motor will be burned. When the frequency and voltage are to be changed in proportion, the flux of the motor is kept to a certain extent to avoid the phenomenon of weak magnetic field and magnetic saturation.

Slip frequency control -- slip speed control is to change the slip of asynchronous motor to adjust speed. The larger the slip is, the slower the speed is. The winding knee is the series resistance of the motor rotor. The adoption of slip frequency control technology improves the static and dynamic performance of the system to a certain extent. At the same time, it is simpler than vector control method, with simple structure, easy realization and control accuracy It is widely used in the vector control system of induction motor.

Without complicated flux detection and complicated coordinate transformation, as long as the rotor flux size is constant, through detecting the stator current and rotor angular velocity, through the operation of mathematical model, the field orientation can be indirectly controlled. To improve the dynamic performance of the speed control system, it mainly depends on the change rate of the 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, the stator winding current of asynchronous motor is decomposed by vector transformation to form the field current component and torque current component similar to DC motor. As long as the size and phase of the stator winding current of asynchronous motor are controlled, the excitation current and torque current can be controlled, so as to control the AC asynchronous motor The rotating speed of the machine is like the control of DC motor, which has good speed control effect.

Its main features are large low-frequency torque, fast dynamic response and flexible control, which are generally used in the harsh working environment, high-speed response and high-precision electric drive system.