(1) Classification by DC power supply nature
I. Voltage-type – The energy storage element is a capacitor, and the controlled variable is voltage, which is equivalent to providing a voltage source. Its dynamic response is relatively slow. When braking, an anti-parallel inverter needs to be set on the power supply side to achieve energy feedback. It can be adapted to multi-motor drive.
Its inverter output AC voltage is a rectangular wave or a stepped wave, while the current waveform, after being filtered by the motor load, is close to a sine wave, but has a large harmonic component. Since it acts as a voltage source to provide AC power to the AC motor, its main advantage is that its operation is almost unaffected by the load's power factor or commutation; its disadvantage is that when the load is short-circuited or when a load is applied while the inverter is running, overcurrent is likely to occur, and protective measures must be applied within a very short time.
II. Current-type – The energy storage element is a reactor. Its DC internal resistance is relatively large, which is equivalent to providing a current source. It has a fast dynamic response and can directly realize regenerative braking. The current-type variable frequency speed control system for induction motors can frequently and quickly achieve four-quadrant operation, which is more suitable for single-machine operation mode where one inverter supplies power to one motor.
Its advantages include four-quadrant operation capability, which facilitates motor braking. Disadvantages include the need for forced commutation of the inverter bridge, resulting in a complex device structure and difficult adjustment. Furthermore, because the grid side uses thyristor phase-shifting rectification, the input current harmonics are relatively high, which can have a certain impact on the power grid when the capacity is large.
(2) Classification based on working principle
I. V/f control --- V/f control frequency converters ensure that the output voltage is proportional to the frequency. This keeps the magnetic flux of the motor constant and avoids the occurrence of weak magnetic field and magnetic saturation. It is often used for energy saving in fans and pumps and is implemented using a voltage-controlled oscillator.
The torque of an asynchronous motor is generated by the interaction between the motor's magnetic flux and the current flowing through the rotor. At the rated frequency, if the voltage decreases, the frequency increases, the magnetic flux increases, and the magnetic circuit tends to saturate, which can burn out the motor in severe cases. Therefore, the frequency and voltage must be changed proportionally to keep the motor's magnetic flux constant and avoid weak magnetic field and magnetic saturation.
II. Slip Frequency Control---Slip speed regulation is to adjust the speed by changing the slip of the asynchronous motor. The larger the slip, the slower the speed. The slip is the series resistance of the motor rotor. The adoption of slip frequency control technology improves the static and dynamic performance of the variable frequency speed regulation system to a certain extent. At the same time, it is simpler than the vector control method and 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 motor.
Without complex flux detection and cumbersome coordinate transformations, indirect field-oriented control can be achieved by detecting stator current and rotor angular velocity and performing calculations through a mathematical model, provided that the rotor flux linkage remains constant. To improve the dynamic performance of the speed control system, the main focus is on controlling the rate of change of rotational speed; clearly, this can be achieved by controlling the slip angular frequency.
III. Vector Control – Based on the characteristics of DC motor speed control, the stator winding current of the asynchronous motor is decomposed into magnetic field current components and torque current components similar to those of a DC motor using vector transformation. By controlling the magnitude and phase of the stator winding current of the asynchronous motor, the excitation current and torque current can be controlled. In this way, controlling the speed of the AC asynchronous motor is just like controlling the DC motor, resulting in good speed control effect.
Its main features are high low-frequency torque, fast dynamic response, and flexible control. It is generally used in harsh working environments and electric drive systems that require high-speed response and high precision.
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