The difference between VFD (Variable-frequency Drive) and reducer
VFD (Variable-frequency Drive)
VFD (Variable-frequency Drive) is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable speed operation of the motor. The control circuit completes the control of the main circuit, and the rectifier circuit converts the AC power. The DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit inverts the DC power into AC power. For VFDs (Variable-frequency Drives) that require a large amount of calculations, such as vector control VFDs (Variable-frequency Drives), sometimes a CPU for torque calculation and some corresponding circuits are needed. Variable frequency speed regulation achieves the purpose of speed regulation by changing the frequency of power supply to the motor stator winding.
Frequency conversion technology was born in response to the need for stepless speed regulation of AC motors. After the 1960s, power electronic devices have experienced SCR (thyristor), GTO (gate turn-off thyristor), BJT (bipolar power transistor), MOSFET (metal oxide field effect transistor), SIT (static induction transistor) , SITH (electrostatic induction thyristor), MGT (MOS controlled transistor), MCT (MOS controlled thyristor), IGBT (insulated gate bipolar transistor), HVIGBT (high voltage withstand insulated gate bipolar thyristor) development process, device update promotion The continuous development of power electronic conversion technology. Since the 1970s, research on pulse width modulation variable voltage variable frequency (PWM-VVVF) speed regulation has attracted great attention. In the 1980s, the PWM mode optimization problem, which is the core of frequency conversion technology, attracted people's strong interest, and many optimization modes were derived, among which the saddle wave PWM mode has the best effect. Since the second half of the 1980s, VVVF VFD (Variable-frequency Drive) in developed countries such as the United States, Japan, Germany, and the United Kingdom has been put on the market and has been widely used.
There are many classification methods for VFD (Variable-frequency Drive). According to the working mode of the main circuit, it can be divided into voltage type VFD (Variable-frequency Drive) and current type VFD (Variable-frequency Drive); according to the switching method, it can be divided into It is divided into PAM controlled VFD (Variable-frequency Drive), PWM controlled VFD (Variable-frequency Drive) and high carrier frequency PWM controlled VFD (Variable-frequency Drive); according to the working principle, it can be divided into V/f controlled VFD (Variable -frequency Drive), slip frequency control VFD (Variable-frequency Drive) and vector control VFD (Variable-frequency Drive), etc.; according to the classification of uses, it can be divided into general-purpose VFD (Variable-frequency Drive), high-performance dedicated VFD (Variable -frequency Drive), high-frequency VFD (Variable-frequency Drive), single-phase VFD (Variable-frequency Drive) and three-phase VFD (Variable-frequency Drive), etc.
VVVF: changing voltage, changing frequency CVCF: constant voltage, constant frequency. The AC power supply used in various countries, whether used in homes or factories, has a voltage and frequency of 400V/50Hz or 200V/60Hz (50Hz), etc. Generally, a device that converts alternating current with a fixed voltage and frequency into alternating current with a variable voltage or frequency is called a "VFD (Variable-frequency Drive)". In order to produce variable voltage and frequency, the device first converts the alternating current from the power supply into direct current (DC).
VFD (Variable-frequency Drive) used for motor control can change both voltage and frequency.
Working principle of VFD (Variable-frequency Drive)
We know that the synchronous speed expression of an AC motor is:
n＝60 f(1－s)/p (1)
in the formula
n————The speed of asynchronous motor;
f——frequency of asynchronous motor;
s————motor slip;
p————The number of pole pairs of the motor.
It can be seen from equation (1) that the rotation speed n is proportional to the frequency f. As long as the frequency f is changed, the rotation speed of the motor can be changed. When the frequency f changes in the range of 0 to 50 Hz, the motor speed adjustment range is very wide. VFD (Variable-frequency Drive) realizes speed regulation by changing the motor power frequency. It is an ideal high-efficiency and high-performance speed regulation method.
VFD (Variable-frequency Drive) control method
The low-voltage general frequency conversion output voltage is 380~650V, the output power is 0.75~400kW, and the operating frequency is 0~400Hz. Its main circuit adopts AC-DC-AC circuit. Its control method has gone through the following four generations.
1U/f=C sinusoidal pulse width modulation (SPWM) control method
It is characterized by a simple control circuit structure, low cost, and good mechanical properties and hardness. It can meet the smooth speed regulation requirements of general transmission and has been widely used in various fields of industry. However, at low frequencies, due to the low output voltage of this control method, the torque is significantly affected by the voltage drop of the stator resistance, which reduces the maximum output torque. In addition, its mechanical characteristics are not as good as those of DC motors, and its dynamic torque capability and static speed regulation performance are not satisfactory. Moreover, the system performance is not high, the control curve changes with the load, the torque response is slow, and the motor rotates slowly. The torque utilization rate is not high. At low speeds, the performance decreases and the stability deteriorates due to the existence of stator resistance and inverter dead zone effect. Therefore, people have developed vector control frequency conversion speed regulation.
2 Voltage space vector (SVPWM) control method
It is based on the overall generation effect of three-phase waveforms and aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulation waveforms at one time and controls them in a way that the inscribed polygon approximates a circle. After practical use, improvements have been made, namely the introduction of frequency compensation, which can eliminate speed control errors; estimate the flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speed; close the loop of output voltage and current to improve dynamic accuracy and stability. However, there are many control circuit components and no torque adjustment is introduced, so the system performance has not been fundamentally improved.
Vector control (VC) method
The method of vector control variable frequency speed regulation is to convert the stator current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system into the AC current Ia1Ib1 in the two-phase stationary coordinate system through three-phase-two-phase transformation, and then through According to the directional rotation transformation of the rotor magnetic field, it is equivalent to the DC current Im1 and It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), and then simulates the DC motor's The control method is to obtain the control quantity of the DC motor, and then realize the control of the asynchronous motor through the corresponding inverse coordinate transformation. Its essence is to equate an AC motor to a DC motor, and independently control the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current to obtain the two components of torque and magnetic field, orthogonal or decoupled control is achieved through coordinate transformation. The proposal of vector control method has epoch-making significance. However, in practical applications, since the rotor flux linkage is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is complex, making it difficult for the actual control effect to achieve the ideal analysis. result.
Direct torque control (DTC) method
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology has solved the above-mentioned shortcomings of vector control to a large extent, and has developed rapidly with novel control ideas, simple and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC transmission for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the flux and torque of the motor. It does not need to equate AC motors to DC motors, thus eliminating many complex calculations in vector rotation transformation; it does not need to imitate the control of DC motors, nor does it need to simplify the mathematical model of AC motors for decoupling.
Matrix cross-cross control method
VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all types of AC-DC-AC frequency conversion. Their common shortcomings are low input power factor, large harmonic current, DC circuit requiring large energy storage capacitor, and regenerative energy cannot be fed back to the grid, that is, four-quadrant operation cannot be performed. For this reason, matrix AC-AC frequency conversion came into being. Since the matrix AC-AC frequency conversion eliminates the intermediate DC link, large and expensive electrolytic capacitors are eliminated. It can achieve a power factor of l, a sinusoidal input current and four-quadrant operation. The power density of the system is high. Although this technology is not yet mature, it still attracts many scholars to conduct in-depth research. Its essence is not to indirectly control the current, flux linkage, etc., but to realize the torque directly as the controlled quantity. The specific method is:
——Control the stator flux and introduce the stator flux observer to realize the speed sensorless method;
——Automatic identification (ID) relies on accurate motor mathematical models to automatically identify motor parameters;
——Calculate the actual values corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. to calculate the actual torque, stator flux, and rotor speed for real-time control;
——Realize Band-Band control to generate PWM signals according to the Band-Band control of flux linkage and torque to control the switching state of the inverter.
Matrix AC-AC frequency conversion has fast torque response