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Function of frequency converter
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Function of frequency converterInverter is the power frequency (50Hz or 60Hz) transform into various frequency AC power and to realize the motor speed operation of equipment, which control circuit complete control of the main circuit, rectifier circuit will be converted into AC DC, DC intermediate circuit of the rectifier circuit output of smoothing filter, inverter circuit will DC and then reverse into alternating current. For a large number of inverters, such as vector control inverter, it is sometimes required to carry out a torque calculation of the CPU as well as some of the corresponding circuit. Frequency conversion speed regulation is achieved by changing the frequency of the motor stator winding power supply to achieve the purpose of speed.
There are many types of frequency converter, according to the classification of the main circuit mode can be divided into voltage type inverter and current source inverter; in accordance with the switch mode classification, can be divided into for PAM inverter control, PWM control inverter and the high frequency PWM inverter control; according to the working principle of the classification can be divided for the V / F control transducer, a slip frequency control inverter and vector control inverter etc.; according to the use of classification can be divided into general inverter, high performance special frequency converter, frequency converter, a single-phase inverter and three-phase inverter.
VVVF: change the voltage, changing the frequency of CVCF: constant voltage, constant frequency. The use of AC power supply, whether for home or for factories, its voltage and frequency are 400V/50Hz or 200V/60Hz (50Hz), and so on. In general, the voltage and frequency of the constant alternating current conversion to a variable voltage or frequency of the device called "inverter". In order to produce a variable voltage and frequency, the device first of all to turn the AC power into DC power (DC).
For motor control of the inverter, both can change the voltage, but also can change the frequency.
Working principle of inverter
As we know, the expression level of the synchronous motor's speed:
N = 60 f (1s) /p (1)
Type
N - speed of asynchronous motor;
F - frequency of asynchronous motor;
S -- motor slip;
P - - - - - - - - - - - - -.
By (1) known, speed n and the frequency f is proportional to, as long as change frequency f can change motor speed, when the frequency f in the 0 ~ 50 Hz range changes, motor speed adjustment range is very wide. Inverter is to change the frequency of the motor power frequency to achieve speed adjustment, is an ideal method for high efficiency, high performance.
Converter wiring diagram:
Inverter control mode
Low voltage general frequency conversion output voltage is 380 ~ 650V, the output power is 0.75 ~ 400kW, the operating frequency is 0 ~ 400Hz, its main circuit uses AC - direct circuit. The control mode has experienced the following four generations.
1U/f=C SPWM (sinusoidal pulse width modulation) control mode
The control circuit is characterized by simple structure, low cost, and good mechanical properties, which can meet the requirements of smooth speed regulation of the general transmission, and have been widely used in various fields of industry. However, this control mode at low frequency, because the output voltage is low, the torque is significantly affected by the stator resistance voltage drop, so that the maximum output torque is reduced. In addition, the mechanical properties after all, there is no DC motor, dynamic torque capacity and static timing performance is not satisfactory and low performance of the system, control curve will change with the change of the load, torque response is slow, motor torque utilization rate is not high, low speed due to the presence of stator resistance and dead time effect of the inverter and performance degradation, stability becomes poor. So people have studied the vector control frequency conversion speed regulation.
Voltage space vector (SVPWM) control method
It is the whole three-phase waveform generation effect as the premise, to approximate the motor air gap ideal circular rotating magnetic field trajectory for the purpose, a generation phase modulated waveform, inscribed polygon approximation circle control. The practice after use and improved, namely the introduction of frequency compensation, can eliminate the error of speed control; through the estimation of flux linkage amplitude feedback to eliminate influence of stator resistance at low speed; the output voltage and current loop in order to improve the dynamic accuracy and stability. However, the control circuit is more, and the adjustment of the torque is not introduced, so the system performance has not been fundamentally improved.
Vector control (VC) method
Vector control variable frequency speed regulation is the asynchronous motor in phase coordinates stator current IA, IB, IC, through three - phase transformation, equivalent to the two-phase stationary coordinate system of alternating current Ia1Ib1, by according to the rotor field oriented rotating transformation, equivalent to a synchronous rotating coordinate system under the direct current Im1, It1 (Im1 equivalent to the excitation current of the DC motor; It1 quite proportional to the armature current and torque), and imitate control method of DC motor, DC motor control variable is obtained, the corresponding coordinate through inverse transform, for asynchronous motor control. The essence of the AC motor is equivalent to a DC motor, respectively, the speed of the magnetic field of the two components of independent control. By controlling the rotor flux, and then decomposing the stator current to obtain the torque and magnetic field of the two components, the coordinate transformation, the realization of the orthogonal or decoupling control. The vector control method has the significance of the epoch. However in the practical application, owing to the impossibility of the accurate observation of rotor flux and system characteristics by the influence of motor parameters is larger, and in the equivalent DC motor control used in the process of rotation vector transform is more complex, making the practical control effect is difficult to achieve the ideal analysis results.
Direct torque control (DTC) method
In 1985, Germany's Ruhr university professor DePenbrock proposed the direct torque control of frequency conversion technology. The technology to a large extent
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