The technical grade of the frequency converter is gradually developed and formed. With the improvement of the use requirements and the development of technology, the quality of the device, the expansion of functions, and the advancement of automation technology are gradually advanced. Generally, the following ten points are generally considered in determining the quality of the product, as follows:
1, the control mode of the inverter
The control method is the first to determine the performance of the inverter, it depends on the software programming technology. With the advancement of the times and technological development, the inverter control methods are:
(1) V/f=C is an open loop control;
(2) SVPWM space voltage vector control, which is open loop control;
(3) Vector control VC, which is closed-loop control, with current I, magnetic flux φ, torque T, and rotational speed n; generally inner ring is φ, T, I, and outer ring is n (or w) .
(4) Direct Torque Control DTC is a closed-loop control. It can have 400% Te at n=0, the above four commonly used.
After 2000, there are inverters without feedback devices in foreign countries, that is, there is no need to install additional sensors or encoders, and the current, voltage, magnetic flux, and rotational speed changes of the inverter during operation are passed, and device devices such as detection and decoupling are fed back. See Figure 1, Table 1, and Table 2 for the input. Since its inception, it has been well received and favored by users. It is mainly convenient to use and functions as a frequency converter with speed sensor feedback device. At present, the control mode of high-voltage inverters is mostly A and B. No feedback device is in trial production, C is in development.
2. Unique control methods for several inverters
(1) Optimized PWM vector control
The sinusoidal pulse width modulation SPWM has the advantages of simple circuit and good linearity, but the output voltage is not high. The maximum linear output voltage amplitude is only /2=0.866 of the input voltage, which is essentially the phase voltage control method. The optimized PWM vector control mode adopts a dual modulation process, that is, the amplitude of the triangular wave phase and the modulation signal (0 to 50 Hz) are adjusted together to control the switching operation state of the IGBT by the switching angle α (30° to 60°) and modulation. Depth α = sine wave amplitude / triangle wave amplitude < 0.95. The characteristic is that the total harmonic current loss is small and the pulsation torque is small. When α=0.92~0.95, the harmonic loss is the smallest, especially when the carrier frequency is low, it can fully show its superiority, which shows that the output voltage is increased by about 20%, the harmonic is reduced, but the signal wave modulation depth is The linearity between the switch angle and the switch angle is affected. As the α increased voltage control becomes smaller, the total harmonic current distortion THD value is still small, so it is often used.
(2) PWM modulation method control
1 synchronous modulation
The number of triangular waves included in one modulation signal period is constant, and the carrier period is changed proportionally while changing the signal period, so that ≥3 and must be an odd number. When the switching frequency is low, the symmetry of the output waveform can be guaranteed, but at a lower frequency, the number of carriers appears sparse, resulting in an increase in the ripple of the current waveform, so it is only applicable to the frequency range of f>20Hz, the current This is the case with the V/F method.
2 asynchronous modulation
The carrier period remains unchanged while the modulation signal changes periodically. That is, the modulation characteristic is that the harmonic loss and the torque ripple are greatly improved during low frequency operation, and the f<20Hz low frequency working segment is applicable.
3 random modulation
According to the period of the modulation signal and the voltage value of the output, the modulation mode, the modulation depth a and the switching angle α can be automatically selected to adapt to the closed-loop system with large operating frequency variation amplitude, large load torque or power variation and impact. This modulation method fundamentally solves the shortcomings of SPWM in the low-frequency working segment, that is, the shortcomings of torque, torque ripple and harmonics. However, there are certain difficulties in software technology, and there is no such modulation method at home and abroad. The product. near
(3) Control according to the U-shaped characteristic curve of the motor and the V/f matching mode
According to the control method of the U characteristic curve of the motor and V/f, it is known that the V/f control is static voltage regulation, that is, when f is constant, the voltage is also fixed, and it is not according to the motor load factor β. For optimal control. Japan’s AREX company won the world-recognized super energy-saving controller 10 years ago. It has an additional link, which can automatically search (through ASIC) according to the actual load rate β, and output the minimum working voltage and minimum working current (U). Shape characteristic curve), therefore it is the most energy-saving, it can save 8%~10% more power than the general frequency converter under the same working conditions, and cos is a dynamic power adjustment controller, especially suitable for fans and pumps. . In the energy-saving operation, the effect is very good, the early products are SMX series. In recent years, in order to expand the scope of use, it is suitable for speed regulation, and the new variety is VMX series. Later, Hitachi J300 series inverters also have this function during energy-saving operation. The U-shaped characteristic curve of the motor is shown in Figure 2.
(4) Fuzzy control mode
Fuzzy control (FUZZY)
This method is controlled by the fuzzy control method according to the motor parameters and operating state, and is especially suitable for the application of the general-purpose single-chip microcomputer such as 8031 as the CPU. The fuzzy control software simplifies the mathematical model, and can control the multi-variable real-time. The random variable can reach a certain precision. It is required to be widely used in household appliances such as refrigerators, air conditioners, washing machines, and microwave ovens. Of course, fuzzy control is also applicable to the control of the frequency converter. For example, the Vacon inverter uses fuzzy current vector control (see Figure 3).
(5) Self-tuning (motor parameter online measurement control)
(6) Recently, green harmonics have come out