Ten points to judge the pros and cons of inverter products!

Ten points to judge the pros and cons of inverter products!

The technical grade of the frequency converter is gradually developed and formed. With the improvement of use requirements and the development of technology, the quality of the device, the expansion of functions, and the progress of automation technology, it will gradually advance. Generally speaking, the following ten key points in judging the quality of a product must be considered first.

　　Inverter control method

　　The control method is the first to determine the performance of the inverter, and it depends on the software programming technology. With the advancement of the times and technological development, inverter control methods include:

　　(1) V/f=C belongs to open loop control;

　　(2) SVPWM space voltage vector control, which belongs to open loop control;

(3) Vector control VC, which belongs to closed loop control, has four types: current I, magnetic flux φ, torque T, and speed n; generally the inner loop is φ, T, I, and the outer loop is n (or w) .

　　(4) Direct torque control DTC is a closed-loop control, 400% Te is available when n=0, the above four are commonly used.

After 2000, inverters without feedback devices appeared in foreign countries, that is, there is no need to install additional sensors or encoders. The current, voltage, magnetic flux, and speed changes of the inverter during operation are used to feedback through detection and decoupling devices. To the input terminal, see Figure 1, Table 1, and Table 2. Since its inception, it has been welcomed and favored by users. It is mainly convenient to use, and its function is no less than the inverter with speed sensor feedback device. At present, the control modes of high-voltage inverters are mostly A and B. The non-feedback device is in trial production, and C is under development.

　　 Several unique inverter control methods

　　 (1) Optimize PWM vector control

　　 Sinusoidal pulse width modulation SPWM has the advantages of simple circuit and good linearity, but the output voltage is not high, and the maximum linear output voltage amplitude is only /2=0.866 of the input voltage. Its essence is a phase voltage control method. Optimized PWM vector control mode adopts dual modulation process, that is, the triangular wave phase pre-adjustment and the amplitude of the modulation signal (0～50Hz) are jointly adjusted to control the switching state of the IGBT. The method is through the switching angle α (30°～60°) and modulation Depth α=sine wave amplitude/triangular wave amplitude<0.95. It is characterized by low total harmonic current loss and low ripple torque. 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 is manifested as the output voltage is increased by about 20%, the harmonics are reduced, but the signal wave modulation depth The linearity with the switching angle is affected. As α increases, the voltage controllability becomes smaller, but the total harmonic current distortion THD value is still small, so it is often selected.

　　 (2) PWM modulation method control

　　①Synchronous modulation

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 increased current waveform pulsation harmonics, so it is only suitable for working in the frequency range of f>20Hz. This is the case with the V/F method.

　　②Asynchronous modulation

　　The characteristic of this modulation is that it can greatly improve the reduction of harmonic loss and torque ripple during low frequency operation, and it is suitable for f<20Hz low frequency operation.

　　③Random modulation

　　According to the period of the modulation signal and the output voltage value, the modulation method, modulation depth a and switching angle α can be automatically selected to adapt to closed-loop systems with large fluctuations in operating frequency, large changes in load torque or power, and impact. This kind of modulation method fundamentally solves the inherent shortcomings of SPWM in the low-frequency operation section, that is, the shortcomings of reduced torque, torque ripple, and greater harmonics. However, there are certain difficulties in software technology, and there is almost no such modulation method at home and abroad. The product.

　　 (3) Control according to the U-shaped characteristic curve of the motor and the V/f coordination mode

According to the control method of the motor U characteristic curve and V/f coordination-as everyone knows, V/f control is a static voltage regulation, that is, when f is constant, the voltage is also constant, and it is not based on the size of the motor load rate β Get the best control. Japan's AREX company was awarded the world-recognized super energy-saving controller 10 years ago. There is one more link, which can automatically search according to the actual load rate β (through the application-specific integrated circuit), and output the minimum operating voltage and minimum operating current (U Shape characteristic curve), so it is the most energy-saving, 8%-10% more energy than the general frequency converter under the same working conditions

　　 (4) Fuzzy control method

This method is controlled by the fuzzy control method according to the motor parameters and operating status. It is especially suitable for applications where general-purpose single-chip computers such as 8031 ​​are used as CPUs. The fuzzy control software simplifies and does not require mathematical models. It can also control multi-variables in real time. Random variables can reach a certain accuracy. Requirements, widely used in refrigerators, air conditioners, washing machines, microwave ovens and other home appliances. Of course, the fuzzy control is also applicable to the control of the inverter. For example, the Vacon inverter adopts the fuzzy current vector control.

　　 (5) Self-tuning (on-line measurement and control of motor parameters)

　　 (6) Recently, green without harmonics came out

　　Several control methods for future development

　　 (1) Intelligent control method:

　　① Based on the maximum of the three efficiency products of the inverter, motor, load (fan, pump, etc.), after passing the fuzzy control technology, input to the inverter, see block diagram 4 (Beijing Sifangyuan).

　　②The installation of passive filters mainly eliminates the 5th and 7th reverse-sequence harmonics, so that the THD% index value of the frequency converter is lower than the harmonic standard, almost close to zero.

　　③The final effect. Under different load rate conditions, the system efficiency is as high as 90% or more. Advanced filtering technology is adopted to make the output harmonic content lower than the national standard. The power factor is above 0.96, and the dynamic response time is not more than 30ms. In short, under the same working conditions, it can save more than 10% of electricity.

　　 (2) Double PWM control method

　　 Nowadays, the main circuit of voltage type AC and DC is widely used. SPWM modulation is only used in the inverter part, and the rectifier is three-phase uncontrollable. Due to the presence of harmonics in the input, the cosφ is also low and the loss is large, which has a certain impact on the power grid. With the wide application of frequency converters, the issue of harmonic pollution in the power grid has been brought up on the agenda. Experts have discussed that it is effective to use dual PWM control, that is, the rectifier bridge also uses controllable IGBT SPWM method. From theoretical analysis, cosφ=1, there is no harmonic, it has the reputation of green inverter, and can realize feedback regenerative braking.

　　(3) Matrix switch control mode is adopted-refers to the inverter DC/AC converter adopts matrix switch mode. Matrix inverter has the advantages of adjustable cosφ, no intermediate DC link, adjustable output voltage amplitude, phase and frequency, etc. It has good frequency conversion ability, input cosφ≈l, output current waveform is good, and harmonics are small. Matrix inverter is A direct cross-to-cross transformation. The switching device is a full control device. The SCR control mode is a chopping control mode. The AC-AC frequency converter adopts phase-shifting control SCR to realize the direct conversion of power between AC current and AC motor, which can realize the bidirectional flow of energy. Frequency conversion efficiency is high, but there is the disadvantage of low output operating frequency, which is similar to dual PWM performance. Japan Fuji has products available in 2010, mainly used in high-speed elevators, MXT type, which belongs to PG vector control, precision analog input is ±0.1% at the highest speed, digital input ±0.005. Monitoring PG is pulse encoder with rotation Moment/speed command, self-tuning function, distance estimation. THD<5%, cosφ>0.99 loss is reduced by 1/3, and the original large dielectric capacitors, AC reactors, and filters are not required.

　　(4) Soft switching control mode is adopted-refers to the inverter DC/AC converter is connected at voltage U=0, and disconnected at current I=0. In this way, the IGBT power loss is the smallest, the ideal value is zero, and the junction temperature is the lowest, which greatly reduces the heat generation during the working process, reduces the radiator and the air volume to the minimum, and greatly improves the capacity.

　　 power module

That is, the inverter DC/AC conversion device is an important part of the inverter. The quality of its module is very important to the inverter. The value of its device accounts for about 50% of the total value of the inverter, so it is worth causing The concern of manufacturers and users is a matter of course. At present, the commonly used power modules for low-voltage or high-voltage inverters include IGBT, IGBT-HV, IGCT, SGCT, IEGT, SiC, etc. Pay attention to the following on this issue:

　　 (1) The package type of the module can be single (medium, high power) or a combination of IPM and PIM.

(2) The safety factor used is 3 to 5 times the rated voltage of the voltage, and 3 to 5 times the rated current of the current, so that the device has a small burden during static operation, and there is sufficient margin during dynamic operation to ensure that the use of Safety and reliability, this is the case in foreign products, so the damage rate of the power module of the main circuit is almost zero, but the domestic voltage is only 2 to 3 times the rated voltage, and the current is 2 to 3 times the rated current. At this time, it is fine in static work, but in dynamic work, due to the small safety factor, the possibility of device damage is high, about 2 to 3%, especially for high-voltage inverters, with high voltages of 3kV, 6kV, and 10kV. The IGBTs are connected in series and parallel. At this time, attention must be paid to the static voltage equalization and dynamic voltage equalization. The safety factor of the withstand voltage, current and power value of the resistor and capacitor is too small, which not only causes the protection circuit device Most of the domestic IGBTs are only packaged, especially high-voltage, high-power devices, most of which are imported from abroad. Such as American IR company (International Semiconductor Company, Texas Company, etc.). Japan's Toshiba, Mitsubishi, Hitachi, etc. Siemens and Siemens in Germany. Chinese sunwoele and so on. The brand products of general large companies have high reliability, good reputation, and can be used with confidence. The fact is true, the ancients said: "I am not afraid of not knowing the goods, I am afraid of comparing goods."

　　 rectifier module

　　 The rectifier module is used for AC/DC rectification. Although it is relatively simple, there are also the above-mentioned points that should be noted.

　　 Microprocessor or CPU

　　The microprocessor or CPU is the core device of the frequency converter, and its importance is self-evident, like the human brain and heart are very important. In the past, most domestic frequency converters used 16-bit 87C196MC from American Intel Corporation, and Japan's Hitachi HD6433037F, MB90F462, MB6475328F10 Toshiba, etc. Germany is relatively rare. In recent years, special IC chips are generally not used, ASICs are used instead of signal microprocessors. One is that there are more sources, the other is low in price, the third is better, and the fourth is 32-bit, which has stronger anti-interference ability and faster transmission speed. , Good real-time performance and greater amount of information, which is also one of the important indicators to judge the quality of the inverter.

　　 filter capacitor

　　 In the main circuit of the AC drive, the filter capacitor is also one of the key components, and it is also one of the vulnerable components, with a normal life span of 3 to 5 years. When the capacity of the capacitor is less than or equal to 85%, it must be replaced when it is found to be swollen, leaking, or over-temperature (yellow casing) during use, otherwise it will affect the use, and the performance of the DC voltage will become small and unstable, and the output torque will decrease after loading , Instability, this is related to the quality of capacitors. In the past, most manufacturers used German or Hitachi dielectric capacitors, but the price was more expensive. Now they use China Nantong Haili brand with good quality and moderate price. It is produced by a Sino-Japanese joint venture. popular

　　Manufacturing craftsmanship

　　The same circuit structure, the same quality of components. Because the manufacturing process is not in place, it will also affect the product quality, which is manifested in many faults, especially for high-voltage, high-current, and high-power inverters. The process is also very important. The performance is as follows:

　　 (1) A safe distance between phases and the ground.

　　 (2) Copper bars are better than aluminum bars. Aluminum is easy to oxidize, has low conductivity, is easy to generate heat and overheat, and the junctions are in poor contact.

　　 (3) The connecting bolts should be blued or galvanized to prevent oxidation.

　　 (4) For high-current bolts, use a torque wrench and tighten them according to the specified torque. If necessary, attach a temperature test piece to the connection to measure the actual temperature, prevent loosening, and check it regularly.

　　 (5) The high-current copper or aluminum bar adopts a double-stacked structure to reduce the distributed inductance, which is beneficial to reduce stray losses and stabilize operating parameters. Many foreign manufacturers have adopted it.

　　 (6) The radiator and ventilator should be regularly dusted and the distribution of the ventilation ducts is reasonable.

　　(7) The connectors are coated with silver or gold in foreign countries. In China, it is often coated with zinc or pure copper, which is easy to oxidize, especially in the humid and high temperature environment, and the main circuit terminal should also be paid attention to.

　　Printed board PCB quality

　　Printed boards are also an important part of quality. At present, most manufacturers use SMT components for SMT assembly, wave soldering, and hand soldering of individual components. The voiding of the printed board in use is particularly important. Otherwise, it will easily cause poor contact of the voiding due to the increase in temperature during use, so that the voids on the front and back, and the middle fracture will also occur. The printed board should be made of epoxy resin or ABS engineering plastics. The lines should be smooth, uniform in thickness, and free of burrs. It should be treated with nickel melting, and the surface should be bright and smooth. The two sides of the printed board should be sprayed with a layer of moisture-proof coating, especially hot and humid. There should be moisture-proof treatment in the environment, and the entrance of the ventilator should be dried. Domestically, there are generally double-sided boards, while abroad there are three- or even four-layer boards.

　　 Stability and reliability are first

The control circuit of the frequency converter is the same as other electronic circuits. It is low voltage and milliampere current, so it is relatively delicate. In order to ensure its stable and reliable operation, it has certain requirements on the working environment, temperature and humidity. It is also very important to strengthen regular maintenance. Therefore, carelessness should not cause the deterioration and enlargement of the accident. The consequences will be unimaginable due to small losses and large losses (such as IGBT burnout, rectifier diode burnout, motor burnout, etc. It seems that the occurrence of such an accident will be beyond regret at that time. It is really "I knew today, why bother in the first place".