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Discussion On Frequency Conversion Speed Regulation Of High Voltage Motor

Aug 26, 2020

Discussion on Frequency Conversion Speed Regulation of High Voltage Motor

With the continuous development of power, electronic technology, and computer technology, AC motor frequency conversion technology has been increasingly perfected and developed rapidly. Small and medium-sized low-voltage frequency conversion (under 690V) have been widely used. In addition to energy saving and consumption reduction, it has become an indispensable key equipment in the fields of machinery, papermaking, metallurgy, petrochemical, mining, building materials, and electric power. However, for the variable frequency speed regulation of medium and large capacity and high-voltage AC motors, in addition to price factors, reasonable transformation or design schemes, as well as comprehensive investment and cost-effectiveness, have also become factors restricting its widespread application.


  1. Current situation of high voltage motor frequency conversion speed regulation scheme


The high-voltage and large-capacity inverter is a high-tech with high technical content and difficulty. Its development and production are difficult: because the power supply voltage of the high-voltage inverter is high, the withstand voltage level of the power electronic components in the world is relatively high. Low; high-voltage and high-power frequency converters are technically difficult and require high manufacturing technology. However, it is mainly used for energy-saving operation of fans, water pumps, etc. It requires low investment, and the investment is repaid by saving electricity.


   2. Classification and characteristics of high voltage frequency conversion schemes


  2.1 High-low-high scheme


This solution is based on a universal low-voltage inverter, that is: add a step-down inverter to the input side of the low-voltage inverter, and reduce the 6KV/3KV to the input voltage of the low-voltage inverter; add it to the input side of the inverter A step-up transformer to match the output voltage with the rated voltage of the motor. The high-low-high frequency conversion speed control scheme is more common in the domestic market and is currently the most widely used. The market is mainly based on Siemens products. In this scheme, it can be divided into 6-pulse rectifier unit and 12-pulse rectifier unit according to the form of DC power supply rectifier unit. The system structure of the 6-pulse rectifier unit is simple, and the cost of the hardware composition is low; the 12-pulse rectifier unit is composed of a three-winding transformer with a difference of 30 degrees on the secondary side and two identical rectifier units. The secondary side must be connected to the incoming line reactor, so its relative cost is higher.However, from the perspective of harmonic components, the effect of 12-pulse rectification is better than that of 6-pulse, and the harmonic components of 6-pulse are 5, 7, and The harmonic components of 11, 13, 17, 19, 23, 25, 12 pulsation are 11, 13, 23, 25.


  This program is most suitable for renovation projects. Advantages: (1) The program is mature. Taking Siemens' products as an example, the number of sets that have been used in China has been issued. (2) The original motor cable does not need to be modified. (3) It is convenient to realize the switching function. (4) Compared with the high-high scheme, the investment is lower. Its shortcomings: There are more high-low or high-high system components, because of the loss of the filter and step-up and step-down transformers, the long-term operating cost is relatively high.


  2.2 High-low-low scheme


The core of the    high-low-low scheme is still based on low-voltage inverters. A double-winding step-down transformer is installed on the input side of the inverter to turn the high-voltage motor into a usable low-voltage variable frequency drive voltage level. This program currently has more users in China. Siemens products are used in many applications. The system is mainly composed of dual-winding step-down frequency converters and transformers. This system has a relatively simple structure and is especially suitable for new projects. For retrofit projects, replacing the motor will increase project expenditure. The harmonic components are mainly 5, 7, 11, 13, 17, 19, 19, 23, 25 times.


  The advantages of this program are: (1) The program is mature. Low-voltage IGBT or IGCT power devices are very mature and have a wide range of applications. (2) The structure is simple, and the maintenance is few. (3) Less investment and quick results. Low-voltage inverters are cheap and the market price is quite transparent. (4) The loss of the step-up transformer and the long-term operation cost are low. The shortcomings of this scheme: When used in a renovation project, the motor needs to be replaced, which increases the workload of the renovation, which also increases the project investment and prolongs the construction period.


  2.3 High-High Plan


  High-High frequency converter is the high voltage power supply directly sent to the high voltage frequency converter after the high voltage switch cabinet, after the high voltage frequency conversion, the voltage is directly supplied to the stator of the high voltage motor.


The medium and high voltage variable frequency speed regulation system mainly has 4 forms: (1) power components (GTO, SCR, SGCT) in series; (2) mid-point clamp is a position type, that is, three-point flat type (GTO, IGBT); (3) Multi-level type, among which four-level, five-level, etc. (IGBT) are commonly used; (4) Single-phase inverter series type. These high-voltage inverters have all been introduced in recent years, with advanced technology, high efficiency, and small footprint. They are the main development direction of high-voltage inverters.


  High-High scheme adopts the way of unit series connection (IGBT), using the principle of phase shifting and superposition (phase shifting transformer), effectively suppressing harmonic interference, making the output waveform almost sinusoidal. It has the following advantages: No need to add any output filtering or power compensation device; the use efficiency can be as high as 97%, and the power factor is higher. Without any compensation device, when the load is> 20%, the power factor is greater than 95% load >20%; There is no pulsating torque caused by harmonics, which can avoid load resonance and extend the service life of the motor and mechanical equipment; it can prevent the main motor and cable insulation from being damaged by dv/dt stress; the motor is not subject to common mode The influence of voltage; there is no limitation on the length of the motor cable within the allowable range of the voltage drop; no pollution to the power grid; the main component is highly reliable due to the redundant design and can adapt to extreme working conditions; the power unit has an automatic bypass function, It can improve the reliability of power supply; it has the function of non-stop for instantaneous power failure. When the power supply voltage drops instantaneously or fails (within 5s), the inverter will not stop. After the power supply voltage is restored, it will immediately accelerate again and return to its original state. When the power supply stops momentarily, the inverter will not stop running within 300ms.


  3, the selection principle of high voltage inverter


   When choosing a high-voltage inverter, you should make a comprehensive comparison of the high-voltage inverters of various manufacturers. Don't be misled by certain advertisements and unrealistic articles, let alone be affected by habitual factors. When choosing a high-voltage and large-capacity inverter, you should pay attention to the following principles: first, reliability and after-sales service; second, price; third, the main technical indicators; finally, the delivery period and the reputation of the manufacturer.


   4. Conclusion


  The frequency conversion speed regulation device is a high-tech product of mechatronics. From the perspective of the development process and development trend of the frequency converter device, it is still mainly represented by power devices. Frequency conversion speed regulation has higher efficiency, wider range and higher accuracy, and complete specifications, which can meet various needs and has a wide range of applications. It is the most promising and ideal speed regulation method.