The application of the hottest frequency converter

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Application of frequency converter and handling of problems during operation

1 application of frequency converter

the chemical equipment plant has many large-scale machining equipment, such as M1380 grinder, b2010 gantry planer, and ¢ 2500 vertical lathe. Due to process requirements, the motor must have excellent speed regulation performance. In the past, the AC motor was difficult to regulate speed, and the motors with high speed regulation performance required to adopt DC speed regulation. However, the DC speed regulator has a complex structure, which not only requires DC generator set and DC expansion unit, but also has large volume, large noise, difficult maintenance and large power consumption. Now, with the development of variable frequency speed regulation technology, AC variable frequency speed regulation is gradually replacing DC speed regulation. The adoption of variable frequency speed regulation can not only improve the control accuracy, production efficiency and product quality of production machinery, but also help to realize the automation of production process. Moreover, the variable frequency speed regulation system also has excellent control performance and remarkable energy-saving effect

most of the equipment in our factory left the factory earlier, with serious aging of the electrical part and frequent equipment failures. The electrical part of a large number of equipment needs to be transformed by frequency conversion and speed regulation technology. The M1380 cylindrical grinder in the metalworking workshop has been technically reformed without the original DC speed regulating device. Frequency converter speed regulation technology is adopted. The reformed equipment uses frequency converter to drive the motor, and the starting current is small. Soft start and stepless speed regulation can be realized, which greatly improves the starting performance. Constant power speed regulation can be carried out above the rated speed, which is easier to operate and more convenient for acceleration and deceleration control, so that the motor can achieve high performance and save 30% ~ 70% of electric energy, with remarkable effect. On the basis of successfully using the frequency conversion speed regulation technology to transform the M1380 grinder, the frequency converter has been widely promoted and applied in the technical transformation and self-made equipment of our factory. This technology has been used to transform the b2010 gantry planer and applied to the self-made 150t turntable and self-made boring machine, and good results have been achieved

2 problems and solutions during operation

with the expansion of the application scope of frequency converter, more and more problems occur in the operation of frequency converter, mainly including high-order harmonic, noise and vibration, load matching, heating and other problems. This paper analyzes the above problems and puts forward corresponding countermeasures

2.1 harmonic problems and solutions

the main circuit form of general frequency converter is generally composed of rectifier, inverter and filter. The rectifier part is a three-phase bridge uncontrollable rectifier, the middle filter part uses a large capacitor as the filter, the inverter part is an IGBT three term bridge inverter, and the input is PWM waveform. The output voltage contains other harmonics except the fundamental wave. The lower harmonic has a great impact on the motor load and will cause torque ripple; The higher order harmonics increase the leakage current of the inverter output cable and make the motor output insufficient. Therefore, the high and low order harmonics output by the frequency converter must be suppressed. The following methods can be used to suppress harmonics

(1) increase the internal impedance of the inverter power supply

generally, the internal impedance of the power supply equipment can buffer the reactive power of the DC filter capacitor of the inverter. The larger the internal impedance, the smaller the harmonic content, and this internal impedance is the short-circuit impedance of the transformer. Therefore, when selecting the inverter power supply. Select the transformer with large short-circuit impedance

(2) install reactors

connect appropriate reactors in series at the input and output ends of the frequency converter or install LC type harmonic filter to absorb harmonics and increase power supply or load impedance, so as to achieve the purpose of suppressing harmonics

(3) the transformer is used for multiple operations

the general frequency converter is a six pulse rectifier, so the harmonic generated is large. If multi-phase operation of transformer is adopted, the phase angle will be 30 ° different from each other. For example, the transformer composed of y- △ and △ - △ has the effect of 12 pulse transition. Part 1: look, it can reduce the low-order harmonic current and play a good role in restraining harmonics

(4) set a special filter

a special filter can be set to detect the frequency converter and phase, and generate a current with the same amplitude as the harmonic current and the opposite phase to the frequency converter, so as to effectively absorb the harmonic current

2.2 noise and vibration problems and solutions

the use of frequency converter speed regulation will produce noise and vibration, which is the impact of high-order harmonic components in the output waveform of the frequency converter. With the change of operating frequency, the fundamental component and higher harmonic component will change in a wide range, which is likely to cause resonance with all parts of the motor

2.2.1 noise problems and solutions

when a motor is driven by a frequency converter, the noise increases because the output voltage and current contain high-order harmonic components and the high-order harmonic magnetic flux in the air gap increases. The electromagnetic noise has the following characteristics: the small sample taken in the research field of new ceramic materials in the output of frequency converter is generally in the low-order harmonic component of [3~5] (3~5) (20~30) mm3, which resonates with the natural mechanical frequency of the rotor, so the noise near the natural frequency of the rotor increases; If the high-order harmonic component in the output of the frequency converter resonates with the core housing, bearing frame, etc., the noise near their respective natural frequencies increases

noise generated by frequency converter drive motor, especially harsh noise. It is related to the switching frequency of PWM control, especially in the low frequency region. The following measures are often taken to stabilize and reduce noise: connect AC reactor at the output side of frequency converter; The electromagnetic torque has margin, and the u/f can be reduced; When a special motor is used and the low-frequency noise volume is serious, check the resonance between the harmonic component in the output waveform of the frequency converter and the natural frequency of the shaft system (including load)

2.2.2 vibration problems and solutions

when the frequency converter is working, the magnetic field caused by the high-order harmonic in the output waveform generates electromagnetic force on many mechanical parts, and the frequency of the force is always close to or coincide with the natural frequency of these mechanical parts, resulting in vibration caused by electromagnetic reasons. The high-order harmonics that have a great impact on vibration are mainly the lower order harmonic components, which have a great impact in PAM mode and square wave PWM mode. However, when sine wave PWM mode is adopted, the lower order harmonic components are small, so the impact becomes smaller

the methods to reduce or eliminate vibration are as follows: an AC reactor is connected to the output side of the frequency converter to absorb the high-order harmonic current components in the output current of the frequency converter; If PAM mode or square wave PWM mode frequency converter is used, sine wave PWM mode frequency converter can be used to reduce pulsating torque; In terms of mechanical system formed by connecting motor and load, in order to prevent vibration, the whole system must be free from electromagnetic force harmonic generated by motor

2.3 load matching problems and solutions

there are many kinds of production machinery with different performance and process requirements, and different torque characteristics. Therefore, before applying the frequency converter, it is necessary to first understand the nature of the load carried by the motor, that is, the load characteristics, and then select the frequency converter and the electric motor. There are three types of loads: constant torque load, fan, pump load and constant power load. The load type uses "future" materials to meet the needs of more future industries. Different types of frequency converters should be selected

(1) constant torque load

constant torque load is divided into friction load and potential energy load. The starting torque of friction load is generally required to be about 150% of the rated torque, and the braking torque is generally required to be about 100% of the rated torque. Therefore, frequency converters with constant torque characteristics, large starting and braking torques and large overload time and overload capacity shall be selected, such as fr-a540 series

potential energy load requires large starting torque and energy feedback function, and can quickly realize forward and reverse rotation. Therefore, the frequency converter with four quadrant operation capability shall be selected, such as fr-a241 series

(2) fan and pump loads

fan and pump loads are typical square torque loads. At low speed, the load is very small and proportional to the square of speed. The combination of general frequency converter and standard motor is the most appropriate. This kind of load does not require high performance of the frequency converter, only requires economy and reliability. Therefore, the frequency converter with u/f=const control mode can be selected, such as fr-a540 (L). However, for such load torque, when the output frequency of the frequency converter is increased to above the power frequency, due to the sharp increase in power, if it exceeds the capacity of the motor frequency converter, the motor will overheat or fail to operate. Therefore, attention should be paid not to increase the output frequency to above the power frequency

(3) constant power load

constant power load refers to the load whose torque is inversely proportional to the speed, but the power remains constant, such as coiler, machine tool, etc. For the load with constant power characteristics, the problems that should be paid attention to when using frequency converter are: within the frequency range above the power frequency, the output voltage of the frequency converter is controlled by a fixed value. Therefore, the torque generated by the motor is of constant power characteristics. At this time, there is no problem using the combination of standard motor and general frequency converter; In the frequency range below the power frequency, the output voltage of the frequency converter is controlled by the u/f fixed value, and the torque generated by the motor tends to be opposite to the load torque. At this time, the combination of the standard motor and the general frequency converter is difficult to adapt. Therefore, the frequency converter should be specially designed

2.4 heating problems and solutions

the heating of the frequency converter is caused by internal losses. The main circuit accounts for about 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, it is necessary to effectively heat the frequency converter. The main methods are as follows:

(1) fan cooling the internal fan of the frequency converter can take away the heat dissipation inside the frequency converter box

(2) reducing the ambient operating temperature the frequency converter is an electronic device, which contains electronic components and electrolytic capacitors, so the temperature has a great impact on its service life. The ambient operating temperature of the general frequency converter is generally required to be between -10 ~ 50 ℃. If the ambient operating temperature of the frequency converter can be reduced this year, which is the 65th anniversary of the establishment of diplomatic relations between Mongolia and China, it can not only prolong the service life of the frequency converter, but also make the performance of the frequency converter more stable

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