Problems and Countermeasures in the operation of t

2022-08-09
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Problems and Countermeasures in the operation of frequency converter

with the improvement of frequency conversion technology, AC motors are more and more widely used. The use of frequency conversion speed regulation can improve the control accuracy, production efficiency and product quality of production machinery, which is conducive to the automation of production process. AC drive system has excellent control performance, and has significant energy-saving effect in many production occasions

application of frequency converter

the power consumption of motors in China accounts for 60%~70% of the national power generation, and the annual power consumption of fans and water pump equipment accounts for 1/3 of the national power consumption. The main reason for this situation is that the traditional speed regulation method of fans, pumps and other equipment is to adjust the air and water supply volume by adjusting the opening of baffles and valves at the inlet or outlet. Its input power is large, and a large amount of energy is consumed in the process of intercepting the flow of baffles and valves. Because most fans and water pumps are torque loads of bungalows, and the shaft power has a cubic relationship with the speed, when the speed of fans and water pumps decreases, the power consumed also decreases greatly, so the energy-saving potential is very large. The most effective energy-saving measure is to use frequency converters to regulate the flow, and the power-saving rate of frequency converters is 20%~50%, with remarkable benefits

due to the needs of technology, many machines require that the motor can adjust speed. In the past, due to the difficulty of AC motor speed regulation, DC speed regulation was used in occasions with high speed regulation performance requirements. However, DC power has complex structure, large volume and difficult maintenance in winter. Therefore, with the maturity of frequency conversion speed regulation technology, AC speed regulation is gradually replacing DC speed regulation, and it is often necessary to carry out quantitative and direct torque control to meet the requirements of such repeated rolling and sampling processes

using the frequency converter to drive the motor, the starting current is small, and it can realize soft starting and stepless speed regulation. It is convenient to carry out acceleration and deceleration control, so that the motor can obtain high performance and greatly save electric energy. Therefore, the frequency converter has been more and more widely used in industrial production and life

existing problems and Countermeasures

with the expansion of the application scope of frequency converters, there are more and more problems in operation, mainly manifested in: high-order harmonics, noise and vibration, load matching, which is suitable for relatively large tension matching, heating and other problems. This paper analyzes the above problems and puts forward corresponding measures

harmonic problems and Countermeasures

the main circuit form of general frequency converter is generally composed of rectification, inversion and filtering. The rectifier part is a three-phase bridge uncontrollable rectifier, the middle filter part uses a large capacitor as the filter, and 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 harmonics usually have a great impact on the motor load and cause torque ripple; The higher harmonics will increase the leakage current of the output cable of the frequency converter and make the motor output insufficient. Therefore, the high and low harmonics output by the frequency converter must be suppressed. The following methods can be used to suppress harmonics. The pattern supplemented by plastic granulator heated by coal and natural gas

(1) increase the internal impedance of the power supply of the frequency converter

generally, the internal impedance of the power supply equipment can buffer the reactive power of the DC filter capacitor of the frequency converter. The larger the internal impedance is, the smaller the harmonic content is. This internal impedance is the short-circuit impedance of the transformer. Therefore, when choosing the power supply of frequency converter, it is best to choose the transformer with large short-circuit impedance

(2) install reactor

connect a suitable reactor in series at the input and output ends of the frequency converter, or install a harmonic filter. The filter is composed of LC type, which absorbs harmonics and increases the power supply or load impedance to achieve the purpose of suppression

(3) the transformer is used for multiple operations

the general frequency converter is a six pulse rectifier, so the harmonics are large. If the transformer is operated in multi-phase, the phase angle will be 30 ° different from each other. For example, the transformer with Y - △, △ - △ combination can form the effect of 12 pulse wave, which can reduce the low-order harmonic current and suppress the harmonic well

(4) set special harmonics

set special filters to detect the frequency converter and phase, and generate a current with the same amplitude as the harmonic current and the opposite phase, which is connected to the frequency converter, so that the harmonic current can be effectively absorbed

noise and vibration and Its Countermeasures

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 high-order harmonic component change in a wide range, which is likely to cause resonance with all parts of the motor

(1) noise problems and Countermeasures

when the motor is driven by the frequency converter, the noise increases because the output voltage and current include the formula: m=n/3 n is the slope of the spectral line (db/octave), which has high-order harmonic components, and the high-order harmonic magnetic flux of the air gap increases. Electromagnetic noise is characterized by the following: since the low-order harmonic component in the output of the frequency converter resonates with the natural mechanical frequency of the rotor, the noise near the natural frequency of the rotor increases. The higher harmonic component in the output of the frequency converter resonates with the bearing frame of the iron core housing, and the noise near the respective natural frequencies of these components increases

the noise generated by the inverter drive motor, especially the harsh noise, is related to the switching frequency of PWM control, especially in the low-frequency region. Generally, the following measures are taken to stabilize and reduce the noise: connect the AC reactor at the output side of the frequency converter. If the electromagnetic torque has margin, the u/f can be set lower. When a special motor is used and the low-frequency noise volume is serious, the resonance with the natural frequency of the shaft system (including load) shall be checked

(2) vibration problems and Countermeasures

when the frequency converter works, the magnetic field caused by the high-order harmonic in the output waveform produces 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 higher harmonics that have a great impact on vibration are mainly the lower harmonic components, which have a great impact in PAM mode and square wave PWM mode. However, when using sine wave PWM mode, the low-order harmonic component is small and the influence becomes small

in order to reduce or eliminate the vibration, an AC reactor can be 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. When PAM mode or square wave PWM mode inverter is used, sine wave PWM mode inverter can be used to reduce pulsating torque. For the mechanical system formed by connecting the motor with the load, in order to prevent vibration, the whole system must be free from electromagnetic force harmonics generated by the motor

load matching and Countermeasures

there are many kinds of production machinery with different performance and process requirements, and their torque characteristics are different. Therefore, before applying the frequency converter, we must first find out the load carried by the motor

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