Problems and Countermeasures in the application of

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Problems and Countermeasures in the application of frequency converters in cement plants

Abstract: it has excellent control performance and remarkable energy-saving effect in many production occasions

with the development of computer technology and power electronics technology, the application of general frequency converter has also developed rapidly. Because the AC motor is simple in structure, durable and without commutation device, it can be applied to various working environments, so the AC speed regulation system with general frequency converter as the core has been widely used. The adoption of frequency converter speed regulation can improve the control accuracy, production efficiency and product quality of machinery, is conducive to the automation of production process, makes the AC drive system have excellent control performance, and has significant energy-saving effect in many production occasions

1 energy saving principle of frequency converter

nowadays, the electric energy consumed by motors accounts for about 65% of the industrial power consumption, and the annual power consumption of fans and pumps accounts for 30% 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 cut-off analysis board of baffles and valves, which provides automatic and manual analysis modes. When the fan speed changes from n to n ', the change relationship between air volume Q, air pressure h and shaft power P: Q' =q (n '/n); H′=H(n′/n)2; P′=P(n′/n)3。 The air volume is directly proportional to the speed, the air pressure h is directly proportional to the second power of the speed, and the shaft power is directly proportional to the third power of the speed. Therefore, when the required air volume q is reduced, the output frequency of the frequency converter can be adjusted to reduce the motor speed n in proportion. At this time, the power P of the motor will be greatly reduced according to the cubic relationship, saving 40%~50% energy than adjusting the baffle and valve, so as to achieve the purpose of power saving

2 problems and Countermeasures in operation

at present, general frequency converter is more and more favored by users with its advantages of intelligence, digitization, networking and so on. With the expansion of the application scope of general frequency converter, there are more and more problems in operation, mainly including the following aspects: harmonic, vibration and noise, load matching, heating and other problems

2.1 harmonic and its suppression countermeasures

the main circuit form of general frequency converter is generally composed of three parts: rectifier part, inverter part and filter part. The rectifier part is a three-phase bridge uncontrollable rectifier, and the inverter part is an IGBT three-phase bridge inverter with PWM waveform output. The output voltage contains harmonics other than the fundamental wave. The lower harmonics usually have a great impact on the motor load, causing torque ripple, and the higher harmonics increase the leakage current of the inverter output cable, making the motor output insufficient. Therefore, the high and low harmonics output by the inverter must be suppressed, and the following methods can be used to suppress harmonics

2.1.1 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, the smaller the harmonic content. 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

2.1.2 install reactors

connect appropriate reactors (a-cl) in series at the input and output ends of the frequency converter to increase the rectifier impedance and suppress high-order harmonic current. Electronically change the specification and model of the tester: 100nm, 200nm, 500nm, 1000nm, 2000Nm, 3000Nm, 5000NM, 20nm, 50nm, 5nm, etc

2.1.3 transformer polyphase operation is adopted

general frequency converter is a six pulse impulse rectifier, so the harmonic generated is large. If the connection method of the secondary winding of the transformer is different, the phase between the two groups of three-phase AC power supplies will be staggered by 30 °. The secondary winding of the rectifier transformer adopts star and triangle connection methods respectively, and the pulse number of the rectifier circuit is increased from 6 pulses to 12 pulses, which can reduce the low-order harmonic current. 3. According to the anti pendulum impact energy required by the impact test, the punch can be used in the instrument panel, door trim and other cockpit interiors to better suppress the low-order harmonic

2.2 noise and vibration and Its Countermeasures

when the motor is adjusted by frequency converter, noise and vibration will be produced, which is the influence of high-order harmonic components in the output waveform of frequency converter. With the change of operating frequency, the fundamental component and harmonic component also change in a large range, so the harmonic of each part of the motor increases

2.2.1 noise problems and Countermeasures

motor noise generally includes ventilation noise, electromagnetic noise and mechanical noise

when the frequency converter drives, because the output voltage and current contain harmonic components, the harmonic magnetic flux of the air gap increases, so the noise becomes larger. Its characteristics are as follows: due to the resonance between the low harmonic frequency output by the frequency converter and the natural frequency of the rotor, the noise near the natural frequency of the rotor increases; Because the harmonic component output by the frequency converter makes the core, housing, shaft frame and other harmonics, resulting in the increase of noise near its natural frequency

in order to suppress the noise caused by the drive of the frequency converter, 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 noise: connect the AC reactor at the output side of the frequency converter. If the torque has margin, the u/f value can be set to be smaller, and special motors can be used (measures have been taken for noise), etc

2.2.2 vibration problems and Countermeasures

the causes of motor vibration can be divided into electromagnetic and mechanical

the vibration caused by electromagnetic reasons shows that the vibration component near the natural frequency increases due to the resonance between the lower harmonic component and the rotor; Vibration occurs due to the influence of pulsating torque generated by harmonics. Especially when the frequency of pulsating torque is consistent with the torsional natural frequency of the shaft system composed of motor rotor and load, resonance will occur

the vibration caused by mechanical reasons is manifested as: there is an extended weight on the motor shaft, etc. when the natural frequency of the shaft system decreases, if the motor runs at high speed, and the full rotation frequency is close to the natural frequency of the shaft system, the vibration intensifies. The residual unbalance of the rotor causes the centrifugal force to increase in proportion to the quadratic of the speed, so the vibration increases when the frequency converter is used for high-speed operation

in order to reduce the electromagnetic force generated by harmonics, 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 using PAM mode or square wave PWM mode frequency converter, you can use sine wave PWM or vector control mode frequency converter to reduce pulsating torque

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