Query on the principle of the hottest thyristor ph

2022-10-21
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Questioning on the principle of thyristor phase-shifting soft start

the starting current of three-phase asynchronous motor is large, and the harm to the power supply, the same equipment and the motor itself is self-evident. Over the years, people have made continuous research and practice in solving the problem of asynchronous motor starting, and have achieved a lot of scientific and practical experimental time: 665.766724 s, ingenious and effective methods and equipment

in recent years, due to the development of power electronics technology, the scientific, simple, effective and practical frequency conversion equipment that people have long expected has become a reality, and the technology of variable frequency speed regulation and variable frequency soft start of asynchronous motors at will. This paper mainly introduces the static load experimental machine technology, which is widely used in automation, intelligent control and automation equipment. "Soft start" has become the most fashionable synonym of modern industrial control technology

thyristor phase-shifting DC voltage regulation and AC voltage regulation technology have been widely used in DC motor speed regulation, AC and DC welding and many other aspects for many years. In the past, no one thought of using it in the starting of asynchronous motor, because everyone knows that asynchronous motor uses sinusoidal alternating current

but now there are thyristor phase-shifting soft start equipment and related manufacturers in the industrial control market. They apply the thyristor phase-shifting AC voltage regulation technology to the start of asynchronous motors, and are crowned with the most fashionable synonym of soft start. What's more puzzling is that the deceptive words in advertisements and brochures that do propaganda are fooled by the fact that there is no distinction between black and white, and the advantages and disadvantages are reversed

from the working principle of three-phase asynchronous motor: only three-phase symmetrical sinusoidal alternating current generates a circular rotating magnetic field; The electromagnetic torque produced is continuous and smooth; With the rapid development of social economy, the main factors affecting the current of asynchronous motor are: ① slip ratio S; ② Power supply voltage um; ③ Impedance Z; ④ Power factor cos φ。

the frequency converter with the latest and most advanced power electronic technology can control the motor current very small during the soft start process of asynchronous motor

iq ≤ in,

the reason is that it can smoothly and arbitrarily change and control the frequency f of three-phase symmetrical sinusoidal AC power supply, so as to smoothly change the speed n1 of the rotating magnetic field in the stator circle and make it follow the speed N2 of the rotor, Always maintain a very small slip ratio

s ≤ Sn,

the commonly used traditional step-down starting equipment is to reduce the power supply voltage um to start the motor at low voltage, reduce the motor starting current, and reduce the starting current to

iq = () in

in the starting mode of high-voltage squirrel cage asynchronous motor, the stator circuit string reactor, the stator circuit string liquid resistance, the rotor circuit string resistance are in the silicon metal impurities, oxygen content Focus on surface defects and geometric dimension control; The basic research on epitaxial growth dynamics, doping dynamics, defect composition and control law of the third generation semiconductor materials is carried out by changing the motor impedance Z and improving the power factor cos φ To reduce the starting current

in a word, the traditional step-down starting method does not change the basic properties of a single sinusoidal alternating current (i.e. the frequency remains unchanged), ensures that the fundamental properties of the asynchronous motor with circular rotating magnetic field and continuous and smooth electromagnetic torque remain unchanged, and the original mechanical characteristics of the asynchronous motor do not change during the starting process

the thyristor phase-shifting soft start is to change the waveform of sinusoidal AC voltage into non sinusoidal pulsed AC, and change the average voltage of AC by adjusting its duty cycle (as shown in the figure). Its average voltage is controllable and changes smoothly. The magnetic field generated by it on the winding of asynchronous motor is a rotating magnetic field with high-order vibration amplitude; Its rotor electromagnetic torque is an impulsive destructive torque with non smooth changes of large and small high-order vibrations. In terms of machinery, it produces vibration (rotor high-frequency mechanical impulse, whose impulse frequency f machinery = 2F power supply), causing mechanical damage to the motor and production machinery; Overvoltage is generated in the electromagnetic conversion of stator, which directly damages the insulation of motor and reduces the service life of motor; From the perspective of electric energy utilization efficiency, the motor is in the state of low power factor, high reactive power loss and high active power loss (rotor high-frequency mechanical vibration) during startup. Especially, the lower the average voltage (i.e. control angle α The larger the conduction angle β The smaller) the more serious the above problems are

some manufacturers claim that their soft start boxes have power-saving and energy-saving functions. In fact, this non sinusoidal pulse AC with changing duty cycle acts on the asynchronous motor. Compared with the traditional starting equipment, the power factor cos φ Lower, greater reactive power loss. Moreover, when the load rate is low, the duty cycle of the output waveform of thyristor phase-shifting AC voltage regulation is smaller, and the amplitude proportion of high-order harmonics increases. The above situation is more obvious, which can not save electricity, but the loss is more severe

one of the main purposes of reducing the starting current of asynchronous motor is to reduce the impact of large current on electricity and reduce the impact on the same equipment. However, in non sinusoidal pulse AC with varying duty cycle output from thyristor phase-shifting AC voltage regulating circuit, high-order harmonics have harmful pollution to electricity; It will affect the normal operation of electrical equipment, communication, television and other systems. Not only does it not work well, it works badly

in Karamay oilfield enterprises, many units bought this so-called soft start box at a high price. It didn't take a few days, and it was soon eliminated. All of them were replaced with real soft start equipment frequency converter

thyristor phase-shifting soft start, an immature product that has not been scientifically demonstrated and actually tested by the national authority, violates the basic laws and regulations of the country on the production and sales of mechanical and electrical products. Its popularity will have a negative impact on the normal production of enterprises and cause a huge waste of social wealth

References:

electrical engineering, published by machinery industry press, edited by Harbin Electrical machinery manufacturing school

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