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Keywords: thyristor, motor, trigger, soft start
1. introduction
AC asynchronous motor has been widely used in China's coal mines, petroleum, chemical industry, steel, power generation, water supply, cement building materials and other fields due to its simple structure, high operating reliability, strong environmental adaptability and excellent drag performance . However, the fatal disadvantage of AC asynchronous motors is that the starting shock is large, which will adversely affect the system. The main impact is reflected in two aspects:
(1) Electrical problems:
It can reach 5-7 times the rated current when starting, which causes the motor winding to overheat due to the current, thereby accelerating its insulation aging, causing voltage fluctuations in the power supply network, and affecting the normal operation of other equipment under the same power grid.
(2) Mechanical issues:
Excessive starting torque produces mechanical impact, which causes large impact on the driven equipment, shortens the service life, and affects accuracy. Such as damage to the coupling, tearing the belt, etc. It causes abnormal wear and impact of mechanical transmission components, accelerates aging, shortens life and increases maintenance workload.
This problem was encountered on the air compressor of the nitrogen production line of the Shougang Cold Rolling Plant. Due to the relatively large motor power (10KV / 3730KW and 10KV / 2250KW), the mechanical impact on the motor and compressor during the starting process is relatively large. Increased the workload for system maintenance; secondly, the motor fell more seriously when the motor was directly started, and other equipment could not work normally when the voltage drop of the grid reached 85%. In view of the above reasons, decided to use PowerEasy thyristor soft start device.
2. The composition and principle of thyristor series high voltage soft start
2.1 Main circuit structure
The thyristor series soft start device is a fully digital intelligent starting device that combines power electronic technology, optoelectronic technology control technology and micro-processing technology. The wiring diagram of the main circuit of thyristor series soft start is shown in Figure 1. It is mainly composed of high-voltage thyristor series valve group and bypass contactor. It is composed of series and parallel connection, supplemented by absorption and equalization clamping circuits to ensure its reliability in high-voltage environment. When the incoming line is powered, the conduction angle of the thyristor is controlled to chopp the AC three-phase power supply, and the amplitude of the output voltage is controlled. And after the starting process is completed, the bypass contactor is closed, the soft start device is switched to the bypass state, and the thyristor is closed at the same time. 
Figure 1 Wiring diagram of the main circuit of thyristor series soft start
As shown in Figure 2, the control circuit of the thyristor series high-voltage soft start device is generally composed of a main control circuit, a trigger device, a signal acquisition circuit, and a human-machine interface. 
Figure 2 Soft start control block diagram
The main control circuit is mainly composed of a digital circuit composed of a DSP microprocessor and a CPLD large-scale gate array. Its main function is to generate a pulse command to trigger the thyristor, and the trigger command of the thyristor is issued through a digital signal processor (DSP). The main control circuit can adjust the conduction angle of the thyristor according to the voltage and current signals collected by the signal collection circuit, so as to adjust the motor current, and realize the closed-loop control of the motor, that is, current limiting start. While sending out the trigger pulse, it can also monitor the operating state of the motor and the soft starter itself through feedback signals such as voltage, current and temperature, protect the entire system, and ensure the safe operation of the system.
2.2.2 Trigger system
The trigger system is a key part of the system's stability and reliability. It must have the ability to resist noise interference and be able to turn on the thyristor in time and accurately. There are two main technical problems here:
1. Trigger pulse can trigger thyristor in time and accurately
Thyristor is a current-controlled bipolar semiconductor element, which requires the drive unit of the gate to be similar to a current source and can provide a particularly steep spike current pulse to the thyristor to ensure accurate and reliable triggering at any time. Thyristor. Therefore, the gate trigger pulse characteristics of the thyristor have a very strong influence on the normal operation of the thyristor.
When the thyristors are used in series, we require that the thyristors connected in series are turned on as much as possible, because the slower turn-on may withstand overvoltage and damage the components. At this time, the turn-on time difference of the thyristors in the same group is required to be as small as possible.
The turn-on speed of the thyristor is mainly related to the amplitude of the gate trigger pulse and the steepness of the pulse rising edge. In other words, the greater the amplitude of the gate trigger pulse, the steeper the rising edge of the pulse, and the shorter the turn-on time of the thyristor. Therefore, in the field of motor soft start, most manufacturers use a strong trigger method to trigger the thyristor. That is, the current amplitude of the trigger pulse IG is greater than or equal to 10IGT, and the pulse rising time tr≤1μs. In short. In order to ensure the reliability of the thyristor valve string operation, IG is much larger than IGT.
2. Trigger mode of trigger thyristor
There are three trigger systems for high voltage isolation, pulse transformer, direct optical trigger and optical fiber trigger. The pulse transformer has a simple structure and low cost, but it has poor electromagnetic compatibility and is not suitable for use in high-voltage environments; although the direct light trigger system is reliable and accurate but expensive, it is rarely used; the fiber trigger system has good reliability, although it is more reliable than direct light The trigger system adds some complexity to the circuit, but the relative cost is much lower and the trigger is accurate and reliable, so it is the first choice for high-voltage applications. The trigger systems of high-voltage soft starters on the market basically use fiber optic trigger systems.
In high-voltage soft start, the working power of the trigger system is generally obtained by high-level energy acquisition, that is, the working power of the trigger circuit board is supplied by the capacitor on the dynamic voltage equalization (ie, RC absorption circuit), which avoids the use of high-voltage transformers. The high cost comes, and electrical isolation is achieved to avoid interference of the trigger signal. It should be pointed out that this method of high-level energy acquisition is only applicable during the soft start process, while in the soft stop process designed for some pump loads, due to the current in the main loop increasing in the second half of the stop process If it is small, it will affect the acquisition of power on the trigger board, thus affecting the triggering of the thyristor. Therefore, the thyristor soft start device of some manufacturers will separately provide additional power to the trigger system when producing products with soft stop function, which increases the cost. The soft start devices of some foreign companies such as AB are more complete in this regard. They use a technology called low-level energy acquisition, that is, a low-voltage high-power transformer is used on the low-voltage side, and a trigger board is used. A set of current sensing device, the transformer wire is passed through the current sensing device on the trigger board, and the induced current is obtained on the trigger circuit board to act as a power source, so that the trigger board can obtain enough power to continue triggering in the late stage of soft stop The thyristor can make the later parking process of soft parking more stable, and reduce the impact of the water hammer effect of the pump load on the pipeline and the impeller.
2.2.3 Signal acquisition circuit and human-machine interface
The main function of the signal acquisition circuit is to collect various signals and perform preprocessing, and then feed back the processed signals back to the main control circuit. At present, there are two main collection signals, voltage signal and current signal, and some soft start manufacturers also collect temperature and speed signals. The voltage and current signals are generally collected through the transformer, and after processing, they are fed back to the main control circuit to judge the operating status of the motor and soft start device based on the feedback data, and trigger pulses are issued or judged by the feedback signal when the overall system operation is abnormal. Failure and issue protection commands.
The man-machine interface circuit is generally composed of an LCD screen and a keyboard, which are used to complete the user's parameter setting, select the starting mode, and display the operating status of the system. Under some operating conditions that require remote control, some soft start devices are also equipped with a remote communication module.
3. Starting mode of thyristor soft start
3.1 Full pressure start
In this state (Figure 3), the soft-start device is equivalent to a solid-state contactor, and the motor is subjected to all current shocks and torque shocks like direct starting. In general, the full-open time of the thyristor is controlled within 0.25s: 
Figure 3 Full pressure start
This mode is a relatively long start mode. By reducing the impact of the starting torque, it realizes a smooth and continuous stepless acceleration of the motor, thereby reducing the friction of the gear, the coupling and the belt to a minimum. The user can adjust the initial torque of the motor. During the acceleration ramp time, the input voltage of the motor rises linearly from the voltage corresponding to the set initial torque, changing the traditional step-down start to stepless, so that the motor can be smooth Starting, reducing the mechanical impact. 
Figure 4 Voltage ramp start
Current limiting start, as the name implies, is to limit the starting current to a certain set current value during the motor starting process. It is mainly used in relatively light load starting and has certain requirements for grid impact. Its input voltage increases rapidly from zero until its current reaches the preset current limit, and then the output current is not greater than the current In the case of limit values, change the conduction angle of the thyristor and gradually increase the voltage until the rated voltage. At the same time, the speed of the motor also gradually increased to reach the rated speed. The advantage of this kind of starting is that the starting current is small, the impact of the motor starting on the grid can be minimized, and the current limit value can be set as needed. But when setting the current limit, it must be set according to the initial torque of the motor. Otherwise, if the setting is too small, it will fail to start or burn the motor. The starting time of this starting method is relatively long. 
Figure 5 Current limiting start
This starting method is mainly used in a relatively heavy working environment. On the basis of torque control, a sudden torque is used to overcome the static torque of the load at the moment of starting, and then the torque is gradually increased until the motor reaches the normal working state. The advantage of this starting method is that it can shorten the starting time and start the heavier load, but it will have a certain impact on the grid when starting, affecting the work of other loads under the same grid. 
Figure 6 Jump start
The soft stop is actually equivalent to the opposite soft start process. The main function is to eliminate the anti-inertial impact of the system. For the pump load, it overcomes the "water hammer" effect. The main process is that the bypass contactor of the soft start device is opened when the motor is performing soft stop, and at the same time the thyristor starts to work, so that the voltage of the motor gradually decreases, and the rotation speed decreases to achieve the effect of soft stop. (Figure 7) 
Figure 7 The whole process of soft start and soft stop
Due to the relatively special mechanical characteristics of pump loads, some soft start manufacturers have specially designed pump-controlled starting and stopping methods for the characteristic curves of pump loads. This type of starting method can smoothly accelerate and decelerate the motor by the motor, so that the centrifugal pump is in Reduce water hammer impact during startup and shutdown. The main control circuit analyzes various operating parameters of the motor by collecting signals, and at the same time controls the state of the motor by changing the conduction angle of the thyristor, thereby reducing the possibility of the system being subjected to surge shock. (Figure 8) 
Figure 8 Start-stop process of pump control
In the load system of high-pressure high-power compressors (10KV / 3730KW and 10KW / 2250KW) of Shougang Cold Rolling Plant, the direct start of the motor will not only have an impact on the power grid, but also put forward high requirements for the mechanical maintenance of the entire system. After using PowerEasy soft start, the effect is good and the expected purpose is achieved. The main application effects are reflected in the following aspects:
1) Compared with the on-site 10KV / 3730KW compressor, its rated current is 253A, and the current will reach about 7 times the rated current during direct start. After using PowerEasy soft start, according to the site conditions, the start mode is set to current limit start, the current limit multiple is 3.2 times, the current is about 810A at the start, and the current control is accurate and stable. This not only reduces the impact of motor starting on the power grid, but also improves the safe operation level of the power grid; from another aspect, the power grid capacity is relatively reduced, the investment cost is reduced, and the construction cycle is shortened.
2) While reducing the impact of the compressor's direct start on the motor itself, it also reduces the mechanical impact of the start on the bearings, pipes and the entire system. Because the motor starts directly and accelerates too fast, it is a very severe test for the life and reliability of the transmission system. The soft start process is smooth and stable, which can effectively eliminate the adverse effects on the transmission system and pipelines, reduce the maintenance workload, and extend the working life.
3) The main circuit of the PowerEasy soft starter adopts imported thyristors in series, the power components have good consistency, and there is sufficient margin in the design. The temperature rise is relatively small when it is started continuously for 3-5 times. Under the working environment can guarantee high reliability.
4) PowerEasy soft starter mainly uses current closed-loop mode to control and monitor the motor and the entire system. The starting process has good consistency under reasonable voltage fluctuations. And the soft start itself has a relatively complete protection system, which can protect and monitor the entire system at any time according to the feedback signal, which fully guarantees the safe operation of the system.
5) During the commissioning process, various protection parameters and starting parameters can be set according to the on-site working environment, which can better adapt to the on-site working conditions.
6) The volume is relatively small, the mechanism design is reasonable, and the noise is low.
5. Conclusion
Thyristor series high-voltage soft starter can effectively change the starting characteristics of the motor and reduce the starting current in the application of compressor equal square torque loads. With its superior performance, it provides comprehensive services for the motor and ensures the entire transmission system. Reliability of operation. While reducing the overall maintenance workload of the system, the thyristor soft start device itself can be maintenance-free, which reduces the overall production cost, has a better return on investment and social benefits, and meets the requirements of the times. trend.
references
[1] Liu Hongwei, Wang Yi, Zhao Kaiqi, Xu Danguo, etc. Research on asynchronous motor soft starter [J]. Small and medium motors, 2002.1.
[2] Xu Furong, Cui Li. Research on soft starting and optimized energy-saving control technology of AC asynchronous motor [J]. Electrical drive automation, 2003.1.
[3] Huang Jun, Wang Zhaoan. Power electronic converter technology [M]. Beijing: Mechanical Industry Press, 1999.12.
[4] Wang Yi, Xu Danguo. Motor soft starter for start-stop control of fan load [J]. Fan Technology, 2001.
[5] Gao Yuenong. Primary intelligence of soft starter [J]. Automation Expo, 2002.1.
[6] Xu Shizhang. Electrical Engineering [M]. Version 3. Beijing: Mechanical Industry Press, 1996.
[7] Gao Yuenong. Discipline of motor soft start [J]. Electrical Drive Automation, 2005.1
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Design and application of high-pressure soft start on compressor load
Abstract: High-voltage high-power motors have a relatively large impact on the load and the grid when starting. Thyristor series high-voltage soft starters are widely used for their superior starting performance and other advantages. In this paper, through the application of high-pressure soft starter on the scene of the high-power compressor load, the working principle is analyzed, the starting function is introduced, and a high-level soft start of thyristor series connection is introduced in a comprehensive manner.