Foreword
With the rapid development of science and technology and the maturity of computer control technology, in recent years, a new intelligent controller "motor soft starter" with a computer as the core and a bidirectional thyristor as the main control loop has been in the industrial production field. It has emerged in the middle, it is flexible and easy to control, has little impact on the power supply system and electrical equipment, and the control elements are not easy to damage and easy to maintain. Many advantages are gradually replacing traditional control devices. The birth of this new technology has made an epoch-making change in the start-stop technology of electric motors.
In many production fields, because three-phase asynchronous motors have the characteristics of simple structure, reliable operation, easy maintenance, and reasonable price, more than 90% of electric drive machinery is driven by three-phase asynchronous motors. According to conventional practice, star-angle start, reactor start or auto-coupling decompression start are generally used for starting three-phase asynchronous motors with larger capacity. Because these technologies are relatively mature, they are still widely used in industrial and agricultural production. But no matter what method is used for starting, the starting current of the three-phase asynchronous motor will immediately form a very high inrush current, (the direct starting current value is 4 to 8 times the rated current of the motor) This gives the power supply equipment or the power supply voltage in the power grid Within a certain range, a transient voltage reduction phenomenon is formed, and the larger the capacity of the motor, the more serious this phenomenon is. At the same time, the hard start will also cause some damage to the power supply system and electrical equipment. The problem of starting high-power three-phase asynchronous motors has a long history. Electrical technicians have been trying to find a way to solve the problem completely.
The three-phase asynchronous motor soft start device is also called non-contact constant current start. It connects a set of high-power bidirectional thyristors in series between the motor and the input power supply. The control circuit uses electronic intelligent control to change the conduction of the thyristor. The pass angle makes the motor voltage increase steadily, and controls the starting current of the motor to be between 1 and 2.5 times the rated current of the motor, and is continuously adjustable, which will reduce the damage of the inrush current to the motor and the power supply equipment or power grid. Improves the stability of the power supply system.
At the same time, the motor soft starter implements information search and detection on various parameters of the three-phase asynchronous motor during startup or operation, and adjusts the trigger pulse of the thyristor in time, which can timely feedback to the motor overload, overcurrent, and lack of equal fault Identification and processing in the processor effectively protects the motor and electrical equipment. The three-phase asynchronous motor soft starter has high technology content, relatively complete protection function, and strong starting ability. It can start all three-phase asynchronous motors from 5.5 to 500KW. Its application field can cover the motor drive equipment in industrial and agricultural production, and it is an ideal replacement product of the traditional starting method. At present, there are more than a dozen domestic motor soft starters in production, and the working principles are similar. The author has conducted some discussions based on years of application in prospecting projects and combined with CMC series motor soft start devices.
2 Working principle of motor soft starter
CMC series motor soft starter is mainly composed of high-power bidirectional thyristor, AC contactor and electronic control part. The core of the control part is a CPU computer chip [80C 196KC] (see Figure 1 for electrical principle). Three pairs of thyristor bridges are connected in series on the three-phase power supply line of the motor. The electronic switching characteristics of the thyristor are used to control The size of its triggering conduction angle changes the opening degree of the thyristor, thereby changing the size of the motor input voltage and input current, and has reached the control of the starting characteristics of the motor.
It is the initial voltage corresponding to the minimum torque required when starting the motor. According to the capacity and load of the motor, adjust the initial starting voltage so that the set initial voltage is equal to the initial voltage that can drive the load. When the motor starts, the output voltage of the soft starter quickly rises to the setting value U1, and then gradually increases the rising voltage at the set rate (rise time) until it reaches the line voltage Ue.
Current limit soft start control mode: As shown in Figure 3, when the motor starts, its output voltage increases rapidly from zero until the output current reaches the set current limit value I m, and then the output current is guaranteed to be not greater than this value The voltage gradually increases and the motor gradually accelerates to complete the start-up process.
3 Characteristics of motor soft starter
Here is an example of the application of the electric motor soft starter, combined with the copyright of the example control engineering network of the air compressor station in the prospecting project. The power of the air compressor is a three-phase asynchronous motor (75KW). The structure of the control system is shown in Figure 4. It mainly includes a motor soft starter, PLC programmable controller, pressure sensor, intake solenoid valve, monitoring alarm and shutdown. Protective function. Its working principle is that when the pressure of the gas tank drops to the starting value of the air compressor, the PLC programmable controller sends a start command to the soft starter of the motor. The soft starter controls the starting voltage and current of the motor through the thyristor to make the air compressor The system starts smoothly to no load. When the voltage reaches the rated value, the contactor pulls in and the thyristor short-circuits. The three-phase power supply is directly added to the motor. The soft starter starts to complete and sends a start completion signal to the programmable controller. After a delay of 10 seconds, the programmable controller turns on the intake solenoid valve, the air compressor runs under load, and pumps air into the air tank; when the air pressure of the air tank reaches the shutdown value, the programmable controller cuts off the intake solenoid valve, Put the air compressor system into the no-load state, and send a stop command to the motor soft starter after 3 seconds. Under the control of the soft starter, the motor gradually decelerates to a complete stop.
(1) In the structure, the motor soft starter is used as the control output actuator. The control logic is implemented with a PLC programmable controller, which makes the system structure simple and clear. The use of digital monitoring and digital settings improves the reliability of the control system. Easy to maintain. At the same time, it has an external control function, which can be connected according to the usage, which is convenient for control.
(2) The motor soft starter provides a smooth and gradual starting process for the motor, reducing the impact of the starting current on the power grid or power generation equipment, controlling the starting current within a safe range, and improving the original control system due to the large starting current impacting the factory power supply And affect the normal operation of other equipment.
(3) The start-up process uses bidirectional thyristor. After the start-up process is completed, the contactor short-circuits the control mode of the thyristor, which avoids the direct control of the motor by the contactor to make the contacts easy to draw arc, adhesion, burnout and other failures. CONTROL ENGINEERING China is copyrighted and saves energy.
(4) Soft start and soft stop modes reduce equipment vibration and noise, reduce mechanical stress, and extend the service life of power generation equipment and mechanical transmission system.
(5) It has multiple protection functions such as over-current, overload and power shortage. At the same time, it can detect various undesirable operating conditions of the system involved in the load (such as air compressor), which is conducive to the safe operation of the protection equipment.
(6) The display function on the control panel makes it easy to fully understand the operation of the equipment on site.
(7) Digital parameter setting and display functions are intuitive, convenient and time-saving.
4 Conclusion
The development stage of the electric motor soft starter began in the 1980s. After so many years of development, the technology has been continuously mature and the functions are becoming more and more perfect. In addition to being able to fully meet the basic requirements of a smooth motor start, it also has many advantages, such as high reliability, low maintenance, complete protection of the motor, and simple parameter settings.
After many years of practice in the prospecting project, the author found that the current motor soft starter still has a defect, that is, it cannot be used for a long time for the motor drive device with high starting torque requirements. This limitation is mainly because the soft starter is actually The above is to complete the work by ramping up its own voltage to the maximum value (while the set voltage is gradually reduced to stop during the shutdown process). The torque is proportional to the square of the voltage, which prevents the motor from reaching the maximum torque from the beginning. Therefore, the current motor soft starter is more suitable for some no-load or light-load starting equipment. At the same time, CONTROL ENGINEERING China is copyrighted. At present, the prices of the motor soft starters produced are very high, and the market price is not easy to be accepted. This will hinder the future popularization and application.
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