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Application and Improvement of Soft Starter for Large and Medium Motors

Mar 24, 2021

Application and Improvement of Soft Starter for Large and Medium Motors

Application and improvement of soft starter for large and medium-sized motors?


Briefly describe the principle, function and characteristics of the soft starter of the new type of control starter for large and medium-sized motors, as well as its application in Danjiangkou Hydropower Station, and put forward suggestions and opinions for improving the soft starter. Control functions and popular applications in this field will play a positive role.


  The traditional large and medium-sized motor control device adopts the contactor and magnetic starter to start and stop directly. The disadvantage is that the control method is simple, inflexible, has a large impact on the system, and the control components are easily damaged, and the maintenance workload is large.


With the maturity of computer control technology, in recent years, a new type of intelligent controller "soft starter" with a computer as the core and a triac as the main control loop has been widely used in this field. It has flexible control methods. Many advantages, such as simplicity, low impact on the system, less damage to control components, and convenient maintenance, are gradually replacing traditional control devices. Of course, the application of new products will always expose some shortcomings. Based on the soft starter used in Danjiangkou Power Plant, this article discusses its operation in three years and the problems exposed, and puts forward several suggestions for improvement from the perspective of production. The control function is more complete, for reference by manufacturers and colleagues.


  1 Function and characteristics of soft starter


  1.1 Application situation


Danjiang Power Plant’s 20m3 air compressor system is the air source for unit phase modulation, maintenance and other gas equipment. It consists of 2 air compressors and 4 air storage tanks. The working pressure is 0.6~0.8MPa. The motor of the air compressor The capacity is 132kW and the rated current is 272A. Two independent control systems each control one air compressor, which has been in operation for more than 20 years, with serious aging of equipment and high failure rate. Restricted by the level of science and technology at that time, the logic control loop is composed of relays, the starting method is the traditional frequency-sensitive resistor step-down method, and the starting motor's actuator is a contactor. The control system has complicated wiring, many fault points, incomplete protection of the motor, large starting current, and a large impact on the power supply of the factory, which is not conducive to the stable operation of other equipment. When the system was reformed in early 1997, a soft starter was applied. The new control system solves many problems in the original control system, and is conducive to the new computer monitoring system interface that is undergoing a comprehensive transformation. It lays the foundation for improving the comprehensive automation level of the whole plant and reducing personnel and increasing efficiency.


  1.2 soft starter


  The soft starter is mainly composed of a main control board with a CPU of 8096, a drive board, a main motor control circuit and a control panel. The soft starter has control functions, motor protection functions, as well as display, alarm, and parameter setting functions.


   1.2.1 Control function mode


   After receiving the external start and stop command, it will realize the control of the motor according to the preset start and stop mode. The selectable start and stop control modes are as follows:


(1) Current-limiting soft-start control mode: As shown in Figure 1, when the motor starts, its output voltage increases rapidly from zero until the output current reaches the set current limit value Im, and then ensure that the output current is not greater than this value In the case of a gradual increase in voltage, the motor gradually accelerates to complete the starting process.


   (2) Voltage ramp start control mode: As shown in Figure 2, U1 is the initial voltage corresponding to the minimum torque required for starting the motor. When the motor starts, the output voltage of the soft starter quickly rises to the set value U1, and then gradually increases at the set rate until the grid voltage is reached, the contactor is closed, and the start process is completed.


   (3) Stop mode: The motor will gradually decelerate at the set rate until it stops completely.


   (4) Free stop: The motor freely stops by inertia without control.


   (5) Jog function: Under the control of this mode, the output voltage of the soft starter rapidly increases to the initial voltage U1, and the output voltage value is maintained.


  1.2.2 Motor protection function


  The motor protection functions of the soft starter include: phase sequence protection, phase loss protection, starting overcurrent protection, running overcurrent protection, running overload protection and protection of the motor unable to complete the starting process for a long time.


   When the protection function of the soft starter is activated, the soft starter will produce a shutdown output, and the reason will be displayed directly on the control panel.


   1.2.3 Display, alarm and parameter setting function


  On the control panel of the soft starter, it can display the motor current, alarm signal and set parameter value; set the motor protection value and motor operation mode in digital form; manually start and stop the motor.


  The working principle of the soft starter is: after the soft starter receives the start command, it will perform relevant calculations according to the set start mode to determine the trigger output voltage signal of the thyristor to control the starting process of the motor. When the starting process of the motor is completed, the soft starter will control the AC contactor to close and short-circuit all thyristors, so that the motor can be directly put into the network and run to avoid unnecessary energy loss. Figure 3 is a schematic diagram of the soft starter.


  1.3 Soft starter used in Danjiangkou Power Plant


  1.3.1 System structure


   The control system structure is shown in Figure 4, which mainly includes a soft starter, a programmable controller, a pressure sensor, and an intake solenoid valve.


  1.3.2 Main functions of the system


  (1) Control function


   Automatic/manual control the start and stop of the air compressor. The control mode is selected by the work mode switch.


   In the automatic working mode, the air compressor is automatically started and stopped according to the change of the air tank pressure. In the manual mode, the programmable controller exits operation, and the air compressor is manually started and stopped on the soft starter control panel. According to the actual situation of the 20m3 air compressor system, the start-stop mode of the motor is selected to limit the soft start and soft stop.


  (2) Protection function


  The protection function involves the entire air compressor system. When any abnormality occurs in the first level cylinder air pressure is too high/low, oil cylinder pressure is too high/low, exhaust temperature is too high, phase failure, phase sequence error, motor overcurrent, overload, etc., the protection action will automatically stop.


  (3) Monitoring and alarm function


   Real-time monitoring of the operating conditions of the system, the main circuit current and voltage indication and display system operating status on the control panel, when the system fails, there will be a detailed fault alarm display on the control panel, and an alarm signal will be sent to the central control room at the same time.


   1.3.3 The working principle of the system


When the air pressure of the air tank drops to the start value of the air compressor, the programmable controller sends a start command to the soft starter, and the soft starter controls the starting voltage and current of the motor through the thyristor, so that the air compressor system starts smoothly to no-load. . When the voltage reaches the rated value, the contactor is closed, the thyristor is short-circuited, and the three-phase power is directly applied to the motor, the soft starter is started and the start-up completion signal is sent to the programmable controller. After a delay of 9s, the programmable controller turns on the intake solenoid valve, the air compressor runs on load, and pumps air to the air tank; when the air tank pressure reaches the stop value, the programmable controller cuts off the air intake solenoid valve to make the air pressure The motor system enters the no-load state, and a shutdown order is issued to the soft starter after 2s. Under the control of the soft starter, the motor gradually decelerates to a complete stop.


  2 Operation effect and improvement suggestions


   The three-year operation of the new control system shows that its reliability, function and performance have been significantly improved and improved compared with the original control system. Mainly reflected in the following points:


   (1) In the structure, the soft starter is used as the control output actuator, and the control logic is realized by the programmable controller, which makes the system structure simple and clear, improves the reliability of the control system, and is also easy to maintain.


   (2) The current-limiting starting mode of the soft starter controls the starting current within a safe range, which improves the condition that the original control system affects the normal operation of other equipment due to the large starting current impacting the power supply.


(3) The start-up process adopts bidirectional thyristor. After the start-up process is completed, the contactor is shorted to the control method of thyristor, which avoids the occurrence of faults such as easy arcing, adhesion and burnout of the contactor to directly control the motor contacts. It also saves energy.


   (4) Soft start, soft stop mode, reduce mechanical stress, protect equipment, and extend its service life.


   (5) The protection function involves various bad operating conditions of the air compressor system, which is beneficial to improve the health of the equipment.


   (6) The display function on the control panel facilitates a comprehensive understanding of the equipment operation on site.


   (7) The digital parameter setting and display function are intuitive, convenient and time-saving.


The anti-interference ability of the soft starter is an important indicator that affects its reliability. During the 3 years of operation of the soft starter, the phenomenon of the device's false start at the moment of power-on has occurred. For this reason, the current soft starter has For problems, the following suggestions and improvement measures are hereby provided for manufacturers’ reference:


(1) In order to prevent the occurrence of false start after the device is powered on, it is recommended that the manufacturer modify the control software and add a delay function block to the final exit signal circuit of the device software. The delay time should be as large as to avoid interference. Signal is the premise, which requires on-site debugging and confirmation, or adding a delay relay to the final signal exit loop of the hardware, can also effectively solve the interference problem.


(2) In order to facilitate maintenance, it is hoped that the soft starter can be easily exited and put into operation during normal operation of the load. That is to say, during normal operation, if the power supply of the corresponding soft starter control device is turned off, the system operation mode remains unchanged. , At this time, the faulty soft starter device can be overhauled. After the overhaul is completed, the soft starter control device power supply is directly turned on, and the requirements of the operating mode (ie, the software of the automatic follower device) are the same. This function is very suitable for on-site daily maintenance .


  (3) It is recommended to modularize the soft starter device for easy maintenance.


  4 Conclusion


In hydropower stations and many industrial air compressors, pumps, fans and other auxiliary equipment control fields, the traditional control structure has many shortcomings. For large loads, the problems become more prominent. The soft starter not only overcomes the traditional control structure Insufficient, and make the control function more perfect. With the continuous advancement of technology and the continuous improvement of soft starters, soft starters will become more mature. Therefore, it is an inevitable trend that soft starters are widely used in large and medium-sized motors.