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Application And Improvement Of Soft Starter For Large And Medium Motors

Sep 16, 2020

Application and Improvement of Soft Starter for Large and Medium Motors

[Abstract] 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 improvement on the problems of soft starter. The starter's control function and popular application in this field will play an active role.

    Traditional large and medium-sized motor control devices use contactors and magnetic starters to start and stop directly. The disadvantages are 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 elements, and convenient maintenance, are gradually replacing traditional control devices. Of course, the application of new products will always reveal 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 perfect, for reference by manufacturers and colleagues.

1 Function and characteristics of soft starter

1 . 1 Application

    The 20 m 3 air compressor system of Danjiang Power Plant is the air source for phase modulation, maintenance and other gas equipment of the unit. It is composed of 2 air compressors and 4 air storage tanks, and the working pressure is 0. 6 ~ 0. 8MPa, the motor capacity of the air compressor is 132 kW, and the rated current is 272 A. Two sets of independent control systems each control an 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 the 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 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 factory power, which is not conducive to the stable operation of other equipment. When the system was modified in early 1997, a soft starter was used. 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, laying a foundation for improving the comprehensive automation level of the whole plant and achieving reduction in personnel and efficiency.

1 . 2 Soft starter

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

1 . 2 . 1 Control function mode

After receiving the external start and stop command, the motor is controlled 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 I m, and then it is ensured that the output current is not greater than this In the case of value, the voltage gradually increases and the motor gradually accelerates to complete the starting process.

(2) Voltage ramp start control mode: As shown in Figure 2, U 1 is the initial voltage corresponding to the minimum torque required to start the motor. When the motor starts, the output voltage of the soft starter quickly rises to the set value U 1, and then gradually increases at the set rate until the grid voltage is reached, the contactor is closed, and the starting 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 U 1, 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, start-up over-current protection, running over-current protection, running overload protection, and protection that the motor cannot complete the starting process for a long time.

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

1 . 2 . 3 Display, alarm and parameter setting functions

On the control panel of the soft starter, the motor current, alarm signal and the set parameter value can be displayed; the motor protection value and motor operation mode are set in digital form; the motor is started and stopped manually.

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 SCR 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 shows the 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 Fig. 4, which mainly includes soft starter, programmable controller, pressure sensor, 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 gas tank pressure. In 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 20m 3 air compressor system, the start-stop mode of the motor is selected to be current-limiting soft start and soft stop.

(2)  Protection function

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

(3)  Monitoring, alarm function

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

1 . 3. 3 System working principle

When the air pressure of the air tank drops to the starting 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, 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 9 s, 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 switches off the intake solenoid valve to make the air The press system enters the no-load state, and after 2 s, a shutdown order is issued to the soft starter. 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 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 short-circuited with the thyristor control method, 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 and soft stop modes 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) Digital parameter setting and display functions 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 For problems, the following suggestions and improvement measures are proposed for manufacturers’ reference:

(1) In order to prevent the accidental startup of the device immediately after power-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 the normal operation of the load, that is to say, if the power supply of the corresponding soft starter control device is turned off during the normal operation, the system operation mode remains unchanged. , At this time, the faulty soft starter device can be overhauled. After the overhaul, the soft starter control device power supply is directly turned on, and the operation mode (ie, the automatic follower software) requirements 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 Inadequate, and make the control function more perfect. With the continuous advancement of technology and 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.