Sep 22, 2020
Application of soft starter in cold shear
Abstract: This article introduces the principle and operating characteristics of soft start, as well as the role and function of the MCC control cabinet.
Keywords: soft starter; AC motor; motor startability; MCC; control cabinet
The cold shear in the continuous rolling mill of the Guangzhou Iron and Steel Plant is driven by a 90kW motor. Before the transformation, the motor was started by a frequency-sensitive resistor. The starting current was above 380A, and the starting time was above 3 minutes. 70A, due to the large starting current and long time, the contacts of the motor rotor slip ring, carbon brush, terminal, frequency-sensitive resistor and main contactor are often damaged. Because of the large inrush current (generally Ie 5-8 times of the load). At the same time, due to the large starting stress, the service life of the load equipment is reduced.
2 Performance and characteristics of SAFESAVE SJR3 soft starter
The SJR3 soft starter detects the motor current, controls the output voltage to be linearly increased to the full voltage, limits the excitation starting current, and realizes the soft start of the motor. It has strong anti-interference ability and control ability, and can avoid being affected by Disturbance of high voltage and strong electrons. The soft starter is triggered by digital control. During the soft start, the constant current smoothly accelerates, avoiding the impact on the power grid. The starting current can be continuously within the range of 30%～70%Ue (Ue is the rated voltage) according to the needs of the field load. Adjustable. The parameters of the soft starter can be adjusted to obtain the best torque with the minimum current. The mechanical advantage of the soft starter is that it can reduce mechanical stress and extend the service life of the motor and auxiliary machinery. The starting time can be set according to different loads, and the starting time can be optimally optimized. Within this time range, the motor speed rises slowly, with all-round protection for the motor such as phase loss, three-phase unbalance, overload, and overcurrent. High cost performance, simple operation, small size, light weight, easy installation and debugging, with SCR overheating and overvoltage protection.
2. 1 The problems of the equipment before the soft starter
The cold shear in the tandem mill adopts (star/delta) conversion starting device. The flywheel with heavy load and large diameter of the cold shear in the continuous rolling mill has a large inertia during starting and stopping, and a high current spike appears at the moment of switching. The inrush current of the motor at the start also has a great impact on the voltage fluctuation of the power grid. At the same time, it causes great damage to the control equipment. Contactors are often burned and need to be replaced frequently, which seriously affects production. Failure of the cold shears in the continuous rolling mill will cause the entire line to stop production. It is obviously not advisable to increase the power distribution capacity just to start the motor. The traditional step-down start does not fundamentally solve the problem.
2. 2 The soft starter used after the transformation starts
The soft starter is a motor control equipment that integrates soft start, soft stop, light load energy saving and multi-function protection. The motor can be started smoothly without impact during the whole starting process, and various parameters in the starting process can be adjusted according to the characteristics of the motor load, such as current limit value, starting time, etc.
3 Working principle and operating characteristics
The starting torque Ma of a three-phase AC asynchronous motor is directly related to the quadratic power of the applied voltage, that is, as long as the voltage on the motor terminal is reduced, these values will be affected. The working principle of soft start is to increase the terminal voltage of the motor from a preset value to the rated voltage by controlling the conduction angle of the three-phase parallel thyristor connected in series between the power supply and the controlled motor.
3. 1 The main starting method of soft start
(1) Voltage double-slope start. During the start-up, the output torque of the motor increases with the increase of the voltage. An initial starting voltage Us is provided at the start. Us can be adjusted according to the size of the load. Adjust Us to be greater than the load The static friction torque produces the best starting characteristics. At this time, the output voltage rises at a certain slope from Us, and the motor continues to accelerate. When the output voltage reaches the speed voltage Ur, the motor basically reaches the rated speed. The soft starter automatically detects the up-speed voltage during the startup process, and when the motor reaches the rated speed, the output voltage reaches the rated voltage.
(2) Current limit start: It is the soft start method that limits the starting current of the motor to not exceed the set value during the start of the motor. The output voltage increases rapidly from zero, knowing that the output current reaches the preset current limit Im, and then gradually increase the voltage while keeping the output current I <Im, until the rated voltage, so that the motor speed gradually increases to reach the rated Rotating speed. The soft starter used in the cold shearing machine in the continuous rolling mill uses a current-limiting start, which reduces the large and long-term starting current in the traditional way during the starting process. When the starting voltage reaches the rated voltage, the soft starter is disconnected. Bypass contactor of the starter. The advantage of this starting method is that the starting current is small and can be adjusted as required. The impact on the power grid is small, and its disadvantage is that it is difficult to know the starting voltage drop when starting, and the voltage drop space cannot be fully utilized.
(3) Kick start: At the beginning of the start-up phase, the thyristor will be fully turned on in a very short time and then quickly fall back. This time can be adjusted artificially and then linearly rise according to the original set value, and enter the constant current start. The method is suitable for starting occasions with heavy load and friction.
4 Application of soft start
Asynchronous motors can be used in various applications that do not require speed regulation. They are suitable for various fan loads and require soft start and soft stop. For changing the load conditions, the motor is running at light load for a long time, and it is only at full load for a short time or moment, the application of soft starter (without bypass contactor) has the effect of light load and energy saving.
By understanding the characteristics and performance of the SAFESAVE soft starter SJR3 series, the use in the continuous rolling mill is compared with the traditional step-down starting equipment. Due to the inherent weaknesses in some special occasions, the traditional control method can no longer meet the modern equipment. Therefore, choosing the soft starter of intelligent motor starting equipment will be the inevitable development direction.