Soft starters generally have the following starting methods:
(1) Ramp boost soft start: This kind of starting mode is the simplest. It does not have current closed-loop control. It only adjusts the conduction angle of the thyristor to increase its relationship with time. The disadvantage is that, due to the unlimited flow, during the starting process of the motor, sometimes a large inrush current is generated to damage the thyristor, which has a great influence on the power grid, and is rarely used in practice.
(2) Ramp constant current soft start: This starting mode is to gradually increase the starting current during the initial stage of motor starting, and keep constant when the current reaches the preset value (t1 to t2 phase) until the starting is completed. During the starting process, the rate at which the current rises and changes can be adjusted according to the motor load. When the rate of current rise is large, the starting torque is large and the starting time is short. The starting method is the most widely used starting method, and is especially suitable for the starting of fans and pump loads.
(3) Step start: Start, that is, in the shortest time, the starting current will quickly reach the set value, which is the step start. The quick start effect can be achieved by adjusting the starting current setting.
(4) Pulse shock start: At the start stage of the start, let the thyristor turn on for a period of time after a large current is turned on for a short period of time, and then rise linearly according to the original set value, and connect to the constant current start. The starting method is less applicable in general loads and is suitable for starting situations where heavy loads are required to overcome large static friction. The soft start is different from the traditional decompression start mode. The traditional decompression start mode of the cage motor includes Y-q start, auto-decompression start, and reactor start. These starting methods are all graded decompression starting, and there are obvious disadvantages, that is, secondary inrush current occurs during starting.
(5) Voltage double ramp start: During the starting process, the output torque of the motor increases with the voltage, and an initial starting voltage Us is provided at the start. The Us is adjustable according to the load, and the Us is adjusted to be greater than the static friction torque of the load to make the load Can start to turn immediately. At this time, the output voltage rises from Us at a certain slope (the slope is adjustable), and the motor continues to accelerate. When the output voltage reaches the speed-up voltage Ur, the motor also basically reaches the rated speed. The soft starter automatically detects the speed-up voltage during the starting process, and when the motor reaches the rated speed, the output voltage reaches the rated voltage.
(6) Current limiting start: It is the soft start mode that limits the starting current of the motor to a certain set value (Im) during the starting process of the motor. The output voltage rises rapidly from zero until the output current reaches the preset current limit Im, and then the output current I is maintained. This has the advantage that the starting current is small and can be adjusted as needed. The impact on the power grid is small, and the disadvantage is that it is difficult to know the starting pressure drop at the time of starting, and the pressure drop space cannot be fully utilized.
(7) Kick start: At the start of the start, let the thyristor fall back after all the conduction in a very short time, and then rise linearly according to the original set value, enter the constant current start, the starting method is suitable for heavy load and need A starting situation that overcomes large static friction.
5, the inverter:
The frequency converter is the motor control device with the highest technical content, the most complete control function and the best control effect in the field of modern motor control. It adjusts the speed and torque of the motor by changing the frequency of the power grid. Because it involves power electronics technology and microcomputer technology, it is costly and has high requirements for maintenance technicians. Therefore, it is mainly used in areas where speed regulation is required and speed control is required.
