Abstract: with the rapid development of power electronics technology, intelligent soft starters are widely used. The intelligent soft starter is a new motor control equipment which integrates soft start, soft parking, light load energy saving and multi-function protection. It is also called Soft Starter. It not only realizes the shock and smooth starting motor in the whole starting process, but also adjusts the parameters in the starting process, such as the current limit and starting time, according to the characteristics of the load of the motor. In addition, it also has a variety of motor protection functions, which fundamentally solves the drawbacks of the traditional buck starting equipment.
1. Characteristics of starting performance of soft starting
(1) the starting voltage can be adjusted to ensure the minimum starting torque of the motor and avoid overheating and waste of energy.
(2) control the smooth start of the motor and reduce the impact of starting current.
(3) the starting current can be adjusted according to the load condition to reduce starting loss and produce the best torque with the minimum current.
(4) the starting time can be adjusted. During this time, the motor speed increases gradually to avoid the impact of speed.
(5) mechanical protection of transmission machinery, eliminating torque surge and reducing impulse current.
(6) constant acceleration and deceleration does not require speed sensors, even when motor load changes.
(7) optional parking and soft parking options, soft parking speed can be adjusted.
(8) there are phase sequence, phase missing, overheating, start-up process overcurrent, overrunning and overload detection and protection during operation, and the overcurrent and overload values can be adjusted.
2, the main starting way of soft start
The starting mode of the soft starter is the way to make the motor from stationary state to stable running state. For example, current limiting starting, voltage ramp starting, current belt impact starting and alternating or combined starting modes of these modes.
(1) voltage double slope starting: in starting process, the output torque of the motor increases with the voltage, provides an initial starting voltage at starting, and will be adjusted to the load static friction torque according to the load, so that the load can start to rotate immediately. At this time the output voltage rises from the beginning to a certain slope (the slope can be adjusted), the motor is constantly accelerating, when the output voltage reaches the speed voltage, the motor also basically reaches the rated speed. The soft starter automatically detects voltage during the starting process. When the motor reaches the rated speed, the output voltage reaches the rated voltage.
(2) current limiter starter is a soft start mode that limits the starting current of a motor not exceeding a set point (J) during the starting process of the motor. The output voltage increases rapidly from zero to zero until the output current reaches a predetermined current limit of J. Then the output current is kept up to J (J), and the voltage is gradually raised until the rated voltage, so that the motor speed is gradually increased until the rated speed. The advantage of this starting mode is that the starting current is small and can be adjusted as required. It has little influence on the power grid, and its disadvantage is that it is difficult to know the starting pressure drop when starting, and can not make full use of the pressure drop space.
(3) jump start: this start stage, let the thyristor fall back in a very short time, then rise linearly in accordance with the original value, and enter the constant current starting. The starting method is suitable for heavy load and need to overcome the starting situation of friction. If this method is adopted, the vibration at startup can also be reduced.
(4) combination starting mode: the actual application of Schneider and SIEMENS soft starter often use the combination starting mode, that is to meet the given voltage and current conditions, the main board according to the load changes, through calculation, the output of a best starting mode. The combined starting mode has higher requirement for the main board, but the starting effect is better, and is suitable for places with large load and high starting requirements.
3, the difference between soft start and traditional decompression starting mode is
(1) no shock current. When starting the motor, the soft starter increases the starting current of the motor from zero to set value by increasing the thyristor conduction angle. No impact on the motor, improve the reliability of the power supply, smooth start, reduce the impact torque on the load machinery, and extend the service life of the machine.
(2) it has the function of soft parking, that is, smooth deceleration and gradually stopping, it can overcome the malady of the instant power cut down, lighten the impact on heavy load machinery, avoid the water hammer effect of the high water supply system and reduce the damage of the equipment.
(3) the starting parameters can be adjusted, and according to the load condition and the relay protection characteristics of the power grid, it can be adjusted freely to the optimal starting current.
4. Main problems existing in actual operation of soft starters.
(1) SCR breakdown: the highest failure rate of soft starter should be thyristor breakdown. The causes of SCR breakdown may be as follows: first, the influence of wet environment, because of wet environment, is one of the main causes of silicon controlled breakdown. With the improvement of operating environment, the failure rate is obviously reduced. Less; second, too high operating temperature is also a major cause of thyristor breakdown. Third. Poor quality of SCR is also a cause of thyristor breakdown. Fourth. Because of the sharp pulse caused by the resonance of the power grid, the instantaneous high voltage breakdown and the gate short circuit. Once thyristor is broken, thyristor is equivalent to 3 series diodes, which lose their electronic switching characteristics. Starting at this time, the motor will withstand a great impact on the starting current and will burn the motor seriously. So the circuit should be protected.