The soft starter has the advantages of small size, adjustable torque, stable starting, low impact and has a soft stop function has got more and more applications, is to replace the traditional self coupling decompression, star angle transform step-down start trend. Due to the soft starter is a newly developed start-up equipment in recent years, the lack of the corresponding design and use experience, cage asynchronous motor is the most widely used electrical equipment. Due to the impact of the current motor starts directly greatly, especially the large capacity motor starts directly will cause the safe operation of power grid to interference or damage and other loads, so according to different conditions, using many kinds of decompression starting mode. In the early stage, there were series reactance or resistance, series auto transformer, star delta conversion and so on. From 1970s onwards, the project began to promote the use of thyristor AC voltage regulator technology to produce soft starter. This kind of soft starter is a novel motor control device, which is composed of motor soft start, soft stop, light load energy saving and multiple protection functions. It is called soft starter. The soft starter is mainly composed of a series of three phase anti parallel thyristors connected in series between the power supply and the controlled motor, and the electronic control circuit. The working principle of soft starter control circuit, using different methods to control the three-phase anti parallel thyristor conduction angle, so that the motor input voltage gradually increased from zero to a preset function, until the start end, give full voltage soft start motor. During the soft start, the starting torque of the motor increases gradually, and the speed of the motor increases gradually. On the basis of the above, the power factor control technology is combined, and the microprocessor is used to replace the analog control circuit.
2 soft starter technology
2.1 the necessity of soft starter application
Start by autotransformer, star angle transform decompression starting motor, which is the most commonly used star angle transform step-down start way, the way of torque motor and voltage applied to the stator of the electric motor is proportional to the square, step-down Start refers to the reduction in motor voltage of stator windings in the startup process in U =0.5Ue, assuming the starting voltage, torque motor startup 0.25Mm, which starts the motor torque only the maximum torque of 1/4. If at this time will increase the voltage U to the motor rated voltage of Ue, the motor torque is suddenly jump from 1/4 to Mm, the startup process is jumping, not smooth, so also called hard start, step-down start control technology of poor reliability, instability, stop will cause on the power grid and mechanical equipment the impact of each opening, triggered a series of problems. For example: in this mode of control, the pump motor must be at the start of its export valve closed, in the low load to start, otherwise it will cause the switch trip, affecting the normal operation of the motor.
Generally speaking, there are several problems in the traditional start-up mode:
(1) the impact on the power grid is large, which affects the quality of power supply;
(2) the impact of mechanical equipment, reduce equipment life;
(3) the switching time of Y - Delta start is generally set according to the experience, and it is not suitable for the occasion of the production process requiring steady start.
Soft start is the use of voltage regulator in the specified start time, the automatic starting voltage will be continuous, smooth rise, until the rated voltage. At this point the motor torque will be increased smoothly until the torque is the maximum value of Mm, the end of the start process. Soft start
The utility model has the advantages that the motor can be started and stopped freely, and the utility model has the advantages of reducing idling and saving energy
According to different loads, different starting modes are adopted to improve the acceleration / deceleration characteristics
Modify the parameters matching load of different objects; the intelligent PLC, and can communicate with each other.
2.2 soft starter technology
The soft start technology of the motor has several different ways, such as magnetic controlled soft start, SCR soft start and liquid resistance soft start, in which the SCR soft
The most widely used startup. Its startup type is:
(1) no current soft start, start, so that the starting current to a certain slope continues to rise until the end of the start-up period, no restrictions on the start-up current. This start-up mode because there is no limit on the starting current, so the impact on the grid is large, generally do not use, to adapt to heavy load start.
(2) constant current soft start. The starting mode is that the starting current is gradually increased at the initial stage of the motor starting, and when the current reaches the preset value, the current is kept constant until the start is completed. During the start-up, the rate of change of current rise can be set according to the motor load regulation. The current rise rate is large, the starting torque is large and the starting time is short. This start-up mode is the most widely used, especially for the start of fan and pump load.
(3) pulse impulse start. Start in a very short time, so that the thyristor close to full conduction, and then restored to a small conduction angle, the normal constant current soft start. Suitable for the occasion of larger static friction torque.
(4) step start. Step start mode is in the boot to the shortest possible time to start the current to achieve the set value, by adjusting the starting current setting value, you can achieve the effect of quick start.
2.3 SCR soft start technology principle
The soft starter combines the power electronic technology, the automatic control technology and the single chip microcomputer technology. It is a kind of full digital intelligent starting device specially designed for three-phase asynchronous motor. The basic principle is to realize the control of the power device through the control of the silicon