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Full voltage starting of motor is the simplest, most economical and most reliable starting mode, but its direct starting current is usually about 5-7 times of the rated current. When the motor power is large, the surge current caused by direct starting will have a great impact on the power grid and affect the work of other loads in the same power grid. Therefore, considering the limitation of power supply and distribution equipment in power grid and substation, most high-power motors adopt step-down starting to limit starting current.

The traditional step-down starting methods include Y-delta commutation starting, series resistance (reactance) starting, auto compensator starting, extended triangle starting, etc. Although the voltage drop of the above several step-down starting methods is reduced, the starting voltage is jumping, and the starting current will also cause certain impact on the power grid and power supply and distribution equipment, and will bring certain damage to the motor and load, affecting their service life.

Since the 1980s, many domestic research institutions have developed soft start technology, which can solve some of the above problems. With the gradual popularization of soft start technology, various soft start technologies have appeared.

1. Principle and classification of soft starter

Soft starter is a new kind of motor control device, which can realize the soft start and soft stop of motor. It can also realize light load energy saving and various protection functions. Its basic structure is a three-phase anti parallel thyristor connected in series between the power supply and the controlled motor and its electronic control circuit. Through parameter setting, the CPU controls the conduction angle of the thyristor, so as to control the output voltage and current of the soft starter, so that the input voltage of the controlled motor changes according to different requirements, so as to realize different functions.

Soft starter is divided into four categories: solid state thyristor soft starter, liquid soft starter (commonly known as liquid resistance cabinet, water resistance cabinet), magnetic control soft starter and variable frequency speed control starter.

2. Function of soft starter

2.1 soft start

In the starting process of the motor, the conduction angle of the thyristor is controlled by the electronic control circuit of the soft starter, so that the input voltage of the motor gradually rises from zero in a set functional relationship until the end of the starting process, and the motor is given full voltage, which is called soft starting. In the process of soft starting, the starting current and starting mode of the motor can be arbitrarily selected and adjusted according to the requirements, so that the motor is always in the best starting process, the starting torque increases gradually, the speed also increases gradually, and the power loss in the starting process is reduced. The soft starting of AC motor mainly includes the following ways:

2.1.1 ramp voltage soft start. This soft start mode is the simplest. Most of the early soft starters used the starting voltage as the control object. In the whole starting process, they only controlled the conduction angle of the thyristor, which increased the voltage as a function of time, but did not have the current closed-loop control. Its disadvantage is that in the starting process of the motor, the current is not limited, sometimes it will produce a large impact current, which not only damages the thyristor, but also has a greater impact on the power grid, so it has been rarely used in practice.

2.1.2 pulse constant current starting. At the beginning of starting, there is a large impulse current to produce a large impulse torque to overcome the resistance torque of the load, so that the equipment can start; Then it enters the constant current starting stage until the end of the starting process. The magnitude and duration of impulse current can be set arbitrarily. This starting mode is suitable for heavy load starting, such as belt conveyor.

2.1.3 constant current soft start. After starting, the starting current can reach the set value quickly and keep constant in the shortest time until the end of the starting process, that is, the starting current is limited in the whole starting process. The current limit is usually 1.5-4.5 times of the rated current of the motor. The starting time of the motor can be controlled by changing the limit value. This method is generally suitable for fast starting of load with large inertia.

2.1.4 slope constant current soft start. At the beginning of the motor starting, the current increases steadily at a certain rate. When the starting current reaches the set value, it remains constant until the end of the starting process. The rising rate and constant current value of starting current can be set arbitrarily, generally according to the load and production requirements. This kind of soft starting mode is most widely used in practice, especially for the starting of fans and pumps.

2.2 energy saving under light load

The power factor and efficiency of the motor are relatively low in no-load or light load operation, but the soft starter can solve the above problems. By monitoring the change of the motor load, the soft starter can change the conduction angle of the thyristor according to the change, so that the working voltage of the motor will automatically reduce when it is no-load or light load, so as to reduce the loss, improve the power factor, and achieve the purpose of light load energy saving.

2.3 soft parking

In the water supply system of high-rise buildings, the instantaneous stop of the motor will produce a huge "water hammer" effect, which will damage the pipes and pumps. Therefore, when the motor driving the water pump load stops, it often makes the thyristor gradually transition from the full on state to the full off state through the electronic control circuit after 0 ~ 120 s delay, so as to realize the safe stop of the motor.

2.4 fault detection and protection

After the introduction of current closed-loop system into the soft starter, it can detect the change of motor current, give an alarm to overload, lack of equal fault, and make corresponding protection, so as to greatly improve the reliability of the motor.

3. Application examples of soft starter in coal mine production

Soft starter is safe, reliable, small volume, low price, maintenance free, which brings great convenience to users, and is widely used in coal mines.

3.1 application in drainage pump

Underground drainage is also one of the main work in coal mine production. Drainage system is a large power consumer in coal mine production, accounting for 13% - 18% of the total production power. Therefore, it is of great significance to effectively control the drainage system and make it run efficiently, economically and reliably. In recent years, with the increase of coal mining depth, many coal mines need to be rebuilt and expanded, and the underground power load has doubled. However, due to the small capacity of the ground power supply system, it is often difficult to meet the starting requirements of underground high-power (more than 1 000 kW) high-voltage motors such as high-voltage drainage pumps.

Using soft starter to control high-pressure drainage pump has obvious technical advantages. Using soft starter in the operation process of drainage pump has the advantages of small starting current, stable starting speed, reliable starting performance and small impact on power grid. At the same time, it also has soft shutdown, low-speed braking, light load energy saving and multiple protection functions, It provides guarantee for safe and reliable operation of mine drainage system.

3.2 application in fan

Because of the particularity of coal mine production, the ventilation system of roadway plays a vital role in the safety production of coal mine. Whether the ventilation of mine roadway is smooth and whether the air volume is sufficient or not is directly related to the safety production of the mine. With the continuous extension of mining and tunneling, the required air volume and fan power will increase. With the alternation of four seasons and the change of hot and cold, the required air volume also needs to be constantly adjusted. The traditional regulating system is realized by adjusting the damper according to the required air volume, which not only wastes electric energy, but also is inconvenient to adjust. The original electric control system generally uses direct or step-down starting, for high-power motor (generally more than 55 kW), the starting time is long, the starting current is large, which also causes great impact on the power grid, and even causes the power supply system to trip. And the protection of the motor is not complete, it is easy to burn the motor.

The fan can be started with the help of soft starter or frequency converter. At present, due to the high price of frequency converter, the soft starter gives full play to its advantages. The soft starter can not only realize smooth starting, but also has remarkable energy-saving effect when adjusting the air volume. At the same time, it also integrates many protection functions, such as phase loss, over-current, short circuit, three-phase current imbalance and so on. This not only greatly improves the safety of mine production, reduces the production cost, improves the degree of automation, but also saves money and is convenient for workers.

4. Conclusion

With the continuous development of motor soft start technology and power electronic devices, there are various motor soft start methods, such as magnetic control soft start, SCR soft start and liquid resistance soft start. Nowadays, in order to control conveniently, soft starter, switch, control circuit and metering equipment are often designed in an integral cabinet to form motor control center (MCC), so as to realize various control and fault monitoring and alarm of motor. Soft starter can be combined with PLC to make motor control more convenient, reliable and flexible, and realize digital control; It can also be combined with computer network technology to make motor control intelligent and networked. With the gradual establishment of various monitoring and control systems in coal mines, the automation level of coal mining enterprises continues to improve. Now people can detect and control underground fans, pumps and other equipment in the central control room.