Problems needing attention in selecting the hottes

Problems that should be paid attention to when selecting low-voltage soft starter

Abstract: low-voltage soft starter has been used more and more because of its small size, adjustable torque, stable startup, small impact and soft shutdown function. This paper briefly introduces the types and characteristics of commonly used soft starters at present, and summarizes the main problems and empirical formulas that should be paid attention to when selecting soft starter in engineering field

key words: soft start impulse current selection application soft starter has been used more and more with the advantages of small volume, adjustable torque, stable start, small impact and soft shutdown function, which has a great trend to replace the traditional auto coupling and decompression, star angle and other starters. As the soft starter is a newly developed starting device in recent years, there is still a lack of guiding norms and regulations in the design, installation, commissioning and use. We also encountered some practical technical problems in the installation and commissioning of the soft starter. For example, the selection of soft starters with different starting loads, the coordination of soft start impulse current and overcurrent protection setting, and the relationship between soft start equipment capacity and transformer capacity. 1. At present, the common soft starters in the market mainly include electronic type, magnetic control type and automatic liquid resistance type. Most electronic types are thyristor voltage regulating type. Frequency converter is also a kind of soft starter in a sense, and it is a starter that can really realize soft start, but the cost is higher. The thyristor soft starter is three groups of thyristors connected in series between the power supply and the motor in parallel, and the AC voltage regulation is realized by controlling the trigger conduction angle by microcomputer. The starting modes of thyristor soft starter include ramp voltage type, sudden jump plus ramp voltage type and current limiting type. Magnetic control soft starter is a soft starter made of three-phase saturated reactors connected in series between power supply and motor by using the principle of magnetic amplifier. When starting, adjust the exciting current of the magnetic amplifier control winding through the digital control board, change the reactance value of the saturation reactor, adjust the starting voltage drop, and realize the soft start of the motor. Whether thyristor soft starter or magnetic control soft starter can only adjust the output voltage during startup, so as to control the voltage drop during startup and limit the starting current. The general soft starter can not adjust the power frequency, so it can not start the motor from zero frequency and zero voltage like the frequency converter to realize shock free starting. In fact, the soft starter still needs to produce a certain impulse current when starting the equipment; The voltage and current change curve of the ramp voltage type control soft starter during startup is shown in Figure 1. When the thyristor soft starter is started with a ramp voltage, the soft starter should output an initial voltage at the beginning (the initial voltage can be adjusted between 80~280v), so that the motor can produce an initial torque sufficient to overcome the static friction of the mechanical equipment, and the driving equipment starts to rotate with the starting current of is. Under the control of microcomputer, continue to increase the output voltage to accelerate the motor. When the output voltage of the soft starter approaches the rated voltage, the motor has reached the rated speed and is is reduced to the load current in. Our company sends professional technicians (certified engineers) to the site for free online commissioning. At the end of dynamic time T1, the soft starter outputs the rated voltage and sends a bypass signal, so that its position faces the circular shaft to close the bypass contactor, the soft starter stops outputting voltage, and the electric motor turns into normal operation. The initial torque of soft start can be adjusted by giving the initial voltage and starting time, and the starting current is controlled within 2~4.5 times the rated current of the motor. The parking modes of low-voltage soft starter mainly include free parking, soft parking and braking parking. The traditional motor parking mode often uses free parking, but there are many applications, free parking will cause great problems, such as the pump system of high-rise buildings, if free parking is adopted, it will produce a huge "water hammer" effect, causing damage to pipelines and pumps. Therefore, the use of soft parking can eliminate the anti inertia impact from the import of free parking belts to their own production. When the stop command is issued at the stop time T2, the terminal voltage of the motor slowly drops from UN. At the moment of the voltage drop, the motor current will have a small current impact, and then the motor current will drop with the voltage drop until the motor stops. 2. Problems that should be paid attention to when selecting low-voltage soft starter 2.1 electrical wiring and component arrangement of low-voltage soft starter cabinet generally, the electrical wiring of low-voltage soft starter cabinet is shown in Figure 2. The electrical components in the cabinet are arranged in the order shown in the figure, so that the main wiring is short and not crossed, which is convenient for copper bar connection. The soft starter is installed on the right side of the contactor and is not affected by the heating of other components. The distance between the soft starter and the side wall of the control cabinet and other elements shall be ≥ 100mm to facilitate the heat dissipation of the soft starter. 2.2 in addition to taking this opportunity to compare technology, performance and price, the selection of soft starter should also consider the specific conditions such as the capacitance of the equipment on site, the weight of the equipment starting load, and the starting frequency. For the equipment with light starting load of water pump, the soft starter with simple function, low price and convenient operation can be selected. This kind of equipment can meet the needs by selecting the soft starter with the same capacity specified in the sample according to the rated power of the motor. For large fans, crushers and other equipment with heavy starting load, soft starters with more starting functions, limited current starting functions and complete self-protection should be selected. Especially for equipment with high power (above 200kW), it is best to choose high-performance soft starter with full starting function. 2.3 the isolator and fuse shall be the isolator in the soft start cabinet, and the disconnector or molded case circuit breaker with isolation function can be selected. The knife fuse switch of disconnector fuse combination should be selected for the low-power soft start cabinet. It not only plays the role of isolation and protection, but also reduces the project cost. The rated current of the disconnector is greater than the rated current of the motor to meet the operation requirements. Because the surge Joule integral (i2t) value of thyristor in soft starter is limited, selecting circuit breaker as short-circuit protection device can not effectively protect thyristor components. It is recommended to select quick fuse as short-circuit protection device. The fast fuse can be AR or NGT