Low voltage soft starter selection

Low-voltage soft starter selection summary: Low-voltage soft starters have been used more and more for their advantages of small size, adjustable torque, stable start, small impact and soft stop function. This paper briefly introduces the commonly used soft starters. The types of starters and their characteristics are summarized, and the main problems and empirical formulas that should be paid attention to when choosing a soft starter in the engineering field are summarized.

Soft starters have been used more and more with the advantages of small size, adjustable torque, stable start, small impact and soft stop function, and have a great tendency to replace traditional auto-decompression, star-angle and other starters . Since the soft starter is a newly developed starting device in recent years, there is still a lack of guiding norms and procedures in the design, installation, debugging and use. We also encountered some practical technical problems in the installation and debugging 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 settings, the relationship between soft start equipment capacity and transformer capacity, etc. 1. Introduction of soft starter

At present, the common soft starters on the market mainly include electronic type, magnetic control type and automatic liquid resistance type. The electronic type is mostly the thyristor voltage regulating type. In a sense, the frequency converter is also a soft starter, and it is a starter that can truly 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 in forward and reverse directions. The trigger conduction angle is controlled by a microcomputer to achieve AC voltage regulation. The starting methods of thyristor soft starters include ramp voltage type, sudden jump plus ramp voltage type and current limiting type.

The magnetic control soft starter is a soft start device composed of three-phase saturable reactors connected in series between the power supply and the motor by using the principle of magnetic amplifier. When starting, the exciting current of the magnetic amplifier control winding is adjusted by the digital control board, and the reactance value of the saturable reactor is changed to adjust the starting voltage drop, so as to realize the soft start of the motor. Regardless of whether the thyristor soft starter or the magnetic control soft starter can only adjust the output voltage during startup, the purpose of controlling the voltage drop during startup and limiting the startup current is achieved. The general soft starter cannot adjust the frequency of the power supply, and it cannot start the motor from zero frequency and zero voltage like a frequency converter, so as to realize a shock-free start. In fact, the soft starter still needs to generate a certain inrush current when starting the equipment; the voltage and current change curves of the ramp voltage control soft starter when starting are 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 power

The motive generates an initial torque sufficient to overcome the static friction of the mechanical equipment, and the drag equipment starts to rotate with a starting current of Is. Under the control of the microcomputer, continue to increase the output voltage to accelerate the motor. When the output voltage of the soft starter is close to the rated voltage, the motor has reached the rated speed, and Is is reduced to the load current In. When the starting time t1 ends, the soft starter outputs the rated voltage and sends a bypass signal to close the bypass contactor, the soft starter stops outputting the voltage, and the motor turns into normal operation. The initial torque of soft start can be adjusted by given initial voltage and starting time, and the starting current is controlled within 2 to 4.5 times the rated current of the motor.

The parking methods of the low-voltage soft starter mainly include free parking, soft parking and braking parking. The traditional motor parking method often uses free parking, but there are many applications where free parking will cause great problems. For example, in the pump system of high-rise buildings, if free parking is used, it will produce a huge "water hammer" effect, causing damage to pipes and pumps . Therefore, the use of soft parking can eliminate this anti-inertia impact caused by free parking. At the stop time t2, a stop command is issued, and the terminal voltage of the motor drops slowly 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 starters

2.1 Electrical wiring and component arrangement of low-voltage soft-start cabinet

The electrical wiring of the general low-voltage soft-start 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 does not cross, which is convenient for the connection of copper bars. 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 components is required to be ≥100mm, which is convenient for the soft starter to dissipate heat.

2.2 Soft starter selection

In addition to technology, performance, and price comparison, specific conditions such as the power grid capacity of the equipment site, the severity of the equipment startup load, and the frequency of startup should also be considered.

For equipment with lighter starting loads such as pumps, soft starters with simple functions, low prices and easy operation can be selected. According to the rated power of the motor, this kind of equipment can meet the needs by selecting the soft starter of the same capacity specified in the sample. For equipment with heavy starting loads such as large fans and crushers, soft starters with more starting functions, limited current starting functions, and more complete self-protection should be selected. Especially for equipment with relatively large power (above 200KW), it is best to choose a high-performance soft starter with complete starting functions.

2.3 Selection of isolators and fuses

For the isolation electrical appliances in the soft-start cabinet, you can choose an isolating switch or a molded case circuit breaker with an isolating function. The low-power soft-start cabinet should choose a knife-fuse switch with a combination of isolating switch and fuse. It not only plays the role of isolation and protection, but also reduces the project cost. The rated current of the isolating switch is greater than the rated current of the motor to meet the operating requirements.

Due to the limited surge Joule integral (I2t) value of the thyristor in the soft starter, choosing a circuit breaker as a short-circuit protection device cannot effectively protect the thyristor element. It is recommended to use a fast fuse as a short-circuit protection device. The fast fuse can choose aR or NGT type semiconductor protection fuse. The selection of fast fuse generally does not need to do breaking capacity verification, because the rated breaking capacity of aR fuse is 50KA, and the breaking capacity of NGT fast fuse is 120KA, which can meet the needs of general power distribution projects. In addition, the fuse also has a current limiting function, and the protection of the thyristor is more reliable than that of the circuit breaker.