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Generally, the inverter is not suitable to switch in the output contactor and other devices. The main reason is that the control of the inverter is too complex. When the motor is switched, there may be a big difference between the phase when the motor is idling and the output phase of the inverter. When the motor is switched, there will be a big impact, which will affect the IGBT life of the inverter.

The soft starter itself is designed to consider the switching. It will pay attention to these details and use some PLL technology to track, but it does not have these functions. Because the SCR is cheap and reliable, it can be made up by the SCR with high voltage withstand point. However, IGBT is too expensive, and the converter generally does not consider the application of switching to power frequency, Therefore, the soft starter in these places may be more ideal than the inverter.

Soft starter in some control areas can be "alone", the following I analyze the reasons.

The first is from the use of occasions, in some do not need to speed control, only need to step down the start of large load occasions, you can choose to use soft starter.

The second point is that in terms of control input, the price of soft starter is much cheaper than that of frequency converter, which is more obvious for high-power load control occasions requiring large capacity controller.

Third, the inverter needs to be equipped with special inverter motor in some high-speed operation demanding strict control occasions. If the ordinary asynchronous motor is used, it will cause great harm to the motor. Therefore, the soft starter control is the best choice in the occasions where the power frequency can be used.

Fourth, from the perspective of the whole set of matching input, it is best to use soft starter control for those who can use soft starter, and then use frequency conversion control for those who are not suitable for soft starter. In this way, it will be very ideal in terms of economic and social benefits by using soft starter and frequency converter for high and low matching.

In principle, cage asynchronous motor can be used in all kinds of applications where speed regulation is not required.

At present, the application range is 380V (660V), and the motor power is from several kilowatts to 800KW. The soft starter is especially suitable for various pump loads or fan loads, where soft start and soft stop are required.

Similarly, the soft starter (without bypass contactor) has the effect of light load and energy saving when the motor is in light load operation for a long time under variable load condition and only in heavy load for a short time or instantaneously.

Five problems in using soft starter

1. There is no short circuit protection for soft starter. In order to protect the thyristor, fast fuse should be used. Fast fuse shall be selected according to rated current of soft starter. It should be pointed out that the low voltage circuit breaker is not suitable for thyristor protection due to its long breaking time (about 0.1s).

2. When the soft starter uses the motor brake to stop, only because the thyristor is not conductive, the input voltage of the motor is 0V, but there is no electrical isolation between the motor and the power supply, so the power supply must be cut off when repairing the motor or circuit. Therefore, a circuit breaker should be added between the soft starter and the power supply.

3. When the power of soft starter is large or the number of soft starters is large, the generated high-order harmonics will have adverse effects on the power grid and interfere with electronic equipment. When the motor starts smoothly to the normal speed, the bypass contactor is closed and the soft starter is short circuited. That is, after the start-up, the high-power thyristor will no longer work, so as to eliminate the high-order harmonic on the power grid and electronic equipment.

4. The soft starter has a variety of built-in protection functions. In the specific application, the protection functions should be selected or disabled by programming according to the actual needs. For example: in the case of sudden power failure, the loss caused by overload is greater than that caused by sudden power failure, the overload protection should act as a signal rather than cut off the circuit.

5. The soft starter needs high environment, so ventilation and heat dissipation should be done well. When it is installed, a certain space should be left above and below it, so that the air can flow through its power module. When the rated current of soft starter is large, fan cooling should be used.