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    When do You Need a Soft Starter?

    The VFD is generally not suitable for switching back and forth between devices such as output contactors, because its control is complicated. When the motor is switched, it may cause the motor to idle, and its phase is very different from that of the variable frequency drive output. There is a relatively large impact, which will affect the life of the IGBT of the inverter. The soft starter itself is designed with these in mind, using some phase-locked loop techniques to track.

    When do you need a soft starter?

    Soft starter diagram

    1. In terms of usage, soft starters can be selected for large loads that do not require speed regulation and only require step-down start.
    2. In terms of control input, the price of soft starters is much cheaper than that of frequency converters, which is more obvious for large-capacity controllers for high-power load control occasions.
    3. VFD needs to be equipped with a special frequency conversion motor in some control occasions with strict high-speed operation requirements. If an ordinary induction motor is used, it will cause great damage to the motor. Therefore, it is best to use a soft starter control in the occasions where power frequency can be used.

    Generally, soft starter can be applied to various applications where squirrel induction motors do not require speed regulation. The current application range is AC 380V (also 660V), and the motor power ranges 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, for variable load conditions, when the motor runs under light load for a long time, and is only under heavy load for a short time or momentarily, the application of a soft starter (without bypass contactor) has the effect of light load energy saving.

    Tips for using soft starter

    • The soft starter itself has no short-circuit protection. In order to protect the thyristor, a fast fuse should be used. The fast fuse should be selected according to the rated current of the soft starter. It should be pointed out that due to the long breaking time of low-voltage circuit breakers (about 0.1S), it is not suitable for the protection of thyristors.
    • When the soft starter uses the motor to brake to stop, the input voltage of the motor is 0V because the thyristor is not conducting, but there is no electrical isolation between the motor and the power supply, so when the motor or circuit is repaired, it must be cut off. Power supply. For this purpose, a circuit breaker should be added between the soft starter and the power supply.
    • When the power of the soft starter is large or the number of units is large, the generated high-order harmonics will cause adverse effects on the power grid and interfere with the electronic equipment. When the motor starts smoothly to the normal speed, the bypass contactor is closed, and the soft starter is shorted. That is, after the start is completed, the high-power thyristor will no longer work, thereby eliminating the dry pollution of the power grid and electronic equipment by the high-order harmonics.
    • The soft starter has a variety of built-in protection functions. The specific application should select the protection function or disable some protection functions through programming according to the actual needs. For example, in the case where the loss caused by sudden power failure is greater than that caused by overload, the overload protection should act on the signal and not be used to cut off the circuit.
    • The use environment of the soft starter is relatively high, and ventilation and heat dissipation work should be done well. When installing, it should be installed on it, and a certain space should be left under it to allow air to flow through its power module. When the rated current of the soft starter is large, the fan should be used to cool down.
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