Necessity and difference of built-in and external bypass contactor of soft starter
The contactor connected in parallel with the soft starter is called a bypass contactor, because the long-term use of the soft starter to start will cause a lot of pressure on the thyristor.
Excessive current of the thyristor for a long time is not good for it. At the same time, it is also easy to cause problems by relying on the circuit board to trigger the thyristor. It is better to use a traditional contactor to maintain it, because the soft start is only useful during the motor starting process and runs at the power frequency. It is the same as the usage of ordinary motors, so it is still reliable to use traditional contactors.
Due to the shortcomings and technical difficulties of the online operation of the motor soft starter, the manufacturer has directly developed a bypass soft starter, that is, the motor is bypassed to run on the contactor after the motor is started. Its advantage is that it avoids the shortcomings of on-line operation of thyristors, especially without mechanical air cooling.
However, it also brings disadvantages: the circuit is complicated and the system reliability is reduced; the powerful intelligent controller cannot be fully utilized, and some cannot protect the motor;
Soft start is a device that can reduce the starting current of the AC motor when starting. When the motor completes the starting process, the device can start the bypass contactor according to the set time to remove the soft start from the main circuit of the motor, and the bypass contactor is used. The main contactor of the motor and the motor is connected to the motor power circuit.
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There are also two types of soft starters. One does not use an external contactor and runs directly after starting. It is a built-in bypass contactor.
The other is that an external bypass contactor is required, which is cheaper and automatically switches to contactor operation after startup. This soft start can start multiple motors at the same time, so that one machine can be used for multiple purposes.
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