The soft starter is only used for starting, continuous use is not allowed, so a bypass contactor should be used after starting.
The bypass soft starter must be used together with the contactor to work normally. After the soft starter starts, the main current is switched to the bypass contactor, and the current during normal operation does not go inside the soft starter, but this does not mean that this is the case. When the soft starter is useless.
There is a high-precision transformer inside the soft starter, the main current will pass through it, and the soft start detects the current and voltage during operation through the transformer. The contactor will command the coil of the bypass contactor to de-energize, and the contactor will disconnect the main current, thus playing a variety of protection functions.
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 the contactor to run 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; the volume and cost of the complete set of devices are increased; the difficulty of maintenance and repair is increased.
After a comprehensive comparison, the bypass operation mode is still used in most of the market. Even if the soft starter with online operation mode is selected, the designer still installs a set of bypass operation contactor to make the soft starter operate in bypass, avoiding possible problems. Defects in SCR online operation.
How to wire the control circuit between the soft starter and the external contactor?
1.understand the secondary control loop of soft start. Generally, soft start has bypass contactor output node.
2. Know the load start-up time again. The best time to cut the bypass contactor is after the start-up process is completed.
3. The main circuit of the bypass contactor should be connected in parallel with the soft starter.
4. String the output node of the bypass contactor into the control loop of the bypass contactor, preferably a self-locking loop. Among them, the external contactor is used for direct starting, which is in a parallel relationship and requires electrical interlocking.
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