The circuit breaker can directly start the equipment, can the contactor be omitted?

Why use contactors and buttons to control the motor instead of directly using circuit breakers?

Some equipment can obviously use a circuit breaker to start the equipment directly. Why use contactors?

I took the question to many old electricians, all of them looked surprised, then smiled awkwardly. To put it bluntly, everyone knows a little about this, but they can't fully summarize the reasons.

The following from several aspects to analyze, circuit breakers and contactors can choose one of the two:

The main function of the circuit breaker is to provide protection to the circuit, and the function of the contactor is mainly to control the circuit.

Functions such as motor forward and reverse rotation, remote control, and local control involve many interlocks, which cannot be met by circuit breakers at this time.

Even if it is a simple start control, the circuit breaker is not enough, for the following reasons:

1. Life: The circuit breaker is suitable for continuous current flow, and automatically disconnects when the circuit is abnormal to provide protection for the circuit. The contactor is suitable for frequent start-stop electrical circuit and is required to meet at least 300000 on-off operations.

Therefore, for motors that start and stop several times a day, it is estimated that circuit breakers need to be replaced every month (large machinery plants and steel mills need circuit breakers to be opened and closed frequently due to the particularity of their equipment. Friends who have worked in these factories all know that it is normal to replace circuit breakers once or twice a month in the factory).

By using contactors to control electrical equipment, zero voltage protection can be achieved. When the production machinery is working, if the air circuit breaker is used to directly control the motor, if the equipment is suddenly cut off during operation and the motor stops running, but the circuit breaker is in the closed position, the motor will start running on its own when the power supply voltage is restored, which may sometimes lead to personal and equipment accidents. The protection implemented to prevent such situations is called zero voltage protection.

2. Safety: When the motor is started, it tends to generate a large current, which will generate an arc when closing.

Because the circuit breaker is manual closing, coupled with the molded case circuit breaker is relatively heavy, the closing speed is slow, it is easy to make the current breakdown the air gap between the contacts, resulting in arc, causing danger to the closing personnel.

Due to the fast closing speed, strong arc extinguishing function and hidden in the distribution box, the contactor will not cause arc threat to the closing personnel. And the contactor is mostly used in the secondary circuit, using weak current to control strong current, so its safety is greatly increased.

Economy: The circuit breaker is not completely unable to eliminate the arc problem, but the cost of a good circuit breaker is higher. In addition to the life problem, the cost of the circuit breaker in use is much higher than that of the contactor.

4. Convenience: The contactor is operated outside the box, which is simple and easy to operate, and is more suitable for non-professionals. However, the circuit breaker (motor control often uses molded case circuit breaker) is large in size, complicated and laborious in operation, and the risk of electric shock may occur when non-professionals open the door.

5, automation: in the era of automation, more and more single-chip, PLC control. The contactor therefore its own structural characteristics, easier to control by computer. The opening and closing of the circuit breaker can only be controlled manually.