Why use a contactor with a button to control the motor instead of directly using a circuit breaker?

Some devices can clearly be started directly with a circuit breaker, so why use a contactor?

I brought the question to many experienced electricians, and they all looked surprised and then awkwardly smiled. To put it simply, they all have a vague understanding but cannot fully summarize the reasons.

Let's analyze from several aspects whether a circuit breaker and a contactor can be chosen between.

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

Functions such as forward and reverse rotation of motors, remote control, and local control involve many interlocking situations, which cannot be satisfied by a circuit breaker.

Even for simple start control, a circuit breaker cannot meet the requirements for the following reasons:

1. Lifespan: Circuit breakers are suitable for continuous current flow and automatically disconnect when an abnormality occurs, providing protection for the circuit. Contactors are suitable for frequently starting and stopping the circuit, required to meet at least 300,000 on-off operations.

Therefore, for motors that start and stop multiple times a day, using a circuit breaker would likely require replacement every month (in large machinery factories and steel mills, due to the special nature of their equipment, circuit breakers need to be frequently opened and closed; friends who have worked in these factories know that it is quite normal to replace circuit breakers once or twice a month).

By using contactors to control electrical equipment, zero voltage protection can be achieved. When production machinery is working, if an air circuit breaker directly controls the motor and the power suddenly goes out while the equipment is running, the motor will stop, but if the circuit breaker is in the closed position, when the power voltage is restored, the motor will start running by itself, which may lead to personal and equipment accidents. The protection implemented to prevent such situations is called zero voltage protection.

2. Safety: When a motor starts, it often generates a large current, which can create an arc when closing.

Because circuit breakers are manually closed, and plastic shell circuit breakers are relatively heavy and slow to close, it is easy for the current to break through the air gap between the contacts, creating an arc, posing a danger to the person closing the circuit.

Contactors, due to their fast closing speed, strong arc extinguishing function, and being hidden inside the distribution box, do not pose an arc threat to the person closing the circuit. Moreover, contactors are often used in secondary circuits, controlling high voltage with low voltage, thus greatly increasing safety.

3. Economy: Circuit breakers are not completely unable to eliminate arc problems, but good circuit breakers are relatively expensive. Coupled with lifespan issues, the cost of using circuit breakers is much higher than that of contactors.

4. Convenience: Contactors are operated outside the box, are simple and easy to operate, making them more suitable for non-professionals. In contrast, circuit breakers (motor control often uses plastic shell circuit breakers) are larger, more complex, and labor-intensive to operate, and non-professionals may risk electric shock when operating the box door.

5. Automation: In the age of automation, more and more microcontrollers and PLCs are being used. Due to the structural characteristics of contactors, they are easier to control via computers. The opening and closing of circuit breakers often can only be controlled manually.

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