It is said that to learn mathematics and chemistry well is not afraid to travel all over the world. In the field of electrical engineering, learning to start, protect and stop circuit can also be competent for general electrician posts in petrochemical industry.

The so-called start protect stop circuit is actually start, protection and stop circuit. The circuit mainly includes circuit breaker, contactor, thermal relay (integrated protector), intermediate relay, button and indicator light. Today, we will explain the selection of the main electrical components in the circuit with examples.

For example: a 22KW three-phase asynchronous motor, Take Changshu switch factory's electrical components as an example, to see how to choose circuit breaker, contactor and thermal relay?

1. Calculate the rated current of motor;

P=1.732×UI×cosφ ；

The results showed that I = 42.2a;

2. Select moulded case circuit breaker;

The rated current of the circuit breaker is determined according to the rated current of the motor, and the circuit breaker with the maximum tripping current is selected. If it is not started frequently, according to experience, the motor rated current of imported circuit breaker is 1.2 ~ 1.5 times of motor rated current, while domestic circuit breaker is generally 1.5 ~ 2 times of motor rated current. If it starts frequently, it is recommended to select a higher gear.

If it is 1.5 times, then:

42.2A*1.5≈63A；

The rated current is 63A and the motor protection type is cm2-63m / 33202-63a.

3. Select AC contactor

AC contactor is selected according to 1-1.4 times of motor rated current. If it is 1.3 times, then:

44.2A*1.3≈55A；

The rated current of AC contactor is 65A and the model is sc-e3n511-c.

4. Select thermal relay

The thermal relay should be 1.0 ~ 1.15 times of the rated current of the motor. The actual setting current is selected according to the load characteristics, and is generally set to be 0.9 ~ 1.15 times of the rated current of the motor.

42.2A*1.1≈46A；

The rated current of thermal relay is 34 ~ 50A and the model is tk-n3pg-c;

5. Cable selection

According to the current carrying capacity of the cable, 3 * 25 + 1 * 16 can be selected.

The above selection basis is simply based on empirical formula, the real choice needs very high professional technology and many considerations. Therefore, the selection of electrical components can not be generalized, mainly depends on the load working conditions, such as: light load, heavy load, frequent startup, cable length, laying method, etc.