Circuit breaker and contactor selection

The rated current of the circuit breaker and contactor should be greater than or equal to the rated current of the motor being protected and controlled. Generally speaking, because the start-stop motor is controlled by a contactor, and the overload of the motor is generally realized by a thermal relay or a motor protector with a contactor, and the circuit breaker is only used for short-circuit protection, and rarely operates, so the general contactor The choice is to be one to two, and the circuit breaker only needs to meet the short circuit protection requirements. The rated current of a 15kW motor is about 30A, the general contactor can be 40A or 50A, and the circuit breaker can be 32A or 40A.

The circuit breaker needs to be selected according to 1.6 times the actual current of the motor. The contactor and thermal relay need only be selected according to the sample you need. Generally, how many KW is used for the contactor. However, the single-phase 220V can not be calculated according to the three-phase.

There is no doubt that the rated current of a four-pole three-phase asynchronous motor is twice the rated power. The key problem lies in the setting value of the thermal relay. Generally, the setting value is between 1.15 and 1.25 of the rated current, so that the motor can work normally.

(1) The voltage level of the AC contactor should be the same as the load, and the type of contactor to be used should be compatible with the load.

(2) The calculated current of the load must conform to the capacity level of the contactor, that is, the calculated current is less than or equal to the rated operating current of the contactor. The contact current of the contactor is greater than the starting current of the load. The breaking current is greater than the current required for the breaking of the load. The calculated current of the load should take into account the actual working environment and working conditions. For a load with a long starting time, the peak current of half an hour cannot exceed the agreed heat. Current.

(3) According to short-term dynamic and thermal stability check. The three-phase short-circuit current of the line should not exceed the dynamic and thermal stable current allowed by the contactor. When the contactor is used to disconnect the short-circuit current, the breaking capacity of the contactor should also be verified.

(4) The rated voltage, current and the number of auxiliary contacts and current capacity of the contactor attracting coil should meet the control circuit wiring requirements. To consider the length of the line connected to the contactor control loop, the recommended operating voltage value, the contactor should be able to work at 85 ~ 110% of the rated voltage. If the line is too long, the contactor coil may not reflect the closing command because the voltage drop is too large; it may not work for the trip command because the line capacitance is too large.

(5) Verify the operating frequency allowed by the contactor based on the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled.

(6) The short circuit protection component parameters should be selected in conjunction with the contactor parameters. For the selection, please refer to the sample manual. The sample manual generally gives the matching table of the contactor and the fuse.