Selection of AC contactors under different loads

　　Low voltage contactor as a device to switch off the load power supply, the selection of contactor should meet the requirements of the controlled equipment, in addition to the rated working voltage and the rated working voltage of the controlled equipment is the same, the load power of the controlled equipment, the type of use, the control mode, the working frequency, the service life, the installation mode, the installation size and the economy are the basis for selection.

　　In order to prevent contact adhesion and ablation and extend the life of the contactor, the contactor should avoid the maximum load starting current, and take into account the length of the starting time and other unfavorable factors, so the load of the contactor on-off operation should be analyzed, according to the electrical characteristics of the load and the actual situation of the power system, the different load start-off current is calculated and adjusted.

　　Selection principles are as follows:

　　The voltage level of the AC contactor should be the same as the load, and the type of contactor selected should be adapted to the load.

　　The calculated current of the load should conform to the capacity class of the contactor, that is, the calculated current is less than or equal to the rated working current of the contactor. The connecting current of the contactor is greater than the starting current of the load, and the breaking current is greater than the current required when the load is running. The calculation current of the load should consider the actual working environment and working conditions. For the load with a long starting time, the peak current of half an hour cannot exceed the agreed heating current.

　　Check against short-term dynamic and thermal stability. The three-phase short-circuit current of the line should not exceed the dynamic and thermal stable current allowed by the contactor. When using the contactor to disconnect the short-circuit current, the breaking ability of the contactor should also be verified.

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