The contactor is used as a device for switching on and off the load power supply. The selection of the contactor should meet the requirements of the controlled equipment. Except that the rated working voltage is the same as the rated working voltage of the controlled equipment, the load power, use category, control Mode, operating frequency, working life, installation method, installation size and economy are the basis for selection. The selection principles are as follows:
(1) The voltage level of the AC contactor should be the same as that of the load, and the type of contactor should be suitable for 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 switching current of the contactor is greater than the starting current of the load, and the breaking current is greater than the breaking current 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 half-hour peak current cannot exceed the agreed heat generation current.
(3) Calibrate according to 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 break the short-circuit current, the breaking capacity of the contactor should also be checked.
(4) The rated voltage and current of the contactor attraction coil and the number and current capacity of auxiliary contacts shall meet the wiring requirements of the control circuit. To consider the length of the line connected to the contactor control circuit, the generally recommended operating voltage value, the contactor must be able to work at 85 to 110% of the rated voltage. If the line is too long, the contactor coil may not respond to the closing command due to the large voltage drop; due to the large capacitance of the line, it may not work on the tripping command.
(5) Check the allowable operating frequency of the contactor according to the number of operations. If the operating frequency exceeds the specified value, the rated current should be doubled.
(6) The parameters of the short-circuit protection components should be selected in conjunction with the parameters of the contactor. For selection, please refer to the catalog manual, which generally provides the matching table of contactors and fuses.
The cooperation between the contactor and the air circuit breaker should be determined according to the overload coefficient and short circuit protection current coefficient of the air circuit breaker. The agreed heating current of the contactor should be less than the overload current of the air circuit breaker, and the on and off current of the contactor should be less than the short circuit protection current of the circuit breaker, so that the circuit breaker can protect the contactor. In practice, the contactor agrees that the ratio of heating current to rated operating current is between 1 and 1.38 at a voltage level, while the circuit breaker has many inverse time overload coefficient parameters, which are different for different types of circuit breakers, so it is difficult to cooperate between the two There is a standard, which cannot form a matching table, and requires actual accounting.
(7) The installation distance of contactors and other components must comply with relevant national standards and specifications, and maintenance and wiring distances should be considered.
3. Selection of AC contactors under different loads
In order to avoid contact adhesion and ablation of the contactor and prolong the service life of the contactor, the contactor must avoid the maximum current of the load starting, and also consider unfavorable factors such as the length of the starting time, so it is necessary to control the load of the contactor on and off. According to the electrical characteristics of the load and the actual situation of the power system, the start-stop current of different loads is calculated and adjusted.
Post time: Jul-10-2023