I. Selection of AC contactors
The rated parameters of the contactor are mainly determined according to the voltage, current, power, frequency and working system of the charged equipment.
(1) The coil voltage of the contactor is generally selected according to the rated voltage of the control line. Considering the safety of the control line, it is usually selected according to the low voltage, which can simplify the line and facilitate the wiring.
(2) The selection of the rated current of the AC contactor should be considered by the load type, use environment and continuous working time. The rated current of the contactor refers to the maximum allowable current of the contactor under long-term operation, with a duration of 8 h, and is installed on the open control board. If the cooling condition is poor, the rated current of the contactor is selected by 110%~120% of the rated current of the load. For long-working motors, because the oxide film on the surface of the contact has no chance to be cleared, the contact resistance increases, and the contact heat exceeds the allowable temperature rise. In the actual selection, the rated current of the contactor can be reduced by 30%.
(3) Load operation frequency and working condition have a great impact on the selection of AC contactor capacity. When the operating capacity of the load exceeds the rated operating frequency, the contact capacity of the contactor shall be appropriately increased. For frequently starting and disconnected loads, the contact capacity of the contactor should be increased accordingly to reduce the contact corrosion and extend the service life.
2. Common fault analysis and maintenance of low-voltage AC contactor
AC contactors can break frequently during work and can wear the contactor contacts during use. At the same time, sometimes improper use, or use in a relatively harsh environment, will also shorten the life of the contactor, causing failure, therefore, in the use, but also to choose according to the actual situation, and in the use should be maintained in time, to avoid greater losses after the failure. In general, the common faults of AC contactors are contact faults, coil faults and other electromagnetic mechanical faults.
(1) Contact melt welding
In the process of dynamic and static contact suction, the contact surface contact resistance is relatively large, causing the contact point after melting and welding together, can not be broken off, called the contact melt welding. This situation generally occurs in the operation frequency is too high or overload use, load end short circuit, contact spring pressure is too small, mechanical jam resistance, etc. When these situations occur, they can be removed by replacing the appropriate contactor or reducing the load, eliminating short-circuit faults, replacing the contact, adjusting the surface pressure of the contact, and causing the jam factor.
(2) Contact points to overheat or burn
It means that the calorific heat of the working contact exceeds the rated temperature. This situation is generally caused by the following conditions: the spring pressure is too small, the contact with oil, the environmental temperature is too high, the contact for the long-term working system, the working current is too large, resulting in the contact disconnection capacity is not enough. It can be solved by adjusting the contact spring pressure, cleaning the contact surface, the contactor, and changing the contactor with a large capacity.
(3) The coil is overheated and burned down
The general situation is due to the coil interturn short circuit, or when the use of parameters and the actual use of parameters are inconsistent, such as the rated voltage and the actual working voltage does not meet. There is also a possibility of iron core mechanical block, in this case, to remove the block fault.
(4) The contactor is not closed after energizing
In general, you can check whether the coil is broken first. In the case of power failure, the multimeter can be used to measure whether the coil is within the specified range.
(5) Lack of suction
When the power supply voltage is too low or fluctuates too high, or the rated voltage of the coil itself is greater than the actual control circuit voltage, the suction of the contactor will also be insufficient. The voltage can be adjusted to match it with the actual rated voltage of the contactor. At the same time, if the movable part of the contactor is blocked, causing the core to tilt, which may also lead to insufficient suction, the stuck part can be removed and adjust the position of the core. In addition, the reaction force spring is too large, but also may lead to insufficient suction, the need to adjust the reaction force spring.
(6) The contacts cannot be reset
First of all, you can observe whether the static and static contacts are welded together. If this happens, generally you can recover by replacing the contacts, and also observe whether there is something stuck in the movable parts.
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Post time: Jul-12-2022
