AC/DC contactor selection method and precautions

AC/DC contactors (hereinafter referred to as "contactors") are commonly used electrical components and are one of the main control devices in the circuit. Correct selection, selection (contactor rated current capacity), reasonable use, to achieve the purpose of high reliability, long life operation of the contactor is very important.

1. Load category of the contactor

According to the operation requirements of low-voltage electrical appliances, the rated working current, rated working voltage and power factor are usually used to characterize the contact and breaking ability of the contactor.

The rated current of the AC contactor is calibrated under the condition of AC three-wire system. When using the AC four-wire system, the rated current should be properly corrected according to the product specification. Otherwise it will seriously affect its life and cause an accident.

2. Main factors affecting the life of the contactor

(1) Conditions of use. Different conditions of use can cause the contactor to withstand different currents, causing different thermal effects of the main contacts. The greater the overload factor, the higher the temperature of the contacts. The amount of electrical erosion and wear of the contact is approximately proportional to the square of the breaking current within a certain range, which has a great influence on the service life.

(2) Operating frequency. Refers to the number of actions per hour. It varies with the control object, generally divided into 300 times / h, 600 times / h, 1200 times / h and so on. Increasing the operating frequency will degrade the arc extinguishing condition of the contact, accelerate the heating rate and increase the amount of electrolytic corrosion, and at the same time, easily cause the electromagnetic coil to burn out. The life of the contactor is not simply proportional to the increase in operating frequency. For contactors that frequently operate, when the life expectancy is not met, the capacity should be reduced.

(3) Installation environment. The pros and cons of the installation environment will indirectly affect the life of the contactor. For example, in hot and humid areas, wet heat contactors are required. In addition, if there is acid gas or conductive dust, high humidity, high ambient temperature, poor power supply quality, high voltage fluctuation, high harmonic content, etc., the life of the contactor will be shortened. The specifications of the contactor are corrected.

3. Ways to improve the life of the contactor

The load category and operating frequency of the contactor are determined by the application conditions and needs and are immutable. When selecting a contactor, in addition to selecting the contactor of the corresponding function according to the requirements, it is necessary to increase the life of the contactor and reduce the number of times of replacing and repairing the contactor. The most effective way is to reduce the capacity. In the case where the installation conditions permit, the larger the derating factor, the larger the life improvement factor.

4. Notes

Since the contactor is installed at a different location than the controlled load, the working conditions and the level of work are not the same. When selecting a contactor, it is necessary to have a comprehensive understanding of the operation of the controlled load and the performance of the contactor, so that the contactor can be correctly selected. The focus is on the type and capacity of the contactor based on the load category.

(1) When used for AC single-phase and AC three-wire system, the contactor that is slightly larger than the rated working current can be selected according to the full load current of the load at the specified operating frequency. Other parameters are selected according to the load parameters.

(2) Working on a heavy-duty motor load, or the contactor does not control a single type of load, but instead is connected to an AC three-wire system, and sometimes to an AC four-wire hybrid state, or from time to time, reverse brake, reverse It can be disconnected at low speed, or the operating frequency can reach 100 times/h or even higher, and the motor power is generally below 20 kW. It can be selected according to the rated current of the motor 3-4 times, and its life can be 5 according to the AC four-wire system. ~10 times rough calculation.

(3) Working on special heavy-duty motor loads, such as long-term use in AC four-wire system or phase loss conditions, frequent jog, reverse braking and reversible operation, the operating frequency can reach 600-1200 times/h or even 3,000 times. /h, special attention should be paid to the selection of contactors with a higher service life. When applied, the starting current of the motor can be roughly selected as the rated current of the contactor.

(4) Verify the allowable operating frequency of the contactor based on the number of actions.