Four phenomena of improper selection of AC contactors

- Jul 19, 2019-

AC contactor (electrical schematic code KM) is the main electrical device in the circuit to control power supply and load on/off, but also our electrician practitioners in the usual work needs to face. In the work, the author from time to time found that there are individual peers in the selection of AC contactors in the process, there are various omissions resulting in improper selection, resulting in follow-up problems. Here, the author will be the most common four typical phenomena to show to everyone, hope that we can prevent micro-du gradually.

 First, some electrical colleagues in the load according to the selection of suitable AC contactors, often only according to the load of the rated current to choose, so that the actual operation of the main contact steam of AC contactors often appear burned, burned and so on! The reason for this fault phenomenon is that the method of selecting AC contactors (main contact capacity) according to the rated current is only applicable to purely resistive loads such as electric wire. For the three-phase asynchronous motor and other inductive loads, because in the start-up process before the smooth operation, its start-up current is subject to the start-up mode, drag load type, start-up frequency, etc. , the start-up current is about 4-7 times the rated current. When selecting AC contactors for this purpose, it is also important and necessary to take into account the load start current.

 Second, with the safety of electricity and safety operation norms awareness, but also in order to reduce unnecessary personal shock accidents, the current AC contactor coil voltage priority adoption - safety voltage level (AC36V) has become the general trend. To this end, in the design, selection, assembly of AC contactors process, we should first use coil voltage level AC36V products. Try to avoid the co-existence of the voltage level of AC380V, AC220V and other coils in the line.

Third, in order to reduce the number of other auxiliary devices (such as intermediate relays, etc.) and reduce the volume of the electronic control system, in the selection of AC contactors should also be based on the AC contactorin the number of auxiliary contacts required in the line, to consider the selection of AC contactor types. For example, in the line need to ac contactor auxiliary contacts more cases, choose can be attached 2 pairs or 4 treatment of auxiliary contacts CJX series AC contactors, than CJT series AC contactors to come more wise.

 Fourth, taking into account the above three factors, the final need for compensation description is about the AC contactor (coil) control mode of the problem. At this stage, it is easy to centralize the regulation of plc and other industrial control equipment is increasingly widely used, in this process, there are many peers directly AC contactor coil to the PLC output terminal, resulting in PLC internal output devices (relays, transistors, thyristors) damage cases. Its failure phenomenon is caused by nothing more than, contactor coil suction process current value is greater than the PLC output device's current capacity caused!

Therefore, when using PLC control AC contactors, the Electrical appliances are used as a control transition.