50 Essential Knowledge of Electrical Control 1. According to the use classification, low-voltage electrical appliances are divided into control electrical appliances, main order electrical appliances, protective electrical appliances, distribution electrical appliances and executive electrical appliances. 2. According to the executive function, low-voltage electrical appliances are divided into contactless and contactless appliances. 3. According to the working principle, low-voltage electrical appliances are divided into electromagnetic and non-electric quantity control devices. 4. Electromagnetic mechanism is composed of attracting coil, core and armature. 5. Electromagnetic appliances can be divided into two categories: DC and AC, both of which are based on the principle of electromagnets. 6. When the knife switch is connected, the power supply should be connected to the inlet end of the static socket side, and the electrical equipment should be connected to the outlet of the movable knife side. 7. When using iron shell switch, the shell should be grounded reliably to prevent electric shock accident caused by external leakage. 8. Contactors are mainly composed of electromagnetic system, contact system and arc extinguishing device. 9. DC contactor does not produce eddy current and hysteresis loss, so it does not heat. 10. Relay is an automatic switching device that controls circuit switching based on external input signals. 11. Relay and contactor have no arc extinguishing device or main and auxiliary contacts because they are used to switch small current control circuit and protection circuit. 12. Speed relay is mainly used for the reverse braking control of cage asynchronous motor. 13. The proximity switch is also called the contactless stroke switch. 14. The fuse is mainly composed of melt and a melt- mounted fuse tube. 15. Circuit breakers are mainly composed of contacts, arc extinguishing systems and trippers. 16. Commonly used short circuit protection devices are fuses and automatic air circuit breakers. 17. The common overload protection device is thermal relay. 18. The hardware of PLC is composed of host computer, I/O extension module and various external devices. 19. The software system of PLC consists of system program and user program. 20. The process of PLC execution is divided into three stages: input sampling or input processing, program execution and output refresh or output processing. 21. Usually a PLC instruction consists of three parts: step number, mnemonic and component number. 22. Relays in the equivalent circuit of PLC are not actual relays. In order to distinguish them from actual relays, they are usually called "soft relays". 23. In the state transition diagram, each state provides three functions: driving load, specifying transition conditions, and positioning new states. 24. Low Voltage Electrical Apparatus refers to all kinds of electrical appliances whose working voltage is below DC 1500V or AC 1200V. 25. Contactors can be divided into DC contactors and AC contactors according to the type of current through their main contacts. 26. The control circuit of relay contactor is composed of various buttons, stroke switches, relays, contactors, fuses and other components. It realizes the control and protection of starting, speed regulation, braking and reverse of electric drive system to meet the requirements of electric drive control in production.Technology 27. Thermal relay is a protective device which uses the principle of thermal effect of current to work. It is mainly used for overload protection of three-phase asynchronous motor in circuit. 28. Speed relay is mainly used for reverse braking control. 29. Time relay refers to the device used to realize the control of delayed connection or delayed disconnection of contacts. 30. The electrical control system of machine tools is made up of many electrical components connected according to certain requirements to realize the automatic control of machine tools.In order to facilitate the design, analysis, installation, debugging, use and maintenance of the control system, it is necessary to connect the electrical components with each other, and to express them in words, symbols and graphics prescribed by the state.This diagram is the electrical control system diagram.Electrical control system diagram includes: electrical schematic diagram, electrical components wiring