Electrical engineer knowledge points

Update date:2018-04-10 Source:MAXGE

Electrical engineer knowledge points

1. Classified by purpose, low-voltage electrical appliances are divided into control appliances, main appliances, protection appliances, distribution appliances and executive appliances.

2. According to the classification of executive functions, low-voltage electrical appliances are classified into contactless appliances and non-contact appliances.

3. Classified by working principle, low-voltage electrical appliances are divided into electromagnetic appliances and non-electricity control appliances.

4. Electromagnetic mechanism consists of attracting coils, cores, armature and other parts.

5. Electromagnetic appliances are divided into two major categories, namely DC and AC, which are made using the principle of electromagnets.


6. When the knife switch is connected, the power inlet should be connected to the inlet side of the static receptacle, and the electrical equipment should be connected to the outlet of the movable knife.

7. When using the iron shell switch, the shell should be reliably grounded to prevent electric shock caused by external leakage.

8. The contactor is mainly composed of an electromagnetic system, a contact system, and an arc extinguishing device.

9. The DC contactor does not generate eddy currents and hysteresis losses, so it does not generate heat.

10. The relay is an automatic switching device that controls the circuit on and off based on external input signals.

11. The relay and the contactor are used to switch the control circuit and the protection circuit of the small current compared to the relay, so the relay does not have the arc extinguishing device, also does not divide the main and auxiliary contact points.

12. Speed relays are mainly used for reverse brake control of cage asynchronous motors.

13. Proximity switch is also called non-contact travel switch.

14. The fuse is mainly composed of a melt and a melt pipe with a melt.


15. The circuit breaker consists of contacts, arc extinguishing systems and trip units.

16. Common short-circuit protection devices are fuses and automatic air circuit breakers.

17. The commonly used overload protection device is a thermal relay.

18. Plc's hardware consists of a host, I/O expansion modules, and various external devices.

19. The PLC software system consists of system programs and user programs.

20. The process of PLC execution program 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. The relays in the PLC equivalent circuit are not actual relays. In order to distinguish them from actual relays, they are often referred to as "soft relays."

23. In the state transition diagram, each state provides three functions: drive load, specified transition condition, and set new state.

24, low-voltage electrical refers to the operating voltage in the _ DC 1500V or _ AC 1200V below the various electrical appliances.

25. The contactor can be divided into DC_contactor and AC_contactor according to the type of current through its main contact.

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