1. 50 pieces of electrical control knowledge
1. According to the classification of uses, low-voltage electrical appliances are divided into control electrical appliances, main command electrical appliances, protective electrical appliances, distribution electrical appliances and executive electrical appliances.
2. According to the classification of executive functions, low-voltage electrical appliances are divided into contact electrical appliances and non-contact electrical appliances.
3. According to the working principle, low-voltage electrical appliances are divided into electromagnetic electrical appliances and non-electricity controlled electrical appliances.
4. The electromagnetic mechanism consists of attraction coil, iron core, armature and other parts.
5. Electromagnetic electrical appliances are divided into two categories: DC and AC, both of which are made using the principle of electromagnets.
6. When wiring the knife switch, the power incoming wire should be connected to the incoming wire on the static socket side, and the electrical equipment should be connected to the outgoing wire on the movable contact knife side.
7.When using an iron case switch, the case should be reliably grounded to prevent electric shock accidents caused by external leakage.
8. The contactor mainly consists of an electromagnetic system, a contact system, and an arc extinguishing device.
9. DC contactors do not produce eddy current and hysteresis losses, so they do not generate heat.
10. A relay is an automatic switching electrical appliance that controls the on and off of a circuit based on external input signals.
11. Compared with contactors, relays are used to switch control circuits and protection circuits with small currents. Therefore, relays have no arc extinguishing device and no distinction between main and auxiliary contacts.
12. Speed relay is mainly used for reverse braking control of cage asynchronous motor.
13. Proximity switch is also called contactless travel switch.
14. The fuse is mainly composed of a melt and a fuse tube in which the melt is installed.
15. The circuit breaker mainly consists of contacts, arc extinguishing system and tripper.
16. Commonly used short-circuit protection appliances are fuses and automatic air circuit breakers.
17. A commonly used overload protection device is a thermal relay.
18． The hardware of PLC is composed of host computer, I/O expansion module and various external devices.
19. The software system of PLC consists of system program and user program.
20. The process of PLC executing the program is divided into three stages: input sampling or input processing, program execution and output refreshing or output processing.
twenty one. Usually a PLC instruction consists of three parts: step number, mnemonic and component number.
twenty two. The relays in the PLC equivalent circuit are not actual relays. In order to distinguish them from actual relays, they are usually called “soft relays”.
twenty three. In the state transition diagram, each state provides three functions: driving the load, specifying conversion conditions, and setting a new state.
24. Low-voltage electrical appliances refer to various electrical appliances with an operating voltage below _DC 1500V or _AC 1200V.
25. Contactors can be divided into DC contactors and AC contactors according to the type of current passing through their main contacts.
26. The relay contactor control circuit is composed of various buttons, travel switches, relays, contactors, fuses and other components to realize the control and control of starting, speed regulation, braking, reverse, etc. of the electric drive system. protection to meet the requirements for electric drag control in the production process.
27. Thermal relay is a protective appliance that works based on the principle of thermal effect of current. It is mainly used as overload protection of three-phase asynchronous motor in the circuit.
28. The speed relay is mainly used for reverse braking control.
29. Time relay refers to an electrical appliance used to realize the control of delayed closing or delayed opening of contacts.
30. The machine tool electrical control system is composed of many electrical components connected according to certain requirements to achieve automatic electrical control of the machine tool. In order to facilitate the design, analysis and research, installation, debugging, use and maintenance of the control system, it is necessary to express the electrical components and their interconnections with nationally prescribed words, symbols and graphics. This kind of diagram is an electrical control system diagram. Electrical control system diagrams include: electrical schematic diagram, electrical component wiring diagram, and electrical component layout diagram.
31. In the electrical control of machine tools, _fuse_ is used for short-circuit protection; thermal relay_ is used for overload protection; and over-current relay is used for overcurrent protection.
32. Each scanning process of PLC is divided into three stages: input sampling stage, program execution stage and output refresh stage.
33. Electrical control diagrams are generally divided into two parts: the main circuit and the control circuit.
34. The structural forms of contacts include bridge contacts and finger contacts.
35. Contactor is an automatic switching appliance used to frequently connect or disconnect AC and DC main circuits and large-capacity control circuits over long distances.
36. The rated breaking capacity of a contactor refers to the current value that its main contact can reliably make and break under specified conditions.
37. A relay is a device that uses changes in various physical quantities to convert changes in electrical or non-electrical quantities into electromagnetic force or to cause step changes in the output state, thereby promoting the same circuit or another circuit through its contacts or mutation amounts. A control element for the operation of other devices or devices.
38. The adjustment device of the relay is used to change the operating parameters of the relay.
39. Low-voltage electrical appliances are divided according to their uses: low-voltage distribution appliances, low-voltage control appliances, etc.
40. The electromagnetic relay reflects the electrical signal. When the coil reflects the voltage signal, it is a voltage relay; when the coil reflects the current signal, it is a current relay.
41. The electromagnetic voltage relay coil is connected to the circuit voltage in parallel to reflect the circuit voltage.
42. The electromagnetic current relay coil is connected in series in the circuit to reflect the size of the circuit current.
43. When starting the three-phase squirrel-cage asynchronous motor Y-△ step-down starting current, the starting current is 1/3 times of the direct starting current. This method can only be used for motors with stator windings using the △ connection method.
44. The starting method of CMC-MX soft starter: current limited starting, voltage ramp starting; the stopping method: free stop, soft stop.
45. Xichi’s integrated high-voltage solid-state soft starter device combines the switch cabinet and the starter cabinet into one.
46. When selecting a contactor, you should start from its working conditions. AC contactors should be used to control AC loads; DC contactors should be used to control DC loads.
47. When selecting a contactor, the rated operating voltage of its main contact should be greater than or equal to the voltage of the load circuit, the rated operating current of the main contact should be greater than or equal to the current of the load circuit , and the rated voltage of the attraction coil should be the same as that of the control circuit. voltage matches.
48. The function of the intermediate relay is to turn an input signal into multiple output signals or to amplify the signal.
49. Commonly used braking methods for cage asynchronous motors are: electrical braking and mechanical braking.
50. According to national standards, the “stop” button must be red and the “start” button must be green.
2. Electrical practical speed algorithm
1. The current at the low-voltage side of the transformer is 1.5 times the capacity (1.44 times to be precise)
2. The three-phase short-circuit current of the low-voltage outlet of the transformer is 150 times the capacity divided by the percentage of the short-circuit impedance of the transformer (144 times when accurate)
3. The rated current of the 10kV side of the transformer is 6% of the capacity (5.8% when accurate); the rated current of the 6kV side is 10% of the capacity (9.6% when accurate)
4. The rated current of the 0.4kV three-phase compensation capacitor is 1.5 times the capacity (1.44 times when accurate)
5. The rated current of a 380V three-phase motor is twice the capacity and
6. Under normal circumstances, the power factor of the low-voltage side is compensated to 0.95, and the capacitor capacity is about 1/3 to 2/5 of the transformer capacity. Suitable for general use places, except for special occasions, such as places with large capacity of resistance furnaces or gas discharge lamps.
7. Copper discharge flow estimation table
The cross-section of the copper bar is S (mm2), and the allowable carrying capacity is I (A). The cross-section of the copper bar is S (mm2), and the allowable carrying capacity is (IA).
S≤100I=3.5S; 200＜S≤600I=2.5S
100＜S≤200I=3S; S＞600I=2S
Note: The smaller the thickness of the copper bar, the greater the current carrying capacity per mm2.
8. Calculation of rated and starting current protection of three-phase 380V cage motor
The rated current ampere is twice its capacity in kilowatts, and its starting current is about 12 to 14 times its capacity. For directly started motors, the instantaneous current of the protective short-circuit plastic case switch is 17 to 24 times its capacity; for light loads For motors that do not start frequently, the fuse current should be 2.5 to 3 times the rated current of the motor; for heavy-duty starting motors, the fuse current should be 4 to 5 times the rated current.
9. How to choose the capacity of low-voltage compensation capacitor? How to choose its protection fuse?
Generally speaking, the total compensation number is the transformer capacity kVA and 30% to 40%. The current ampere of a single unit is 1.5 times its capacity kVA. The fuse current of the protection fuse is not less than the rated current of the capacitor . 1.5 times.
Estimation example:
A 1250kVA transformer, short-circuit impedance is 6%, low-voltage 0.4kV rated current Ie=1.5*1250=1875A; low-voltage short-circuit current Ik=1250*150/6=31250=31.25kA; high-voltage 10kV side current I, I=1250* 6%=75A;
The compensation capacity of low-voltage capacitor is approximately Q=1250*1/3=420kVar.
Therefore, the high-voltage current transformer is selected as 75/5;
Low voltage current transformer 2000/5, 2500/5;
The circuit breaker can be selected with a current breaking capacity of not less than 35kA;
There are two compensation capacitor cabinets, one with 200kVar and one with 220kVar.
Note: If the dynamic stability of the high-voltage current transformer does not meet the requirements, the transformation ratio can be increased or another current transformer can be selected.
Keywords related to this article: Network IO controller