I. 50 Essential Electrical Control Tips
1. According to their uses, low-voltage electrical appliances are classified into control appliances, master control appliances, protection appliances, power distribution appliances, and actuators.
2. According to their function, low-voltage electrical appliances are divided into contact electrical appliances and non-contact electrical appliances.
3. According to their working principle, low-voltage electrical appliances are classified into electromagnetic electrical appliances and non-electrical quantity control electrical appliances.
4. The electromagnetic mechanism consists of several parts, including an attraction coil, an iron core, and an armature.
5. Electromagnetic electrical appliances are divided into two main categories: direct current and alternating current, both of which are made using the principle of electromagnets.
6. When wiring a knife switch, the power supply line should be connected to the inlet terminal on the stationary socket side, and the electrical equipment should be connected to the outlet terminal on the moving contact knife side.
7. When using iron-cased switches, the casing should be reliably grounded to prevent electric shock accidents caused by external leakage.
8. A contactor mainly consists of an electromagnetic system, a contact system, and an arc-extinguishing device.
9. DC contactors do not generate eddy currents or hysteresis losses, so they do not generate heat.
10. A relay is an automatic switching electrical device that controls the on/off state 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 do not have arc extinguishing devices and do not have a distinction between main and auxiliary contacts.
12. Speed relays are mainly used for reverse braking control of squirrel-cage induction motors.
13. Proximity switches are also known as contactless limit switches.
14. A fuse mainly consists of a fusible element and a fusible tube for mounting the fusible element.
15. Circuit breakers mainly consist of contacts, an arc-extinguishing system, and a trip unit.
16. Commonly used short-circuit protection devices are fuses and automatic air circuit breakers.
17. The commonly used overload protection device is the thermal relay.
18. The hardware of a PLC consists of a host computer, I/O expansion modules, and various external devices.
19. The software system of a PLC consists of system programs and user programs.
20. The process of PLC program execution is divided into three stages: input sampling or input processing, program execution, and output refresh or output processing.
21. A typical PLC instruction consists of three parts: step number, mnemonic, and component number.
22. The relays in the PLC equivalent circuit are not actual relays. 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 setting the new state.
24. Low-voltage electrical appliances refer to various electrical appliances with a working voltage of less than 1500V DC or 1200V AC.
25. Contactors can be classified 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 components such as buttons, limit switches, relays, contactors, and fuses. It realizes the control and protection of the starting, speed regulation, braking, and reversing of the electric drive system to meet the requirements of the production process for electric drive control.
27. A thermal relay is a protective electrical device that works by utilizing the thermal effect of electric current. It is mainly used in circuits for overload protection of three-phase asynchronous motors.
28. Speed relays are mainly used for reverse braking control.
29. A time relay is an electrical device used to achieve delayed connection or delayed disconnection of contacts.
30. A 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. To facilitate the design, analysis, installation, debugging, use, and maintenance of the control system, it is necessary to represent the electrical components and their interconnections using nationally standardized text, symbols, and graphics. This type of diagram is called an electrical control system diagram. Electrical control system diagrams include three types: electrical schematic diagrams, electrical component wiring diagrams, and electrical component layout diagrams.
31. In machine tool electrical control, short circuit protection uses fuses; overload protection uses thermal relays; and overcurrent protection uses overcurrent relays.
32. Each scanning process of a 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. A contactor is an automatic switching device used to frequently connect or disconnect AC/DC main circuits and high-capacity control circuits over long distances.
36. The rated breaking capacity of a contactor refers to the current value that its main contacts can reliably make and break under specified conditions.
37. A relay is a control element that uses changes in various physical quantities to convert changes in electrical or non-electrical quantities into electromagnetic force or to cause a step change in the output state, thereby causing other devices or devices in the same circuit or another circuit to operate through its contacts or sudden changes.
38. The adjustment device of a relay is used to change the operating parameters of the relay.
39. Low-voltage electrical appliances are classified by their purpose: low-voltage power distribution appliances, low-voltage control appliances, etc.
40. Electromagnetic relays respond to electrical signals. When the coil responds to a voltage signal, it is a voltage relay; when the coil responds to a current signal, it is a current relay.
41. The coil of an electromagnetic voltage relay is connected in parallel with the circuit voltage to reflect the magnitude of the circuit voltage.
42. The coil of an electromagnetic current relay is connected in series in the circuit to reflect the magnitude of the circuit current.
43. When a three-phase squirrel-cage induction motor is started with Y-Δ reduced voltage, the starting current is 1/3 of the direct starting current. This method can only be used for motors with stator windings connected in delta configuration.
44. Starting methods of CMC-MX soft starter: current limiting start, voltage ramp start; stopping methods: free stop, soft stop.
45. The Xichi integrated high-voltage solid-state soft starter combines the switch cabinet and starter cabinet into one unit.
46. When selecting a contactor, its operating conditions should be taken into account. An AC contactor should be selected to control an AC load, while a DC contactor should be selected to control a DC load.
47. When selecting a contactor, the rated operating voltage of its main contacts should be greater than or equal to the voltage of the load circuit, the rated operating current of its main contacts should be greater than or equal to the current of the load circuit, and the rated voltage of the attraction coil should match the voltage of the control circuit.
48. The function of an intermediate relay is to convert one input signal into multiple output signals or to amplify a signal.
49. Common braking methods for squirrel-cage induction motors include electrical braking and mechanical braking.
50. According to national standards, the "Stop" button must be red and the "Start" button must be green.
II. Practical Quick Method for Electrical Engineering
1. The low-voltage side current of the transformer is 1.5 times its capacity ( 1.44 times for precision).
2. The three-phase short-circuit current at the low-voltage outlet of the transformer is 150 times the capacity divided by the percentage of the transformer's short-circuit impedance (144 times for precision).
3. The rated current on the 10kV side of the transformer is 6% of its capacity ( 5.8 % when precise); the rated current on the 6kV side is 10% of its capacity ( 9.6 % when precise).
4. The rated current of the 0.4kV three-phase compensation capacitor is 1.5 times its capacity ( 1.44 times for precision).
5. The rated current of a 380V three-phase motor is twice the capacity.
6. Under normal circumstances, the power factor on the low-voltage side is compensated to 0.95 , and the capacitor capacity is approximately 1/3 to 2/5 of the transformer capacity. This is suitable for general applications, except in special cases such as resistance furnaces or gas discharge lamps requiring large capacities.
7. Copper busbar current carrying capacity estimation table
The copper busbar cross-section S (mm²) and its allowable current carrying capacity I (A) are given.
S≤100I= 3.5S ;200＜S≤600I= 2.5S
100＜S≤200I=3S; S＞600I=2S
Note : The smaller the thickness of the copper busbar , the greater the current carrying capacity per mm² .
8. Calculation of rated and starting current protection for a three-phase 380V squirrel-cage motor
The rated current in amperes is twice the capacity in kilowatts, and the starting current is approximately 12 to 14 times the capacity. For direct-start motors, the instantaneous short-circuit protection current of the molded case switch is 17 to 24 times its capacity. For lightly loaded motors that are not frequently started, the fuse current is 2.5 to 3 times the motor's rated current. For heavily loaded starting motors , the fuse current is 4 to 5 times its rated current .
9. How to select the capacity of the low-voltage compensation capacitor ? How to select its protective fuse ?
Generally, the total compensation in kilovolt-amperes is 30% to 40% of the transformer capacity in kilovolt-amperes, the current in amperes of a single capacitor is 1.5 times its capacity in kilovolt-amperes, and the fuse current of the protective fuse is not less than 1.5 times the rated current of the capacitor.
Example of estimation :
A 1250kVA transformer has a short-circuit impedance of 6% . The rated current on the low-voltage (0.4kV ) side is Ie = 1.5 * 1250 = 1875A; the low-voltage short-circuit current is Ik = 1250 * 150 / 6 = 31250 = 31.25kA ; the high-voltage (10kV) side current is I<sub>0</sub> , I<sub>0</sub> = 1250 * 6% = 75A.
The low-voltage capacitor compensation capacity is approximately Q = 1250 * 1/3 = 420 kVar .
Therefore , a 75/5 high-voltage current transformer is selected;
Low-voltage current transformers 2000/5 , 2500/5;
The circuit breaker should have a breaking capacity of not less than 35kA.
There are two capacitor banks , one with a capacity of 200kVar and the other with a capacity of 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.
