1. Low-voltage electrical appliances
Electrical appliances that function as switches, protectors, controllers, or regulators in circuits with AC rated voltage of 1200V and DC rated voltage of 1500V and below.
2. Control electrical appliances
Electrical appliances used to send control commands in an automatic control system.
3. Fuse
It is a simple short-circuit or severe overload protection device, the main body of which is a molten metal made of low-melting-point metal wire or metal sheet.
4. Time relay
A control appliance that uses a contact to make or break the circuit with a time delay.
5. Electrical schematic diagram
An electrical schematic diagram is a circuit diagram used to represent the connection relationship and working principle of conductive parts in various electrical components of a circuit.
6. Interlocking
The "interlocking" circuit is essentially a combination of two blocking circuits. When K1 is activated, it blocks the energization of K2, and when K2 is activated, it blocks the energization of K1.
7. Self-locking
A self-locking circuit uses the output signal itself to interlock and maintain the output action.
8. Zero-pressure protection
The protection mechanism used to prevent motors from starting automatically when power is restored after a power grid failure is called zero-voltage protection.
9. Undervoltage protection
When the power supply voltage drops below the allowable value, measures need to be taken to cut off the power supply in order to prevent the control circuit and motor from malfunctioning. This is called undervoltage protection.
10. Star connection
There are three windings, each end of which is connected to one phase of the three-phase voltage, and the other end is connected together.
11. Triangle connection
The three windings are connected end to end, and three-phase voltages are connected to the three connection terminals respectively.
12. Pressure reduction start-up
When the motor capacity is large, the power supply voltage is reduced and connected to the stator winding of the motor to start the motor.
13. Main circuit
The main circuit is the circuit from the power source to the motor or the end of the line, and it is a circuit through which a strong current flows.
14. Auxiliary circuit
The auxiliary circuit is a circuit through which a small current flows.
15. Speed relay
A non-electrical signal detection device that takes rotational speed as input can output a switching signal when the measured rotational speed rises or falls to a predetermined value.
16. Relay
A relay is a control element that uses changes in various physical quantities to convert electrical or non-electrical signals into electromagnetic force (with contact type) or to cause a step change in the output state (without contact type).
17. Thermal relay
It is a protective electrical appliance that works by utilizing the thermal effect of electric current.
18. AC relay
A relay whose attraction coil current is AC.
19. Full-pressure start
When the motor capacity is small, the stator winding of the motor is directly connected to the power supply and started at the rated voltage.
20. Voltage
The potential difference between the two ends of the circuit.
21. Contacts
A contact, also known as a contact point, is an actuator in electromagnetic electrical appliances, used to connect and disconnect circuits.
22. Electromagnetic Structure
An electromagnetic mechanism is a sensing element in an electromagnetic electrical appliance. It converts electromagnetic energy into mechanical energy, thereby driving the contacts to move.
23. Electric arc
An electric arc is actually a discharge phenomenon generated by the gas between contacts under the action of a strong electric field.
24. Contactor
A contactor is an automatic control switching device suitable for remote control, frequent operation of AC and DC main circuits and high-capacity control circuits in low-voltage power distribution systems.
25. Temperature relay
A protective electrical device that uses an overheating element to indirectly reflect the winding temperature and activates is called a temperature relay.
26. Jog circuit
Press the jog button, the coil is energized and attracted, the main contacts close, the motor is connected to a three-phase AC power supply and starts rotating; release the button, the coil is de-energized and released, the main contacts open, the motor stops rotating.
27. Electrical control system
An electrical control system is composed of electrical control components connected according to certain requirements.
28. Variable Pole Speed Regulation
In asynchronous motor speed control, the method of changing the number of stator pole pairs is used.
29. Electrical Component Location Diagram
An electrical component layout diagram is a diagram used to show the actual installation positions of various components in an electrical system.
30. Wiring diagram for electrical components
An electrical installation wiring diagram is a concrete implementation of an electrical schematic diagram. It is drawn using prescribed graphic symbols according to the actual positions and wiring of electrical components.
31. Variable frequency speed control
In asynchronous motor speed control, the speed control method involves changing the power supply frequency.
32. The principle of energy-saving braking of a three-phase asynchronous motor
Energy-consumption braking involves switching the stator windings to DC power while the motor stops and disconnects the three-phase power supply, generating a static magnetic field. The interaction between the rotor induced current and the static magnetic field produces a braking torque for braking.
33. Working principle of reverse braking of a three-phase asynchronous motor
Reverse braking is achieved by changing the phase sequence of the three-phase power supply to the stator windings when the motor stops, causing the rotating magnetic field of the stator windings to reverse, and the rotor to be subjected to braking torque in the opposite direction of rotation, thus stopping the motor quickly.
34. What is the difference between short-circuit protection and overload protection?
A short circuit generates a large short-circuit current and electrodynamic force, which can damage electrical equipment. It is necessary to quickly disconnect the power supply. Commonly used short-circuit protection components include fuses and automatic switches.
Motors can tolerate short-term overloads, but prolonged overload operation will cause the winding temperature to rise beyond the allowable value, necessitating power cut-off to protect the motor. A commonly used overload protection component is a thermal relay.
35. The starting current of the motor is very large, so why doesn't the thermal relay trip?
Because the thermal element of a thermal relay has thermal inertia, it will not deform quickly. When the motor starts, the current is very large, but the starting time is very short. The large current is not enough to deform the thermal element and cause the contacts to operate.
36. Under what conditions can an intermediate relay replace an AC contactor?
They can be replaced when the number of contacts is the same, the rated voltage of the coil is the same, and the control is for low current.
37. What are the different types of commonly used relays based on their operating principle?
Electromagnetic, magnetoelectric, inductive, electric, photoelectric, piezoelectric, time and temperature (thermal) relays, etc.
38. In the main circuit of an electric motor, since a fuse is already installed, why is a thermal relay also needed? What is the difference between them?
Fuses can only be used for short-circuit protection, not overload protection; while thermal relays can only be used for overload protection, not short-circuit protection. Therefore, it is necessary to install both in the main circuit.
39. The function of a thermal relay
Thermal relays are electrical appliances that work by utilizing the heating effect of electric current. They are mainly used for overload protection of motors, phase loss protection, and control of the heating status of other electrical equipment.
40. What are the different types of rated duty systems?
Standard working hours: 8-hour workday, long-term workday, short-term workday, intermittent workday.
41. Under what circumstances are reverse braking and regenerative braking of a three-phase AC motor applicable?
Reverse braking is suitable for small-capacity motors (below 10kW) that do not require frequent starting and braking. Energy-consumption braking is suitable for larger-capacity motors that require smooth and accurate braking and frequent starting.
42. What are some commonly used master control switches?
Control buttons, limit switches, proximity switches, universal changeover switches, master controllers and other master control electrical appliances (such as foot switches, reversing switches, emergency switches, toggle switches, indicator lights, etc.).
43. What is the basis for electrical control analysis?
Based on the equipment manual, electrical control schematic diagram, general wiring diagram of electrical equipment, and layout and wiring diagram of electrical components.
44. What are the different types of relays based on the nature of the input signal and their working principle?
According to the nature of the input signal, it can be classified as: voltage, current, time, temperature, speed, pressure, etc.
According to their working principle, they can be classified as electromagnetic, induction, electric, thermal, electronic, etc.
45. What is the difference between an intermediate relay and a contactor? Under what conditions can an intermediate relay replace a contactor?
Contactors have large main contact capacity and are mainly used in main circuits; intermediate relays have a large number of contacts and are mainly used in control circuits. When the circuit current is small (less than 5A), intermediate relays can be used instead of contactors.
46. What are the basic rules for drawing electrical schematic diagrams?
(1) Electrical schematic diagrams are generally drawn in two parts: the main circuit and the auxiliary circuit.
(2) All electrical components shall use the unified graphic symbols and text symbols of the national standard.
(3) The positions of the conductive parts of each electrical component should be arranged according to the principle of easy reading and analysis. Different parts of the same electrical component may not be drawn together.
(4) All electrical components’ contacts are drawn in the open/closed state when there is no power or external force.
(5) Connection points of cross wires with direct electrical connection should be indicated by black dots.
(6) Electrical components should generally be arranged in the order of operation from top to bottom and from left to right, and can be arranged horizontally or vertically.
47. What are the characteristics of reverse braking and energy consumption braking for a three-phase AC motor?
When the power supply is reversed for braking, the relative speed between the rotor and the stator rotating magnetic field is close to twice the synchronous speed of the motor. Therefore, the reverse braking current flowing through the rotor windings at this time is equivalent to twice the starting current of the motor when starting at full voltage. As a result, the reverse braking torque is large and the braking is rapid.
In regenerative braking, there are two types of control methods depending on the DC power supply: time-based control and speed-based control. Both methods require a DC power supply and a transformer, resulting in slow braking.
48. In the "forward-reverse-stop" control circuit of the electric motor, the compound button already plays an interlocking role. Why is it necessary to use the normally closed contact of the contactor for interlocking?
Because if the main contacts of a contactor are "welded" together by a strong electric arc or if the contactor mechanism malfunctions and the armature is stuck in the engaged state, a short circuit will occur if the other contactor operates. Interlocking the normally closed contacts of the contactors can prevent short circuits from occurring in such situations.
49. What is self-locking control? Why is it said that the contactor self-locking control circuit has undervoltage and loss-of-voltage protection?
A self-locking circuit uses the output signal itself to interlock and maintain the output action.
When the power supply voltage is too low, the contactor coil is de-energized, and the self-locking contact returns to disconnect the coil circuit. When the voltage rises again, the coil cannot be energized, thus forming undervoltage and loss-of-voltage protection.
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50. What are the different methods for designing electrical schematic diagrams? Which method is commonly used for simple machine tool control systems? Write out the design steps.
There are two types: experience-based design and logic-based design. Experience-based design is more commonly used. The design steps are: main circuit → control circuit → auxiliary circuit → interlocking and protection → overall inspection → iterative modification and improvement.
51. What is the speed range when the contacts of a speed relay actuate?
The operating speed of a typical speed relay contact is around 140 r/min, and the reset speed is 100 r/min.
52. According to their operating principle, what types of time relays are there?
Time relays include electromagnetic, air-damped, electric motor, and electronic types.
53. What are the different categories of low-voltage electrical appliances according to their uses?
1) Control electrical appliances 2) Distribution electrical appliances 3) Actuating electrical appliances 4) Communicable low-voltage electrical appliances 5) Terminal electrical appliances
54. Selection Principles of Time Relays
When selecting a time delay device, factors such as delay duration, delay accuracy, control circuit voltage level and current type, delay method, and contact type and quantity should be considered.
55. Principle of unidirectional reverse braking control circuit for electric motors
Pressing SB2 activates KM1 and engages self-holding, causing the motor to run. When the speed reaches 140 rpm or higher, the KS contact closes. Pressing SB1 de-energizes KM1 and activates KM2 for reverse braking. When the speed drops below 100 rpm, the KS contact opens, braking ends, and the motor slowly stops rotating.
56. Working principle of jog control circuit
Pressing SB energizes the KM coil, closes the KM contacts, and causes the motor to rotate; releasing SB de-energizes the KM coil, opens the KM contacts, and stops the motor.
57. Working principle of starting and self-holding control circuit
Pressing SB2 energizes the KM1 coil, causing KM to engage and connecting the main contacts to the motor power supply, initiating motor operation. Simultaneously, the auxiliary contacts close, connecting and maintaining the control circuit. Releasing SB2 allows the auxiliary contacts to continue connecting the control circuit, enabling the motor to continue running. Pressing SB1 de-energizes KM1, opens the auxiliary contacts, disconnects the main contacts from the motor power supply, and the motor gradually stops rotating.
58. Working principle of multi-point control circuit
Pressing any one of the buttons SB2, SB4, or SB6 will energize the KM coil, causing KM to snap in and self-hold, and the motor to run; pressing any one of the buttons SB1, SB3, or SB5 will de-energize the KM coil, causing KM to disconnect, and the motor to gradually stop.
59. Working principle of the contacts of a speed relay operating counterclockwise.
Its rotor shaft is connected to the shaft of the controlled motor, and the stator is loosely fitted around the rotor. When the motor is running, the rotor of the speed relay rotates with the motor shaft, the permanent magnet forms a rotating magnetic field, the cage bars in the stator cut the magnetic field to generate an induced electromotive force, forming an induced current. Under the action of the magnetic field, it generates an electromagnetic torque, causing the stator to rotate in the same direction as the rotor. However, due to the return lever blocking it, the stator can only rotate a certain angle in the same direction as the rotor. When the stator deflects to a certain angle, the normally closed contact opens and the normally open contact closes under the action of lever 7.
60. Working principle of forward, stop, and reverse circuits
When starting in forward rotation, pressing the forward start button SB2 energizes and latches the KM1 coil, causing the motor to start and rotate in the forward direction. When starting in reverse rotation, pressing the reverse start button SB3 energizes and latches the KM2 coil, causing the motor to start and rotate in the reverse direction. In the control circuit, the normally closed auxiliary contacts of the KM1 and KM2 forward and reverse contactors are connected in series in the circuit of the other coil, forming a mutually restrictive control. If the motor has entered forward rotation after pressing the forward start button SB2, to change the direction of the motor, the stop button SB1 must be pressed first, and then the reverse start button must be pressed.
