The electrical control system is generally called the secondary control loop of electrical equipment. Different equipment has different control loops, andHigh voltage electrical equipmentAndLow voltage electrical equipmentThe control mode of is also different.Specifically, the electrical control system refers to the combination of several electrical components used to control one or some objects, so as to ensure the safe and reliable operation of the controlled equipment. Its main functions include: automatic control, protection, monitoring and measurement.
The so-called electrical control system refers to the combination of several electrical components used to control one or some objects, so as to ensure the safe and reliable operation of the controlled equipment.The main functions of the electrical control system are: automatic control, protection, monitoring and measurement.It consists of three parts: input part (such assensor, switches, buttons, etc.), logic parts (such asrelay、contactAnd executive parts (such asSolenoid coil, indicator light, etc.).[1]
process planning
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The purpose of electrical control system process design is to meet the manufacturing and use requirements of electrical control equipment.Completed atElectrical schematic diagramAfter the design and selection of electrical components, the overall configuration of the electrical control equipment can be carried out, that is, the design of the general assembly drawing and the general wiring diagram. Then the electrical assembly drawing and wiring diagram of each part can be designed, and the component catalog, incoming and outgoing line numbers, list of main materials and other technical data of each part can be listed. Finally, the operation instructions can be prepared.[2]
1. The design principle of the electrical system has the following four points:
(1) Maximize the requirements of production machinery and process on electrical control circuit;
(2) On the premise of meeting production requirements, strive to make the control circuit simple and economical;
(3) Ensure the reliability and safety of the control line;
(4) Easy operation and maintenance.[1]
2. Basic contents of electrical control system design include:
(2) Determine the electric drive scheme and control mode;
(3) Select motor capacity and structure;
(4) Design electrical control schematic diagram and calculate main technical parameters;
(5) Select electrical components and prepare a list of electrical components;
(6) Prepare design calculation instructions.[3]
Among them,Electrical schematic diagramIt is the central link of the whole design, because the electrical schematic diagram is the basis of process design and other technical data.[3]
major function
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In order to ensure the reliability and safety of primary equipment operation, a lot of assistance is neededelectricalThe combination of several electrical components for which the equipment serves and can realize a certain control function is called control loop or secondary loop.These devices shall have the following functions:
(1) Automatic control function.High voltage and high current switchequipmentThe volume is very large. Generally, the operating system is used to control opening and closing. Especially when the equipment fails, the switch needs to automatically cut off the circuit. There should be a set of automatically controlled electrical operating equipment to automatically control the power supply equipment.[4]
(2) Protection function.Electrical equipment and lines will fail during operation, and the current (or voltage) will exceed the allowable working range and limit of the equipment and lines, which requires a set of protective equipment to detect these fault signals and automatically adjust (disconnect, switch, etc.) the equipment and lines.[4]
(3) Monitoring function.Electricity is invisible to the eyes. It is impossible to distinguish whether a piece of equipment is electrified or powered off from the outside, so it is necessary to set various audio-visual signals, such as lights and sound, to electrically monitor the primary equipment.[4]
(4) Measurement function.Light and sound signals can only qualitatively indicate the working state of the equipment (power on or power off). If you want to know the working condition of the electrical equipment quantitatively, you need to have various instrument measuring equipment to measure various parameters of the circuit, such as voltage, current, frequency and power.[4]
In the process of equipment operation and monitoring, most of the traditional operating components, control appliances, instruments, signals and other equipment can be replaced by computer control systems and electronic components, but they still have a certain range of applications in small equipment and local control circuits.This is also the circuit realization microcomputerAutomatic controlThe foundation of.[4]
system composition
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The basic circuit of commonly used control lines is composed of the following parts.
(1) Power supply circuit.The power supply of power supply circuit includes AC 380V, 220V and DC 24V.[4]
(2) Protection circuit.The working power supply of the protection (auxiliary) circuit includes single-phase 220 (AC), 36V (DC) or DC 220 (AC), 24V (DC), etc. It protects electrical equipment and lines against short circuit, overload, voltage loss, etc. It is composed of fuse, thermal relay, voltage loss coil, rectifier assembly, voltage stabilizing assembly, etc.[4]
(3) Signal circuit.Circuits that can timely reflect or display the normal and abnormal working status information of equipment and lines, such as signal lights with different colors, audio equipment with different sounds, etc.[4]
(4) Automatic and manual circuits.In order to improve work efficiency, electrical equipment is generally equipped with automatic links, but in installation, commissioning and emergency handling, manual links need to be set in the control line for commissioning.Automatic and manual conversion can be realized through combination switch or transfer switch.[4]
(5) Brake parking circuit.Cut off the power supply of the circuit, and take some braking measures to make the control links of the motor stop quickly, such as energy consumption braking, reverse power braking, reverse pull braking and regenerationElectric brakingEtc.[4]
(6) Self locking and locking circuit.After the start button is released, the circuit remains energized, and the electrical link that the electrical equipment can continue to work is called the self-locking link, such as the moving contact of the contactor is connected in series in the coil circuit.For two or more electrical devices and components, in order to ensure the safety and reliability of equipment operation, only one can be powered on to start, and the other can not be powered on to start the protection link, called locking link.For example, the dynamic breaking contacts of two contactors are connected in series with each other in the coil circuit.[4]
System View
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Electrical control system diagram
The electrical control system diagram is the theoretical basis for the installation, commissioning, use and maintenance of electrical circuits, mainly includingElectrical schematic diagramElectrical installation wiring diagram and electrical element layout.The electrical control principle of the electrical equipment used in the system is used to guide the installation of electrical equipment and the commissioning and operation of the control system.
Reading method of electrical control system diagram
1. Read the machine first, then read the electricity
To read the machine first is to first understand the basic structure, operation, process requirements and operation methods of production machinery, so as to have a general understanding of the structure and operation of production machinery.After reading electricity, it is to make clear the control requirements for electric drive on the basis of understanding the machinery, so as to make preparations for analyzing the circuit.[4]
2. Read the main text first, then the auxiliary text
Reading the main circuit first means reading the diagram from the main circuit first.First of all, it is necessary to see clearly how many motors are used to drive the machine tool equipment, and the role of each motor. Combined with the processing technology and the main circuit, it is necessary to analyze whether the motor has step-down starting, whether there is positive and reverse control, and what kind of braking mode is used.Secondly, it is necessary to find out what electrical components control the electrical equipment. Some are manually controlled by knife switches or combination switches, and some are automatically controlled by buttons and contactors or relays.
3. Break up the whole into parts[4]
Finally, the overall inspection is carried out. After "breaking the whole into parts", the working principle of each local circuit and the control relationship between each part are gradually analyzed.Then check the whole control circuit with the method of "zero integration".To avoid omission.In particular, it is necessary to further check and understand the links between control links from the overall perspective.[4]
Common protection links
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The electrical control system must meet the production process requirements on the premise of safety and reliability.Therefore, in the design and operation of the electrical control system, the possibility of various faults and abnormal conditions of the system must be fully considered, and corresponding protection devices must be set in the control system.Protection link is an indispensable part of all electrical control systems.Common protection links for low-voltage motors are as follows:
Short circuit protection
It will occur when the insulation of the appliance or circuit is damaged, the load is short circuited, and the wiring is wrongshort circuitPhenomenon.The instantaneous fault current generated during short circuit can reach ten to ten times of the rated current, which may damage electrical equipment or distribution lines due to overcurrent, or even cause fire due to arc.Short circuit protection is required to have instantaneous characteristics, that is, it is required to cut off the power supply in a very short time.Common methods for short circuit protection includeFuseProtection andLow voltage circuit breakerProtection.[5]
Overcurrent protection
Overcurrent protection is a kind of current type protection different from short circuit protection.so-calledOvercurrentIt refers to the operating state of motor or electrical components that exceed their rated current, which is generally smaller than the short circuit current, and does not exceed 6 times the rated current.In the case of overcurrent, electrical components are not damaged immediately, as long as the current value can return to normal before reaching the maximum allowable temperature rise, it is still allowed.However, excessive impact load will cause the motor to flow through excessive impact current, resulting in damage to the motor.At the same time, the excessive electromagnetic torque of the motor will also damage the mechanical rotating parts, so the power supply must be cut off instantaneously.It is more likely that the motor will generate overcurrent than short circuit during operation, especially in the motor with frequent starting, forward and reverse rotation, and repetitive short-time operation.[5]
Common overcurrent relay andContactorIn cooperation, the coil of the overcurrent relay is connected in series in the protected circuit, and the normally closed contact of the overcurrent relay is connected in series in the coil circuit of the contactor.When the circuit current reaches its setting value, the overcurrent relay acts;The normally closed contact is disconnected, the contactor coil is powered off and released, and the main contact of the contactor is disconnected to cut off the power supply of the motor.This overcurrent protection link is commonly used forDC motorAnd three-phase wound rotorasynchronous motorIn the control circuit.If the action power supply of the overcurrent relay is 1.2 times the starting current of the motor, it can also realize the short-circuit protection function of the circuit.[5]
overload protection
Overload protection is one kind of overcurrent protection.Overload means that the running current of the motor is greater than its rated current, but within 1.5 times of the rated current.There are many reasons for motor overload, such as sudden increase of load, phase failure operation or power supply voltage reduction.If the motor is overloaded for a long time, the temperature rise of its winding will exceed the allowable value, which will cause insulation aging and damage.Overload protection device is required to have inverse time limit characteristics and will not act instantaneously under the influence of short-time overload impulse current or short circuit current of motor, so thermal relay is usually used for overload protection.When more than 6 times of the rated current passes through the thermal relay, it needs 5 seconds before it acts, so the heating element of the thermal relay may burn out before the thermal relay acts, so when the thermal relay is used for overload protection, it must also be equipped withFuseorLow voltage circuit breakerShort circuit protection device of.Because the overload protection characteristics are different from the overcurrent protection, the overcurrent protection method cannot be used for overload protection.[5]
It can also be used with open phase protectionThermal relayTo realize overload protection.[5]
Voltage loss protection
The motor can only work normally at a certain rated voltage. If the voltage is too high or too low, or if the power is cut off suddenly due to non human factors during the operation, the production machinery may be damaged or personal accidents may occur.Therefore, in the electrical control circuit, voltage loss protection, overvoltage protection and undervoltage protection should be set according to the requirements.[5]
If the motor stops because the power supply voltage disappears, once the power supply voltage recovers, it may start by itself, causing personal accidents or mechanical equipment damage.The protection set to prevent the motor from self starting or the electrical components from self starting when the voltage recovers is called voltage loss protection.Start and stop devices controlled by contactors and buttons have the function of voltage loss protection.This is because when the power supply voltage disappears, the contactor will automatically release and cut off the motor power supply;When the power supply voltage recovers, the self-locking contact of the contactor has been disconnected and will not start automatically.If the contactor is not controlled by buttons but by manual switches and travel switches that cannot be reset automatically, a special zero voltage relay must be used for voltage loss protection.In case of voltage loss during operation, the zero voltage relay will release and its self-locking circuit will be disconnected;When the power supply voltage recovers, it will not start automatically.[5]
Under voltage protection
When the motor is running, the decrease of power supply voltage causeselectromagnetic torque When the load torque is constant, the speed decreases and the motor current increases.In addition, the voltage drop causes the control appliance to release, resulting in abnormal circuit operation.Therefore, when the power supply voltage drops to 60%~80% of the rated voltage, it is necessary to cut off the motor power supply and stop working. This protection is called undervoltage protection.[5]
In addition to the above contactor and button control modes, which use the undervoltage protection function of the contactor itself, undervoltage protection can also be used. The pull in voltage of the undervoltage relay is usually set to (0.8~0.85)
The release voltage is usually set to (0.5~0.7)
。The voltage relay coil is connected to the power supply, and its normally open contact is connected to the contactor coil circuit in series. When the power supply voltage is lower than the release value, the voltage relay acts to release the contactor coil, and its main contact disconnects the motor power supply to achieve undervoltage protection.[5]
Overvoltage protection
ElectromagnetElectromagnetic chuckHigh inductance load, DC electromagnetic mechanism, DC relay, etcInduced electromotive force, so that the electromagnetic coil insulation is broken down and damaged.Therefore, overvoltage protection measures must be taken.Usually, overvoltage protection is to connect a resistor in parallel at both ends of the coil, connect the resistor in series with the capacitor or connect the diode in series with the resistor to form a discharge circuit to realize voltage protection.[5]
Field weakening protection
DC motorExcessive reduction of magnetic field will cause motor overspeed, which needs to be setWeak magnetic fieldProtection.This protection is realized by connecting an undercurrent relay in series in the motor excitation coil circuit.When the motor is running, if the excitation current is too small, the undercurrent relay will release, its contact will disconnect the motor armature circuit line contactor coil circuit, the contactor coil will be powered off and released, the main contact of the contactor will disconnect the motor armature circuit, and the motor will disconnect the power supply to protect the motor.[5]
Other protection
In addition to the above protection, there are overspeed protection, travel protection, oil pressure (water pressure) protection, etc. These are all connected in series with a normally open contact or normally closed contact controlled by these parameters in the control circuit to control the control circuit.These devices include centrifugal switchesTachometer generator, travel switch, pressure relay, etc.[5]
Troubleshooting
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If the electrical control circuit fails, the electrical equipment can not work in the light, affecting production, etc., and personal injury accidents can be caused in the serious case.Therefore, it is required to find out the cause in time and eliminate it quickly after the failure occurs.During troubleshooting, the following steps can generally be followed:
Observe and investigate the fault
Electrical faults are various, for example, the same fault may have different fault phenomena, and different types of faults may be the same fault phenomenon.The identity and diversity of this fault phenomenon make it difficult to find faults.However, the fault phenomenon is the basic basis for finding electrical faults and the starting point for finding electrical faults. Therefore, it is necessary to carefully observe and analyze the fault phenomenon, find out the most important and typical aspects of the fault phenomenon, and find out the time, place, environment, etc. of the fault.[6]
Analyze the cause of the fault
It is the key to find the electrical fault to analyze the cause of the fault according to the fault phenomenon.The basic electrician theory, which is the basis of the analysis, is a full understanding of the structure, principle and performance of electrical equipment, and a combination of actual faults.There may be many reasons for a fault. It is important to find the main reason among many reasons.[6]
When analyzing electrical equipment faults, the following methods are often needed:
Determining the fault location is the ultimate purpose of finding the fault of electrical equipment.Determining the fault location can be understood as determining the fault point of equipment, such as short circuit point, damaged element, etc., and also can be understood as determining the variation of some operating parameters, such as voltage fluctuationThree phase imbalanceEtc.[6]
The determination of the fault location is based on careful investigation and analysis of the fault phenomenon.In this process, a variety of means and methods are often used, including the following:
