Electrical control systems are generally referred to as secondary control circuits for electrical equipment. Different equipment has different control circuits, and the control methods for high-voltage electrical equipment and low-voltage electrical equipment are also different.
Functions and components of electrical control systems
Main functions
To ensure the reliable and safe operation of primary equipment, numerous auxiliary electrical devices are required. A combination of several electrical components capable of performing a specific control function is called a control circuit or secondary circuit. These devices must have the following functions:
(1) Automatic control function. High voltage and high current switching equipment is very large in size. Generally, an operating system is used to control the opening and closing of the circuit. Especially when the equipment fails, the switch needs to automatically cut off the circuit. There needs to be a set of automatic control electrical operating equipment to automatically control the power supply equipment.
(2) Protection function. Electrical equipment and lines may fail during operation, and the current (or voltage) may exceed the allowable range and limit of the equipment and lines. This requires a set of protection equipment to detect these fault signals and automatically adjust (disconnect, switch, etc.) the equipment and lines.
(3) Monitoring function. Electricity is invisible to the eye. Whether a device is powered or de-energized cannot be distinguished from its appearance. Therefore, it is necessary to set up various audio-visual signals, such as lights and sounds, to monitor the primary equipment.
(4) Measurement function. Light and sound signals can only qualitatively indicate the working status of the equipment (power on or off). If you want to quantitatively know the working status of electrical equipment, you also need various instruments and measuring devices to measure various parameters of the circuit, such as voltage, current, frequency and power.
In equipment operation and monitoring, traditional operating components, control electrical appliances, instruments, and signaling devices can mostly be replaced by computer control systems and electronic components. However, they still have a certain range of applications in small equipment and circuits for localized control. These are the foundations for circuits to realize microcomputer-based automated control.
System Composition
The basic circuit of a commonly used control circuit consists of the following parts.
(1) Power supply circuit. The power supply circuit can be supplied with various voltages such as AC380V and 220V.
(2) Protection circuit. The working power supply of the protection (auxiliary) circuit is available in various types such as single-phase 220V, 36V or DC 220V, 24V, etc. It provides various protections for electrical equipment and lines such as short circuit, overload and undervoltage. It is composed of protection components such as fuses, thermal relays, undervoltage coils, rectifier components and voltage regulator components.
(3) Signal circuit. A circuit that can promptly reflect or display information about the normal and abnormal working status of equipment and lines, such as signal lights of different colors, audio equipment with different sounds, etc.
(4) Automatic and manual circuits. In order to improve work efficiency, electrical equipment is generally equipped with automatic circuits. However, during installation, commissioning and emergency handling, manual circuits are also required in the control circuits. Automatic and manual modes can be switched through combination switches or changeover switches.
(5) Braking and stopping circuit. This is a control circuit that cuts off the power supply to the circuit and takes certain braking measures to stop the motor quickly, such as energy consumption braking, power supply reverse connection braking, reverse connection braking and regenerative braking.
(6) Self-locking and interlocking circuits. A self-locking circuit is an electrical link that keeps the circuit energized and allows the electrical equipment to continue operating after the start button is released. For example, the normally open contact of a contactor is connected in series in the coil circuit. An interlocking circuit is a protective mechanism that ensures the safe and reliable operation of two or more electrical devices and components, allowing only one to be energized and the other to be prevented from starting. For example, the normally closed contacts of two contactors are connected in series in the coil circuit of the other contactor.
What is electrical interlocking? Self-locking?
Electrical interlock
In electrical control, interlocks are primarily designed to ensure the safe operation of electrical appliances. They are mainly formed by the mutual control of two electrical components. There are three main methods of achieving this: electrical interlocking, mechanical interlocking, and electromechanical linkage interlocking.
Electrical interlocking: The normally closed contacts of two relays are connected to the coil control circuit of another relay. This way, when one relay is energized, a closed circuit cannot be formed on the coil of the other relay. However, this action can also be achieved using a mechanical linkage. Thirdly, there is electromechanical interlocking. For example, in a high-voltage cabinet, if the switch is not disconnected, the isolating switch cannot be opened; if none of the above can be opened, the grounding switch cannot be closed; and if the grounding switch cannot be opened, the high-voltage cabinet door cannot be opened, preventing switch checks and other operations. Electrical interlocking is achieved through the contacts of relays and contactors. For example, when a motor rotates forward, the contacts of the forward contactor cut off the electrical path between the reverse button and the reverse contactor. Mechanical interlocking is achieved through mechanical components. For example, if two switches cannot be closed simultaneously, a mechanical lever can be used to prevent the other switch from closing when one is closed. Electrical interlocking is relatively easy to implement, flexible, and simple; the two interlocking devices can be installed in different locations, but its reliability is relatively poor. Mechanical interlocking has high reliability but is more complex and sometimes even impossible to implement. Usually, the two interlocking devices should be installed in close proximity.
After the main power supply is restored, it can automatically switch back to the main power supply (or it can remain connected). This electrical function is called electrical interlocking. In many applications, motors need to rotate in both directions. For example, the opening and closing of a door is controlled by the motor's forward and reverse rotation. The forward and reverse rotation of the motor is achieved by reversing the phase sequence of the power supply. If reverse rotation is activated while the motor is running in forward rotation, it will cause a short circuit between phases, burning out electrical equipment. To avoid this, during forward rotation, the normally closed auxiliary contact of the AC contactor is connected in series in the motor's reverse rotation control circuit, and the auxiliary contact of the AC contactor for reverse rotation is connected in series in the motor's downward rotation control circuit. When the motor is running in forward rotation, the normally closed auxiliary contact of the AC contactor cuts off the control circuit for the reverse motor, preventing it from operating in reverse.
When operating in reverse, the normally closed auxiliary contact of the AC contactor is used to cut off the control circuit for forward rotation of the motor, so that the forward rotation operation is ineffective.
A circuit is divided into a main circuit, also called a primary circuit (the wiring of the power supply), and a control circuit, also called a secondary circuit. The secondary circuit controls the primary main circuit.
An AC contactor is a control element containing a control coil. It can operate on either AC 220V or AC 380V. When energized, it closes, connecting the main circuit and enabling the motor to operate. The circuit that controls the switching of the control coil is called the control circuit.
When an electrical component is not energized, the closed contacts are called normally closed contacts, and the open contacts are called normally open contacts. The contacts in the main circuit can carry a large current. Different sizes of AC contactors are selected according to the size of the motor. The auxiliary contacts are connected in the control circuit, so the current is limited to 5A.
Self-locking electrical control circuit
Characteristics of a contactor: A contactor typically has six terminals, three of which are normally open contacts, two are normally closed contacts, and one is the coil. When the coil is energized, all normally open contacts close and all normally closed contacts open.
In this diagram, the left side is the main circuit, and the right side is the secondary circuit (for clarity, we have omitted the connection between the main and secondary circuits). We will only focus on the secondary circuit here. SB2 is a normally open button, KM below is the contactor coil, and KM above is the normally open contact of the contactor.
Without the contactor (i.e., without all the points marked KM in the diagram), the circuit is energized when SB2 is pressed and de-energized when it is released (a characteristic of normally open buttons; start buttons all use normally open buttons). Therefore, we connected a contactor coil and connected the normally open contact in parallel with SB2. This results in the coil being momentarily energized when SB2 is pressed, thus closing the normally open contact and ensuring that the circuit remains energized when SB2 is released.
The most common self-locking circuit
start up
When the motor starts, close the power switch QS to connect the power supply to the entire control circuit.
When the start button SB2 is pressed, its normally open contact closes, energizing the contactor coil KM and causing it to engage. Simultaneously, the auxiliary normally open contact connected to both ends of SB2 closes.
In the main circuit: the main contacts close to connect the motor to the three-phase AC power supply and start it to rotate.
In the secondary circuit: when SB2 is pressed, it sends power to the KM coil. When the KM auxiliary contact is closed, it also supplies power to the KM coil, thus forming two power supplies.
When the SB2 start button is released, although the SB2 circuit is disconnected, the KM coil still keeps the coil energized through its auxiliary contacts, thus ensuring that the motor continues to run.
This method of keeping the coil energized by the contactor's own normally open auxiliary contacts is called contactor self-locking, also known as electrical self-locking. The normally open auxiliary contacts that provide this self-locking function are called self-locking contacts, and this circuit is called the self-locking circuit.
stop
To stop the motor, press the SB1 button. The contactor KM coil will be de-energized and released, and both the main and auxiliary contacts of KM will open, cutting off the power supply to the motor's main circuit and control circuit, thus stopping the motor operation.
When the SB1 button is released, the normally closed contact of SB1 closes again under the action of the reset spring. Although it returns to the original normally closed state, the original KM self-locking contact has already opened when the KM coil is de-energized, and the contactor can no longer be energized by the self-locking contact.
Circuit protection
Fuses FU1 and FU2 provide short-circuit protection for the main circuit and control circuit, respectively. The thermal relay FR provides long-term overload protection for the motor.
Commonly used protection mechanisms in electrical control systems
In addition to meeting the requirements of production machinery processing technology, electrical control systems should also ensure the long-term, safe, reliable, and trouble-free operation of equipment, protecting power supply equipment and motors in the event of various faults or abnormal operation. Therefore, protection components are an indispensable part of all electrical control systems, used to protect motors, power grids, electrical equipment, and personal safety.
1. Short circuit protection
Short circuits can occur when the insulation of motors, electrical appliances, or wires is damaged, or when there is a circuit fault. Large short-circuit currents and electrodynamic forces can damage electrical equipment. Therefore, it is required that the control circuit can quickly disconnect the power supply in the event of a short circuit. Commonly used short-circuit protection components include fuses and low-voltage circuit breakers. The components for motor short-circuit protection should be installed according to the following requirements:
1) In a system with a directly grounded neutral point, it should be installed on each phase.
2) In a system with an ungrounded neutral point, when using fuses for protection, they should be installed on each phase; when using low-voltage circuit breakers for protection, they should be installed on at least two phases.
2. Overload protection
Prolonged overload operation of an electric motor will cause the winding temperature to rise beyond its allowable value, resulting in aging of the insulation material, reduced lifespan, and in severe cases, damage to the motor. The larger the overload current, the shorter the time to reach the allowable temperature rise. A commonly used overload protection element is a thermal relay. For high-power, critical motors, an inverse-time overcurrent relay should be used.
Due to thermal inertia, thermal relays will not trip instantaneously due to short-term overload current or short-circuit current of the motor. Therefore, when using thermal relays for overload protection, short-circuit protection must also be provided, and the rated current of the fuse selected for short-circuit protection should not exceed four times the rated current of the heating element of the thermal relay. Because the characteristics of overload protection are different from those of overcurrent protection, overcurrent protection methods cannot be used for overload protection.
3. Overcurrent protection
Overcurrent protection is widely used in DC motors or wound-rotor induction motors. For three-phase squirrel-cage induction motors, since short-time overcurrents do not have serious consequences, overcurrent protection is not required.
Overcurrent protection is often caused by incorrect starting and excessive load. It is generally smaller than short-circuit current. Overcurrent is more likely to occur during motor operation than short circuit, especially in repetitive short-time duty motors that frequently start in both forward and reverse directions.
It must be emphasized that although short-circuit, overcurrent, and overload protection are all current protections, they cannot replace each other due to differences in fault current, operating values, protection characteristics, protection requirements, and the components used.
4. Undervoltage protection
If the power supply voltage disappears for any reason while the motor is operating normally, and the motor restarts on its own when the power supply voltage is restored, it may damage the production equipment or cause personal injury. Furthermore, the simultaneous spontaneous starting of numerous motors and other electrical equipment on the power grid can also cause unacceptable overcurrent and instantaneous voltage drops. The protection system designed to prevent motors from starting on their own or electrical components from automatically activating when the voltage is restored is called undervoltage protection.
The start/stop control circuit using contactors and push buttons has a loss-of-voltage protection function. When the power supply voltage disappears, the contactor automatically releases, cutting off the motor power; when the power supply voltage recovers, the contactor's self-locking contacts have already opened, preventing automatic restart. If a non-resettable manual switch or master controller is used to control the contactor, a dedicated zero-voltage relay must be used. During operation, if a voltage loss occurs, the zero-voltage relay releases, its self-locking circuit disconnects, and it will not automatically restart when the power supply voltage recovers.
5. Undervoltage protection
When a motor is running normally, an excessive drop in power supply voltage can cause some electrical components to trip, leading to malfunctions in the control circuit and even accidents. When the mains voltage is too low, if the motor load remains constant, the motor current will increase, causing the motor to overheat and potentially burn out. Furthermore, low power supply voltage can also cause the motor speed to decrease or even stop. Therefore, when the power supply voltage drops below the permissible value, protective measures are needed to promptly disconnect the power supply; this is called undervoltage protection. This is typically achieved using an undervoltage relay.
Requirements and steps for electrical control system design
1. Design Purpose
The main purpose of electrical design is to understand the general electrical control system design process, requirements, tasks, and methods through the design practice of electrical control devices for a specific production equipment. Design also helps review and consolidate previously learned knowledge, enabling flexible application. Electrical design must meet the requirements of the production equipment and processes; therefore, before designing, it is essential to understand the equipment's purpose, structure, operating requirements, and technological processes, cultivating a holistic perspective in the design work.
Electrical design should emphasize the cultivation of abilities. While completing design tasks independently, attention should also be paid to the cultivation and improvement of other abilities, such as independent work ability and creativity; the ability to comprehensively apply professional and basic knowledge to solve practical engineering and technical problems; the ability to consult books, product manuals and various reference books; engineering drawing ability; and the ability to write technical reports and compile technical documents.
2. Design Requirements
To ensure the successful completion of the design task, the following points should also be observed:
(1) After accepting the design task, a design task book and work schedule should be prepared according to the design requirements and the design content to be completed, and the workload to be completed in each stage should be determined and the time should be properly arranged.
(2) During the process of determining the scheme, students should proactively raise questions to obtain assistance from their supervisors, and engage in extensive discussions to ensure sufficient evidence. During the specific design process, students should think deeply, especially regarding key parameters, which should be calculated and verified.
(3) All electrical drawings must comply with the relevant national standards, including lines, graphic symbols, project codes, circuit numbers, technical requirements, title blocks, component lists, and the folding and binding of drawings.
(4) The instruction manual should be written in a fluent and concise manner, with neat and legible handwriting.
(5) All design tasks should be completed within the stipulated time. (This text is from www.eadianqi.com)
(6) If conditions permit, you should conduct tests and demonstrations on your own design circuit and consider the possibility of further improvement.
3. Design Task
The course design requirements should be expressed in the form of a design task statement, which should include the following:
(1) A brief introduction to the name, purpose, basic structure, operating principle and process of the equipment.
(2) Dragging method, sequence of motion of moving parts, requirements of each action and control requirements.
(3) Interlocking and protection requirements.
(4) Auxiliary requirements such as lighting, indication, and alarm.
(5) The drawings to be drawn.
(6) Requirements of the instruction manual.
The central task of schematic design is to draw electrical schematics and select electrical components. The purpose of process design is to obtain the construction drawings required for the manufacturing of electrical equipment. There are many types and quantities of drawings; the design primarily focuses on practicing with overall electrical equipment configuration diagrams, electrical board component layout diagrams, wiring diagrams, control panel layout diagrams, electrical boxes, and major machined parts (electrical mounting base plates, control panels, etc.). Each designer only needs to complete one part. Schematic diagrams and process drawings should be drawn according to requirements. Component layout diagrams should indicate overall dimensions, installation dimensions, and relative positional dimensions. Wiring diagram numbering should be consistent with the schematic diagram, and all incoming and outgoing wire numbers, wiring specifications, and connection methods (using terminal blocks or connectors) should be indicated.
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