The electrical control principle circuit design is the core content of principle design, through which various design indicators are realized, it is also the basis of process design and various technical data.
There are two main methods of electrical control circuit design: analytical design and logic design.
1. Analysis and design method
The analysis and design method is to select the appropriate basic control link (unit circuit) according to the requirements of the production process or combine more mature circuits according to their interlocking conditions, and after supplementation and modification, they are integrated into a complete circuit that meets the control requirements. When there is no ready-made typical link, it can be designed while analyzing according to the control requirements.
The advantage of the analysis design method is that the design method is simple and there is no fixed design procedure. It is based on proficiency in the basic links of various electrical control circuits and the ability to read and analyze electrical control circuits, which is easy for beginners to master. For electrical technicians with certain work experience, they can complete design tasks faster, so they are widely used in electrical design; the disadvantage is that the designed plan is not necessarily the best plan. When the experience is insufficient or the consideration is not Full time will affect the reliability of line work.
To this end, the circuit working conditions should be reviewed repeatedly, and simulation tests should be conducted when conditions are found, and problems should be corrected in time until the circuit actions are accurate and meet the production process requirements.
2. Logic design method
The logic design method is to use logic algebra for circuit design. Starting from the drag requirements and process requirements of the production machinery, the contactor and relay coil in the control circuit are powered on and off, and the contacts are closed and opened. The main command The connection and disconnection of electrical appliances are regarded as logical variables, and the relationship between them is expressed in a logical relationship according to the control requirements, and then simplified, and the corresponding circuit diagram is made.
The advantage of the logic design method is that it can obtain an ideal and economical solution, but this method is more difficult to design, the entire design process is more complicated, and involves some new concepts. Therefore, it is rarely used alone in general conventional design. The specific design process can refer to the special discussion materials, no further introduction will be given here.
The basic steps of electrical schematic design are:
(1) The functional block diagram of designing the system according to the determined drag scheme and control method.
(2) Design the specific circuit of each part in the functional block diagram. The design is carried out in the order of main circuit, control circuit, auxiliary circuit, interlocking and protection, overall inspection, repeated modification and perfection.
(3) Draw the general principle diagram.
(4) Appropriate selection of electrical components, and formulation of component lists.
In the design process, the above steps can be appropriately selected according to the simplicity of the control circuit.
3. General requirements in schematic design
In general, the electrical control schematic diagram should meet the requirements of the production machining process. The circuit must be safe and reliable, easy to operate and maintain, and have low equipment investment. For this reason, the control circuit must be properly designed and the electrical components should be selected reasonably.
The schematic design should meet the following requirements:
1. The electrical control principle should meet the requirements of the process
Before designing, the working performance, structural characteristics and actual processing conditions of the production machinery must be fully understood, and on this basis, the control methods, starting, reverse, braking and speed adjustment requirements must be considered, and various interlocks and protective device.
2. The type of control circuit power supply and the requirement of voltage value
For a relatively simple control circuit, and when there are not many electrical components, AC 380V or 220V power supply is often used directly without controlling the power transformer. For more complex control circuits, a control power transformer should be used to reduce the control voltage to 110V or 48V or 24V. This solution is beneficial to maintenance, operation, and reliable operation of electrical components.
For the control circuit of the DC power drive that is operated more frequently, the 220V or 110V DC power supply is commonly used. The control circuit of DC electromagnet and electromagnetic clutch is usually powered by 24V DC power supply.
The voltage of the AC control circuit must be one or more of the following specified voltages:
6V, 24V, 48V, 110V (preferred value), 220V, 380V, 50Hz.
The voltage of the DC control circuit must be one or more of the following specified voltages:
6V, 12V, 24V, 48V, 110V, 220V.
3. Ensure the reliability and safety of the electrical control circuit
In order to ensure that the electrical control circuit works reliably, the following aspects should be considered:
(1) The work of electrical components should be stable and reliable, meet the environmental conditions of use, and the coordination of action time should not cause competition.
In a complex control circuit, under the action of a certain control signal, the circuit changes from one stable state to another stable state, and the state of several electrical components often changes at the same time. Considering that electrical components always have a certain operating time, yes For sequential circuits, you will get several different output states. This phenomenon is called “competition” of the circuit. As for the switching circuit, due to the release delay effect of the electrical component, the possibility that the switching element does not output according to the required logical function will also appear. This phenomenon is called “adventure”.
Both “competition” and “risk” phenomena will cause the control circuit to fail to act as required, resulting in control failure. Generally, the analysis of the action of the control circuit electrical appliances and the contact on and off are static analysis, and the action time of the electrical components is not considered. In actual operation, due to the electromagnetic inertia, mechanical inertia, and mechanical displacement of the electromagnetic coil Factors such as the contactor or relay from the energization of the coil to the closing of the contact have a period of attraction; when the coil is powered off, there is a period of release time from the power off of the coil to the opening of the contact. These are called the actions of electrical components Time is the inherent time of electrical components, different from the artificially set delay, the inherent action delay is uncontrollable, and the artificial delay is adjustable.
When the action time of electrical components may affect the action of the control circuit, it is necessary to accurately analyze the action time with a method that accurately reflects the action time of the element and its mutual cooperation (such as the time graph method) to ensure the normal operation of the circuit.
(2) The connection of coils and contacts of electrical components shall comply with relevant national standards
The graphic symbols of electrical components shall comply with the provisions in GB4728, and the layout shall be arranged reasonably when drawing. For example, the main circuit is generally arranged on the left or above, the control circuit or auxiliary circuit is arranged on the right or below, and the component catalog is arranged above the title. For the convenience of reading the diagram, sometimes the on-off of the master switch, the operation requirements of the solenoid valve, the control flow, etc. are shown on the drawing in the form of an action state table or process chart, and it can also be marked on each side of the control circuit. Out of control purposes.
In the actual connection, the following points should be noted:
① Connect the electrical coil correctly. AC voltage coils cannot usually be used in series. Even two voltage coils of the same type cannot be connected in series and connected to an AC power source with twice the rated voltage of the coil to avoid unreliable work due to uneven voltage distribution.
② Reasonably arrange the position of electrical components and contacts. For the series circuit, when the position of electrical components or contacts are interchanged, it does not sound its working principle, but in actual operation, it affects the safety of the circuit and is related to the length of the wire.
③ Prevent parasitic circuits. The parasitic circuit refers to an accidental connection circuit that is not due to a misoperation during the operation of the control circuit.
④ Minimize the number of connecting wires and shorten the length of connecting wires.
⑤ When the control circuit is working, the number of energized appliances should be reduced as much as possible to reduce the possibility of failure and save electrical energy.
⑥ When using small-capacity relay contacts in the circuit to open or close the large-capacity contactor coil, analyze the size of the contact capacity. If it is not enough, you must increase the relay capacity or increase the intermediate relay, otherwise the work is unreliable .
4. Should have the necessary protection links
In the event of an accident, the control circuit should be able to ensure the safety of operators, electrical equipment, and production machinery, and effectively prevent the expansion of the accident. For this reason, certain protective measures should be taken in the control circuit. Commonly used measures include leakage switch protection, overload, short circuit, overcurrent, overvoltage, loss of pressure, interlock and stroke protection. If necessary, corresponding indication signals can also be set.
5. Convenient operation and maintenance
The control circuit should proceed from the work of the operation and maintenance personnel, and strive to be simple to operate and easy to maintain.
6. The control circuit strives for simplicity and economy
On the premise of meeting the technological requirements, the control circuit should be simple and economical. Use standard electrical control links and circuits as much as possible, reduce the number of electrical appliances, use standard parts and use the same type of electrical appliances as much as possible.