How to learn to read and draw electrical schematics

Electrical diagrams

The main purpose of electrical diagrams is to reflect, with sufficient completeness and clarity, the interconnection of individual devices, automation equipment and auxiliary equipment that are part of the functional units of automation systems, taking into account the sequence of their work and principle of operation . Basic electrical diagrams serve to study the principle of operation of the automation system, they are necessary during commissioning and in operation of electrical equipment.

Basic electrical diagrams are the basis for the development of other design documents: electrical diagrams and tables of shields and consoles, external wiring connection diagrams, connection diagrams, etc.

In the development of automation systems for technological processes, schematic electrical diagrams of independent elements, installations or sections of an automated system are usually performed, for example, an actuator valve control circuit, an automatic and remote pump control circuit, a tank level alarm circuit, and etc. .

The main electrical circuits are compiled on the basis of automation schemes, on the basis of the specified algorithms for the functioning of individual control, signaling, automatic regulation and control units and general technical requirements for the object to be automated.

On schematic electrical diagrams, devices, devices, communication lines between individual elements, blocks and modules of these devices are depicted in conventional form.

In general, schematic diagrams contain:

1) conventional images of the principle of operation of one or another functional unit of the automation system;

2) explanatory inscriptions;

3) parts of individual elements (devices, electrical devices) of this circuit used in other circuits, as well as elements of devices of other circuits;

4) schemes of switching contacts of multi-position devices;

5) list of devices, equipment used in this scheme;

6) list of drawings related to this scheme, general explanations and notes. To read schematic diagrams, you need to know the algorithm of circuit operation, understand the principle of operation of devices, devices on the basis of which the schematic diagram is built.

Schematic diagrams of monitoring and control systems by purpose can be divided into control circuits, process control and signaling, automatic regulation and power supply. Schematic diagrams by type can be electrical, pneumatic, hydraulic and combined. Electric and pneumatic chains are currently the most widely used.

How to read a wiring diagram

The schematic diagram is the first working document, based on which:

1) make drawings for the manufacture of products (general views and electrical diagrams and tables of boards, consoles, cabinets, etc.) and their connections with devices, actuators and with each other;

2) check the correctness of the connections made;

3) set the settings for the protective devices, means of control and regulation of the process;

4) adjust travel and limit switches;

5) analyze the circuit both in the design process and during commissioning and operation in case of deviation from the specified operating mode of the installation, premature failure of any element, etc.

Thus, depending on the work being done, reading the circuit diagram has different purposes.

Also, if reading schematics is all about figuring out where and how to install, place, and connect, then reading a schematic is much more difficult. In many cases, this requires in-depth knowledge, mastery of reading techniques and the ability to analyze the information received. Finally, the mistake made in the schematic diagram will inevitably be repeated in all subsequent documents.As a result, you will again have to go back to reading the circuit diagram to find out what mistake was made in it or what, in a particular case, does not correspond to the correct circuit diagram (for example, the software with many contacts, the relay is connected correctly, but the duration or sequence of switching contacts set during setup does not match the task) …

The tasks listed are quite complex, and consideration of many of them is beyond the scope of this article. Nevertheless, it is useful to clarify their essence and list the main technical solutions.

1. Reading a schematic diagram always begins with a general familiarization with it and the list of elements, find each of them on the diagram, read all notes and explanations.

2. Define the power system for electric motors, magnetic starter coils, relays, electromagnets, complete tools, regulators, etc. To do this, find all the power supplies on the diagram, identify the type of current, rated voltage, phasing in AC circuits and polarity in DC circuits for each of them, and compare the data obtained with the rated data of the equipment used.

Common switching devices are identified according to the diagram, as well as protective devices: circuit breakers, fuses, overcurrent and overvoltage relays, etc. Determine the settings of the devices through the captions of the diagram, tables or notes, and finally, the protection area of ​​each of them is evaluated.

Familiarity with the power system may be necessary to: identify the causes of power outages; determining the order in which power should be supplied to the circuit (this is not always indifferent); checking the correctness of phasing and polarity (incorrect phasing can, for example, in redundancy schemes lead to a short circuit, a change in the direction of rotation of electric motors, damage to capacitors, violation of circuit separation using diodes, damage to polarized relays and others.); assessing the consequences of a blown fuse.

3. They study any circuits of any electrical receiver: electric motor, magnetic starter coil, relay, device, etc. But there are many electrical receivers in the circuit, and it is far from indifferent which of them begins to read the circuit - this is determined by the task at hand. If you need to determine the conditions of its operation according to the diagram (or check that they correspond to the specified ones), then they begin with the main electrical receiver, for example, with the valve motor. The following electricity consumers will reveal themselves.

For example, to start the electric motor, you need to turn on magnetic switch… Therefore, the next electrical receiver should be the coil of the magnetic starter. If its circuit includes a contact of an intermediate relay, it is necessary to take into account the circuit of its coil, etc. But there may be another problem: some element of the circuit has failed, for example, a certain signal lamp does not light up. Then she will be the first electric receiver.

It is very important to emphasize that if you do not adhere to a certain purposefulness when reading the chart, then you can spend a lot of time without deciding anything.

So, studying the chosen electrical receiver, it is necessary to trace all its possible circuits from pole to pole (from phase to phase, from phase to zero, depending on the power system). In this case, it is necessary to first identify all contacts, diodes, resistors, etc. included in the circuit.

Please note that you cannot view multiple circuits at once. First you need to study, for example, the circuit for switching the coil of the magnetic starter «Forward» during local control, adjusting in what position the elements included in this circuit should be (the mode switch is in the «Local control» position, the magnetic starter «Back» is turned off), which you need to do to turn on the coil of the magnetic starter (press the button of the button «Forward»), etc. Then you need to mentally turn off the magnetic starter. After examining the local control circuit, mentally move the mode switch to the «Automatic control» position and study the next circuit.

Familiarity with each circuit of the electrical circuit aims to:

a) determine the conditions of operation that the scheme satisfies;

b) error identification; for example, a circuit may have series-connected contacts that must never close simultaneously;

v) determine the possible causes of the failure. A faulty circuit, for example, involves the contacts of three devices. Given each of them, it is easy to find a defective one.Such tasks arise during commissioning and troubleshooting during operation;

G) install elements in which time dependencies can be violated either as a result of incorrect setting or due to an incorrect assessment by the designer of the actual operating conditions.

Typical shortcomings are too short pulses (the controlled mechanism does not have time to complete the started cycle), too long pulses (the controlled mechanism, after completing the cycle, begins to repeat it), violation of the necessary switching sequence (for example, the valves and the pump are turned on in the wrong order or sufficient intervals between operations are not observed);

e) identify devices that may be misconfigured; a typical example is an incorrect setting of a current relay in the control circuit of a valve;

e) identify devices whose switching capacity is insufficient for switched circuits, or the nominal voltage is lower than necessary, or the operating currents of the circuits are higher than the nominal currents of the device, etc. NS.

Typical examples: the contacts of an electric contact thermometer are inserted directly into the circuit of a magnetic starter, which is completely unacceptable; in a circuit for a voltage of 220 V, a reverse voltage diode of 250 V is used, which is not enough, because it can be under a voltage of 310 V (K2-220 V); the nominal current of the diode is 0.3 A, but it is included in the circuit through which a current of 0.4 A passes, which will cause unacceptable overheating; the signal switching lamp 24 V, 0.1 A is connected to a voltage of 220 V through an additional resistor of the type PE-10 with a resistance of 220 Ohm.The lamp will glow normally, but the resistor will burn out, because the power released in it is about twice the nominal;

(g) identify devices subject to overvoltage switching and evaluate protective measures against them (e.g. damping circuits);

h) identify devices whose operation may be unacceptably affected by adjacent circuits and assess means of protection against influences;

i) to identify possible spurious circuits both in normal modes and during transient processes, for example, recharging of capacitors, energy flow in a sensitive electrical receiver, released when the inductance is turned off, etc.

False circuits are sometimes formed not only with an unexpected connection, but also with a non-closure, a contact blown by one fuse, while the others remain intact. For example, an intermediate relay of a process control sensor is turned on by one power circuit, and its NC contact turns on through the other. If the fuse blows, the intermediate relay will release, which will be perceived by the circuit as a mode violation. In this case, you cannot separate the power circuits, or you have to draw a diagram differently, etc.

Incorrect circuits can be formed if the sequence of supply voltages is not observed, indicating poor design quality. With properly designed circuits, the sequence of supplying the supply voltages, as well as their recovery after disturbances, should not lead to any operational switching;

To se) assess in sequence the consequences of insulation failure at any point in the circuit.For example, if the buttons are connected to the neutral working wire, and the starter coil is connected to the phase wire (it is necessary to turn it back), then when the switch of the Stop button is connected to the ground wire, the starter cannot be turned off. If the wire closes to ground after the switch with the «Start» button, the starter will automatically turn on;

l) evaluate the purpose of each contact, diode, resistor, capacitor, for which they proceed from the assumption that the element or contact in question is missing, and evaluate the consequences of this.

4. The behavior of the circuit is established during partial power off as well as recovery. Unfortunately, this critical problem is often underestimated, so one of the main tasks of reading the diagram is to check that the device can go from some intermediate state to an operational state and that unexpected operational switches will not occur. Therefore, the standard prescribes that circuits should be drawn under the assumption that the power supply is switched off and that the devices and their parts (eg relay armatures) are not subject to forced influences. From this starting point, it is necessary to analyze the schemes. Timing diagrams of the interaction, reflecting the dynamics of the circuit's operation, and not just its steady state, are of great help in circuit analysis.