For the star-delta starting circuit, those who are engaged in electrical and industrial control are certainly no strangers. Generally, such circuits are constructed with time relays, intermediate relays and AC contactors.

The following figure is the motor star-delta starting control circuit constructed with PLC:

Let’s look at its main program:

In the above figure, TON is the power-on delay time module inside the PLC. The upper left side of each module is the switch input, and the lower left is the delay time, which is 10 milliseconds in the figure; the upper right is the corresponding register, and the lower right is the time delay termination.

There are many switch input terminals in the above figure, such as circuit breaker status switch, control button switch, AC contactor status switch and so on. The reason for the delay is to eliminate the jitter when switching input.

The following figure is the control program:

The working principle of the program in the figure will not be explained in this article.

From the above pictures, we can see that the control of PLC is very different from the circuit constructed by relay.

1. The functions that the relay control circuit can achieve, the PLC can do; conversely, the functions that the PLC can achieve, the relay control circuit may not be able to achieve.

For example, the contact anti-shake program in Figure 2, although we can also use time relays to achieve, but to do anti-shake operations for so many switches, a large number of time relays are required, the cost is high, and the reliability is not high. This is the first advantage of PLC, adding many special functions.

2. Compared with the relay circuit, the control function of PLC is more strict and the performance is better. However, there are not many people who can understand the PLC program, so once there is a problem with the PLC, maintenance is difficult.

3. Compared with the relay circuit, PLC can easily realize the instantaneous collection of data and data exchange.

Let’s look at the picture below:

The above picture is the time stamp SOE program of switch variable displacement and fault. The measurement accuracy of the SOE program is 1 millisecond. SOE function is difficult to achieve with relays.

Look at the picture below:

Similar to the function in the picture above, it is impossible for a relay to do it.

4. Compared with the relay circuit, PLC is of course more expensive. However, its high reliability and stability are unmatched by relay circuits.

Mainly these. Another point, some people think that the single-chip microcomputer will gradually replace the PLC in some fields, this view is wrong. PLC is actually a single-chip microcomputer application system, but the technical parameters of PLC are many times higher than that of single-chip microcomputer. What we see at the industrial control site are various PLCs, not specially developed single-chip microcomputers. Why? The reason is that PLCs have strong anti-interference ability, that is, EMC anti-electromagnetic disturbance ability.

The so-called EMC electromagnetic disturbance has two aspects. The first is that electrical products have a high ability to resist electromagnetic disturbances. The second is that electrical products themselves will not generate strong electromagnetic disturbances to affect the surrounding electromagnetic environment. In the electromagnetic disturbance test, the most troublesome are the electrostatic disturbance and the electromagnetic disturbance of the EFT electric fast pulse group.

The following is an article titled “EFT Electric Fast Burst Test” in Baidu, which introduces the situation of electric fast burst electromagnetic disturbance test:

Part of the original text of the article “EFT Electrical Fast Burst Test”:

Most electronic products are required to pass standard tests for items such as Electrical Fast Transient (EFT) and Electrostatic Discharge (ESD). EFT and ESD are two typical burst disturbances. The peak voltage of a single pulse of EFT signal can be as high as 4kV, and the rising edge is 5ns. The peak voltage of the ESD signal during the contact discharge test can be as high as 8kV, and the rise time is less than 1ns. These two kinds of burst interference have the characteristics of burst, high voltage, and broadband.

Electrical fast transient bursts are caused by inductive loads (such as conduction interference generated by relays and contactors, radiation interference generated by high-voltage switch switching, etc.) The reason is the transient disturbance generated at the disconnection. When the inductive load is repeatedly switched on and off for many times, the pulse group will be repeated many times with the corresponding time interval. This kind of transient disturbance energy is small and generally does not cause damage to the equipment, but due to its wide spectrum distribution, it will affect the reliable operation of electronic and electrical equipment.

The purpose of the electrical fast variable burst test is to test the performance of electronic and electrical equipment when subjected to such transient disturbances. A repetitive fast transient test is a test in which a burst of many fast transient pulses is coupled to the power, signal and control ports of electrical and electronic equipment. The main points of the test are the short rise time, repetition rate and low energy of the transient.

This test is a burst test consisting of many fast transient pulses coupled to power lines, control lines, and signal lines. This waveform is not the actual waveform when the inductive load is disconnected (the disturbance amplitude is increasing when the inductive load is disconnected), and the waveform used in the experiment makes the experimental level more severe. The electrical fast pulse group is composed of continuous pulse trains with an interval of 300ms. Each pulse train lasts for 15ms and consists of several non-polar single pulse waveforms. The rising edge of a single pulse is 5ns, the duration is 50ns, and the repetition frequency is 2.5KHz (for 4KV test level) or 5KHz (for other levels). According to the Fourier transform, its spectrum is discrete spectral lines from 5K-100M, and the distance of each spectral line is the repetition frequency of the pulse. Apply interference to the power terminal selection coupling/decoupling network with a coupling capacitor of 33nF. Select special capacitive coupling clips to interfere with I/O signals, data and control ports, and the equivalent coupling capacitance is about 50-200pF.

The former is to connect the reference ground of the electronic product with the ground of the instrument, and then load a high voltage on the casing of the electronic product to see how the electronic product is resistant. In many cases, even if the power filter capacitor is slightly higher, the electronic product will be blown up on the spot.

As for the electrical fast burst test, industrial products must pass the test of the three-level burst pulse. If the ordinary microcontroller system is burned on the spot, and the PLC is nothing.

In order to improve the anti-interference ability of the PLC, the basic program of the PLC operating system adopts the principle of reading one and executing one, and will not wait in place. In this way, it is difficult for the basic program of PLC to have an infinite loop, which in turn improves the anti-interference ability of the PLC.

As for the ladder diagram and modular programming languages, they are all standard programming high-level languages ​​under IEC61131 and have nothing to do with the basic language of the PLC operating system.

The above are the basic performances that PLC must meet, and have nothing to do with programmers! As for replacing PLC with single-chip microcomputer, it is purely a naive idea and understanding of laymen.

What is the difference between PLC control and relay control?

The emergence of PLC control is to overcome the shortcomings of relay control in programming and maintenance. Their differences are mainly reflected in the following points:

1. Logic control method

(1) Relay control: The series and parallel combination of mechanical contacts of various electrical components are used to form logic control; hard-wired connections are used, and the connections are many and complex, making it difficult to modify and add functions in the future.

(2) PLC control: it is stored in the memory in the form of a program, and the logic can be changed by changing the program; less wiring, small size, convenient and reliable.

2. Sequence control method

(1) Relay control: use the hysteresis action of the time relay to complete the sequential control of the timing: the mechanical structure inside the time relay is easily affected by changes in ambient temperature and humidity, resulting in low timing accuracy.

(2) PLC control: timers composed of semiconductor circuits and clock pulses generated by crystal oscillators have high timing accuracy; users can set the timing values ​​in the program according to their needs, with great flexibility, and the timing time is not affected by the environment influences.

3. Control the speed

(1) Relay control: relying on the pull-in action of the mechanical contact to complete the control task, the working frequency is low and the working speed is slow.

(2) PLC control: use program instructions to control semiconductor circuits to achieve control, which is stable and reliable, and the running speed is greatly improved.

4. Flexibility and Extensibility

(1) Relay control: After the system is installed, due to the limited number of electrical equipment contacts and complicated wiring, the system will have poor flexibility and scalability in the future.

(2) PLC control: with dedicated input and output modules; less wiring, good flexibility and scalability.

5. Counting function

(1) Relay control: does not have the function of counting.

(2) PLC control: There is a specific counter inside the PLC, which can realize the step-by-step control of the production equipment.

6. Reliability and maintainability

(1) Relay control: A large number of mechanical contacts are used, and arcs will be generated when the contacts are opened and closed, causing damage and accompanied by mechanical wear, short service life, poor operational reliability, and difficult maintenance.

(2) PLC control: using microelectronic technology, the internal switching actions are completed by non-contact semiconductor circuits; small size, long life, high reliability, and can be displayed to the operator at any time to monitor the execution of the control program in time condition, to facilitate on-site debugging and maintenance.

The difference between PLC ladder diagram and relay control system diagram

By comparing the PLC ladder diagram and the relay control system diagram, it can be seen that the graphic symbols in the ladder diagram are very similar to the symbols in the relay circuit diagram, the structures of the two diagrams are also very similar, and the logical relationships expressed are the same. It is evolved from the relay control diagram, but there are certain differences in use, so the ladder diagram and the relay control system diagram have both the same and different points. The differences are as follows:

①The relays used in the relay control diagram are all actual physical relays; in the PLC ladder diagram, the name of relay is still used, such as: input relay, output relay, intermediate relay, etc., but these are not real physical relays, but The internal register of PLC is called “soft relay”.

②The connection between the electrical components in the relay control diagram must be connected by hard wiring. To change the control function, the actual wiring of the control circuit must be changed; while the PLC wiring is a “soft connection” realized by the program, only need to change the user program, not The control function can be changed by changing the external wiring.

③ The number of contacts in the relay control diagram is limited, and the service life is also limited; and each programming element of the PLC corresponds to an internal register, and its state can be read repeatedly in the program. It can be considered that the number of relay contacts of the PLC is infinite. , there is no life limit.

The ladder diagram is mainly composed of busbars, contacts and coils.

① Bus: The left vertical line of the ladder diagram is called the starting bus, and the right vertical line is called the ending bus (the ending bus can be omitted). The busbar is equivalent to the power line in the circuit, and the ladder diagram starts from the left busbar, passes through the contacts and coils, and ends at the right busbar.

②Contact: There are two types of contacts in the ladder diagram: normally open contact and normally closed contact. These contacts can be external contacts or the state of an internal relay. Each contact has a label, and the contacts with the same label can be used repeatedly. The contacts are placed on the left side of the ladder diagram.

③ Coil: The coil in the ladder diagram is similar to the coil of the contactor and the relay, which represents the result of the logic output. Generally, the coil with the same label can only appear once in use. The coil is placed on the right side of the ladder diagram.

Reviewing Editor: Tang Zihong


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