1 PLC technology
A Programmable Logic Controller (PLC) has gradually become the most widely used industrial controller since its adoption in 1969, and currently has several branches. PLC control consists of a central processing unit (CPU), memory, power supply unit, etc., and has strong anti-interference capabilities. It uses a type of programmable memory and is specifically designed for industrial field applications. It is a product of the combination of microcomputer technology and traditional relay contact control technology, overcoming the shortcomings of relay control systems such as complex wiring, high power consumption, and poor flexibility, while also taking into account the habits of maintenance personnel, and can be flexibly applied in production practice.
Traditional relay control technology uses hardware wiring, which makes maintenance difficult if a fault occurs. PLC control, on the other hand, uses stored logic, with the program stored in memory. If a fault occurs, only the program needs to be checked. The programmable structure of a PLC mainly includes a programmer, input/output units, user program memory, system program memory, and a central processing unit.
In terms of control speed, traditional relay control technology relies on mechanical actions to achieve control, resulting in low efficiency and slow speed (on the order of nanoseconds). It is also prone to jitter during operation, potentially causing damage to electrical components. PLC, on the other hand, is controlled by a program, making it much faster than traditional relay control technology, typically on the order of microseconds, and it does not exhibit jitter during control.
In delay control, traditional relay control technology relies on the lag of the equipment, resulting in poor timing accuracy and difficulty in adjusting the time during operation. In contrast, PLC monitoring and control uses a crystal oscillator to generate clock pulses, allowing for precise time manipulation. Although PLC control is similar to microcomputer technology, their operating methods differ. PLC technology uses cyclic scanning, while microcomputers use keyboard scanning. Figure 1 illustrates the scanning method of PLC technology.
Design of a three-phase asynchronous forward and reverse rotation control circuit for a motor
Many operating systems require motors to be able to rotate in both directions. From the working principle of a motor, we know that swapping any two of the three-phase power supplies will reverse the motor's rotation. Therefore, the essence of achieving forward and reverse rotation is simply changing the power input lines. However, simply swapping the input lines can easily lead to a short circuit, so an interlock must be implemented. Figure 2 shows the schematic diagram of the forward and reverse rotation of a three-phase asynchronous motor. In the diagram, KM1 and KM2 are the main contacts of an AC contactor. When KM1 is engaged, the main contact of the AC contactor KM2 will be disengaged, allowing the motor to rotate forward. If the main contact of the AC contactor KM1 is disengaged, KM2 will be engaged, causing the motor to rotate in reverse. FU1 in the diagram is mainly used to prevent short circuits, and the circle represents the motor M.
As shown in the diagram above, the software interlock function of the PLC program is unreliable during use. Therefore, an interlock needs to be added to the hardware. The address allocation table is shown in Table 1. In addition to adding an interlock to the hardware, a thermal protection device is also required.
Draw the PLC ladder diagram according to the specific functions and requirements of the designed equipment, as shown in Figure 3. Then, analyze it to obtain the programming code.
The designed program is as follows:
0LDX000
1ORY005
2ANIX002
3ANIY004
4OUTY005
5LDX001
6ORY004
7ANIX002
8ANIY005
9OUTY004
10END
In the ladder diagram in Figure 3, the normally open contacts controlled by the external buttons on the PLC are mainly the first and second level X001 and X002 contacts on the left busbar. A single button press closes either X000 or X001, allowing output relays Y005 or Y004 to form a closed circuit. This enables normally open contacts Y005 or Y004 to self-lock and simultaneously achieve forward and reverse rotation of the motor. Stopping is achieved via an external button on the PLC. The button releases the normally open contact X002, de-energizing the relay and stopping the motor.
3. Conclusion
The paper presents the design of a three-phase asynchronous forward and reverse rotation control circuit for a motor. Experiments show that the design can effectively achieve forward and reverse rotation control of the motor. However, there are still areas for improvement in the control process, such as low automation and inconvenience in actual operation.
