Tips for PLC application!

1. PLC communication port damage case JICHENGPEIXUN

We have a project where the PLC port is burnt out. The PLC communication line is led out through the slip ring. Considering that it rained a few days ago, it is suspected that the PLC communication line is short-circuited due to water ingress into the slip ring, which burns out the PLC port. We use a megohmmeter to measure the communication line (both ends of the line are suspended), and we find that there is resistance between the communication lines. Normally, it should be infinite, but when measured, the resistance is between 5M and 10M. Therefore, it is determined that the PLC port is burnt out due to water ingress into the slip ring, and it is normal after replacing the slip ring.

2. Try to avoid calling the same subroutine multiple times JICHENGPEIXUN

There is nothing wrong with calling the same subroutine multiple times in a program, but we should try to avoid this practice, especially when it has formal parameters. The following example will illustrate this.

As shown in Figure 1 below, both networks 13 and 14 call the protection subroutine. At this time, the running status of the protection subroutine when network 14 calls it is shown in Figure 2. We noticed that the value of the parameter #protection when network 14 calls it (1169, the parameter value when network 13 calls this subroutine) is not the value required by network 14 to call the protection subroutine (should be 481). This will cause the result we don't want.

3. Misoperation caused by line capacitance of PLC input

There is capacitance between each conductor of the cable, and a qualified cable can limit this capacitance within a certain range. Even for a qualified cable, when the cable length exceeds a certain length, the capacitance between each line will exceed the required value. When this cable is used for PLC input, the capacitance between the lines may cause the PLC to malfunction, resulting in many incomprehensible phenomena.

The main reasons are: the wiring is correct, but the PLC has no input; the input that the PLC should have is missing, but the input that it should not have is there, that is, the PLC inputs interfere with each other.

Recently, when debugging a PLC system, a phenomenon occurred. The MIC sensor does not work, or after working, the action of another sensor (FLY) affects the MIC sensor, that is, when the MIC works, once the FLY sensor works, the MIC becomes inactive. In other words, the actions of the sensors affect each other. It is suspected that the cable quality is poor and the inter-wire capacitance does not meet the requirements. After directly connecting the MIC sensor to the PLC without using the cable, everything works normally.

Ways to eliminate the influence of line capacitance:

(1) Use cables with twisted cores;

(2) Try to shorten the length of the cable used;

(3) Use separate cables for inputs that interfere with each other;

(4) Use shielded cable.

4. Reasonable PLC programming to eliminate misoperation JICHENGPEIXUN

Eliminate finger tremors: PLC delay or differential instructions can be used to detect the rising edge of the button signal to avoid such misoperations;

Unconscious operation: ① Optimize the display function and use different indicator lights to display various working states: flat light > running state, high-frequency flash (1 time per second) > test state, low-frequency flash (1 time per 3 seconds) > stepping state. ② Input signal interlock

5. Inverter overvoltage treatment case JICHENGPEIXUN

When the given speed is reduced to slow down the motor, the motor enters the regenerative braking state, and the energy fed back to the inverter by the motor is also high. This energy is stored in the filter capacitor, which increases the voltage on the capacitor and quickly reaches the setting value of the DC overvoltage protection, causing the inverter to trip. Take measures to add a braking resistor outside the inverter to consume the regenerative electric energy fed back to the DC side by the motor.

6. Inverter overcurrent processing case JICHENGPEIXUN

We use Yaskawa inverters to drive 120 small motors. When one of the small motors has an overcurrent fault, the inverter will give an overcurrent fault alarm, causing the inverter to trip, thereby causing other normal small motors to stop working, which is not what we want.

Treatment method:

Install a 1:1 isolation transformer on the output side of the inverter. When one or several small motors have an overcurrent fault, the fault current will directly impact the transformer instead of the inverter, thus preventing the inverter from tripping. After the experiment, it works well and the previous fault of normal motors stopping has not occurred.