Fluctuations in frequency values and causes of false shutdowns of kemron VFD and their solutions

Install four low-power Comron brand frequency converters in one control cabinet, using proportional synchronous speed control method for synchronous speed control of cutting, water supply, and belt conveyor in gypsum board production line. For the convenience of operation and monitoring, the control panel of the frequency converter is installed on the front of the cabinet and connected with the signal cable provided by the manufacturer.

During on-site debugging, it was found that the displayed speed values of each frequency converter fluctuated by more than plus or minus 30 revolutions during operation! The user suspects that the speed is unstable and requests a solution.
Firstly, the G terminal of the main circuit was independently grounded, and the speed control signal line was shielded and grounded on one side. The fluctuation of the speed value has been improved, but it has not been eradicated yet. Inquire with the manufacturer and answer that it is caused by interference. It is recommended to carry out grounding treatment. After being ineffective, the manufacturer cannot propose a better solution. During the trial run, it was found that there were still fluctuations in the operation of a single unit. There are more running units, resulting in greater fluctuations. Some of the four machines have larger fluctuations, while others have smaller fluctuations, but after all, the speed values cannot be stabilized. This problem was ultimately not completely solved, but it seemed to have little impact on operation, so it was left unresolved.
Later, a similar equipment was installed in another gypsum board factory. After running for several months, the user reported that one of the 3.7kW water supply frequency converters had repeatedly stopped, and the number of shutdowns in a day reached more than ten times. The user requested an on-site solution.
On site observation: The speed display value of the frequency converter fluctuates greatly during operation, with all three digits flashing, and the FWD indicator light also flashing. The speed value suddenly drops to zero during operation, or it may immediately increase and continue to operate, or it may need to be restarted to operate. Sometimes after shutdown, F000 is displayed and enters the parameter setting state for no reason, as if someone has performed shutdown and parameter adjustment operations. However, this “person” is definitely interfering with the signal, causing the CPU of the frequency converter to receive shutdown instructions or other operation instructions. The running current of the 3kW motor is only 2.6A, and there are no fault codes such as overvoltage, undervoltage, or overload when the frequency converter is shut down. Obviously, both the motor and the frequency converter are not fault protected shutdown. Running the frequency converter alone, the phenomenon still persists; Remove all the connections between the start/stop terminals and the speed control terminals, but the phenomenon still persists; Switching to the control panel for start/stop and speed regulation, the phenomenon still persists; Switching to a 5.5kW frequency converter for operation, the phenomenon still persists!
The above phenomenon was determined to be caused by signal interference. Firstly, conventional grounding treatment was also carried out; 2. After being ineffective, the carrier frequency of the frequency converter was adjusted to the lowest carrier frequency of 2kHz, resulting in slight improvement; 3. After wrapping a layer of tin foil on the connecting cable from the control panel to the frequency converter, the number of shutdowns has been reduced, indicating an improvement; 4. After purchasing a magnetic ring with a moderate diameter and winding 2-3 turns at each end of the connecting wire, the displayed speed value and the flashing FWD indicator light disappeared. The speed value was incredibly stable, and even the fluctuation of 1 revolution disappeared!

Interference is the carrier wave at the output end of the frequency converter after it operates, which enters the I/O port of the CPU through the connection cable on the operation panel! For those with less interference, there is fluctuation in input frequency and display value; If the interference is severe, a random shutdown phenomenon will occur. The effect of wrapping a magnetic ring on the signal line to suppress interference is very good.