What Should We Pay Attention to During the Grounding Anti-Interference Process?

In the field of industrial control, most technicians are quite familiar with the use, parameter function and debugging of frequency converters (hereinafter referred to as -VFD), but we often ignore the safety measures of VFDs, such as input isolation devices, output grounding, etc. Of course, the VFD itself also has overcurrent, overload, undervoltage and other protection measures, but more functions and hardware protection measures are actually of great benefit to our production and maintenance costs, especially safe grounding, just in case. , to avoid personal safety accidents caused by equipment or motor insulation damage.

No matter what kind of control system, VFD, especially high-power VFD, motor and equipment itself, we should formally require grounding (usually the VFDPE terminal), and pay attention to the grounding resistance to be less than 10 ohms, the grounding cable should be as short as possible, and the wire diameter should be as small as possible thick. We can summarize the general criteria:

The 11-15KW motor uses more than 8 mm2 copper wire, the 18.5-37KW motor uses more than 14 mm2 copper wire, the 45-55KW motor uses more than 12mm2 copper wire, and the 75KW motor uses more than 38 mm2 copper wire. In addition, when the length of the main power cable between the motor and the inverter exceeds 50m, it is recommended to install an AC output reactor to avoid damage to the motor insulation, excessive leakage current, or frequent protection of the inverter. Of course, the voltage drop of the AC output reactor must be considered. We can increase the input and output voltage of the VFD, or use motor derating to prevent the motor from burning out.

VFD and servo When all of us require the high performance, brand or cost-effectiveness of these products, don’t forget that we actually provide the most basic safety measures, and there must be corresponding measures at the input and output ends. No matter which product is used, it can be used stably, and our maintenance is also convenient and fast. In addition, with the diversification of VFD applications, the interference problem of VFD will become prominent, and the amount of interference will become an important criterion for considering VFD quality. In terms of interference factors, one is external factors, such as nearby high-voltage equipment or the high-voltage cable VFD of the bridge itself, which is the source of interference. Its power supply and load will affect the circuit network and surrounding electronic equipment. Relatively speaking, the better the performance of the VFD, the smaller the impact on the outside world and the greater the interference to the VFD. Usually the communication and analog signals used by the frequency conversion cabinet are very obvious. Either the communication fails or the analog signal cannot be captured, causing the equipment to operate abnormally. So, how can we reduce or avoid the impact of VFD interference?

To put it simply:

• (1) The cabinet and VFD ground terminals should be effectively grounded. Be careful not to confuse it with the zero line.
• (2) Adjust the carrier frequency. The parameter tuning process should be carefully considered. Listen to the sound of the motor running and make appropriate adjustments.
• (3) VFD adds a reactor and filter at the input end, and a magnetic ring at the output end.
• (4) The power lines and signal lines in the electrical cabinet should be separated. It is best to use shielded cables for signal lines.
• (5) If communication is involved, add terminal resistors and terminal capacitors at both ends of the communication to prevent interference.
• (6) Replace with a new VFD.

Generally speaking, we will stock a certain number of VFDs, but sometimes due to objective factors such as engineering or site conditions, some VFDs are useless for a long time and the inventory time is too long. In particular, the VFD is unpacked in the cabinet and has not been powered on for a long time, so be sure to check the safety factor when powered on. First of all, long-term non-use of the VFD directly leads to the aging of the electronic components in the VFD, such as the electrolyte of the capacitor drying up, the movable pins being oxidized, etc. And sometimes there will be more troublesome soft faults, which will bring great trouble to maintenance. The manufacturer gives the service life of certain components in the VFD, such as cooling fans, electrolytic capacitors, etc. The replacement life given by the manufacturer shows that the fan is a rotating part, and bearings and other rotating parts will always wear out after long-term use. In order to ensure heat dissipation, it is best to replace it with a new one. As the service life of the capacitor just mentioned increases, the electrolyte will gradually dry up, thereby reducing the capacitance. After reaching the service life, even if the VFD is not broken, in principle, the electrolytic capacitor must be replaced to nip it in the bud. Moreover, some customers’ on-site environment is relatively humid, and it is easy to short-circuit and connect electricity. Therefore, it is necessary to keep unused VFDs in a dry place. If possible, switch it on and off once every six months, especially the high-voltage VFD. In addition, there are many professional issues, which if not taken seriously, can easily lead to VFD failure. Therefore, VFDs that have not been used for a long time after unpacking should be avoided to be powered on immediately, and can only be used after being inspected and confirmed by professionals.