Brief introduction to the laying and routing method of wires and cables in VFD (Variable-frequency Drive) control cabinets

VFD (Variable-frequency Drive) consists of main circuit, power circuit, IPM drive and circuit, cooling fan and other departments. Its layout is mostly unitized or modular. Due to inaccurate use or unreasonable settings, VFD (Variable-frequency Drive) malfunctions and failures may easily occur, or the expected operating results cannot be met. In order to prevent problems before they occur, it is especially important to analyze the causes of the problems in advance.

1. Analysis of common faults in the main circuit

The primary and secondary circuits are mainly composed of three-phase or single-phase rectifier bridges, lubricating capacitors, filter capacitors, IPM inverter bridges, current limiting resistors, contactors and other components. Many of these common problems are caused by electrolytic capacitors. The life of the electrolytic capacitor is mainly determined by the DC voltage applied to both ends and the internal temperature. The model of the capacitor has been selected in the retrospective design, so the internal temperature determines the life of the electrolytic capacitor. Electrolytic capacitors will indirectly affect the service life of VFD (Variable-frequency Drive). Generally, the service life is halved every time the temperature rises by 10°C. Therefore, on the one hand, the appropriate temperature should be considered during installation, and on the other hand, measures can be taken to reduce the pulsating current. The use of AC or DC reactors with improved power factor can reduce the pulsating current, thereby prolonging the life of electrolytic capacitors.

In the case of capacitors, the deteriorating environment of electrolytic capacitors is usually judged by the electrostatic capacity that is relatively easy to measure. When the electrostatic capacity is lower than 80% of the rated value and the insulation is below 5M, it is recommended to consider replacing the electrolytic capacitor.

2. Analysis of the typical faults of the main cycle

Fault phenomenon: VFD (Variable-frequency Drive) exhibits overcurrent tripping during acceleration, deceleration or normal operation.

First of all, it should be distinguished because of load or VFD (Variable-frequency Drive). If it is the fault of the VFD (Variable-frequency Drive), the current at the time of tripping can be checked through the historical record, which exceeds the rated current of the VFD (Variable-frequency Drive) or the setting value of the electronic thermal relay, while the three-phase voltage and If the current is balanced, consider whether there is overload or sudden change, such as motor stall. When the load inertia is large, the acceleration time can be appropriately delayed, and this process does not damage the VFD (Variable-frequency Drive) itself. If the current at the time of tripping is within the rated current of the VFD (Variable-frequency Drive) or within the setting range of the electronic thermal relay, it can be judged that the IPM module or related departments are faulty. First of all, it can be judged whether the IPM module is damaged by measuring the forward and reverse resistance between the main circuit output terminals U, V and W of the VFD (Variable-frequency Drive) and the P and N terminals of the DC side, respectively. If the module is not damaged, the driver is faulty. If the IPM module is overcurrent or the VFD (Variable-frequency Drive) trips to the ground during deceleration, it is generally the module of the upper half bridge of the inverter or its driving power failure; and when the IPM module is overcurrent, it is the lower half bridge. The module or its drive motor is faulty. The reasons for these faults are mostly caused by external dust entering the VFD (Variable-frequency Drive) or moisture.

3. Control failure analysis

It is the power supply department that controls the life of the VFD (Variable-frequency Drive), which is the lubricating capacitor and the buffer capacitor in the IPM board. It is a fixed value affected by the load, so its life is mainly determined by temperature and power-on time. Because the capacitors are all welded on the board, it is more difficult to judge the deterioration environment by measuring the electrostatic capacity. Generally, it is estimated whether it is close to its service life according to the capacitor temperature and service time.

The power board supplies power to the control circuit, the IPM drive power, the overview operation display board, and the fan, etc. These power supplies are generally obtained from the DC voltage output from the main power supply and then separately rectified by the switching power supply. Therefore, if a certain power supply is short, in addition to the damage to the rectified power, it may also affect the power supply of other departments. short lead to other power outages, etc. Generally, it is easier to find out by looking at the power board.

The logic control board is the focus of VFD (Variable-frequency Drive), which concentrates large-scale integrated circuits such as CPU, MPU, RAM, EEPROM, etc., and has high reliability. The probability of failure itself is very small, but sometimes due to Turning on the power to make all the control terminals closed at the same time will cause the VFD (Variable-frequency Drive) to show EEPROM failure, which only needs to reset the EEPROM from the beginning.

The IPM circuit board includes drive buffer circuit, as well as overvoltage and lack of phase circuit. The PWM signal from the logic control board inputs the voltage drive signal into the IPM module through the optical coupling, so while detecting the mode speed, the optocoupler on the IPM module should also be measured.

4. Cooling system

The cooling system mainly includes heat sinks and cooling fans. Among them, the cooling fan has a short life. When the service life is approaching, the fan will vibrate, the noise will increase and initially stop, and the VFD (Variable-frequency Drive) will trip due to IPM overheating. The life of the cooling fan is limited by the bearing, which is about 10,000 to 35,000h. When the VFD (Variable-frequency Drive) continues to operate, the fan or bearing needs to be replaced every 2 to 3 years. In order to prolong the life of the fan, the fan of some products only runs when the VFD (Variable-frequency Drive) is running instead of the power supply.

5. External electromagnetic interference

6. Installation

VFD (Variable-frequency Drive) belongs to the installation of electronic devices, and there are requirements for careful installation and utilization in its imitation list. In special environments, if these requirements cannot be met, response measures must be adopted as far as possible: vibration is a secondary cause of mechanical damage to electronic devices. Inert gas and dust will cause corrosion of electronic devices, poor contact, and reduced insulation, which will cause shortcoming. As a preventive measure, the control board should be treated with anti-corrosion and dust-proof, and a closed layout should be adopted; temperature is the main factor affecting the life and reliability of electronic devices. Elements, especially semiconductor devices, should be installed in accordance with the installation requirements or avoid direct sunlight.

In addition to the above points, it is often necessary to check the air filter and cooling fan of the VFD (Variable-frequency Drive) on a regular basis. For special high and cold places, in order to prevent the microprocessor from being unable to work normally due to the low temperature, necessary measures such as setting up an air heater should be adopted.

7. The power supply is very

The power supply is roughly divided into the following three types, namely phase loss, low voltage, power failure, and sometimes their inclusion forms. The main reasons for these extraordinary phenomena are mostly caused by wind, snow and lightning strikes on power lines, and sometimes due to short-to-ground and short-to-phase in the unified power supply system. Lightning strikes vary greatly by region and season. In addition to voltage fluctuations, some power grids or self-generating units also exhibit frequency fluctuations, and these phenomena sometimes appear repeatedly in a short period of time. For the general operation of the equipment, the VFD (Variable-frequency Drive) power supply is also required to respond. .

If there are indirectly started electric motors and induction cookers nearby, in order to prevent the voltage drop when these devices are put into use, the power supply should be separated from the VFD (Variable-frequency Drive) power supply to reduce mutual influence.

For equipment that is required to continue to operate after an instantaneous power failure, in addition to selecting a VFD (Variable-frequency Drive) with a suitable price, the deceleration ratio of the motor load should also be considered afterwards. When the VFD (Variable-frequency Drive) and the external control circuit adopt the instantaneous power failure compensation method, after the voltage loss is restored, the speed measurement of the tachometer motor is used to prevent overcurrent during acceleration.

For equipment that requires continuous operation, a VFD (Variable-frequency Drive) should be installed with an active switching UPS. Like VFD (Variable-frequency Drive) with diode input and single-phase control power supply, although it is in the state of lack of phase, it can continue to work, but the current of individual devices in the rectifier is too large, and the pulse current of the capacitor is too large. Long-term operation will adversely affect the life and reliability of the VFD (Variable-frequency Drive), and should be checked and disposed of as soon as possible.

8. Lightning strikes,

Lightning strikes or impulse voltages caused by lightning strikes can sometimes cause VFD (Variable-frequency Drive) damage. In addition, when the power supply system is equipped with a vacuum interrupter on the same primary side, a high surge voltage will occur during short opening and closing. In order to prevent overvoltage damage caused by impulse voltage, it is usually necessary to press the receiving device such as a pressure sensitive resistor at the input end of the VFD (Variable-frequency Drive). Vacuum interrupters should be added with RC surge receivers. If there is a vacuum interrupter on the primary side of the transformer, the VFD (Variable-frequency Drive) should be disconnected before the vacuum interrupter operates in the control sequence.