When the VFD is viewed regularly, the power supply should be blocked, the operation of the VFD should be stopped, and the cover of the VFD should be removed. The first thing is to check the places that cannot be seen without downtime, or the places where it is difficult to find problems on a daily basis, as well as the inspection and adjustment of electrical characteristics.
After the VFD power supply is blocked, there is still a high charging voltage on the filter capacitor of the main circuit. The discharge requirement is generally 5 - 10 min. Before starting the viewing operation, it is necessary to wait for the charging indicator to turn off and take a test with a voltmeter to admit that the voltage is below the safe value of 25V DC.
1. Check whether the cooling system is normal and clean the dust of the air filter.
First, clean the internal parts of the VFD, use a vacuum cleaner to suck off the internal dust, and wipe the things that can't be sucked off with a soft cloth. Because there may be dust, foreign matter, etc. If it is dropped during operation, it should be cleaned from top to bottom, and the leads of the main circuit components, insulated terminals, and the ends of capacitors should be carefully scrubbed with a soft cloth. The cooling fan system and air duct should be carefully cleaned, and the VFD should be clean inside and the air duct should be dredged.
2. Due to the temperature rise and vibration during the operation of VFD, the main circuit equipment, terminals and leads of the control circuit are often loosened, resulting in corrosion, oxidation, poor contact and wire breakage. Therefore, check whether the screws, bolts and other fasteners are loose, and tighten them if necessary; For soldering parts and crimp terminals, check for corrosion, discoloration, cracks, breakage, etc. Also look at whether the frame structure is loose and whether the wires and wires are damaged.
3. Check whether the connection of the control circuit board is loose, whether the capacitor is leakage, and whether the circuit on the board is corroded or cracked. Generally, it is not possible to measure the practical capacity of the capacitor on the control circuit, and its performance and life can only be inferred from its appearance condition, operation condition and surface temperature rise. If there is no abnormality in the appearance, it can be judged to be normal. Check the resistors, inductance coils, relays, and contactors on the control circuit, and first see whether there is looseness, broken wire, whether the terminal strip is damaged, and whether the contact is rough.
4. Check whether the filter capacitor is leaking and whether the capacitance is decreasing. The high-performance VFD has the function of automatically indicating the capacitance of the filter capacitor. On the panel, the capacitance and initial capacitance values at the factory of the capacitor, as well as the capacitance decline rate and the estimated capacitor life, can be displayed. If the VFD does not have this function, it needs to be measured with a capacitance meter. The measured capacitance should be greater than 85% of the initial capacitance, otherwise it should be replaced. The surge absorption capacitance and resistance of the surge absorption circuit should be checked for anomalies, diode limiters and nonlinear resistors for discoloration and deformation.
5. Check whether the insulation resistance is within the normal range. VFD has been tested for insulation at the factory, and users generally no longer conduct insulation tests. However, after a period of operation, when it is necessary to measure the insulation resistance during repair, the following process should be followed, otherwise the VFD may be damaged. Before measuring, all VFD leads should be removed.
(1) Measure the insulation resistance of the main circuit. When measuring the insulation resistance of the main circuit, it should be ensured that the main power supply is disconnected, and all the main circuit terminals, including the incoming terminals (R, S, T or L1, L2, L3), outlet terminals (U, V, W) and external resistance terminals, are short-circuited to avoid high voltage entering the control circuit. Connect the 500V megohmmeter between the common line and the ground (PE terminal), and the indication value of the megohmmeter is greater than 5 mω is normal. The measurement method of motor cable insulation is to remove the motor cable from the VFD U, V, W terminals and the motor, and measure the interphase and relative insulation resistance, and its insulation resistance should be greater than 5 mΩ. The method of power cable insulation detection is to separate the power cable from the VFD R, S, T or L1, L2, L3 terminals and the power supply, and measure the interphase and relative insulation resistance, and the insulation resistance should be greater than 5mΩ. The motor insulation detection method is to disconnect the motor from the cable and measure the insulation resistance of each winding of the motor between the terminals of the motor junction box. The measured voltage shall not be greater than 1000V, but shall not be lower than the mains voltage, and its insulation resistance shall be greater than 1 mΩ.
(2) Measure the insulation resistance of the control circuit. In order to avoid high voltage damage to electronic components, do not use a megohmmeter or other instruments with high voltage to measure, but use the high resistance of a multimeter to measure the insulation resistance of the control circuit. It is normal for a metric value greater than 1 mΩ.
(3) Measure the insulation resistance of the external circuit. In order to avoid the high voltage of the megohmmeter being applied to the VFD, when measuring the insulation resistance of the external circuit, it is necessary to remove the external circuit from the VFD before the measurement, and pay attention to check whether the high voltage of the megohmmeter can be applied to the VFD through other circuits. If so, all relevant linkages should be removed.
6. After all the above items are completed, the maintenance loop action should be checked. It is important to maintain that the circuit is in a safe working condition. Therefore, it is necessary to look at the reliability of the maintenance function at a given value. In general, the top maintenance functions that should be reviewed are as follows.
(1) Detect overcurrent maintenance function. Overcurrent maintenance is the circuit with the most problems in the VFD control system, and it is also the most important circuit to maintain the main circuit components and equipment. Generally, after imitating the overload and adjusting the action value, the experiment can operate firmly under the set overcurrent value and block the output.
(2) Detection of phase loss and undervoltage maintenance functions. When the power supply is out of phase or the voltage is abnormally reduced, it will cause the power unit to fail to commutate and cause overcurrent problems. It is required to detect out-of-phase and under-voltage signals in real time, and block the trigger signal for maintenance. It can supply power to the VFD through the voltage regulator at the input end of the VFD power supply, imitate the lack of phase, undervoltage and other faults, and investigate whether the relevant maintenance functions of the VFD such as phase loss and undervoltage are correct.
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