Printing Industry VFD (Variable-frequency Drive) Troubleshooting Example

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This article introduces various common VFD (Variable-frequency Drive) faults in the printing industry and their case analysis.

(1) AEG Multiverter122/150-400 VFD (Variable-frequency Drive) trips due to overvoltage in the DC circuit at startup

This VFD (Variable-frequency Drive) does not trip due to overvoltage every time it is started. During inspection, it was found that when the VFD (Variable-frequency Drive) was powered on but there was no closing signal, the DC circuit voltage reached 360V. The positive pole of the DC circuit of this type of VFD (Variable-frequency Drive) was connected in series with a contactor, which was attracted after the pre-charging process when there was a closing signal. Therefore, it was suspected that the IGBT of the pre-charging circuit had poor performance. The situation remained the same after the IGBT of the pre-charging circuit was disconnected. When the VFD (Variable-frequency Drive) output terminal was checked with a multimeter, its resistance to ground was very small. It was found on site that the motor junction box was wet with water. After drying, the VFD (Variable-frequency Drive) worked normally.

Since the motor junction box is wetted by water, the leakage current of the negative pole of the DC circuit to the ground charges the capacitor of the DC circuit through the junction box and the freewheeling diode in the VFD (Variable-frequency Drive) inverter. In this case, the closing of the circuit is usually understood as an overcurrent trip, but it is actually an overvoltage trip. I believe that the output voltage and frequency of the VFD (Variable-frequency Drive) gradually increase during startup. When the motor is wetted by water, the output current will change at a high rate, causing overvoltage in the DC circuit.

(2) The AEG Maxiverter-170/380 VFD (Variable-frequency Drive) that controls the roller motor has a speed feedback value greater than the speed setting value. After observation, it was found that:

a) This situation does not exist during the steel rolling process. It only occurs when the steel leaves the roller;

b) When the speed feedback value is greater than the speed setting value, the DC circuit voltage is 125% of the rated voltage, exceeding the limit setting value of 115%;

c) The incoming line voltage of the VFD (Variable-frequency Drive) has exceeded the upper limit;

During the steel rolling process, the roller motor controlled by the VFD (Variable-frequency Drive) will increase in speed. When the steel leaves the roller, the roller motor speed drops to the original speed. Because this VFD (Variable-frequency Drive) The VFD (Variable-frequency Drive) is not equipped with a braking device. During deceleration, the braking current is limited by the voltage regulator to keep the DC circuit voltage from exceeding the limit setting value of 115% (default value). Due to the high incoming line voltage, the DC circuit voltage exceeds the set limit value. The voltage regulator works during deceleration, resulting in a very small braking current and the motor speed cannot be reduced. During steel rolling, the load of the power grid increases, the DC circuit voltage is lower than the limit setting value of 115%, and the braking function returns to normal. When the power grid voltage could not be reduced at that time, the DC circuit voltage limit setting value was increased to 127%, and the VFD (Variable-frequency Drive) worked normally. During the shutdown and maintenance, we changed the gear of the transformer according to the power grid situation, so that the incoming line voltage of the VFD (Variable-frequency Drive) was within the allowable range, and the VFD (Variable-frequency Drive) worked normally thereafter.

(3) After the AEG Multiverter22/27-400 VFD (Variable-frequency Drive) is powered on, the LCD screen on the operation panel displays normally, but the ready indicator is not on, and the VFD (Variable-frequency Drive) cannot be closed.

When checking the fault record in the VFD (Variable-frequency Drive) menu, no fault was found, but the operation of each button on the operation panel was recorded in the event record. The LED indicators on the A10 main board and A22 power board in the VFD (Variable-frequency Drive) were checked to be normal. The VFD (Variable-frequency Drive) incoming power supply was tested with a test pen and one phase was found to be abnormal. The three-phase results of the multimeter were: Vab=390V, Vac=190V, Vbc=190V. After inspection, it was found that the incoming terminal block had poor contact.

The ready indicator is a comprehensive reflection of various status information in the VFD (Variable-frequency Drive). When it is not on, it can remind maintenance personnel to pay attention to the fact that the VFD (Variable-frequency Drive) is not ready. At this time, when the incoming power supply is abnormal, the fault record of the VFD (Variable-frequency Drive) fails to reflect the reason for not being ready, which may be related to the design of the circuit.

(4) During the debugging process, the Siemens MIDIMASTER Vector (22kW) VFD (Variable-frequency Drive) tripped after starting due to overcurrent

Due to communication reasons, the capacity of the VFD (Variable-frequency Drive) supplier and the supplier of the controlled equipment did not match (the motor power was 30kW). The control mode of the VFD (Variable-frequency Drive) was selected as vector control. When the motor parameters were entered, the VFD (Variable-frequency Drive) automatically limited the rated current of the motor from 60A to 45A. There was no power factor on the motor nameplate. According to the requirements of the VFD (Variable-frequency Drive) manual, it was set to 0. When the motor was started after automatic identification (P088=1), the VFD (Variable-frequency Drive) tripped due to overcurrent. Considering the matching reasons, the control mode was changed to V/F control, and the situation remained the same. When checking the motor parameters later, it was found that the power factor was 1.1. After changing it to 0.85, the VFD (Variable-frequency Drive) worked normally.

Due to capacity mismatch, the VFD (Variable-frequency Drive) will produce incorrect results when calculating based on the input motor parameters. When encountering this situation and temporarily unable to solve the matching problem, be sure to check whether there are inappropriate parameters after automatic identification.

(5) The letter "E" is displayed on the LCD screen of the PMU panel of the Siemens 6SE70 series VFD (Variable-frequency Drive)

When this happens, the VFD (Variable-frequency Drive) cannot work. Pressing the P key and re-stopping the power supply are ineffective. There is no relevant introduction in the operation manual. When checking the external DC24V power supply, it is found that the voltage is low. After solving the problem, the VFD (Variable-frequency Drive) works normally.

The fault countermeasures table in the VFD (Variable-frequency Drive) operation manual introduces all common faults. When some codes not involved appear, the VFD (Variable-frequency Drive) should be fully checked.

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