After repairing a Delta VFD-A series 18.5 KW inverter, the reason for repairing is to stop with a normal load and jump GFF when the frequency rises to about 40 Hz.
A GFF fault is a ground fault or a fuse fault, that is, a protective alarm associated with excessive current protection. The GFF fault can be said to be a common fault of Delta's machines. Usually, the aging of the current sensor causes the static operating point to drift, which increases the error of the current detection. Because it can start and run to about 40Hz before tripping, we must first determine whether the driver circuit failure. Power-on startup The inverter measures the no-load output voltage balance and initially proves that the drive circuit is intact. After dismantling, the oscillating function of the ET521A is used to detect the quiescent output waveforms and the null carrier shape of the three current sensors CS1, CS2, and CS3, indicating that the current sensor has good performance. Then use the ET521A to measure the thyristor trigger terminal waveform of the rectifier module is normal; when measuring the trigger waveform of the 6-channel IGBT, find the driving waveforms of the lower V and W phase legs as shown in the figure below: there is obvious during the conduction flat top Waveform distortion.
The ET521A was converted to DC voltage measurement, and all the upper and lower tubes were driven with the same static and dynamic voltages. It can be seen that this subtle difference is absolutely indistinguishable by the voltage measurement method of an ordinary multimeter. Because Delta's thyristor and inverter module trigger pins are directly soldered to the power/drive board, and most of the electronic components of the drive circuit are mounted on the heat dissipation board, the power/drive board is not removed. When checking the measurement points of this part of the circuit is less, and than the kind of connection of the plug connector only dial and insert contrast, because the demolition welding and welding and other operations will be much more troublesome. Then remove the driver board to check, and finally find out that the pressure drop of the luminous tube voltages of the two PC929 input control terminals actually driven by the V and W phases is only about 1.1V, while the normal ones are more than 1.5V (digital watches of different brands). Readings may vary slightly!) Explain that their internal light emitting tube has been severely deteriorated. When the CPU's drive pulse arrives, the light intensity decreases and the signal transmission capability weakens, which results in a distortion of the rear drive waveform. After replacing the relevant components such as the PC929 and the electrolytic capacitor, the driver circuit is not soldered to the trigger terminal of the module. The effect is not detected because the driver circuit is in an idle state. If the power/drive board is soldered to the trigger pin of the module to check the repair effect, if there is still a problem, do you have to remove the solder once again! In fact, there is no reliable method of welding. The following figure is a schematic diagram of the drive circuit of this machine:
In electronic circuits, we prioritize the maximization of signal power transmission, and the conditions for maximizing signal power transmission are that the output resistance of the signal source is equal to the input resistance of the next stage. The schematic diagram from the upper part of the drive can be analyzed and found that the output resistance of this circuit is about 33Ω - equivalent to saying that the GE input resistance of the IGBT in this circuit is approximately 33Ω! In other words, we only need to connect a 33Ω/2W resistor to the GY-EY terminal to simulate the actual loading effect of the drive circuit.
Connect a 33Ω/2W resistor to the 6-channel driver output. After startup, measure the waveforms of the drive down-channels. The following figure shows the repair waveform of the V-phase down-pipe. Description V, W phase lower tube drive circuit has been repaired.
After installing the machine with a motor test machine, start the inverter to 50Hz work, and slowly increase the load to 35A current when the phase current and the phase-to-phase voltage balance smoothly, the inverter's own detection current reading is similar to the external measurement clamp meter reading, thus Prove that the machine has been successfully repaired.