Delta's VFD-B series frequency converters are Delta's flagship models, featuring vector control and supporting MODBUS communication at a baud rate of 38.4K. They are widely used in many industries.
The customer recently sent in a Delta VFD-B 15KW frequency inverter for repair. Reportedly, it frequently shuts down suddenly during normal operation. When it stops, the inverter displays nothing, but after a few seconds, the display returns and it functions normally again. This frequent malfunction is causing a high scrap rate in production, severely impacting production.
After the user brought the device to our service center, we immediately inspected it. Based on the user's description of the fault symptoms and after actual power-on operation and observation, we confirmed that the customer's fault description was accurate. The preliminary diagnosis was that the inverter had a poor internal power supply.
The internal power supply of this frequency converter is a switching power supply, which is also the common power supply method for frequency converters nowadays. It has the characteristics of high conversion efficiency, small size and wide operating power supply voltage range.
Because the timing of this inverter failure is unpredictable, it has caused us a lot of trouble in our maintenance. It usually takes 1-2 hours of normal operation for the failure to occur, and the failure time is only a few seconds. Therefore, we can only replace the vulnerable components based on experience before testing.
Initially, it was suspected that the switching power supply was overloaded, causing some components to overheat and malfunction, resulting in excessive current and triggering the protection shutdown. Therefore, the cooling fan was first disconnected, and the power was restored for observation. After running for a period of time, the machine still malfunctioned. A careful oscilloscope examination of the output waveform of the UC3842 switching power supply chip revealed it to be normal. However, checking the waveform across the current-limiting resistor showed the current was slightly higher than normal. The normal operating current of this model's switching power supply is typically around 0.3A (peak-to-peak), but this inverter was exceeding 0.5A, confirming the initial diagnosis of overload. However, a thorough inspection of all loads, drive circuits, control boards, and detection circuits showed they were all functioning correctly. The repair reached a stalemate.
Then, after carefully examining the circuit diagram of the switching power supply, it was discovered that at the current detection terminal of the switching power supply, pin 3 of the UC3842 was connected to another protection circuit. The chip's power supply pin was connected to pin 3 via the Zener diode D2. This part of the circuit should be the switching power supply's own overvoltage protection. Static testing showed that the Zener diode's characteristics were good, and measurements with the regulated power supply were also normal, with a regulated voltage of around 21V (power supply voltage 17V). After removing this Zener diode and powering on the machine, the current waveform surprisingly returned to the normal 0.3A. It seemed that the problem was caused by the poor characteristics of this Zener diode. A new 21V Zener diode was installed, and after a morning of trial operation, it worked normally without the previous fault recurring. Finally, it was delivered to the customer, and after several days of actual operation, it worked normally, proving that the inverter was completely repaired.
D2 in the diagram
Therefore, when troubleshooting difficult inverter faults, in addition to conducting more tests, it is also important to be good at analyzing the schematic diagram in order to find the real cause of the fault.
