Tag: VFD repair

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Special situation of Alpha VFD tripping OC fault

Generally speaking, there are two sources of OC faults:

  1. When the operating current of the inverter module is too high, reaching more than three times the rated current, and the voltage drop of the IGBT tube rises to more than 7V, the driving IC returns an overload OC signal to notify the CPU and implement fast shutdown protection;
  2. After collecting a sharp increase in abnormal current from three current transformers at the output end of the frequency converter (some low-power models use two), a voltage comparator (or internal circuit of the CPU) outputs an OC signal to notify the CPU and implement fast shutdown protection.

Of course, when the driving IC or current sampling circuit is abnormal, the frequency converter will falsely report an OC fault.
Small power models often use a shunt resistor directly connected in series at the output end to collect current signals. After being amplified in the front stage, they are isolated by an optocoupler operational amplifier and transmitted to the CPU. The power supply of its preamplifier is taken from the floating power supply of the driving IC. In this way, when the module is damaged (or removed), the power supply branch connected to the inverter module is broken, causing the current sampling circuit to output the highest negative pressure. The CPU mistakenly believes that there is a large current signal and reports an OC fault. In this situation, the frequency converter trips the OC fault as soon as it is powered on, making it impossible to inspect whether the drive IC circuit can output six normal trigger pulses.
In addition, if the peripheral circuit of the driving IC is abnormal or damaged, it can also misreport OC faults. Therefore, during maintenance, it is necessary to distinguish whether it is a fault reported by the current sampling circuit or the driving IC, whether it is a circuit damage misreport or a module damage. Is there really an overcurrent fault? And take measures to clear the alarm status for easy maintenance.
But the OC jumping faults caused by the following reasons often go unnoticed. Overhaul an Alpha frequency converter. Due to damage to the main DC circuit voltage detection circuit, the voltage on terminal 8 was 0 (normally around 3V). The frequency converter experienced an undervoltage fault and could not be put into operation. When the terminal is artificially fed with a voltage of+5V, the frequency converter trips the OC fault when powered on. Through experiments, it has been proven that when the voltage is below 2.5V, an undervoltage fault code will trip, and when the voltage is above 3.8V, an OC fault will trip. Therefore, it is found that when the DC circuit voltage is too high or the DC detection circuit is abnormal, it is another reason for the frequency converter to trip the OC fault.

When conducting maintenance or emergency response, take 5V voltage from the 8 pins of the wiring block CN1 and fix a 3V voltage with a divider resistor. This will facilitate maintenance or emergency operation of the frequency converter.

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A Hidden Cause of Alpha Inverter OC Fault Tripping and Shutdown

During the maintenance of the Alpha low-power inverter, it was discovered that the inverter has a common problem – it is prone to OC failure. It manifests as: often jumping faults during start and stop operations, but sometimes also jumping faults during operation; Sometimes it inexplicably improves and can run for varying lengths of time. When I thought there was no problem, I started to frequently skip OC faults again; When measuring the output voltage of U, V, and W with a probe under no load, it is prone to tripping faults. However, after connecting the motor, it starts running and does not jump again. After a while, the OC fault still jumps when connecting the motor.

The handling of such faults is quite tricky, and it is possible that the fault has been eliminated during the testing process, resulting in no evidence for investigation. Even when testing the hardware circuit (protection circuit) during frequent malfunctions, I couldn’t find any problems and couldn’t figure out the root cause of the problem. This problem puzzled me for over two months.
The hardware protection circuit is mainly completed by two LM393 dual operational amplifier circuits, U22 and U24. The signal is then inverted by a first stage inverter and sent to pin 16 of the CPU. U22 and U24 jointly input two output current signals, one overload OC signal returned by the inverter drive IC, and one DC voltage detection signal, which are respectively added to the input terminals of four operational amplifiers. After open-loop amplification processing (the operational amplifier circuit is actually used as a switch circuit here), Four fault signals were connected in parallel, and after undergoing a first level of phase inversion processing, they were sent to pin 16 of the CPU. I first cut off the overload OC signal returned by the inverter drive IC, and then cut off the “total” fault signal of the inverter output, but both were ineffective, and the fault phenomenon still persists. Is there another way to string in OC signals elsewhere? Impossible!
There may be some inexplicable interference in the circuit, but the source and cause of the interference are difficult to identify. We racked our brains and exhausted all means to install capacitor and resistor filtering elements in the fault signal circuit to improve the anti-interference performance of the circuit, but to no avail. Could it be that during the loading and unloading process of the inverter drive module during the start/stop moment, the fluctuation of CPU power supply caused the malfunction? The measured CPU power supply is 4.98V, which is very stable and meets the requirements.
Without any reason, I had a sudden inspiration and adjusted 4.98V to 5.02V. After conducting a start/stop test, the fault was surprisingly eliminated!
Analyze and speculate the cause of the malfunction as follows: the setting of the static voltage working point outside or inside the CPU is improper or too low, which is exactly at the critical point of signal interference level, making it easy to experience random OC faults that are confusing. After slightly increasing its 5V power supply, the voltage value at its operating point also increases accordingly, avoiding the critical point of interference level, and the frequency converter changes from “neural” to “normal”.

When the machine leaves the factory, if the CPU power supply adjustment value is slightly higher, the machine can run normally for a long time. If the adjustment value is slightly lower, or if there is a slight decrease in 5V due to some reason (such as component variation, temperature drift, etc.) during use, frequent OC tripping faults may occur. Adjusting the 5V power supply can easily solve the problem while ensuring that there are no issues with the hardware protection circuit. If it is not due to an accidental factor, then the depth of the concealment of this fault makes it difficult to “adjust” it properly.