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Variable Frequency Drive (VFD) Repair

Recently we were working on a variable frequency drive and this repair was a hot rush – meaning that an entire line in a factory was down, the need to repair the drive quickly and efficiently was of utmost importance.

Once the drive was received into our repair laboratory we started working on it immediately by diagnosing the issues and ordering the parts.

Once the parts were installed the next thing to do was to power up the VFD (Variable Frequency Drive) to see if the new parts solved the issue. Once powered the bus was not charging, we determined that there must be something else wrong. After a full tear down of the drive our engineers drew schematics to work out how power was getting to the bus. We discovered the voltage was coming through a precharge resistor into the bus which once it was charged the SCRs (Silicon Controlled Rectifier) would fire and the voltage would be on the bus. The problem was we still had no voltage on the bus. Realizing that it was the precharge resistor that had blown we traced this back and found that although the connector looked functional in reality it was blown inside.

Once this was repaired the bus was charged. But then one of the bus capacitors blew which was probably the cause of the blown precharge resistor in the first place. After replacing this resistor from our large inventory of in house stocked components we were able to try running the low voltage portion of the drive the problem being it wasn’t powering up.

After drawing further schematics we found it was through an 8k 70w resistor working as a potential divider that then fed the PWM (Pulse Width Modulation) chip on the SMPS (Switched Mode Power Supply). But the voltage was too low for the switch mode power supply to work. We needed to make a repair.

After disassembling the drive and reverse engineering the low voltage portion of the drive we were able to trace the PWM chip’s voltage input to one of the input connectors and also off to what seemed to be a shut down optocoupler circuit from the control card. Investigating further we found a 400 ohm resistance across VCC and OV leading us to start pulling and investigating components. Surprisingly there was a SOT-23 SMD transistor that was not fully open or shorted across collector and emitter which was measuring 1.2k ohms. This was the culprit, after changing this part the voltage was restored and the low voltage portion of the drive was repaired.

It’s hard to believe that a component (see right in the middle of the picture) the size of half a grain of rice could stop an entire factory from working! This is why you need ENA Electronics to be your Industrial Repair and Reverse Engineering company of choice, we go the extra mile and don’t give up on those extra difficult repairs. For more information on the work we do head to our repair page.

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