Learn what Under Voltage in a VFD is, what the causes are and how to fix the issues that create Under Voltage in a WEG Variable Frequency Drive in this brief hands-on tutorial video using the Free WPS software. This is the best VFD, with a super low cost, but with high end features like Dynamic Braking, Fire Mode, PID, 65,000A SCCR, Zero Stack footprint, Multi-Speed, etc., and is even cULus listed!
The SUB indication or the F-21 fault both tell us there isn’t enough DC bus voltage for the drive to enable the output to the motor. A drive takes an AC input, rectifies it, and runs it across a large capacitor to create a DC signal which is chopped and sent out to a motor. By adjusting the chopping on the three phases, the drive can very accurately control the speed of the motor. These faults are telling us that this DC level is too low for the chopper to do its job. SUB says there isn’t enough DC voltage to enable the output when the motor isn’t running. Fault 21 occurs when the motor is running and the DC bus drops too low. And it’s easy enough to demo. Since this is just a rectified signal, it is directly proportional to the input voltage. The lower the input voltage, the lower the DC bus. So, I just inserted a Variac here, so I can change the amplitude of the voltage into the drive. Let’s monitor parameter 4 which shows us the DC bus voltage. I have this volt meter connected to the line voltage going into this drive. I’ll slowly lower the input voltage, and when it gets too low, the drive issues the SUB indication. Looks like the DC bus was right at 200 volts when that happened. If we look in the programming manual, we see that the undervoltage threshold is 200 volts, which agrees with what we just found. Bingo. If I raise the input voltage the drive automatically comes back. You don’t have to reset anything because the drive wasn’t running when this happened. This time let’s hit RUN. The drive is running at the default min speed of 3 Hz. If I lower the input voltage until the DC bus hits 200 volts, we get Fault 21. Again, this tells us the DC bus fell too low to keep the output enabled, while the drive was running. This time, if I raise the input voltage the fault doesn’t go away. You have to manually reset the fault and then hit run again to get the motor moving, which makes sense. You don’t want the motor to start spinning again until you know everything is safe, right? So that’s it. If you get a SUB or an F21 message, it usually means the input voltage isn’t high enough to get the DC bus to 200 volts. So just put a volt meter across the input to the drive and see if it looks reasonable. I’m using single phase here, so it only took one measurement. If you are using a 3-phase supply, you will want to measure all three phases. While that is usually the issue, there are a couple other things that can give you these under voltage messages. Make sure you have the right drive for your input voltage. For example, you might have a 220 volt drive but a 120 volt line input. So be sure to check the drive’s label. I know, it seems silly to mention it, but sometimes it’s the simple things that trip us up. If you are using a 3-phase supply, there might be something goofy with the phasing of the input voltage. Get an oscilloscope and verify that all three phases look correct relative to each other. If you think you might have power line issues, consider using one of these guys. They provide full power system analysis of single phase, 2-phase, 3-phase 3-wire and 3-phase 4 wire systems. And you can configure and communicate with them via ModBus RTU. And they even have alarm outputs. Very cool. Finally, if none of those are the issue, then use one of AutomationDirect’s Free award-winning support options to contact Tech Support. There could be something wrong with the drive. And click here to learn more about the WEG CFW300 Variable Frequency Drive. Click here to subscribe to our YouTube Channel so you will be notified when we publish new videos.