Anyone who manages, installs, and supports variable frequency drives (VFD’s) knows it is a good day when the equipment is running, and shafts are turning.
What happens when that happy VFD faults out and you see that it shut down due to an overvoltage condition? At that moment having a good understanding of what causes that to occur will be valuable information as you aim to get the VFD back online.
At the core an over voltage fault occurs when the drives DC bus measures a voltage higher than the predetermined overvoltage trip point. When that happens the VFD protection kicks in and will take it offline by means of a fault.
The most common representation of this condition is when the fault screen has “oV” or “F05” among others depending on the brand of drive. As we dig deeper into overvoltage faults here are possible causes of this condition:
- As a motor begins to slow down (think applying brakes in your car), it sends power back onto the drive that is dissipated by the drives braking circuit or an active front end in regeneration applications.
- When the braking time is too short, and the inertia of the load is too great a large spike in energy is put back onto the drive causing the voltage to rise on the bus and thus creating an over voltage spike.
- Once you determine that this is the likely culprit one area to consider adjusting is the deceleration time. By increasing the deceleration time, you are essentially spreading out the energy dissipation and giving the VFD more time to absorb the regenerative voltage increase.
- If that is not an option, you can investigate how to decrease the load on the motor. This will impact the regenerative voltage as when the load is reduced the energy required to stop the load should decrease as well.
- If either of these options are not available, you can add braking resistors to the drive which will help absorb these voltages and prevent the VFD from being damaged by regenerative voltages.
Input Power Spike –
- The power source feeding your VFD can have an impact on your VFD, and you need to know what to look for. This is primarily a function of the utility feeding your facility or it could be from other sources internal to your facility.
- When you determine this is what causes the faults it is time to act. By adding an input reactor on the line side of the VFD you help protect the device from these surges.
- The best way to stay in front of this condition is by monitoring the incoming power from the utility. This will be a clear indicator if the issue is from the utility itself or if it is being caused by other sources inside the facility.
Rapid Load Changes–
- Often when people troubleshoot VFD’s they spend all the time at the drive itself and not enough time evaluating the load. The load has a tremendous impact on how the VFD operates and if it changes rapidly a power spike can be seen on the system. This can be caused by something being added or removed from the system that the VFD is trying to compensate for.
- During this condition, the motor may experience excessive slippage as motor trying to catch up to synchronous speed. During this state the system consumes more power.
- To correct these issues, ensure there are no binding points or slippage occurring in the mechanical linkages around the motor.
- This would be a good point to double check and ensure that the VFD is properly rated for the duty of the application.
Grounding Issues –
- With many electrical devices having a solid grounding system is critical for long life and reliability.
- If there is a floating ground or the ground was not installed correctly from the beginning, you may begin to experience overvoltage faults.
- If the ground is not properly installed, it could be energized and the unwanted voltage that dissipates is not eliminated and thus causes the over voltage condition.
- To ensure this is not an issue take the time to verify the ground circuit with a quick test with your voltmeter. A properly installed ground circuit should not have a voltage present and if that is not the case you found the culprit that needs to be corrected.
The next time your VFD has an overvoltage fault you have these 4 areas to dig into when troubleshooting.
The team at EECO is here to serve you with VFD support and you can connect with us here to learn more.