DC Link Voltage in AC Drive Rise - Cement Mill Classifier Application

Assuming that your input voltage is 400V, mathematically, DC link voltage can not be acceded at 565VDC (roughly 400V x 1.414). So for 780VDC, it means there is regenerative power returning back into the system.

The main fan is driving the classifier via the medium inside, this means this speed differential in these conditions causes regeneration and you have to decide whether you need to handle this or avoid it.

If this didn't happen before, something has changed in the process, the medium or airflow being common reasons.

If you have a VFD on the fan, maybe you can connect the DC bus to share this energy, or add a regenerative supply to the classifier VFD, typically via DC bus connections again.

This is a common scenario in this type of application and braking is often required.

When you go to higher speeds, the differential speed of the classifier to the airflow is less and the classifier returns to 'motoring' or driving region of operation.

Hope this helps.

There is re generation in the drive which is the reason for the voltage rise.

The reason for re generation is due to the process condition which makes the driven equipment run faster than the frequency generated by the drive. To avoid the problem, try tuning the process. To solve the problem, If the proces is running better with the present process conditions, you may have to add a dynamic braking or regenerative breaking unit to the DC link of the drive. Re generative breaking link can save energy by feeding back the excess energy to the grid and may be pay back the investment.