What must be observed with regard to the DC link capacitors when drive controllers are stored for a longer period of time?

Answer:

In principle, drive controllers can be stored for a few years without this having a significant effect on their usability.
If the storage time exceeds 5 years, it is recommended to connect the drive controller to the respective mains voltage for one hour before use. This must be done without load. (no DC bus connection with other components and no controller enable).

Background information:
During long storage without voltage (>5 years) of servo or frequency inverters, the ageing of electrolytic capacitors can lead to their failure in extreme cases.
Over time, the oxide layer, which serves as dielectric within the electrolytic capacitors, gradually dissolves. The oxide layer is regenerated by the so-called leakage current, which flows when voltage is applied. This current supplies the anode with oxygen ions and thus promotes the formation of the oxide layer.
If the oxide layer is partially dissolved by ageing, comparatively high leakage currents must be expected. These place additional stress on the electrolytic capacitor and other components, and voltage asymmetries can also occur in the DC link.
If the drive controller is connected to the supply voltage without load, the capacitors inside the device can regenerate and can then be used again as intended.