There are large capacitors in a typical VFD (Variable Frequency Drive). If the drive has been stored for a period of time the capacitors will build up an oxide coating between the plates. This can cause the VFD to be damaged. Normally this is done by raising the AC Voltage and thus the DC voltage on the large capacitors slowly over time. This removes oxide build up on the capacitors that have not been used for a period of time. You use the Variac to power the VFD without any load and over a period of time (typically hours) you eventually bring the drive to full operating voltage. The VFD manufacturer will often provide instructions for the proper reforming of capacitors in their drives. If the drive is 480VAC nominal then most use the 1520CT-3. It is recommended that fuses be placed on input side to prevent damage to the Variac if you increase the voltage too soon or if connected to a defective drive. Monitoring the output current is also a good idea.
Reforming of capacitors is also recommended for antique radios and other electrical/electronic (particularly older devices) that have not been used for some time. A procedure similar to above is used except using a single phase Variac. For 120VAC nominal rated devices a 3PN1010B transformer is commonly used assuming that the device normally operates within the 10A rating of this transformer.