Variable Frequency Drives and Capacitor Forming

The need to know

Variable frequency drives (VFDs) are one of the most ubiquitous pieces of automation equipment today. Many of our customers use VFDs ranging from cutting-edge modern drives to 25 year old legacy VFDs. However, many people do not know that powering up an old drive without taking certain precautions may risk permanently damaging the drive, costing you money and time. Here's a failed capacitor next to it's replacement in our shop:

When purchasing a VFD, it is important to know when the drive was last powered up. If it has been more than a year (or less in hot and humid storage conditions), then you must apply power slowly through a variable transformer or variable DC supply in order to form the capacitors.

We take pride in providing reliable parts at a great price. Every legacy drive that leaves our facility has its capacitors formed prior to shipping. In addition, we are happy to power up old drives you may have around. Feel free to reach out to us at to discuss your needs. Otherwise, read on for more information on the technical aspects regarding capacitor forming.

Capacitor forming in detail

VFDs use DC bus capacitors to hold energy for the inverter. Aluminum electrolytic capacitors have an aluminum oxide surface between the plates of the capacitor. If left unpowered, an oxide layer can degrade, affecting the capacitance of the capacitor. Applying power quickly to the capacitor can cause the capacitor to permanently fail. However, by applying power slowly, through a current-limited source, the aluminum oxide layer can be dissipated over a period of minutes or hours.

Here's information from various manufacturers discussing the issue:

At Industrial Automation Co, we work on VFDs of all ages. Our team of engineers and technicians use single- or three-phase variable transformers and high voltage DC power supplies to reform bus capacitors. Contact us for more information.