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Electrolytic Capacitor DC Bus Capacitors rupture danger

VFD s use Electrolytic Capacitors in the DC Bus section. This article by Dean Williams is useful information.

Electrolytic Capacitors, The Ticking Bomb
Author: Dean Williams

Inside most industrial electronic equipment resides a component called an electrolytic capacitor. (IF you need to purchase DC Bus Capacitors for your VFD, click the contact icon on the top of this page) Typically, the function of this capacitor is to act as a power supply filter. It’s comprised of two strips of aluminum foil (called plates) separated by a paper insulator. The foil and paper are rolled into a cylinder and immersed in an electrolyte solution consisting of an ethyl glycol fluid. All of this is placed inside an aluminum can and sealed up via some sort of rubber safety vent plug. (image courtesy of learnabout-electronics.org )

The capacitor works by storing a DC charge between the two plates. One of the plates has an insulating oxide layer that is created & maintained when the capacitor is charged up. It is this insulating oxide layer (called a dielectric) that is crucial to proper capacitor operation. If the dielectric was not present, the capacitor would short circuit and draw large amounts of current. When large amounts of current flow into the capacitor, it causes the electrolyte solution to boil and turn into a gas. Once turned into a gas, pressure builds rapidly until, hopefully, the safety vent plug releases pressure.

This rupture can be very dramatic and destructive. Not only is the boiling liquid and gas hot, it is also corrosive and will damage any components covered by the solution. Under controlled laboratory conditions, measurements have been taken during a violent, large capacitor rupture. The equivalent explosive force of half a hand grenade has been measured. For some interesting viewing go to www.youtube.com and type in “capacitor explosion”. You will see perfect examples of violent failures.

The dielectric oxide-insulating layer is created during manufacture. However, it will deteriorate in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will fail if voltage is not applied. Maintaining the integrity of the dielectric requires the steady application of power. When voltage is applied, the oxide layer is rebuilt. The oxide layer thickness is the crucial factor to determine the voltage rating of the capacitor. Otherwise, the oxide layer will break down under voltage and cause a spectacular failure.

By applying voltage to the capacitor on a regular basis, the oxide layer is “reformed”, and the integrity of the insulating characteristic is ensured. In EMA’s experience, the maximum time between power ups should be no longer than 1 year, provided the equipment is kept in a dry location at roughly 70 degrees F. Large temperature swings require less time between power ups.

The procedure to “reform” the capacitors in electronic equipment is to apply rated voltage for a period of at least 1 hour. Realizing after the unit is de-energized, a chemical reaction is still occurring for up to 24 hours, thereby rebuilding the oxide layer during this “resting” period. Should more than 12 months pass between power ups, great caution should be exercised.

At EMA, we constantly see equipment that is sent to our repair shops with damage caused by improper “reforming” of the capacitors. Often, the customer has a spare piece of equipment sitting on a shelf for years with no attention made to reforming the capacitors. During an emergency breakdown, the spare unit is brought out to the machine, installed and powered up, only to have a dramatic failure occur when the capacitors violently fail. This can occur in many different pieces of equipment such as monitors, PLC’s, power supplies, and especially variable frequency drives (VFD’s). VFD’s have a large capacitor bank that is especially prone to becoming “unformed” and failing dramatically upon power up after a long de-energized period.

EMA has the capability to reform these older capacitor banks by using special power supplies that allow for controlled voltage increase, and limited current supply. In some worst case situations it has taken 8 hours to safely bring a capacitor bank to full voltage.

In conclusion, look through your shelves to see if you have any electronic equipment that has not been powered up in the last 12 months. If the answer is yes, you should seriously consider sending it to EMA for an operational checkout under power. If you find that a large amount of items need capacitor reforming, EMA can travel to your site and perform the service at your facility.

To locate your nearest EMA facility, see the listings at the bottom of this page, or click the contact icon at the top right.

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One Response

  1. If a vfd was installed after seven years of storage and operated for 3 years prior to failure would not reforming the capacitors be the probable cause of failure.

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