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Variable Frequency Drive VFD Bus Capacitors life span

It’s well documented that VFD bus capacitors have a limited life span and at some point require replacement….preferably prior to a catastrophic failure.

blowcaps Capacitors can and do fail within the expected service life rating. The expected service life rating can only be used as a general guideline because of the various conditions that play into and affect the Capacitors “actual” life span.

Temperature, over voltage/current, improper mounting/clamping, terminal connections, and shelf life are some conditions that contribute to the actual life span of bus capacitors.

swollencapOver temperature dramatically decreases the life span of the bus capacitors. As the temperature increases it causes an increase in electrolytic loss thus causing the fluid inside the capacitor to prematurely dry out. As the fluids dry out, the capacitor will start to overheat and you may see a discoloration in the capacitor. However, the capacitor can also continue to operate right up until a catastrophic failure with no visual warning signs beforehand.

Capacitor over voltage/current results in a break down of the aluminum oxide layers thus causing a short circuit between the electrodes. As the current flows through the paper spacers the electrolyte heats up and can result in the capacitor leaking, bulging or even bursting. Also consider that over voltage/current with the added increase in temperature is a deadly combination that rapidly accelerates premature capacitor failures.

capacitor bankImproper mounting & clamping and/or under or over tightening of the capacitor terminal connections will also cause issues. The mounting & clamping mechanism aids in reducing capacitor swelling. Terminal connections can be tricky; too tight can strip out the mounting holes in the bus capacitor, while too loose results in a poor mechanical and electrical connection.

It’s even important to use the proper screws. Bus capacitors are commonly mounted to bus work, which can vary in thickness. Using too long a screw can penetrate the capacitor resulting in damage, while too short means a poor connection that will loosen over time.

Capacitor shelf life also has an expiration date. Capacitors with no voltage applied allow the liquid electrolyte solution consisting of ethyl glycol fluid to deteriorate. Capacitors can be reformed by properly applying the correct voltage for a set time to the capacitor and increasing the voltage until the final voltage requirement is met to reform the oxide layer. The process of reforming the capacitors thus ensures the integrity of the insulating characteristics. This is something EMA does regularly for customers, not only in our shop, but out in the field as well.

Performing Preventive/Predictive Maintenance on a routine basis where the bus capacitors are inspected should be implemented. After a certain age, or extreme environmental conditions, the bus capacitors should be replaced.

EMA provides PM Services on various makes and models of Medium Voltage Variable Speed Drives including the Siemens Robicon Perfect Harmony MV Series Drives. During our PM one of the many tasks we complete is to inspect the bus capacitors; replacing them when necessary. Being proactive and scheduling regular preventive/predictive maintenance during a scheduled down time will assist in eliminating unnecessary and costly downtime.

For more information on EMA’s Preventive/Predictive Maintenance, Reforming Capacitors, or spare parts please contact Rebecca Zeeuw @ rebeccaz@emainc.net

OR Contact us by any of the methods on the right of this page. No One, Anywhere, is Better at Drives (or Bus Caps) than we are.


5 Responses

  1. Our Wastewater Treatment Plant receives spare VFD’s from contractors when they complete a project
    We have reformed caps on the drives > 100 HP. We have 5,10 and 25 hp drives that have been on the shelf for years.
    Should all of these be reformed? Is it economically feasible to do this?

    1. John,
      We would suggest you do this to any VFD sitting on the shelf for over a year. On something that small, you can likely get away with using single phase 230 to two of the input terminals for 1 hour and then letting it “rest” for 24 hours. That should be enough to reform the capacitors on something so small.

      1. How would you recommend I reform caps on bigger drives in the 100hp to 300hp range? What hardware do I need? Would a 110v variac into a step up transformer suffice? What do I hook it up to? Thx.

        1. Chuck. You can use a variac with a step up transformer but it needs to be rated high enough to handle the current drawn by the drive. It is typically not much. You would hook it up the line in terminals of the drive. For an easy, ready-to-go solution, I’d recommend Bonitron’s reformer: https://www.bonitron.com/maintenance/m3628acf/

  2. Can the life span of DC Buss capacitors be extended by replacing using 1000uF with 1500uF with same volt rating?

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