Maximize Variable Frequency Drive (VFD) Lifetime with Proper Link Capacitor Care

Maximize Variable Frequency Drive (VFD) Lifetime with Proper Link Capacitor Care
The Variable Frequency Drive (VFD) is used to control a three-phase induction motor’s direction, speed, and torque, plus a variety of other dynamic parameters such as acceleration. Internally, most VFDs feature two stages including an AC to DC converter (rectifier) and a DC to AC converter (inverter). Between the input and output stages, we find one or more DC link capacitors such as the representative Chemi-Con ERHB701LGC122MDC5U device shown in Figure 1. The link capacitors are typically aluminum electrolytic types chosen for high performance, compact size, and low cost. The link capacitors provide filtering and ready energy reserves for the VFD’s output inverter.

This engineering brief describes the care and feeding of the VFD’s link capacitors. This is a critical consideration, as long storage times can degrade the capacitor. Without proper care, the capacitors in a newly installed VFD may experience a rapid disassembly event. The capacitor will forcefully eject its contents, often causing considerable internal VFD damage.

Figure 1: Image of a large aluminum capacitor typically used for the DC link function in a VFD.

Tech Tip: Consult the VFD manual for the capacitor reforming instructions. The procedure may be as simple as applying power to the VFD and waiting a few hours. The procedure may also be complex requiring an external variable DC power supply with reform times up to 10 hours.

Aluminum electrolytic capacitor construction

Recall that a capacitor consists of two metal plates separated by a dielectric. There are several competing design properties to consider including:

  • high voltage requires a thick dielectric

  • high capacitance requires a thin dielectric and a large surface area

The aluminum electrolytic capacitor balancing these engineering constraints by using a thin oxide layer on the anode (positive plate). This oxide layer serves as an insulator and an essential component of the capacitor.

Problem with the oxide layer

The oxide layer in an aluminum electrolytic capacitor is self-forming when a DC voltage is applied. Consequently, there are no concerns for an operational VFD. However, the oxide layer degrades when the capacitor is in storage.

As a case study, consider a plant with several operational VFDs and a few spares in the parts cage. It’s important to understand that the capacitor(s) in the spare VFDs are degrading. After a year or two, the capacitors are in poor shape as the oxide layer decomposes. If you install that spare VFD on a 3 AM service call, you can expect an explosion in the “ready spare” VFD as the high voltage punches through the thin oxide layer, causing the capacitor to quickly overheat. If you are lucky, the capacitor may be the only problem within the VFD. Often, the rapid disassembly event casts conductive foil debris over the VFD internals shorting out critical components.

Solution

Aluminum electrolytic capacitors that have been in storage for an extended period must be reformed by applying a DC voltage. You must apply this forming voltage in a slow, controlled manner to prevent excessive current passing through the degraded oxide layer. The circuit’s operational high voltage may only be applied after the oxide layer has been restored.

As an example, consider the Siemens SINAMICS G120 converters. The capacitor reforming procedure is shown in Figure 2. Here, we see that a VFD stored for over three years requires an 8-hour reforming procedure. By the book, that 3 AM VFD equipment failure will be restored sometime around noon.

Tech Tip: You will be tempted to take shortcuts with the capacitor reform procedure. This may cause the capacitor to immediately pop or cause damage resulting in short VFD life.

Figure 2: DC link capacitor reforming schedule for the Siemens SINAMICS G120 converter. A VFD that has been in storage for more than three years will require an 8-hour reforming procedure.

Parting thoughts

This topic seems counterintuitive.

After all, the VFD is a robust and powerful piece of gear built to withstand harsh environments while driving large AC motors. By comparison, something as simple as shelf life seems trivial. Yet here we are. The critical link capacitors have all the benefits and flaws of the original aluminum electrolytic capacitors found in electronics dating back nearly a century.

Be sure to review the reforming directions in your VFD’s technical manual. Also, take steps to periodically reform your on-the-shelf spares or be prepared to manage the extended down time. Perhaps you should add the spares to your annual preventative maintenance schedule.

Please give this post a thumbs up if you found this information valuable. Also, please help each other by describing your VFD experiences.

Best wishes,

APDahlen

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About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

A collection of Dahlen’s industrial control articles can be found on this index page.

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