Reforming or Testing AC Drive Bus Capacitors, Capacitor Reforming Guidelines

Summary

Reforming or Testing AC Drive Bus Capacitors, Capacitor Reforming Guidelines

Question

• How do I re-form AC drive capacitors that have not been used in many years?
• How long can a drive stay in storage without being powered up before you are at risk of capacitor failure upon power up?
• What is Reforming a Capacitor?
• What is a recommended solution for reforming Bus Capacitors?
• If I am reforming a PowerFlex class drive what capacitor value should I be expecting?
• Where can I find information on the DC bus capacitance on the PowerFlex drives?

Environment

PowerFlex Drives

Answer

In the past, Rockwell Automation has received numerous information requests from customers pertaining to drives that have not had power applied to them for a long period of time. The major concern regarding the Drives would be the affect of time on the Aluminum Electrolytic Capacitors. Bus Capacitors that have not had power applied to them for a period of 2 years or longer should undergo a Bus Capacitor re-forming process.

What is Re-forming of DC Bus Capacitors

Re-forming is a technical term used with capacitors that have been un-powered for a long time. To understand the re-forming process, we must first understand the forming process. To create any capacitor, three items are needed:

1. An Anode (positive end)
2. A Cathode (negative end)
3. A dielectric inserted between the two foils.

An Aluminum Electrolytic capacitor follows this basic formula using aluminum foil both as the anode and the cathode, with an aluminum oxide formed on the foil as a dielectric and liquid electrolyte as a conductor.

Capacitance is directly related to the surface area of the anode foil; the more surface area, the greater the capacitance. To increase the surface area of the anode foil, manufacturers dip the foil into an acid bath. This bath etches grooves into the foil, thus increasing the surface area.

• After this process, a non-conductive, insulating aluminum oxide, Al2O3, is grown on the surface of the aluminum anode and cathode foil, including the etched grooves.
• A thin paper layer is added between the foils to prevent them from rubbing against each other and to hold/absorb the conductive electrolyte that is used to reduce the distance between the anode and cathode.
• The aluminum oxide formed on the foil is the insulator between the positive and negative plates, but the aluminum oxide formed on the foil is a brittle material.
• When this oxide cracks, a leakage current will flow between the anode and cathode. This does not cause the capacitor to fail. In fact, the leakage current causes localized heating, which creates more oxide, thus sealing off the leak.
• After a prolonged period of time, approximately 2 years or greater, with a lack of voltage applied to an aluminum electrolytic capacitor, the oxide will start to breakdown and the leakage current will be very high and the drive will fail eventually.

DANGER: If voltage is applied to the capacitors at this point, the high leakage current (ripple current) could cause enough heating to vaporize the electrolyte in the capacitor causing the part to vent or potentially explode.

To prevent this from happening, the capacitors need to be re-formed. Re-forming is the process of re-growing the oxide in the grooves of the aluminum foil:

• The simple way to re-form an aluminum electrolytic capacitor is to apply a controlled DC voltage.
• The DC voltage level should be stepped for the defined duration at each increment.  See the reforming guidelines below.
• The DC voltage will create the proper amount of leakage current that will form new oxide.

Bus Capacitor Reforming Guidelines:

• For the reforming guidelines, reference Preventive Maintenance Checklist of Industrial Control And Drive System Equipment Service Bulletin, publication DRIVES-TD001.  The Bus Capacitors section covers the guidelines with regard to storage duration and DC voltage ramp-up values.
• Capacitor reformer Bonitron M3628PCF Portable Capacitor Former Manual, Bonitron 3628 Portable Capacitor Former video.
• Automation Capacitor reformer Bonitron M3628ACF Automatic Capacitor Former Manual, Bonitron M3628ACF Automatic Capacitor Former video.
• To verify the Bus Capacitance of the PowerFlex Drives see: The Drives in Common Bus Configurations Application Guidelines, publication DRIVES-AT002.  For PowerFlex 755T drive Bus Capacitance reference Drives in Common Bus Configurations with PowerFlex 755TM Bus Application Technique, publication DRIVES-AT005.

Rule of thumb for the capacitor value for PowerFlex 4 or 5 class products are – 50uf -60uf (-/+ 10%) per Amp for a 460VAC drive.

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Source: Rockwell Automation’s Technote database.