Capacitor Reforming and Storage Protecting Stored VFDs from Startup Failure
Stored VFDs, spare drives, and DC bus capacitor banks can change over time, even when they are not running. When electrolytic capacitors sit unpowered for long periods, applying full voltage immediately can increase the risk of leakage current, overheating, and capacitor failure. Capacitor reforming helps prepare stored equipment for startup by gradually restoring capacitor condition before the drive is returned to service.
In This Article
Use this guide to understand why stored VFDs may need capacitor reforming, what happens to electrolytic capacitors over time, and how maintenance teams can reduce startup risk for spare drives and capacitor banks.
Bonitron Products
Bonitron capacitor forming equipment helps maintenance teams charge, discharge, form, measure, and test electrolytic capacitor banks.
What is Capacitor Reforming?
Capacitor reforming is a maintenance process used to gradually restore the dielectric layer inside electrolytic capacitors before full voltage is applied. In VFDs and other DC bus equipment, this is especially important when the equipment has been stored without power for an extended period.
A VFD relies on DC bus capacitors to store and smooth energy after incoming AC power is converted to DC. If those capacitors have been sitting unused, applying full voltage immediately can place unnecessary stress on the capacitor bank and increase the risk of failure.
Why Long-Term VFD Storage Matters
Many facilities keep spare VFDs in storage so they can respond quickly when production equipment fails. The spare drive may sit on a shelf, in a warehouse, or inside a maintenance room for months or years before it is needed.
The problem is that stored equipment can age even when it is not running. Electrolytic capacitors inside the drive can lose dielectric strength over time. When the drive is finally needed, applying full input power immediately may create a higher risk of capacitor stress, nuisance faults, or failure during startup.
This can turn a planned backup strategy into an unexpected downtime event. A spare drive that has not been maintained may not be ready when production needs it most.
What Happens to Electrolytic Capacitors in Storage?
Electrolytic capacitors depend on an internal dielectric layer that can degrade when the capacitor remains unpowered for long periods. Storage conditions, temperature, humidity, capacitor age, and time out of service can all affect capacitor condition.
When rated voltage is applied too quickly after long-term storage, the capacitor may experience excessive leakage current. That leakage current can create heat, stress the capacitor bank, and increase the risk of failure.
Capacitor reforming helps reduce this risk by applying voltage in a controlled manner, allowing the dielectric layer to recover before the drive or capacitor bank is placed back into normal service.
Common Risk Scenarios
Capacitor reforming is especially important when the storage history is unknown, the drive has been unused for an extended period, or the spare drive is needed for a critical production application.
Stored Spare Drives
Spare drives may sit unused for long periods before being installed during a production emergency.
Unknown Storage History
Drives pulled from inventory may have unclear maintenance records or unknown time since last energization.
Critical Replacement Drives
A spare drive that fails during startup can extend downtime when production already depends on a fast replacement.
Large Capacitor Banks
Larger drives and DC bus systems can represent a higher replacement cost and greater downtime risk.
Warehouse Inventory
Drives stored in uncontrolled environments may be affected by temperature, humidity, and long periods without power.
Aging Equipment
Older capacitor banks may need condition checks before being returned to service or relied on as backup equipment.
Bonitron M3628ACF Automatic Electrolytic Capacitor Former
The Bonitron M3628ACF is designed for maintenance teams that need a controlled way to reform and test capacitor banks. It can be used to charge, discharge, form, measure, and test electrolytic capacitors, helping verify stored drives and capacitor banks before they are returned to service.
This makes it useful for facilities managing spare drives, stored equipment, capacitor banks, and maintenance programs where startup reliability matters.
View the M3628ACF Product Page
Forms Capacitors
Supports controlled capacitor reforming instead of rapidly applying full rated voltage to stored capacitors.
Charges and Discharges
Provides controlled charging and automatic discharge functions for capacitor bank maintenance.
Measures Capacitance
Calculates estimated capacitance after discharge, helping maintenance teams evaluate capacitor-bank condition.
0–1000 VDC Output
Provides variable DC output voltage for a wide range of capacitor bank maintenance applications.
Digital Displays
Includes digital voltage and current displays for monitoring the forming and testing process.
Maintenance Records
USB record storage helps teams document and access capacitor maintenance history over time.
How Capacitor Reforming Works
Capacitor reforming is a controlled charging process. Instead of applying full voltage all at once, the capacitor bank is brought up gradually. This gives the capacitor dielectric time to recover and helps reduce the risk of excessive leakage current, overheating, and failure.
The M3628ACF manual describes a Profile Charge operation that increases voltage incrementally, pauses at each voltage step for a selected hold time, then holds at the final voltage before automatically discharging the capacitor bank and calculating estimated capacitance.
This matters because capacitor maintenance is not only about getting the drive powered again. It is also about understanding capacitor-bank condition. Measuring capacitance over time can help maintenance teams identify aging trends, estimate remaining useful life, and avoid replacing an entire capacitor bank too early.
Stored Drive and Capacitor Reforming Decision Guide
The right maintenance action depends on storage duration, storage conditions, drive criticality, available records, and whether the capacitor bank can be accessed and tested safely.
| Situation | Risk | What to Check | Recommended Action |
|---|---|---|---|
| Drive stored for six months to two years | Electrolytic capacitors may have changed during storage depending on ambient conditions. | Check storage environment, manufacturer guidance, maintenance records, and date of last energization. | Review whether capacitor reforming is required before applying full voltage. |
| Unknown storage history | The capacitor bank may not be ready for immediate full-voltage startup. | Check date codes, warehouse records, drive history, and whether the unit has been periodically powered. | Use a controlled forming process before returning the drive to service. |
| Critical spare drive needed immediately | A stored replacement drive may fail when production needs it most. | Check whether the spare has been maintained, tested, and documented. | Verify the capacitor bank before installation and maintain a record of the result. |
| Aging capacitor bank in service | Capacitance may decrease as the internal chemistry changes over time. | Measure and record capacitance periodically to track condition. | Use capacitance records to help estimate wear and avoid unnecessary replacement. |
| Drive shows DC bus or capacitor-related faults | The capacitor bank may be degraded, stressed, or outside expected condition. | Review fault history, capacitance readings, leakage behavior, and manufacturer recommendations. | Inspect and test the capacitor bank before returning the equipment to production. |
| Large capacitor bank or high-value drive inventory | Replacing capacitor banks too early can add unnecessary cost. | Track capacitance and maintenance records over time. | Use measured data to support planned maintenance decisions. |
Frequently Asked Questions
Bonitron notes that electrolytic capacitors can undergo physical changes during long-term storage, depending on ambient conditions, over a period from six months to two years. Always check the drive manufacturer's storage and reforming guidance before applying full voltage.
If electrolytic capacitors have changed during storage, rapidly applying rated voltage can create excessive leakage current. That leakage current can cause capacitors to overheat and fail.
The M3628ACF is an automatic electrolytic capacitor former. It can charge, discharge, form, measure, and test electrolytic capacitors, and it can store maintenance records for tracking capacitor condition over time.
Yes. Measuring capacitance periodically can help maintenance teams evaluate capacitor wear and estimate remaining useful life, which may help avoid replacing an entire capacitor bank before it is necessary.
Need Help Maintaining Stored Drives or Capacitor Banks?
Bonitron can help review your stored drive maintenance process, capacitor reforming requirements, and spare equipment readiness strategy.
Contact Bonitron