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quality • Reviewed 2026-10-16

Why do surplus or stored VFD inverters explode on power-up, and how do you reform DC bus capacitors safely?

Prevent catastrophic drive explosions. Understand aluminum electrolytic capacitor dielectric degradation and the step-by-step reforming procedure for surplus VFDs.

Direct Engineering Summary

Variable Frequency Drives (VFDs) and servo packs stored without power for more than two years suffer from the chemical degradation of the aluminum oxide dielectric layer inside their large DC bus electrolytic capacitors. Applying full line AC voltage (e.g. 400V/480V) directly to un-reformed capacitors causes excessive leakage current, rapid internal gas heating, and catastrophic capacitor blowout or input rectifier failure. Maintenance engineers must reform capacitors by applying a progressively stepped DC or AC voltage over several hours before placing stored surplus drives into full-load operation.

The Chemistry of Electrolytic Capacitor Shelf Degradation

Aluminum electrolytic capacitors rely on a microscopic aluminum oxide (Al2O3) insulating layer formed on the anode foil. In the presence of electrolyte and operating voltage, this layer continuously self-heals.

When a drive sits idle in a warehouse or maintenance storage room for 24 to 36 months, the liquid electrolyte chemically dissolves the oxide layer. If full line voltage is suddenly switched on, the weakened insulating barrier cannot resist the electric field, leading to dielectric breakdown, intense thermal runaway, internal boiling of electrolyte, and explosive pressure venting.

Step-by-Step Capacitor Reforming SOP

  1. 1 Step 1: Check Drive Manufacturing Date — Examine the VFD nameplate serial number. If the drive has been stored unpowered for more than 2 years, reforming is mandatory before commissioning.
  2. 2 Step 2: Connect Variable AC Supply (Variac) — Disconnect the motor load and control wiring. Connect a single-phase or three-phase variable auto-transformer (Variac) to the drive input terminals (L1, L2, L3).
  3. 3 Step 3: Voltage Stepping Schedule — Step the input voltage up gradually: apply 25% of rated voltage for 30 minutes, increase to 50% for 30 minutes, 75% for 30 minutes, and finally 100% of rated voltage for 60 minutes.
  4. 4 Step 4: Measure DC Bus Leakage Current — Monitor the DC bus voltage across terminals (+/PA and -/PC). Verify that DC voltage stabilizes to 1.414 times the applied AC RMS voltage without abnormal current draw or capacitor bulging.
Storage Duration Without PowerRecommended Reforming MethodProcess Duration
Under 1 Year No special reforming needed; direct power-up permitted Immediate
1 to 2 Years Power up at rated line voltage with no motor load connected 30 Minutes idle on line
2 to 3 Years Step voltage using Variac: 50% for 30 min, 75% for 30 min, 100% for 60 min 2 Hours total stepping
Over 3 Years Controlled DC source with current-limiting resistor (1kΩ 100W) 4 to 6 Hours gradual reforming

Need Urgent Sourcing or Technical Verification?

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