Variable Frequency Drives (VFDs) that sit unused for 12 months or longer can experience deterioration of the dielectric layer inside their DC bus capacitors. Applying full power suddenly can cause excessive leakage current, overheating, or capacitor failure.
Capacitor reforming gradually restores the dielectric layer before the VFD is returned to operation.
Preparation
Inspection Before Capacitor Reforming
- Disconnect the VFD from all motor loads.
- Visually inspect for bulging capacitors or leaking electrolyte.
Reforming Procedure
DC Bus Capacitor Reforming Using a Variac
Setup
Connect a variable AC transformer (0–130VAC or 0–240VAC depending on drive voltage) to the Input L1/L2 (single-phase) or L1/L2/L3 (three-phase) terminals.
Stage 1: Low Voltage
Apply 30–40VAC (for 230V drives) or 60–80VAC (for 480V drives) for 30–60 minutes.
Stages 2–4: Gradual Voltage Increase
Increase the voltage in steps - 25%, 50%, 75%, 100% of nominal input voltage, holding each step for 30–60 minutes.
Final Stage: Full Rated Voltage
Apply full rated AC voltage for 1 hour.
Verification
Monitor the DC bus voltage (labeled DC+ and DC- or + and -) with a voltmeter; it should rise slowly and stabilize without excessive heat or noise from the capacitors.
Alternative Procedure
Reforming Using a Regulated DC Power Supply
Sometimes it is not possible to slowly increase DC bus voltage with a variable AC transformer, in this case capacitors are reformed with direct connection of regulated DC power supply to the DC bus, and voltage is increased in steps - 25%, 50%, 75%, 100% of nominal DC bus voltage, holding each step for 30–60 minutes.
Final Stage
Apply full rated AC voltage for 1 hour.
Brands We Stock & Refurbish
VFD Manufacturers
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