Typical Capacitor Reforming and Maintenance Recommendations Using an AC Input for Drives which have been Stored for Extended Periods

Submitted by glx25v on Tue, 12/01/2020 - 14:16

General Information

A standard variable frequency drive (VFD) takes in AC voltage, converts (or rectifies) it to DC voltage, and then inverts it back to AC to output to the motor. VFDs use capacitors (electrolytic) in the DC circuitry to smooth and store voltage. When drives are stored for long periods of time, they should be subject to a voltage treatment and capacitor reforming process which will reform the dielectric and return the leakage current to the initial level.

When stored above room temperatures for an extended period of time, the anode foil may react with the electrolyte increasing the leakage current. After storage, the application of even normal voltages to these capacitors may result in higher than normal leakage currents. In most cases the leakage current levels will decrease in a short period of time as the normal chemical reaction within the capacitor occurs. However, in extreme cases, the amount of gas generated may cause the safety vent to open.


Typical Drive Storage Maintenance Recommendations Using ISE VARIAC to reform Capacitors

Check with your drive manufacturer for their recommended procedure.  Below is typical instructions suitable for most drives.

You should disconnect the output of the VFD during the reforming process since the VARIAC is not meant to power the motor(s) during this process.

It is typically recommended to power up drives every 6 months for 30-60 minutes. Over longer periods of time without power, the drives electrolytic DC Bus capacitors require reformation especially if stored in an area of high temperatures.

DC Bus Capacitor Reforming Using Adjustable AC Voltage:

  1. Connect the inverter input L1, L2, and L3 to a 3 phase variable transformer (ISE VARIAC). Ensure the variable transformer has the proper voltage (230v, 460v or 600v) for the design drive. Make sure the adjustable output voltage setting is turned down so when power is applied to the Variac, the output will be at or near 0 VAC. Monitor the current with a Clamp-on Ammeter to prevent damage to the drive or the VARIAC.
  2. Apply power to the Variac, listening for abnormal sounds and watching for abnormal visual indications in the drive. If the Variac has a current indication, make sure the current is near zero or a steady output voltage applied.
  3. Slowly turn the Variac up, increasing the Variac's output voltage to 100 volts. Let the output voltage remain at 100 volts for 5 - 10 minutes while keeping close watch for abnormal signs within the inverter. While increasing the variac's output, the current will momentarily increase as current is necessary to charge the capacitors.
  4. Increase the Variac output in 100 volt increments, waiting 5 - 10 minutes in between increments, until rated voltage is achieved. Rated voltage is usually printed on the capacitor label. Once rated voltage is reached, leave rated voltage applied for 15 minutes.
  5. Repeat this process if any abnormal indications occur. Otherwise, the capacitor reformation is complete. Disconnect the inverter from the Variac and proceed to apply rated input voltage as normal.
  6. Contact your drive’s manufacturer for additional recommendations and if reforming is not successful.


VARIAC® is the registered trademark of ISE, Inc.   ISE VARIAC® variable transformers are used worldwide.  Go to VARIAC.com for product information.