Published: 02/12/2003
| Question: | How to reform the DC bus capacitors on an Variable Speed AC drive that has been stored for more than 12 months. |
| Answer: | Shelf-life is considered to be dead time between last operating use and re-energizing of the Variable Speed Drive. When stored for long periods, drives should be kept in a heated area where temperatures do not exceed 131 degrees Fahrenheit, nor below 32 degrees.
When the storage period is much more than one year, it is recommended that start-up be performed by an experienced technician from an Authorized Service Providers.
The electrical characteristics of aluminum electrolytic capacitors are dependent on temperature; the higher the ambient temperature; the faster the deterioration of the electrical characteristics (i.e., leakage current increase, capacitance drop, etc.). If an aluminum electrolytic capacitor is exposed to high temperatures such as direct sunlight, heating elements, etc., the life of the capacitor may be adversely affected. When capacitors are stored under humid conditions for long periods of time, the humidity will cause the lead wires/terminals to oxidize, which impairs their solderability. The drive should be covered completely with durable plastic so all air openings are sealed off to protect it from dampness and humidity.
A voltage treatment reformation process to electrolytic capacitors may have to be applied after a capacitor has been stored for more than 1 or 2 years. If aluminum electrolytic capacitors are stored above room temperatures for long periods 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 in a short period of time. However in extreme cases, the amount of gas generated may cause the safety vent to open.
Capacitors that are stored for long periods of time should be subjected to a voltage treatment/reforming process (Note 1), which will reform the dielectric and return the leakage current to the initial level. Leakage current increase during storage will vary with the working voltage of the capacitors, normally in this order.
NOTE 1: Connect the inverter input L1, L2, and L3 to a 3 phase variac. Make sure the variac voltage setting is turned down so that when input power is applied to the variac the output of the variac will be at or near 0 Volts.
Three Phase Variac
 
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.
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.
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.
If any abnormal indications occur during this process, it is recommended that the process be repeated. Otherwise, this completes the capacitor reforming procedure. Disconnect the inverter from the variac and proceed to apply rated input voltage as normal.
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Product Information:
| Product Group: | Inverters/Drives |
| Product: | Any AC Drive |
| Applications: | |
| Category: | Maintenance |
Other Information:
| Link to related files: |  |
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