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Dc Bus Capacitor Issues


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It's common knowledge nowadays that the Electrolytic Capacitors in the DC Bus or Link of VSDs can undergo chemical changes to the dielectric if left in storage for too long. Unfortunately we find this out the hard way as the drive goes bang when we turn it on.

The recommended procedure for dealing with a drive that's been out of service for a long time (2 years or more) is to soft power the drive with a Variac to "reform" the capacitor chemistry. Sometimes this is done by running the Variac through a 3 phase rectifier directly into the DC Bus to get around drive's SCR front end.

My question is: How effective is this procedure and does it totally "repair" the capacitors chemistry ?



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Hello Dinge


Yes, electrolytic capacitors will often need to be reformed if they have not been charged up for a period of years.


The easiest way to reform the capacitors is to charge them via a large resistor to limit the current.

If you have a bridge rectifier that supplies DC of the right voltage through a resistor, you will find that the voltage will climb slowly to the full voltage. The unformed capacitors will limit the maximum voltage when fed by a resistor until they are formed.


Best regards,


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