Hi there,

Can someone tell me why I need to reform bus capacitors at AC drives with DC link? If I forget (which I was already done) what can I expect?

Regards, Jeff

-- Jeff (jeff_ghrt@yahoo.com), January 16, 2005

Answers

Unused electrolytic capacitors lose some of their internal voltage blocking qualities when sitting idle on the shelf. The result is excessive leakage current when re-exposed to voltages that are well within their intended design limits. Leakage current X circuit voltage = Watts of dissipation, or Heat. What happens next is how bad it is. In some cases the result is a big 'Boom' within seconds or minuites of power re-application. In others, the problem will cure itself without any obvious effects.

If the unit has been in service, the capacitors should never need re- forming. If it has been sitting on the 'spares' shelf for over a year the caps may need reforming. [Follow the recommendations of your manufacturer] A resistor is placed in series with each capacitor to limit current while the voltage is gradulally increased over a period of hours or days. This allows the internal chemistry of the capacitor insulation to re-form without causing excessive heat build-up.

However, capacitors do deteriorate with time, temperature and 'normal' current flow during operation of the equipment. As a result, the DC bus caps on an AC drive may need replacing after several years of service. Again, follow the recommendations of your drive manufacturer regarding expected lifetime of the capacitors. Eventually, they will fail by loss of capacitance, venting, or shorting....sometimes in a spectacular way. DonV

-- Don Vollrath (dvollrath@magnetek.com), January 20, 2005.

What does "reforming" mean, and what does it entail?

-- Lutfi Al-Sharif (al_sharifvtc@compuserve.com), January 24, 2005.

For more information about reforming capacitors see: http://home.insightbb.com/~stephenwmoore/Electronics/Reform.htm DonV

-- Don Vollrath (dvollrath@magnetek.com), January 25, 2005.