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dian
11-06-2013, 12:17 PM
i have this 3-phase vfd (about 5 kw) that i dont know how long it has been sitting without power. so i decided to power it up slowly, in order to not damage the caps. how large could the caps be?

as i only have a sett of electronic (1/4 watt) resistors, i am running a 100 k ohm resistors between each phase. i am measuring 2.2 mA on the phases. weird, because 380v/ 100 000 ohm = 3.8 mA. further, 2.2 mA x 380V = 0.84 watt. but the 1/4 watt resitors dont get warm.

am i doing something wrong? does the vfd have to be switched on to load the caps?

anyways, so at 2 mA, how long will it take the caps to load?

lakeside53
11-06-2013, 12:32 PM
Your caps also have an ESR so your simple math will not not work, and... there's other circuitry other than just the caps on the buss. 100k is way too small anyhow - more like 470 ohm... (high power in case of failure so use a heater or whatever).

My ABB vfds about the same size have a couple of procedures detailed here :
http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/cf2c9be9fc666ebbc1257729002bd7be/\$File/Guide_for_capacitor_reforming_Rev_G.pdf