Ripple Current Rating In Bus Capacitor

[n s k]

Hi,

 

I am designing a Variable speed drive for 7.5KW induction motor. As per the design specification the max input ac voltage is 480VAC and min input voltage is 370VAC, so the Max and min dc bus voltages are 678.82VDC and 523VDC respectively. The allowable ripple is 15%, therefore the Max ripple voltage is 0.15*678.82 = 101.8VDC. I chose the min dc bus capacitance value by a simple formula C = Idc /(6*supply frequency*(Max allowable ripple voltage)) = 554uF (where Idc = motor rating/ Min. bus voltage, Idc = 7500/523 = 14.34A). Now my problem is how to choose the ripple current rating for the DC bus capacitor? can anyone give me a method or formula to find the appropriate ripple current rating for dc bus capacitor.

 

With regards

NSK

[Carl]

Hello

My understanding of the ripple current is that it is a specification of the capacitor, supplied by the manufacturer, usually measured as an RMS value. There is also a direct correlation to the working temperature and ripple rating. This information should be found with the capacitor specs. A word of caution to much AC in the cap will cause it to explode the more expensive caps have a weak point designed in them specifically designed for this. If there is no weak point lets just say it goes off with some energy behind it !!!

 

[n s k]

Hello carl,

 

Thanks for ur reply.

 

Yes, even i noticed ur point in capacitor physical size, with same rating capacitor with different size has different ripple current rating.( i.e.. Bigger the size larger the ripple current rating, smaller the size lesser the ripple current rating).

 

once again thanks

 

with regards

 

Nsk

 

[Carl]

A good article to read is AN - 1044 by international rectifiers. It gives the calculations for teh capacitor