Re: What size capacitor should I use?

> If a series resistor limited the charge rate of the capacitor to

less

> than the average current draw of the system, then you'd have a

problem.

> Otherwise it might not be a bad idea.
>
> CS

I think you have a valid point about average current...But you are
forgetting in a servo system the current can go to 5 to 10 times its
average value...Which only aggravates the problem of a series
resistor...


Wally




--- In CAD_CAM_EDM_DRO@yahoogroups.com, codeSuidae <codesuidae@c...>
wrote:

> cnc_4_me wrote:
>
> >
> >
> >>Are you suggesting that you are able to discharge your entire
> >>
> >>
> >filter
> >
> >
> >>capacitor through your servo in 45 microseconds? If so, I think
> >>
> >>
> >your
> >
> >
> >>steppers are truely enormous.
> >>
> >>regards
> >>CS
> >>
> >>
> >
> >No, i am not...45uS is simply the smallest unit of time in a

average

> >PWM system...
> >
> >
> >
> True, but your analysis suggests that each PWM pulse requires a

full

> recharge of the filter cap.
>
> I don't think the PWM frequency will be of any consequence here
> (presuming the capacitor has low enough ESR to supply the necessary
> current) What's important is the average current draw, any how

much it

> discharges the capacitor before the next AC charge cycle (which is

never

> more than 0.01 seconds away). If your capacitor is too small the
> current and voltage ripple will be quite large. A very large
> capacitance will keep voltage ripple at a minimum, but you'll still

need

> to be sure the current ripple is within the specs for the

particular

> capacitor you are using.
>
> If a series resistor limited the charge rate of the capacitor to

less

> than the average current draw of the system, then you'd have a

problem.

> Otherwise it might not be a bad idea.
>
> CS