Re: What size capacitor should I use?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@v...>
wrote:

> Dave:
>
> The OP asked if he needed to do this "per drive, so that is where

the

> multiple power supplies came from.
>
> Several of us have commented that multiple caps will work just

fine; one

> person did say multiple caps in parallel would be better; I am not

sure why

> that would be, but I am not prepared to dismiss it out of hand

without

> hearing the reasons why it might be better.
>
> Andy Wander
> Verrex Corporation
>
>

There are two reasons to parallel caps (well actually three). Cost.
In our world of scrounged parts it's ususally easy to find caps at
the voltage required at smaller values. When you start looking for a
35,000 to 80,000 mfd cap with a 100 volt rating they get more scarce
($$$$). Next is the physical size. Smaller value caps are typically
shorter and have less cross section per MFD so three 15,000 100VDC
caps will probably end up being somewhat smaller in total. The last
thing is ESR (Effective Series Resistance). All caps have
capacitance (duh), resistance and inductance. The larger the cap the
more ESR it has. Take 3 smaller caps in parallel each have smaller
ESR each and in parallel they have 1/3 the ESR of one. ESR in linear
DC power supplies is of little concern since it would limit the
ability to filter higher frequency waveforms but are used extensively
in switchers as filters. Besides, I wanted to give everyone
something else to stew about (:-)

Oversizing the power supply for the load offers no tangable benefits
other than making the builder feel better. I think even the 66%
number is conservative. I find in the actual world the total worst
case current draw (all motors under normal load) is closer to 50% of
the nameplate values on a chopper system.

Watts is watts. The average current is a factor of the instantaneous
current times the duty cycle.