Re: [CAD_CAM_EDM_DRO] gecko 320 filter capacitor values

Well, like I say- the ESR is a commonly neglected/misunderstood factor.

In fact, probably the majority of consumer electronics failures (TV,VCR, etc) are due to caps whose ESR has grown too high. The ripple increases, the voltage spikes too high, and this may result in a burned out transistor too. The prob is, going for "overkill" low ESR with high reliability can be prohibitively expensive for consumer apps. So they make what they can and move on. That is, cap ESR rating/reliability is often the compromise that ultimately results in the product's death.

The mfg spec sheet should always state ESR/ripple rating. You'll probably find something an inch long with the diameter of a quarter might have a 1A-2A ripple rating.

Capacitors cannot be cooled with the same kind of effectiveness that transistors can. The problem is the heat is generated throughout the capacitor body and it is not extremely good at conducting the heat to the outside, even if the outside is an exposed metal can without a plastic covering. The outside could be fan-cooled with cold air to 75F while the very middle of the cap is 100F. However, the cap will still be better off than turning off the fan and letting the outside rise to 100F. Then the center might be 125F.

Danny


---- Denis Casserly <denisc1@...> wrote:

> HI Danny
> I found the capacitor rating method off Gecko's site. Thanks for the info on
> ESR, I never was aware of it before and the heat dissipation that takes place
> too. Luckily I made accomocation in the cabinet layout for plenty of forced
> ventilation.
> cheers,
> Denis
>
> On Wednesday 25 February 2009 21:28:57 Danny Miller wrote:
> > Well, I can't give you really authoritative recommendations- why don't
> > you contact Gecko??
> >
> > But what I wanted to say is don't just look at capacitance. Caps come
> > with widely varying ESR, and varying ripple ratings. In fact it's
> > typically the most difficult part. Very low ESR and high ripple rating
> > is very expensive, large, and these ratings are subject to age & heat
> > degradation.
> >
> > A cap with a 100milliohm ESR will drop from 72v to 68.2v under a 38A
> > surge- even if its capacitance was absurdly large. The ripple rating
> > comes from a number of factors, one of the main ones being heat- during
> > a 38A current, the 100milliohm cap develops 144.4W of heat. And for
> > every amp-second that flows out of the cap, another amp-second must
> > eventually flow in which will generate a second, identical amount of
> > heat based on ESR. Caps don't dissipate heat especially well, nor do
> > they tolerate high temperatures. But of course the driver is not
> > getting all 38A from the cap, most of it should come from the power
> > supply, but then this becomes a complicated answer based on wire
> > resistance/inductance vs cap capacitance and ESR.
> >
> > Those big "computer" caps are typically kings of ripple rating though.
> >
> > Danny
> >
> > dcassyc1 wrote:
> > > Hi all,
> > > I'm looking for advice on sizing power supply filter capacitors for Gecko
> > > 320 servo drives, used to power 2 separate servo applications.
> > > First application is for 1 servo motor rated with these specs:
> > > terminal voltage: 72 VDC continuous current: 8.5A peak current: 38A.
> > > How large should the filter capacitor be? Would a 100VDC rating be
> > > sufficient for this 72 volt supply? How many MFD?
> > > Second application is for 2 motors with these ratings:
> > > terminal voltage 60 VDC continuous current 3.5A peak current 20A. Would a
> > > 75 VDC capacitor be sufficient for this 60 VDC supply? How many MFD?
> > > Is there a formula that is available to calculate the mfd value?
> > > Is there a recommended maximum ripple voltage?
> > > cheers,
> > > Denis
>
>
>
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