Re: Capacitor sizing question

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@t...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> <davemucha@j...> wrote:
> >
> > Hi all,
> >
> > I'm a little on the fence here and hope someone can offer some
> > guidance.
> >
> >
> > With a 40 volt transformer and a bridge rectifier I should be
> getting
> > 38.8 volts before the cap. After the filter cap, the voltage

would

> > be 54.8 volts.
> >
> > For the motors, the calculation says 8,000uF so with a cap of
> 5,600uF
> > at 63 volts I can parallel two of them and get more than enough
> > capacatance, but am I cutting the voltage too close ?
> >
> > I'll be using Gecko's with a 470uF-100v on each Gecko.
> >
> > Typically the rule of thumb is 20% over for caps and this is only
> 10%
> > over.
> >
> > This is for a plasma table and hopefully high rapids as the norm
> for
> > cutting.
> >
> > Any recomendations for the caps ? am I being greedy for trying

to

> > use a $1.50 capacitor (on sale) or should I get a much larger
> voltage
> > capacitor ?
> >
> > Dave
>
> There are two things you need to worry about Dave. The voltage

from

> your secondary is of course porportional to the input and you need

to

> design using "worst case" numbers, i.e. high line conditions.

While

> the nominal AC voltage is ususally 120VAC it can be as low as

110VAC

> and as hign as 125VAC (with possible surges to 130VAC). Caps have

a

> breakdown voltage that is a function of the dialectric and spacing
> across a temperature range and like all electronics the manufacture
> will build in some safety margin but experience over time has

taught

> us that running any electronic device (passive or active) right at
> the edge of its ratings will shorten its live span.
>
> The other issue is the dreaded back EMF from the motors. If you

were

> to incorporate an active voltage clamp on the motor DC then you

might

> be able to use those caps.
>
> I don't know if you will be using steppers or servos but its nice

to

> have the option to raise the DC voltage if you find your upper end
> performance needs a little boost. If your caps are the 100V

species

> then you can easily raise it to the driver limit without a lot of
> retrofit.
>
> Given a choice I would sacrifice capacitance for breakdown voltage

in

> a motor application.

Thanks,

at 40 volts, the max is about 55 volts, at 130 it is almost the
rating of the cap.

Since these will be on a plasma machine, the decel will be from high
speeds so back EMF will be a major considderation.

Also the transformer is not as high a voltage as desired, but at
$10.00 (40V/5A) they are pretty cheap and sized just right.

so, 100v cap is the goal.

That leaves the room for upping the voltage if opportunity avails
itself.

Thanks again.

Dave