Re: capacitor

Leslie,

That is a good "worst-case" approximation but it is overly
pessimistic. It neglects to take into account the rate of energy
return.

A burning stick of dynamite releases the same amount of energy as one
set off by a blasting cap; the difference is the span of time that
energy is delivered.

It's the same here. Friction and electrical losses in the motor and
drive absorb the returned energy unless the rate of return is so
large (very rapid deceleration of a large mass from a high speed)
that it results in a net negative power supply current.

This phenomena is of more concern with step motor because their "back
EMF" can be many times the supply voltage. The DC servodrive does not
exhibit this effect at all. Deceleration at any rate or load
generates no returned power, ever.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Les Watts" <leswatts@r...> wrote:
> I just do a quick rough calculation where the total energy of
> a regenerative brake at max velocity (1/2mv^2) plus
> the energy stored in the motor field (1/2Li^2) equals the
additional energy
> in the supply cap (1/2C(v+vr)^2 to find
> the voltage rise in that cap. I needed about 20,000 uf
> to keep the rise a few volts maximum without using a
> shunt regulator.
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.rabun.net/~leswatts/wattsfurniturewp.html
> engineering page:
> http://www.rabun.net/~leswatts/shop.html
>
> ----- Original Message -----
> From: "Jon Elson" <elson@p...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Wednesday, March 20, 2002 12:38 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Re: capacitor
>
>
> > mariss92705 wrote:
> >
> > > Jon,
> > >
> > > Good points except for the cap size which is a bit of an over-
kill. I
> > > suggest using the following for sizing caps:
> > >
> > > For 60Hz, C = (80,000 times I) / V
> > > For 50Hz, C = (100,000 times I) / V
> > >
> > > Where V is the power supply voltage, I is the maximum power
supply
> > > current and the answer, C, is in microfarads.
> > >
> > > This scales the ripple voltage to about 10% of the output
voltage and
> > > keeps the power factor reasonable.
> > >
> > > For 10A @ 80VDC, this yeilds 10,000 uF at 60Hz.
> >
> > Well, maybe I'm thinking of Servo power supplies, but I use a
MUCH bigger
> > cap than that in my power supply. But, then, a servo can really
draw
> > substantial current surges. Also, unless you have a braking
resistor
> > and control circuitry, you need enough energy storage to prevent
the
> > DC bus from surging during motor deceleration.
> >
> > Jon
> >
> >