Re: [CAD_CAM_EDM_DRO] Re: capacitor

Mariss,

Yes this energy approximation neglects motor and wire
resistance, magnetic losses,cap ESR, etc. It certainly does
not consider time, being an energy solution. It is pessimistic,
but it is very very safe. Please understand- I am an aerospace engineer! It
will result in a larger capacitor than is needed but not a totally
unreasonable value, particularly
considering that we may be talking about 20 year old
electrolytics from joe's electronic surplus shop.
(disclaimer: I do not use old or used electrolytics in any
project and neither will anyone else that has had a 10,000 uf
250v one blow up in their face as I have ;^) )

Being so conservative I also go for 2.5x working voltage
safety factor on my simple unregulated supplies.
I just had the power company float my neutral and you can guess how many
fuses I blew on 120v stuff. I still have a scope to fix. No caps popped
though!

The supply overvoltage condition during regeneration that Jon and I mention
Is something discussed by my amplifier maker
and I follow their recommendations...except for buying
their expensive shunt regenerative energy dissipators.
I just derate supply voltage by 10 % for ac line conditions
and a few more volts for 1/2C(v+vr)^2 and I know I am
going to be good to go. Sound ok?

In my case I decelerate 1000+ kg effective mass at up to
.5g very worst case so it can be a whopping violent stop.
Certainly detonation category rather than burning dynamite
slowly in a fire. (disclaimer again: Don't do that!!!! ;^) )
Normally it is much much less.


In any event You are the servo amp maker and we are users.
I plan on using your amps in later projects, perhaps for clients.
God knows how you can sell them for such a low price.
I defer to your knowledge on the subject and would like to
hear more of your thoughts. I would like very much to see a detailed
analysis if it is not too esoteric for the group charter.
(guess it is)

We love math!!

Les


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: "mariss92705" <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, March 20, 2002 11:32 AM
Subject: [CAD_CAM_EDM_DRO] 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
> > >
> > >
>
>
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