Re: capacitor

Les, Jon,

I don't mean to belabor the point but once I have my teeth into
something I have to take to its conclusion. That means I have to make
sure I understand the underlying principle or have others show me
where I am mistaken.

I am making two assumptions:

(1) "Energy return" during deceleration is not a given.
(2) There is no difference between acceleration and deceleration.

Both points are tied together in this thought experiment:

Say you are in a car that is rolling backwards at 20 MPH and the
transmission is in "drive". You apply torque (press the gas) to the
drive wheels at a constant rate. The car decelerates, stops, then
accelerates until you are going 20 MPH in the forward direction. Oh
yeah, and your eyes are closed as well (don't try this at home folks,
this is a trained professional driver).

The questions are:

(1) If "energy return" is a given, why are you pressing the gas pedal
during deceleration?

(2) Can you tell when deceleration ends and acceleration begins?
Remember, your eyes are closed (no peeking).

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Les Watts" <leswatts@r...> wrote:
> Jon, Mariss,
>
> I think this is a good discussion.
>
> It's very on topic because many of us build large power supplies,
often with
> surplus components, and
> face the issue of sizing components like filter caps.
>
> No one is wrong here.
>
> It is an established fact that a typical 4 quadrant PWM
transconductance dc
> servo amp can and will pump
> energy into power supply busses and can raise that buss voltage
well above
> nominal- enough to
> engage overvoltage limits or damage components.
>
> That does not mean Mariss' amps will.
>
> A quick Yahoo search yields volumes of analysis and data on that
very
> subject.
>
> The rub is that some servo amp manufacturers and sales reps often
want to
> sell us very expensive "shunt regenerative energy absorbers" to put
across
> our power supplies to prevent the problem. Some of us have probably
shelled
> out the cash for one.
>
> Whether you need to consider one of these gizmos or not can be
precisely
> calculated. There is no uncertainty. It does involve lots of fancy
math like
> laplace transforms and such
> that people like me love but might not be useful to others.
> They just want a system that works.
>
> Now, Mariss' experiment with his drives is very enlightening
> about the robustness of them. If I tried that with my drives
> they would prob break in to oscillation and be toast! Under
> those conditions, with that motor, wire size, inertia,and friction
it is
> just not a problem with his drives. It was a fairly severe test. I
am
> impressed. The amp topology must be very good.
>
> For others I will stick to the advice of using energy calculations
> to size capacitors. Many, many others use this method. Although I
derived a
> linear "equivalent mass" expression,
> it is usually put in terms of equivalent moment of inertia:
>
> C= Jw^2/(v^2-vnom^2) where J is system moment of inertia,
> w is the maximum angular velocity in radians/sec,
> and the term in parentheses is a small voltage that the supply
> is derated by. I just pulled this bit out of a control book and
> can explain further off line if anyone needs to know.
>
> I contend that this simple, conservative approach will:
>
> -Work with my amps
> -work with your amps
> -work when someone fails to recognize the tolerance on large
> capacitors
> -work with low friction systems that can backdrive
> -work with high friction systems that can't
> - probably work even with old surplus caps that are way off
> spec (that I suggested not using)
> -work without using a regenerative energy absorber
>
> It might cost a little more because it is conservative- a few
> dollars perhaps.
>
> 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: "Jon Elson" <elson@p...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Thursday, March 21, 2002 1:31 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Re: capacitor
>
>
> .
> >
> > First, you only have armature inertia there. It is a completely
different
> > story when you have 300+ Lbs of machine table moving. I never
> > exercised my servo amps much on the bench. Just a few quick
checks
> > and I put it on the machine. (I'd been using a modified stereo
amp
> > previously, which didn't work well at all, but it moved the
table.)
> >
> > I can tell you that energy does flow back to the power supply on
my
> > system. I won't put the diode in, because I KNOW it will blow up
> > my amps. I'll see if I can get a reading on it sometime soon.
> >
> > Jon