Re: [CAD_CAM_EDM_DRO] Re: capacitor

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@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
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