spring/shock absorber has "reactance"?

[Farina]

"Airy R.Bean" <Me@privacy.net> wrote in message
news:35civgF4kfve7U2@individual.net...

Yes- you're partly right, and I was misled by
the OP, who perhaps was misguided into referring
to shock absorbers rather than to masses.

However, if you take a shock absorber that is not
mounted in a vehicle and compress it, you'll find that
it will spring back, much as if you compress a bicycle
pump with your finger closing the hole at the sharp end.

It won't spring back as far, nor as readily, however.

Let us not get side-tracked into irrelevancies; what the
OP asked was for an explanation of reactance, and I think
he got a good one from me, EOE, which you have reproduced
below.

"Vilnius Roma" <nospam@nospam.com> wrote in message
news:35cic7F4kjdbdU1@individual.net...
"Airy R.Bean" <Me@privacy.net> wrote in message
news:35c20gF4kjnumU1@individual.net...
Reactance is characterised by the storage of energy.
In the case of the capacitor, you might think that your
AC source is the only voltage source in your circuit, but
after the first 1/4 cycle, the capacitor acts as a voltage source
and starts to give back the energy that it has stored.
The combined result of the two voltage sources, your
AC excitation and the capacitor itself, accounts for
the out-of-phase current waveform.
(This bothered me for years! How could the current
be non-zero if the AC driving voltage was zero?!)
The same analogy applies to springs and to shock absorbers;
the spring stores energy when stretched; the shock-absorber
stores energy when compressed.
Wrong. shock absorber does not store energy, it dissapates it.

Many math books have the analagy as a showing of application to springs and
masses, and to simple electronic circuits.
The shock absorber is modeled as a damper and very small spring which is
added to the larger spring since they are in parallel.
The larger spring dominates completely and the small spring of the shock
absorber can be ignored.

 

[Rowbotth]

In article <BE15A128.3046B%salmonegg@sbcglobal.net>,
Repeating Rifle <salmonegg@sbcglobal.net> wrote:

in article 1106265161.475444.323700@z14g2000cwz.googlegroups.com,
alanh_27@yahoo.com at alanh_27@yahoo.com wrote on 1/20/05 3:52 PM:

They say that the mechanical analogues of capacitors are springs, and
of inductors are shock absorbers. And this does have a strong
intuitive appeal.

But do springs/shock absorbers have any kind of frequency-dependent
behaviors?


These analogies are meaningful because of identical mathematical
description. As part of the formalism, charge is equivalent to position and
rate of charge change is current and analogous to rate of change of
position.

Capacitors store potential electrical energy C*V^2/2 as springs store
potential mechanical energy k*x^2/2. Inductors store kinetic electrical
energy L*I^2/2 as masses store kinetic mechanical energy m*v^2/2.

When you make a lagrangian formulation based upon these energies, the
equations for electrical and mechanical motions are identical and frequency
dependence is identical.

Bill

Bill

I agree with this. It is only an analogy. Just as process / chemical
engineers equate water pressure to voltage and water flow to current,
you can use a spring or shock absorber to describe the VAr.

As long as the analogy is only appropriate to grasp basic principle of
operation, you are OK. If they know enough to get into calculations and
want to start doing the math, then get them an electrical textbook and
get them to accept that a negative number can have a square root....

The spring is how I describe the VAr to our mechies and process types.
It seems to satisfy them, and allows them to understand enough about
what I'm talking about to accept what they see, electrically.

HR.

 

[daestrom]

"Beachcomber" <not_real@xxx.yyy> wrote in message
news:41f122e6.2272312@news.comcast.giganews.com...

On 20 Jan 2005 15:52:41 -0800, alanh_27@yahoo.com wrote:

They say that the mechanical analogues of capacitors are springs, and
of inductors are shock absorbers. And this does have a strong
intuitive appeal.

But do springs/shock absorbers have any kind of frequency-dependent
behaviors?


Yes... Think of a standard spring watch movement. (I believe the
formal term is "escapement"). There is a specific time period between
each direction reversal, just like a clock pendulum.

Exactly. The mass and shape of the wheel determine the moment of inertia of
the wheel. This can be analogous to inductance. The spring, analogous to
capacitance. The combination forms a 'tuned' circuit that oscillates at a
resonant frequency and requires only a little energy input to make up for
frictional losses. Much like how a tuned circuit draws little real power
from the supply to make up for resistive losses.

The escapement mechanically gets a 'nudge' from the pawl system that it
controls as it swings through the center point. Electronically, this is
similar to having a pulse of power applied as the current through the
inductor approaches maximum.

daestrom