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varia: of water resistors & power measurements
Subject: varia: of water resistors & power measurements
Date: Tue, 27 May 97 10:01:24 EDT
From: pierson-at-ggone.ENET.dec-dot-com
To: tesla-at-pupman-dot-com
It is certainly possible to do 'water rheostats', in fact, historically,
they
have been used in a number of applications where high power dissipation
is
needed. (Notably, specialized RR applications, dissipating thousnads of
KW.)
[note to the chop line crew: lines above may be get chopped.]
_sealing_ such a device is a recipe for disaster, tending towards steam
explosions. Doping the water, to provide ions, was routine. Changes
in resistance were had by varying 'water' (actually, solution) level,
or by running the electrodes up and down. I suppose they could be,
designed, for shortish periods of operation, and portability, with
'remove before using' plugs and filled 'back at the lab' then
transported
plugged, to avoid leakage, then unsealed' for operation.
================================
>Subj: Input Power Measurement
>Hi All
>It is obvious to all that any given transformer can be run in excess of
>its rating. Thus, a 1kva transformer may be run with 2kva input and
>live. This does not mean that the TC is being run at 1kva since the
>transformer is rated at 1kva.
Yep. Running for _long_ at large amounts above rating shortens
transformer life.
>How can the actual input power to the TC be measured????????
With care. 8)>>
>We can know the input voltage, I think, since most of us run from the
>mains (except those that insert a variac and transform the mains voltage
>up to 130v or 140v). We can still measure the output voltage of the
>variac, I think, and therefore know the input voltage.
Sort of. Fairly closely, however if the load current is
nonsinusoidal (as seems likely) the line voltage is (to a
lesser degree) distorted as well.
>BUT....how can the input current be measured.
With care. 8)>>
We've discussed previously. There are a lot of ammeters. Each
has its characteristics. SOME are 'true RMS', some not.
>I seriously doubt that it is sinusoidal.
Agree, for a spark gap system. (may also be true for a tube
type
system.)
>There are ac ammeters, several different types which give an indication.
>Is there one type of ammeter, the reading of which could be multiplied
>by the voltage as obtained above,
Preferable a 'true RMS'. Notably hot wire, thermocouple,
i think moving iron is true RMS. (And, in any case, many
'true RMS' meters are only true over a portion of the
frequency range. Meters _marked_ in RMS may or may not
INDICATE RMS. Many (notably 'rectifier type' ONLY indicate
RMS for a Sine Wave...)
>which would give an input power
Volts times amps gives VA Which Is _not_ Power, unless the
phase angle (power factor) is known.
>which all of us could use for comparison purposes?
Rough comparison at best, since the power factor of each can be
different.
The problem is a nontrivial one.
>I have a commercial instantaneous reading wattmeter. Does it provide
>any usable information?
Probably, tho for 'non usual' type loads (eg: spark gap tesla
coil) there are at least two things to be aware of:
1) The stray fields may give it's electronics fits.
Obvious fits like 'no reading' are obvious. Many times
stray fields will have more subtle effects, leading to
wrong values displayed.
2) Depending on what assumptions the designers made as
to what waveforms were espected (hints on this should
be in the manual) the results may be distorted.
>For instance....if I compare the input power to two different systems
>which use different types of input transformers....will I indeed be
>able to say that one system is more efficient than the other?
Uhhhhhhh.
To me, efficiency is input over output. How does one measure
the output _power_ of a tesla coil? 8)>>
If a 'rough and ready' (and thats better than not trying, IMO)
measure of 'getting same spark/discharge from each, then
measure power to each' approach is ok, then, i would say yes.
regards
dwp