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Watts That You Say?



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>Storey Clamp wrote:
>	True Watt Meters are available. During one of my junk yard crawls, I
>ran across some made by the Ideal Electric Company of Mansfield, Ohio. I
>had never seen one before, and thought it was so interesting that I
>decided to describe for the list.
>	 It is operated by oval shaped concentric air wound coils. The current
>coil is the stationary coil, and is wound with perhaps 20 turns of heavy
>wire. The moving coil is the voltage coil, and is wound with many turns
>of very fine wire.

	Yep.

	Thats 'watt' i meant by a 'four wire real wattmeter'.  For well behaved
	sine waves, it can be very accurate.  For modestly distorted waveforms
	its still pretty good, as the coils are air core, so the frequency
	responce holds up.  Spark Gap type Tesla coils can draw some seriously
	intermittent power, but i'd GUESS these would stay withing, say, 10%
	of real power, even so. as the meter does an 'instantaneous' multiply
	of the inphase volts & amps.

============================
>...watts, measured by the voltmeter & ammeter on the control panel....
	Uhhhh.  That allows calculation of VA, not Watts.  there are two things
	at least separating VA from Watts, here:
		1) The meters, especially the ammeter, are seeing distorted
		waveform & how that meter responds is unknown.  [see below].
		2) The load is of 'ill defined'  8)>> power factor so the
		phase shift shift between Volts & Amps is unknown.

	(Its tempting to think of a tesla coil as 'inductive' from all the
	windings.  What happens, in a transformer, is that the load side
	impedance gets reflected to the primary and, under load, the impedance
	seen at the primary IS SET BEY THE LOAD, not the fact that there happen
	to be coils in the way.  To the extent that the 'tank' and 'tesla
	secondary are resonant, and resonant with each other, they Should In
	Theory be resistive.  [resonance is defined as being where XL and XC
	balance, leaving ONLY 'some" R....].  How close this occurs in practice
	i leave for someone with more experience to say.

=====================================
>...ac meters measure RMS.....
	Wellllllllll.

	Its a WHOLE lot more complicated.  [I've got a two volume set on
	'just meters' on my bookshelf.  Its 3" thick, total.}  There are
	LOTS of different kinds of AC meters.  What each measures and, in turn,
	indicates, is a longish story.  (like 3" long, 5 by 7 pages....)

	COMMONLY:
	'rectifier type meter'.  Usually shows up as an AC voltmeter.  A DC
	D'arsonval movement, with rectifier.  ACTUALLY MEASURES _average_,
	but the manufacturer ASSUMES it will be used on a sine wave and
	calibrates it to _read_ RMS.  Gets real unpredictable, real quick if
	not fed a nice sine wave, within its rated frequency....

	'Hot Wire':
	Uncommon, tho (actually) show up in some automobile instruments,
	might be some scrounging there.  Read RMS, as i recall.  More or
	less waveform immune.

	Moving Iron/Iron Vane:
	Iron vane, or core, 'repelled' by field from a fixed coil.  Commonly
	shows up as ac ammeter/milliameter, but can show up as a voltmeter,
	if used with 'multiplier' resistors.  Decent RMS response (as i recall),
	typically up to 1kc or higher, depending on design.  Usually
	recognizable by the nonlinear scaling ont he face, as the response
	is nonlinear.  Gives 'expanded scale' effect.

	(And on to thermocuple meters, etc.  This is just 'off the top'.
	One hint on thermocuople meters, many a time a meter so marked is a
	straight DC D'Arsonval FOR USE WITH an external TC.)

	regards
	dwp
	(I can ramble on, or research, within limits, meterology, but do not
	want to become boring....)






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