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Re: Measuring coil performance (watt meters)



Tesla List wrote:
> 
> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
> 
> >
> 
> Actually, the power measurement may not be that hard.  One can get power
> transducers that would do all the work for you.  These are simple canned
> devices that you apply voltage and current (perhaps through a current
> transformer) into and they output a signal proportional to the power.  Such
> a device can be found in many places but there is a description of one at:
> 
> www.ayainstruments-dot-com/acdcwatt.htm
> 
> These do all that nasty real time non-sinusoidal RMS stuff for you.  Their
> accuracy is very good (0.5% or 0.2%).  I remember them costing $100 to $200
> dollars new.  Such a device should give an almost perfect input power
> measurement for relatively little money and hassle.

What about using a surplus utility watt hour meter? They also
automatically handle the non-sinusoidal waveforms (although they are
bandwidth limited, so you can't get too extreme) by virtue of their
design.

You can pick these up pretty cheap (<$10, if not free from the local
utility) at a variety of places, and for a bit more, mail order from C&H
Sales in Pasadena CA (although I don't recall how much they want for
one).

Granted, it measures watt hours, but watthours and a stop watch do quite
nicely. Measure how long it takes for the rotor to make one turn, and
then do the math. The calibration ratio of rotor turns per kWh is
usually on the front of the meter somewhere, but I just hooked up a
resistive load  (a 300 Watt light), measured rms volts, rms amps, and
timed one turn.


For a more sophisticated approach, there is an old (about 20 yrs)
National Semiconductor Ap note that describes how to build a small
instantaneous watt meter using the log characteristic of a diode and a
bunch of op-amps.