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Re: Input Power Measurement
Subject: Re: Input Power Measurement
Date: Wed, 28 May 1997 16:46:18 -0400
From: "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
To: "Tesla List" <tesla-at-pupman-dot-com>
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: Input Power Measurement
> Date: Tuesday, May 27, 1997 1:46 AM
>
> Subject: Input Power Measurement
> Date: Mon, 26 May 1997 23:42:39 -0700
> From: Skip Greiner <sgreiner-at-wwnet-dot-com>
> Organization: Greiner, Ltd.
> To: tesla list <tesla-at-pupman-dot-com>
>
>
> 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.
>
> How can the actual input power to the TC be measured????????
>
> 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.
>
> BUT....how can the input current be measured. I seriously doubt
that it
> is sinusoidal. 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, which would give an
> input power which all of us could use for comparison purposes?
>
> I have a commercial instantaneous reading wattmeter. Does it
provide any
> usable information? 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?
>
> Any suggestions would be appreciated.
>
> Skip
Skip,
The current can be measured using an RF current meter. Most of these
use a heavy bar of metal (shunt) attached to a thermocouple. The
shunt is usually fairly massive. This helps to automatically average
out the current, and keeps the voltage drop low. This is preferable
to rectification schemes, as these usually involve an initial voltage
drop due to the diodes(s) used. You can usually find such RF ammeters
at those glorious things we call Hamfests. The RF ammeter is not a
linear device, but it also does not have any "dead" spots. Although
they were designed specifically for RF current measurement, they in
fact work just fine all the way down to DC levels.
Hope this helps.
Fr. Tom McGahee