Thanks.  This is very helpful.

Warren

On 4/17/07, Peter P. <plpeter2006@yahoo.com> wrote:
>
> Vasile Surducan <piclist9 <at> gmail.com> writes:
>
> > The overall efficiency in a domestic microwave oven is very small.
> > Not because the magnetron but because the supplying transformer
> > (assuming the oven is not Panasonic and has no inverter) and the way
> > microwaves are absorbed in the food.
> > Need more details ?
>
> I strongly disagree. The efficiency is much higher than 50% (but does not
> reach
> the 90%+ possible with a magnetron acc. to books).
>
> One can calculate the efficiency of the oven as a first approximation
> using
> the water cup method. This is the standard way to test microwaves by
> techs,
> too. Here I describe it again:
>
> Measure mains voltage and put an AC ammeter in the mains circuit. This
> should
> be a clamp ammeter (do not use a shunt one unless it is spike-proof, the
> inrush
> can be huge).
>
> Take a cup of water, weigh it (first empty cup then full, take the weight
> of
> the water alone), and measure its temperature. Put *another* cup in the
> oven
> and nuke 1 minute. Discard this quickly, put in the weighted cup quickly
> and
> again nuke for 1 minute. Remove the cup, stir quickly and take its
> temperature.
> Measure the oven current while it runs (better: use a real power meter).
>
> The heat that went into the water is about SPH = 4.2 Cal/deg C and gram.
> So:
>
>   P1 = Uac * Iac (or readout from real power meter)
>
> (take average Iac, P1 will read high due to the power factor of the
> transformer)
>
>   P2 = SPH * deltaT [deg. C] * (Weight [grams] / 60 [seconds])
>
> Efficiency is:
>
>   eta = P1 / P1
>
> Because of factors entering P1 this eta will read low wrt. the real value.
> To
> improve the reading look into power factor measurement of the transformer
> and
> so on. The measured output should be at least 400 Watts for a 750 Watt
> input
> oven(750 Watt is the nameplate rating). This is equivalent to 53%
> efficiency.
> Not so bad for a simple circuit involving a transformer, a diode, a
> capacitor
> and a fancy vacuum diode.
>
> Peter P.
>
> (the first cup heating serves to take the magnetron heater close to
> operating
> temperature. Then the 2nd 1 minute interval starts with the magnetron
> closer
> to emission start so it does not 'steal' time while heating up - this
> would not
> matter with an integrating energy meter in the mains lead but it matters
> with
> an ammeter as the current changes a lot during the 1st 10 seconds without
> this
> emasure)
>
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