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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