Re: Microwave Transformer Currents


<sck0006
"Phil Allison"
>>
>>> The rms values are
>>
>>( V I P )
>>
>>> 120 3.29 394.8
>>
>>
>>** The primary resistance of that tranny is likely to be less than 1 ohm.
>>
>>So the real "power" figure at 120 AC is more like 10 watts copper loss
>>plus
>>a few watts of iron loss.
>>
>>394.9 is the reactive power draw in VA.
>>
>
> So you're saying that the transformer is mostly reactive, not
> resistive, being that the resistance is low <1ohm...
> and most of the rms power being dissipated is reactive, which my
> multimeter won't actually measure because it doesn't measure a true
> average of power..


** To measure power in watts with a reactive *non linear* load requires a
"wattmeter". Such a meter effectively multiplies the voltage and current
value at each instant to derive instantaneous power, then averages the
result.


> The 10w copper is the resistive power loss due to the resistance of
> copper?

** Yep.

Good old " I squared R " .


> The few watts of iron loss is due to the average value of reactive
> loss? (in a perfect world, the reactive loss would be 0, this is due
> to real world imperfections, the inductive heating of the core...?)


** The iron core dissipates energy in being heavily magnetised and then
re-magnetised in the opposite direction 60 times a second. Makers often
specify this as so many watts per kilogram of iron , depending on the grade.


> So if I had a power analyzer that measured rms power over time, it
> would measure something like 15 or 20W (per second), as opposed to the
> 394VA that I measured?


** I expect a "wattmeter" would read that sort of number.


> Which brings me to my next question...
> Power factor.
> Adding a selected value capacitor would bring this transformer into a
> mostly resistive device, correct?


** Not really, a saturating transformer is a non linear load as well as an
inductive one.


> And finally...
> So the real power figure of this transformer is probably less than
> 20W, correct? At that idle power, would this make a suitable power
> transformer for a supply when wound with a secondary of say, 36
> turns...


** Once you put a load on the secondary, the primary copper losses will
increase but the iron loss reduces. Of course, the secondary gets hot too -
so the overall heat loss will rise to something like 30 watts. I suspect the
secondary can deliver about 500 VA before the whole caboodle gets too hot to
be safe.

Microwave oven trannys are "cut to the bone" designs and need fan cooling to
achieve the rated output continuously. Running one with no fan means
de-rating it by a large margin - maybe 2 or 3 to 1 in terms of VA.

Connecting two with their primaries wired in series means their secondaries
must be wired in parallel and the two units be very close in ratio.



......... Phil





.

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