In order to determine the initial current demand of variable rated
Microwave Ovens I would like to know how such ovens achieve their
different power levels. If, as I suspect, it is by reducing the
voltage pulse width this may reduce the
energy level but will it reduce the instantaneous current surge at
'switch on' and hence overcome the circuit breker tripping problem I
am experiencing?

I would appreciate your comments. 

Many thanks

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Request for Question Clarification by haversian-ga on 13 Nov 2002 11:09 PST

Could you give us a little more information?

What model of microwave oven do you have?  What circuit is it plugged
into? (15A, 20A, 30A, etc.)  Does the breaker trip every time you use
the microwave, or only on some settings?

Usually (it's more complicated but this mostly works) voltage *
amperage = wattage.   So, if you have a 1000W microwave operating at
110VAC, you're drawing about 9A.  If your microwave is plugged into a
15A circuit, this will let you run about 1500W worth of equipment - is
the circuit being used for other appliances at the same time?

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Clarification of Question by jon19-ga on 14 Nov 2002 07:26 PST

Hi,

Thank you for the initial response to my question and the request for
more details.
My request originates from a situation where several microwave ovens
are switching on together and the combimed current  surge is causing
the main circuit breaker (controlling several sub-circuits) to
operate.

Since most microwave ovens have several power settings, e.g. low,
medium, high and defrost, I wondered if the problem could be overcome
by starting the ovens at the lower power setting and then switching to
the higher level as required. However I suspect the surge current
would still be of the same order of magnitude whatever the power
setting at switch on.

You will appreciate that stating a specfic model will not be helpful
in this situation since there are several different models. Also I
appreciate that current equals watts divided by volts for a steady
power demand, but not in the case of 'switch on' where transformers
and magnetrons are involved.

Hence my request for some information regarding the method of
controlling the power levels and if there is a suitable way of
restricting the combined surge current where several ovens 'switch on'
at the same instant.

From your useful comment that the power rating appears to be
controlled by  switching the magnetron on and off for varying
intervals it would seem to me that the current surges will be quite
regular and at the same intensity whatever the power setting. If this
is the case then overcoming my circuit breaker tripping problem seems
bleak!!

I would value any further comments .

John Turner

Every microwave oven I have seen implements the power level by varying
the duty cycle of the magnetron.  For example, a power level of 30%
would mean that the magnetron is on, full power, for 30% of the time,
and off for 70% of the time.  You can observe this yourself - the
sound usually changes when the magnetron switches on and off.  You can
sometimes see it in the appearance of the food as well, such as
bubbling when the power is on, and nothing when it's off.

Oh, I should also add that the "rating" of microwave ovens isn't the
AC power consumption, but rather some measure of emitted microwave
energy.  There is also not a universal standard for how this is
measured - over the years, ratings have edged up in a sort of "grade
inflation".

The surge current is likely to be quite a bit higher than the rated
draw.

Right - you have to look at the sticker on the back of your appliance
to get the total wattage or current draw, which includes power for
cooling, a turntable, electronics, etc.

If you need all the microwaves on at the same time (at high power), I
suspect your only recourse is writing a check to an electrician, or
using extension cords to access multiple circuits.  If you only need
30% power levels, you may have success staggering start times so the
30% duty cycles of 3 ovens don't overlap.

You are correct that the startup current draw may be much higher than
the standard rating (my laser printer, for example, draws over 1000W
for 20ms at startup), though this may or may not be your problem. 
Have you tried starting the ovens at full power a few seconds apart? 
Is it only the startup current draw, or does full operating power
cause the breaker to trip?

You just posted a pretty long reply which I appreciate, but if you
could describe your situation more fully, someone might be able to
help you with a "thinking outside the box" style solution.  What
circuits are available?  Why do the ovens need to start
simultaneously?  What is your budget for this fix?  Anything else you
think might be pertinent?

Otherwise, if the ovens need to startup at the same time, you will
have to find some way to provide them with that much power...

If the problem is to do with the surge on startup why not use a
different circuit breaker - and I do not mean one for a higher current
but one that will not trip on a surge lasting x ms.

I had a similar problem with Halogen lights on a transformer which
surge on startup and trip the breaker. One solution was to use a
different circuit breaker - which would not trip on startup but would
trip on a short circuit.  The difference is how long the surge is
allowed to take place for before it trips.

See http://us.binnacle.com/online/specs/bluesea/blueseafaq3.asp for
some theory

Then it is probably over to an electrician for the work.