From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Inverter and microwave
Date: Tue, 16 Jan 2001 17:16:55 -0500

Young wrote:

> Herch,
>
> A microwave cannot modulate power. That means that it can either
> deliver full power or no power. The way it controls the energy it puts
> into the food is by intermittent operation. You set it to 50% of
> power, and it will be on 50% of the time and off 50% of the time. When
> it cuts in the inverter will see exactly the same peak power demand as
> if it was on all the time.

Wrongo Puppychild, but thanks for playing.  My Amanna RadarRange
does a quite nice job of modulating its magnetron.  In this case, it
does so with phase angle control of the HV supply.  The other
popular method is to vary the filament current which varies the
emission which varies the anode current which varies the power.
While on-off control is popular for low end consumer ovens, phase
angle control is still quite popular with commercial ovens.

> In fact intermittent operation will impose
> more of a load on the inverter than if it was on all the time because
> the main high frequency unit will be imposing a surge each time it
> cuts in. The "gump" sound you here when you first switch it on is an
> indication of a surge of power while the magnetron is being energized.

Nope, sorry, wrong again.  That thump is the momentary inrush that
occurs when the first half cycle of power happens to be of the same
polarity as the last on the previous heating cycle.  The combination
of the residual magnetism left in the transformer core and the
inrush from the incoming power saturates the core and causes high
current to flow for the half cycle (and sometimes longer if very
cheap metal has been used in the core.)

The filament takes a couple of seconds to warm, during which the
power consumption ramps up smoothly.  One can typically hear the
magnetron "chatter" as the filament first starts emitting.

>
> Better to put a few more bucks into the problem and get a small
> generator that will do the job effectively.

Nope, not necessary.

First, the reason the microwave cannot be run on an inadequate
inverter - the magnetron draws power on only a small part of the
peak of each cycle of power.  Pseudo-square wave inverters with less
than 3 steps typically don't supply the necessary peak voltage.  The
microwave will power up and appear to operate normally except that
it either won't heat or will just barely heat, depending on a lot of
minor variables.

If the inverter does supply sufficient peak voltage, it is possible
to reduce the peak power draw by the simple expedient of reducing
the magnetron filament current.  Typically the filament is supplied
from an aux winding on the magnetron transformer.  This winding is
typically a few loose turns of heavily insulated wire.  Unwinding a
half turn at a time until the inverter and oven survive with each
other is possible.  This winding operates at the accelerating
voltage (typ: 2500-4000 volts) so one must be very careful.

I personally would not take this route unless I some reason
compelled me to use a particular microwave oven and/or someone gave
me the inverter.  If the oven is the only heavy load in the RV and
both the oven and the inverter must be purchased, I'd suggest buying
the 12 volt powered oven sold by Flying J, TA truck stops and the
like.  Price is under $300.  It contains a native 12 volt power
supply inside the microwave.  It draws about 90 amps at full power.
The cost of the oven is about the same or a little less than a
regular small microwave and inverter but if this is the only heavy
120 volt load, it is a much better solution.  It is more efficient
since the power supply inverts the 12 volts directly to the high
voltage the magnetron needs.  And when the oven is off, there is no
residual power consumption like there would be with an external
inverter.

BTW, this same brand is now offering 12 volt compressor-based
mini-friges.  I looked one over at a TA this weekend.  Looks quite
nice.  It has a small converter that directly drives the compressor
from 12 volts.