Hello Nick, hello John,
many many thanks (again) for the detailed explanations. I realise,
though, that I have a lot to learn when it comes to AC circuitry. So
maybe it is not such a wise idea to do this all by myself.
I have a meeting in my university's central "factory" (what would I call
this in English? It is where they build all their specialised equipment)
on Wednesday, and there I will talk to some technicians who will
hopefully be able to help me.
I was planning on using a filtered mains socket like this one:
http://de.farnell.com/productimages/farnell/standard/42749305.jpg
I will also think about the bucking winding a little more.
Best regards,
Jens
Am 31.10.2011 13:50, schrieb John Rehwinkel:
Thanks for the insight, but the filtering does not sound trivial to me. As it
appears, MOT spot welders can also be made without any electronics:
Yeah, you don't need any electronics, you can just control the primary with a
foot switch.
How much current is actually flowing in the primary circuit?
As it happens, MOTs don't really have sufficient inductance to current limit on
their rated line voltage. If you remove both secondaries entirely, and hook up
the primary, the transformer will sit there and draw 400-500 watts (I'm not
giving the
primary current, as that depends on the mains voltage in your country). This
is because they're optimized for cheapness,
not energy efficency, and run at the edge of saturation:
http://wiki.4hv.org/index.php/Microwave_oven_transformer
This will vary depending on whether the magnetic shunts are removed (general
wisdom seems to be to leave them in).
When you're driving a secondary load, primary current will tend to rise, as
you're dissipating more power. Most MOTs can deliver 700-1200 watts into the
magnetron, so figure a primary current equivalent to 1800 watts or so max,
possibly more as it's not a resistive load, so VA can be higher than the
wattage, due to phase shift (one of the functions of the magnetic shunts is to
try to reduce the phase shift, when the MOT is operating into its original load
- with a spot welder load, things will be different).
Many people will have some sort of lashup to reduce the primary current. These
are generally one of five varieties - adding a ballast resistance (like a
toaster or space heater) in series, adding some inductance (such as another
MOT, often with a shorted secondary), using a bucking transformer to reduce the
incoming mains voltage, using a bucking winding on the MOT itself to do the
same thing, or using a 220V MOT on 120V mains.
My favourite is the bucking winding on the same transformer (or, you can think
of it as adding some turns to the primary). It is my favourite approach
because it doesn't require additional parts, just more wire. Simply add
several turns of wire sufficent to handle the primary current to the core, and
wire it in series with the primary, observing phasing.
I'd add the additional bucking winding first, keeping it compact, then wind the
heavy secondary turns. Then I'd hook up the bucking winding and primary in
series, and drive the secondary with a few volts. Then swap the leads to the
bucking winding and try again. Use the hookup that gives the most volts out of
the primary + bucking windings while driving the
secondary.
This will nicely reduce your power consumption, and keep your core from getting
quite so hot.
I realized I answered your one-sentence question with several paragraphs of
blather, but when working with hand built mains operated equipment, I really
want people to understand what they're getting in to.
- John
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