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microwave oven transformer rewinding for high current low voltage use?

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LAN_403

alf

Fri Jun 17 2011, 07:57PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hey,

im gonna rewind a microwave oven transformer to supply about 50v at 20a for my flyback transformer,
and i have 2 microwave oven transformers - a small transformer from a microwave oven rated at 700W, and a big transformer from an old microwave oven rated at 400W,

which could supply more power?
im guessing the one from the 400W oven, since the transformer is larger than the other.

thanks. Alf.

Micha0_0

Sat Jun 18 2011, 12:07AM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

hey .

not the answer for you question.
but do you cut off the core to wind a nice spool or do you just put the wire through the holes for the secondary ?

Dr. Slack

Sat Jun 18 2011, 07:03AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It's unlikely that the iron is radically different between the two, so probably yes, more iron means MOAR POWA. You could always wind 10 turns on each and check with a variac, if you have one. In any case, the larger core will be easier to wind onto.

but do you cut off the core to wind a nice spool or do you just put the wire through the holes for the secondary

The answer is yes.

Both have their difficulties, just do whichever you feel you have the competance to finish. I tried cutting a core off once, and found I couldn't keep a bit of weld on both halves to hold them together, the lams fell apart, and it was game over. But others on this forum are more skilled. Poking 10 turns through, then again, then again, then connecting them all in series looks untidy, and gives you a very low copper window utilisation ratio, but gets you there.

Micha0_0

Sat Jun 18 2011, 11:27AM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

i already have a mot without secondary.
i'll try to cut the core off and wind a new secondary.
but , how thick the wirde should be ?

alf

Sat Jun 18 2011, 06:52PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Micha0_0 wrote ...

hey .

not the answer for you question.
but do you cut off the core to wind a nice spool or do you just put the wire through the holes for the secondary ?

hey,

i intended to cut off the secondary and wind a spool of wire, but i may try threading some wire through the holes first lol

thanks. Alf.

alf

Sat Jun 18 2011, 07:03PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Dr. Slack wrote ...

It's unlikely that the iron is radically different between the two, so probably yes, more iron means MOAR POWA. You could always wind 10 turns on each and check with a variac, if you have one. In any case, the larger core will be easier to wind onto.

but do you cut off the core to wind a nice spool or do you just put the wire through the holes for the secondary

hey,

yeah i do have a variac, but wouldnt i have to measure the short circuit current to find out which has more power?
haha i intended to cut off the secondary, i hadnt thought of this happening, i dont actually know what you mean though lol
i thought it would be as simple as cutting off the secondary, removing the filament winding, and the magnetic shunts, and winding some thick wire round the core?
btw what is a copper window utilization ratio? lol

thanks. Alf.

Dr. Slack

Sun Jun 19 2011, 07:08AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

i do have a variac, but wouldnt i have to measure the short circuit current to find out which has more power?

How complicated does "measure current" have to get? If you put 10 turns one a core that will standa 1v/turn(ish) when working unsaturated, put a 12v auto bulb in series with your variac output, and then bring it up from zero, the bulb will be out or only dim while the core is unsaturated and only drawing magnetising current, then glow much more brightly as the core saturates. Or you could use a DMM on AC amps instead of a bulb.

Copper window utilisation? You have a big hole in the iron. A well-wound transformer may fill that with 50% copper, 20% enamel, 30% air, tape, bobbin and contingency. If you thread single wires through the core, you'd be lucky to get 25% to be actual current-carrying copper. Result, higher winding resistance.

klugesmith

Tue Jun 21 2011, 06:07AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

alf wrote ...
im gonna rewind a microwave oven transformer to supply about 50v at 20a for my flyback transformer,
and i have 2 microwave oven transformers - a small transformer from a microwave oven rated at 700W, and a big transformer from an old microwave oven rated at 400W,
which could supply more power?
im guessing the one from the 400W oven, since the transformer is larger than the other.

Getting 1000 watts by rewinding a 400W or 700W transformer will be a neat trick, even considering that in normal service the magnetron input power is more than its output power. Are you going to increase the operating frequency, increase the forced-air cooling, or pack the wire tighter than at the MOT factory? In the case of MOT from 400W oven, you might also need to use wire with lower resistivity than standard copper, perhaps by operating the unit under liquid nitrogen.

Seriously: Can you tell us the core area (square inches of iron in the center leg) and the winding window area (square inches of the hole through which the winding turns pass). Other things being equal, such as flux and current density, and copper packing efficiency, the volt-amp rating is proportional to the product of those areas. (unit = 4th power of linear dimension).
Knocking out core shunts will increase the secondary window areas, so you might want to do the comparison using the primary window areas.

You could also compare the volt-amps by measuring actual volts/turn with nominal voltage on the primary (using for example the existing filament windings, if # of turns is known). Multiply that by window area, which is proportional to maximum ampere-turns whether filled with many turns of thin wire or few turns of thick wire.

For 50 volts, I would suggest trying to fish the secondary wire through the windows many times, at least for a first try, rather than cutting up the core. (if you cut the core you'll also have to make a bobbin that fits, and might need a fancy mandrel to support the bobbin core and flanges against the pressure of many turns under tension).
Start by determining the necessary number of turns, including allowance for voltage drop, and choose the largest wire gauge that will fit. Try a few turns with magnet wire and a few turns with stranded wire, just to get a feel for the stiffness and the difficulty of getting it tight. For each new turn, you have to pull the remainder of your pre-cut wire through two windows without kinking. So it's a hassle proportional to N squared.

alf

Fri Jun 24 2011, 06:29PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

hey,

thanks for your replies, i shall try that :)

thanks again. Alf.

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