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Author Topic: Notes on Microwave Oven Transformers  (Read 4225 times)
WD4HXG
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Posts: 182




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« on: January 16, 2008, 11:31:12 AM »

During a recent search for transformers for a homebrew amp, microwave oven transformers were viewed as a possible solution. If you are looking for reasons to use a MOT (microwave oven transformer) then this will likely be discouraging news.

A "1200 watt" rated oven's transformer was dissected and the following was noted.

1 The steel laminations are E and I parts. Unlike the tradition designs used in older US manufactured parts, the E's were all inserted from one side as a bank of laminations. The I's were similarly banked and placed against the E's from the opposite side.
They were attached to each other using a mechanical notching technique.

The I's have a half round notch on the outer edge midway of the I. When these are place together in a bank the manufacturer seam welds along the groove formed by the notches to hold the I laminations together. A mounting plate is then spot welded to the surface with the groove formed by the notches in four places to attach the I laminations to the mounting plate.

2 There were shunt laminations used between the primary and secondary windings. These were pressed into position by the manufacturer. The lack of space between the primary, secondary and filament windings made removal tedious. The primary was damaged during the attempt to remove the shunts. Although the primary did not physically short during the removal process I was not comfortable with using the transformer with damage to the insulation.

2 The high voltage winding, primary winding and filament windings are not wound on a common bobbin. Actually no bobbin was used on any winding. The windings seem to have been wound on a mandrel, removed from the mandrel and inserted into the E laminations. The windings were wrapped with a single thin layer plastic insulator which was maybe 5 mils thick.

The wire used on the mains primary is heavy gauge, appearing to be between #13 and  #14 (67 mils). The wire on the high voltage secondary was much finer between #24 and #25 gauge (18 mils). The filament wire was just heavy insulated hookup wire, about #22 gauge.

The mains winding was positioned near the open end of the E's while the high voltage winding was placed against the boxed end of the E's. The filament winding was wound between the primary and high voltage secondary and then wedged with what appeared to be phenolic strips on both sides. The whole unit then appeared to have been dipped in a shellac or similar product.

3 After removal of the windings it was obvious that the primary was light weight for the amount of copper wire. To determine the number of turns of wire on the damaged primary, the primary was cut into two parts with a bandsaw. The reason for the light weight became apparent as the wire was manufactured from aluminum.

4 Removal of the shunts may be possible without damage to the windings but my methods failed as the primary insulation was damaged. Removal of the filament winding wire is not practical as the portion between the primary and high voltage secondary is compressed between the winding resulting in a tight friction fit. The wire simply snapped when applying force to pull it from between the windings.

------------------------------------------------------

Given I had damaged the transformer primary I chose to disassemble the device. The I and E laminations are pressed together at the factory assembly floor. The E laminations have a cutout and the I laminations a matching tong which when pressed together lock the lower I laminations as a bank to the upper E laminations. They can be separated by using a bench vice, wood blocks cut from 2x4's and a heavy machine hammer.

Mount the transformer in the vice by placing the box end of the E laminations in the vice. You want it solidly tightened in the vice. Place one end of a 2X4 wood block edge against the I lamination's edge adjacent to the E laminations. Sharply rap the opposite end of the wood block. You may have to do this several times and change to the opposite side of the E & I mating surfaces. However after a few raps the unit I had begin to separate. This can be done without removing the mounting plate welded to the I laminations.

Removal of the windings is more of an effort. The unit I disassembled required using an arbor press. I set the transformer primary winding on two wood blocks and then pressed the E laminations as an assembly out of the windings. The high voltage winding was destroyed as the insulating coating acted like an adhesive holding the secondary winding to the surface of the laminations. Oddly the mains winding suffered no further damage but the damaged insulation from previous efforts to remove the shunts was obvious, otherwise the primary remained intact. Other than the primary's light weight it appeared to be well made and the red-brown enamel insulation hid the fact it was not copper.

If I was forced to use a stock MOT in an amp I would derate the transformer specs by 50%. The use of aluminum wire in the primary winding may be a sound engineering practice but it would worry me. I was puzzled how the wire used in the primary was soldered to the terminals on the wiring plate.

Removing the shunts is another problem which I have not solved to my satisfaction.

Many thanks to Bill Draginis for his insight and suggestions in the disassembly process.









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KE3WD
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Posts: 5694




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« Reply #1 on: January 17, 2008, 07:30:22 PM »

I remember a special (and caustic) liquid flux that allowed us to perform electrical soldering to aluminum with standard 60/40.  Memory escapes me as to the chemical composition right now, sorry, its been years.  

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KC8OJU
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Posts: 31




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« Reply #2 on: August 15, 2008, 02:11:55 PM »

I used to collect MOT's They all used copper wire. The filament wire was at least 14 AWG. The I's were Welded to the E's making a shorted turn around the core. There are 4 shorted turns or welds around the I's and the E's.
I understand it as a spike suppressor. The shorted turns sit there and draw up to 400 watts of power. You must have gotten hold of a really old transformer. They don't make them that way any more.
To break the I's from the E's I have to grind awy the welds,(the shorted turns) from the E to I connection.
Once that is done they come apart really eazy. Only the filament doesn't have a bobbin.

I took 2 E's with the primary and secondary on them and potted them together with a vise to hold them while the epoxy cured. This only eliminates 2 shorted turns,(welds) on the 2 I's that were eliminated. Instead of drawing 800 watts sitting there it only draws 600 watts.

I'm using this set up to power my 4-1000A I'll have to use another MOT for the filament and wind my own filament winding. I think it's 0.75 volts per turn.

73 OM

n8zu
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KC8OJU
Member

Posts: 31




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« Reply #3 on: January 07, 2013, 08:20:46 PM »

mosr newer MOT are using aluminum wire these days to save costs I'm going With charge pump smps to get HV now straight from 115VAC with 10 stages of pumps at 50Khz only need 0.1mfd caps for 2000PEP watts
73 de
n8zu
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