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Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Hi, I have a MOT I've rewound with 4 gauge wire, however I can't seem to get much power out of it at all! I have removed the current shunts, it has about 3 turns on it, primary measures ~2.5 ohm, it's a 240V transformer, it's working because I can hear it buzzing, but even with the output shorted through a piece of fencing wire, my AC current clamp barely reads anything, and the wire struggles to even become red hot!
Anyone got any idea's on what could be going on? Not enough turns is my only thought ...
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
If you just have three turns you could be saturating. Why not try uping the turns, and if need be, up the turns on the secondary. There are formulas for figuring out the minimum number of turns you need.
Registered Member #1221
Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196
I made a transformer some years back with 3-4 turns of a jumper cable as the secondary and it made a tungsten welding rod ( 1.5mm diameter and some 10-15 cm long ) glow white hot. The clamp meter read something around 200-300 Amps.
Dont know whats wrong with your setup though. I also used a 240 volt transformer mot.
I'll see if I can get a pic of my transformer if its any help..
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Maybe my mains input wires are too thin? Although they don't seem to warm up or anything ... I can't really think of anything that may be wrong ... Will look into it some more tomorrow.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Nothing resistive connected to the SECONDARY can cause saturation. Find out the volts/turn ratio. wind enough turns to give a reasonable voltmeter reading open circuited. That will tell you the volts/turn level the core is at. Now how much current you get will be limited soley by the resistance of the primary and secondary windings with that core flux level. I imagine a MOT core runs at around 1.5 T if abused.
What is happening to your setup is exactly what happens in a AC welder that uses a shunt gap to control current. Its set too low.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
As Radiotech said, the first step of troubleshooting is to measure the open-circuit voltage of the secondary. Should be on the order of 1 volt (RMS) per turn. Then I would also measure and report the primary voltage and current with no load on the secondary.
The no-load state is as close as you will get to core saturation. When secondary current is allowed to flow in a resistive load, the magnetic flux density will go down slightly, as IR drop reduces the effective primary voltage. Lesson to remember: Transformers saturate when they get too much voltage, not when too much secondary current is drawn. Increased magnetizing current is a consequence, not a cause.
Your removal of the core shunts (they are not "current shunts") should have increased the no-load magnetizing current, which is high to begin with in unmodified MOT's. Because you need the same flux-change in core through the primary winding, and you have increased the core loop reluctance, so you need more ampere-turns to magnetize it.
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
With 2 1/2 turns, the MOT gives about 2.1VAC on my cheap DMM.
My load is a piece of fencing wire, which should melt instanlty on the MOT.
Ok, just bought a few metres of 8gauge wire, the next step down from 4gauge. Rated for about 58A cont., but will be able to fit a lot more turns on, and it seems to be what everyone else is using.
EDIT: Yep, the higher voltage made the difference. Melting through bolts now
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MOT output pathetic ...
