Microwave oven transformer - How to prevent overheating? - General

Hey guys, I've been testing MOTs recently, mostly for use in "homemade" arc lamps, but the one problem I keep running into is the primaries get really hot. I run it ballasted with the MOT on the left, primaries in series secondaries shorted. It pulls about 10-13 amps under this configuration. The little MOT was from a somewhat small microwave, not sure what wattage. [img]http://s7.postimage.org/s765ip6d7/littlemot.jpg[/img] Is there a solution to this? Would it really get hot enough to burn out? There should be some point where it reaches an equilibrium, but probably not at this power. It seems the best solution is to get a 1000-1200 watt microwave or larger. I don't expect the little MOT to take it, but I'm afraid other MOTs will be similar. The giant one that I use as a ballast was stone cold.. Ballasted with the small MOT is the same way, not even warm at 10 amps. I got it out of a really old microwave, looked like 2500+ watts. absolutely enormous, weighted 40+ pounds, and had a HUGE fan. They just don't make MOTs like they used to make them. Another idea was to cut a MOT up and stack windings on a new core.. maybe make a new primary. Proper high voltage high current, ie pole pigs, costs way too much, so I'd rather try that first. It would be an interesting project.

You could ballast them with lightbulbs. They'll limit the current. Also, yes, they will burn out eventually. I've lost a couple doing the same thing you are. What will happen eventually is you'll begin to see smoke, and then usually within about 30 seconds your breaker will blow. This is caused by the insulation melting away and causing a short. I'd recommend moving to flyback xformers from CRT's or similar, and you can customize their input current/voltage by making your own simple ZVS flyback driver circuit. They'll consume a lot less current, run a lot cooler and you can get much greater voltage out of them. MOT's are good (stock) for high current. They'll typically run at just under an amp of current at high voltage. Because of their robustness, they're fun to rewind where the secondary only has a few windings and pumps out hundreds of amps. Here's a neat application using two MOT's in parallel to double the output current though, if you're wanting to do that. I took some pictures below: [img]https://lh6.googleusercontent.com/-UmjG4oHk-oo/ThvwOkcuRpI/AAAAAAAAAEc/qgdU1mOWbKU/s901/IMGA0013.JPG[/img] [img]https://lh3.googleusercontent.com/-geF25E6dZjU/ThvxJvSoxRI/AAAAAAAAAEw/mcqloa4iXkM/s764/IMGA0001x.jpg[/img] You can also ballast it with the microwave's brushless fan. It's neat because the RPM changes under load: [img]https://lh3.googleusercontent.com/-2W4ppRajiPo/ThvxoHZtdUI/AAAAAAAAAE0/H6BW-EiqpvQ/s901/IMGA0001.JPG[/img]

Nice idea with the fan, you should point it at the MOT though.. then it serves as a variable load fan. I have one but I imagine it doesn't really allow much current. I tried lightbulbs and couldn't really get arcs to form. I used two 250w heat lamps in series, no luck. I use a MOT as a ballast, but the big MOT doesn't limit the current enough for using the little one to last more than 5 minutes. I've considered a ZVS flyback setup, but I'm not sure how powerful they are. I want either 60kv at a few miliamps, or MOTs with a multiplier. Microwave capacitors are dirt cheap (1 bucks online) and diodes are easy to find. I own a couple NSTs , they are fun but again too little current, and to boot they are built within an inch of their rating. I really do need an indestructible source, and a good MOT seems to be quite capable at delivering high voltage AND high currents around 1 KVA. It seems to be just a matter of finding well built MOTs with a ballast suited for it.

I never bothered to ballast MOTs, never ran them for long enough. I do hope you know what your doing. MOTs will kill you. If you can, get a small arc welder. That hopefully should keep you under 2kw or so for your dual MOT setup [editline]4th March 2013[/editline] How are you wiring it? Core to core? [editline]4th March 2013[/editline] [QUOTE=fox '09;39724566]Nice idea with the fan, you should point it at the MOT though.. then it serves as a variable load fan. I have one but I imagine it doesn't really allow much current. I tried lightbulbs and couldn't really get arcs to form. I used two 250w heat lamps in series, no luck. I use a MOT as a ballast, but the big MOT doesn't limit the current enough for using the little one to last more than 5 minutes. I've considered a ZVS flyback setup, but I'm not sure how powerful they are. I want either 60kv at a few miliamps, or MOTs with a multiplier. Microwave capacitors are dirt cheap (1 bucks online) and diodes are easy to find. I own a couple NSTs , they are fun but again too little current, and to boot they are built within an inch of their rating. I really do need an indestructible source, and a good MOT seems to be quite capable at delivering high voltage AND high currents around 1 KVA. It seems to be just a matter of finding well built MOTs with a ballast suited for it.[/QUOTE] I've got a 70kV, 2mA x-ray transformer lying around, got a 15/30 NST too. I really need to finish my Vacuum Tube Tesla Coil

[QUOTE=download;39792690]I never bothered to ballast MOTs, never ran them for long enough. I do hope you know what your doing. MOTs will kill you. If you can, get a small arc welder. That hopefully should keep you under 2kw or so for your dual MOT setup [editline]4th March 2013[/editline] How are you wiring it? Core to core? [editline]4th March 2013[/editline] I've got a 70kV, 2mA x-ray transformer lying around, got a 15/30 NST too. I really need to finish my Vacuum Tube Tesla Coil[/QUOTE] I know what I'm doing, don't worry. I have two MOTs running oppositely phased, with the cores grounded. Ballast is in series after the MOTs. I'd like to run 4 MOTs, 2 sets of 2 in series and also running oppositely phased for 8kv difference. perhaps under oil once I find more.. I read if you knock the shunts out they run cooler, so I will give that a try too. I wish I had a nice x-ray transformer, but I imagine they're hard to find, and expensive on ebay.. a multiplier will probably get me where I need.

The shunts are what stop it from drawing so much it catches fire. I'd leave them there. My xray transformer came from a dental xray head. Cost me $150 or so off Ebay. I had a thread on here detailing me dismantling it I'll dig up if you want

[QUOTE=download;39801728]The shunts are what stop it from drawing so much it catches fire. I'd leave them there. My xray transformer came from a dental xray head. Cost me $150 or so off Ebay. I had a thread on here detailing me dismantling it I'll dig up if you want[/QUOTE] Please do. I ahve that X-ray tube I want to [del]get cancer[/del] screw with.

[QUOTE=MIPS;39804637]Please do. I ahve that X-ray tube I want to [del]get cancer[/del] screw with.[/QUOTE] Pretty sure you posted pics of your own Russian one in the thread... [editline]5th March 2013[/editline] Seems I posted it on my alt... [url]http://facepunch.com/showthread.php?t=1025652[/url] Half the post are people making stupid comments about cancer [editline]5th March 2013[/editline] Wow, re-reading that thread I just cringe at the poor use of "there", "their" and "they're" in my post

[QUOTE=download;39801728]The shunts are what stop it from drawing so much it catches fire. I'd leave them there. My xray transformer came from a dental xray head. Cost me $150 or so off Ebay. I had a thread on here detailing me dismantling it I'll dig up if you want[/QUOTE] I mean shunts on the "working" transformers, with proper current limiting it doesn't matter whether it has shunts or not, right? Keeping them saturated with current limiting would generate unnecessary heat wouldn't it? For high output MOTs from giant microwaves I imagine it doesn't matter when ballasted, but little ones easily saturate and get really hot. I also tried rewinding a MOT I broke (tried to run in series) with one turn of 150A wire. It's fun, I get about 800-1000A out of it at no volts. Should of just used smaller wire and just twisted them, that way I could get more turns. My main use is shorting things out on it, so that would probably melt quickly. 150A wire melts, but probably slower. Oh wow that's just beautiful. Is it midpoint grounded from the center tap or is that something else? I may buy one, but I do have a 4 stage (or 3 stage full wave) cockroft walton I still need to put together. Doorknob caps, good diodes, etc. May be worth trying that for xrays first.

It's not midpoint grounded, but it is a 110v transformer and I live in Australia where we have 240v. My solutions was the split the two primary windings that were running in series then running them in parallel so it was 55v in. I then hooked it up to my arc welder which is 50v out so I could run it. Some of the biggest arcs I've ever seen outside of a Tesla Coil. They easily jumped 10cm and could be drawn out to twice that. I need more insulating oil for it though [editline]9th March 2013[/editline] CAD drawing I did of my VTTC I did ages ago. I really need to get this project going again: [img]http://img829.imageshack.us/img829/4056/baseoverview1.png[/img] [img]http://img84.imageshack.us/img84/3565/baseoverview2.png[/img] [editline]9th March 2013[/editline] Based on Steve Ward's VTTC design: [url]http://www.stevehv.4hv.org/VTTC5.htm[/url]

I'd suggest joining 4hv.org, it's a high voltage electronics forum, they're pretty helpful

Yeah, I think I will. Nice plan, I'd love to see it in action. I solved my problem with this circut: [url=http://postimage.org/][img]http://s10.postimage.org/3y09wseih/mot_setup1.png[/img][/url] Two identical MOTs 180 degrees out of phase. Arcing across the HV gives 1500-1700VA pull from the mains, depending on voltage input. on a 15A breaker of course.. HV to GND gives over 2000 VA, variac complains so I don't enjoy doing it often. It works for more than 15 minutes at 1500 VA, transformers aren't too warm. Variac complains at about 1700+ VA so I'm not to keen on using it constantly. Otherwise as I said, it works quite well. Just takes a bit of setting up for it to do what you want. edit: Single capacitors are naughty.. setup gets really hot obviously, but doesn't burst into flames after 20 minutes of continuous normal and intermittent high current use. Good enough to use it, and at lower powers can easily be ran without much trouble. [img]http://s23.postimage.org/wjimur5y3/arc5.jpg[/img]