Another MOT question... - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: August 07, 2018, 10:47:41 pm
content: So I've been looking round the web for an answer to this question, even found a thread on here, but fell short of what I was looking for.I've done the whole inefficiency thing of rewinding a MOT secondary and using it. What I was looking for is reversing a MOT, I've seen the secondary is connected to the core. I was wandering if that can be detached isolated and powered in reverse. I've not attempted that at all yet, but was interested on what you'd get out of the primary side, if it where it's secondary winding. It's nothing more than a curiosity at this stage, I've no intentions of kicking the bucket early. Is it possible to energize the high voltage winding as in reverse ? I understand the safety implications, I'm not asking for definitelys, just probabilitys.Thanks for reading, any thoughts appreciated.

author: sureshot
author_rank: Regular Contributor
timestamp: August 07, 2018, 11:29:01 pm
content: I gleanened the following from the web " Trying to clear out some 'junk' but have always tended to keepscrapped parts from old (UNREPAIRABLE/NOT-WORTH-FIXING) microwaves!e.g. the transformers, fans, fuse holders, overheat switches. And evenoccasionally have used some of them for repairs to other microwaves!I applied 115.7 volts AC to what had been the HV winding of one ofthese scrapped transf. recently and got about 6 volts on what had beenthe original primary. That established the ratio between the twowindings as approx 20:1.Also a small voltage (about 0.185 volts) across what had been themagnetron (Approx. 3 volt?) heater winding. Which gives ratio of about1:33 to the original primary winding; also a ratio of about 1:700 fromthe original HV winding.So the idea of using the transformer backwards comes to mind?Depending on the unit they are typically rated as follows?Input winding 115 volts at whatever total wattage of the magnetroncircuit is. Assume say 1000 watts requiring about 8.7 amps primarycurrent! Usually intermittent use.The original HV secondary operates at around 2200 volts, so 1000 watts(ignore the magnetron heater current for a moment) would average about400 to 500 milliamps? And pretty peaky because of the half wave highvoltage doubling circuit???More moderately; at 115 volts and say 400 m/a into the old HV windingthe transformer might be able to supply around 6 volts at around 5 to7 amps from what used to be the 115 volt primary?Or at 115v/250 m/a maybe 3 to 5 amps? For 6.3v heaters?Alternatively could such an ex-microwave power transformer be used inthe plate circuit (non-push/pull) of an audio amp using say a 6v6 or6L6 (perhaps triode connected?) with a DC plate current of 45 to 75milliamps?Thus in such an arrangement the 20:1 ratio to the original primarycould provide something of a match for an 8 ohm speaker? (20)^2 x 8 =3200Problems; the laminations of such transformers do not seem to have airspaces as in some audio transformers carrying DC? The bottom (neutral)end of HV winding is sometimes at cabinet 'ground'. Hence a problem;unless the transformer is isolated and even then the transformer framecould be 'hot' whenever the unit is plugged in if that winding nowused as the primary! Therefore unsafe.Have also heard of cutting off the old HV winding etc. and using theremainder of transformer to make battery charger; down rating atypical transformer to say 500 watts (continuous) for battery chargingthat's sort of around 30 to 40 amps at 12+ volts. Not sure what theturns per volt would be but should be possible to poke a few turnsaround the partially stripped core until one gets around 14 to 15volts RMS "Not my writing, but interesting never the less. And started thinking low voltage multiplier...

author: Jwillis
author_rank: Super Contributor
timestamp: August 08, 2018, 06:22:32 am
content: I've rewound a few MOTs Without any problems.All I do is seperate the "I" from the "E" core remove the HV coil ,shunts and heater.Make a new bobbing out of cardboard or plastic rewind secondary to desired voltage on to new bobbing over or under primary it doesn't matter.Sand the "I" and "E" so the y sit flush clamp tight and re weld.I've even separated the laminates and over lapped the E plates filling the gaps if the I plates.This works best and dosen't require welding.Slap on some thinned out epoxy and your done.I'm not sure about reversing the primary and secondary for it would seem you would take a lot of loss.Not very efficient .Best efficiency is matching the winding's to the voltage and the core area. .As for the VA output that is dependent on the core area and not the winding's.I don't quite understand what you mean about carrying DC through a transformer.DC won't transfer through a transformer at all.Transformers are for an alternating current only.

author: sureshot
author_rank: Regular Contributor
timestamp: August 08, 2018, 10:07:04 am
content: Yes the DC thing is mentioned in that text i copied and pasted above. I know it would be inefficient really. I was was just curious what you'd get out of a primary to secondary with that many windings. I've built a 12 Volt MOT, it work ok using a low drop out voltage regulator and a pair of high powered pnp darlington transistors. That runs hot, but i expected that, its also wasteful of power, but it works. I don't use it, just built it to see if it would work. My interest here was to see if it was possible to get a secondary usable voltage if the transformer was reversed. Looking at the original primary winding diameter I'd hazard a guess of somewhere between 5 and 8 Amps maybe. But output voltage unknown. I know one end of the high voltage secondary is fixed to the core, but i was thinking of isolation of the input if it was reversed. Maybe potting the input wires in epoxy. Just a curiosity as i say, our microwave is up for renewal soon, so that transformer will be usable. Thanks for your reply appreciated.My low voltage 12 Volt MOT below.

author: sureshot
author_rank: Regular Contributor
timestamp: August 08, 2018, 06:45:11 pm
content: My maths is not fantastic, but is there a way to calculate the output voltage from winding resistance measurements, or is that not enough to be able to make an estimated conclusion ? Any ideas appreciated.

author: jmelson
author_rank: Super Contributor
timestamp: August 08, 2018, 07:29:04 pm
content: Quote from: sureshot on August 08, 2018, 06:45:11 pmOutput voltage is a simple ratio of primary turns to secondary turns. If you don't know the number of primary turns (likely) then wind, say, 10 turns, apply mains voltage to the primary, and measure voltage on the 10 turns. Divide by 10, and you have the volts/turn for this transformer. So, if you desire 30 VAC, and the above test got 10 V on the 10 turn winding, you have 1 V/turn. To get 30 VAC on your secondary, you need 30 turns there.Winding resistance is pretty useless, it depends on the gauge of the wire and the length per turn, which varies. Each layer of the winding has a longer length.Jon

author: sureshot
author_rank: Regular Contributor
timestamp: August 08, 2018, 09:49:07 pm
content: Yes I see what you mean, but I don't know the turns count for the primary or secondary. I know the current drawn by the primary is 3 Amps with no load on the secondary. I know the secondary is around 750 Volts no load voltage. I'm in the UK so the mains single phase is 240 Volts AC.I'm sure there would be losses reversing the input. I'm just trying to work out what the reverse output voltage from this transformer. I won't float the chassis, I'd isolate the input completely.Thank you for the help.

author: Jwillis
author_rank: Super Contributor
timestamp: August 09, 2018, 04:36:51 am
content: I'm not great at the math either.So download Xformer Designer.It runs on it's own without installation.Measure the width and length of the center core.We want the core area so height doesn't mater. Enter that in the first field of the program,enter the volts of the winding you measured . Enter 50 hz ,Enter 1 Flux for silicon steel .This is close enough. Start with the primary first then secondary .With the number of winding's on the primary and secondary you can calculate the ratio.This program will give you the VA ,number of turns for the winding and turns per volt.I've used it lots and its pretty close .Mine doesn't like standard measure for some reason.If this happens to you use the metric measure.To calculate Amps A = VA / V this is potential amps not actual.It dependent on the gauge of wire . If you want to calculate the safe working amperage of the wire we can discuss that.

author: sureshot
author_rank: Regular Contributor
timestamp: August 09, 2018, 10:33:21 am
content: Thank you for your help, cheers for the application. I will look it up.Thanks again.