Transformer secondarys

[davelectronic]

So I've got what I hope to be a future project, but the single biggest component is a bit different from the norm.
For some time I have wanted to get a MOT (microwave oven transformer) as a usable base for a power supply. What I've tried so far is, a single MOT with high voltage secondary removed and replaced with high current cable. As expected it ran very hot from the primary winding, and a lot of wasted energy.

So recently another second MOT came my way, and I put two transformers primarys in series. And high current wire on both secondary windings. I didn't get quite enough wire on both secondaries, the cable was 6mm Earthing cable with heat shrink tube added for better insulation. My total voltage from both secondary windings is 8.4 Volts RMS, this is not enough voltage for my needs. I liked the idea of running the pair of transformers primarys in series as they run very cool each receiveing about 120 Volts AC for the mains here in the UK. So I am contemplating rewinding both secondaries again with slightly smaller gauge wire, 12 AWG something like that. But the one thing I noticed from the first secondary winding was a bit of sag when I added a 50 watt halogen lamp load. I'm thinking this is because each MOT is only receiveing 120 Volts AC in series with each other.

And so before I attempt rewinding both secondaries in series, should I expect a fairly large secondary windings voltage drop under load conditions ? I really like the idea of running the two transformers in series as they run quite cool, but that is half the rated AC voltage of each transformer. So I need to increase the series secondary voltage to around 15 Volts RMS. I had thought about a voltage multiplier for the series primary input, but I'm not sure that is a practical solution. I did think, what about a voltage multiplier on the secondary series winding, but as that's going to be very high current, perhaps that is even more impractical idea. So in summary, what kind of voltage drop could I expect with rewound secondary windings, to get the voltage up to a reasonable level. If I wanted 15 Volts RMS, would an unloaded secondary of 18 Volts allowing a 3 Volt drop under load down to the 15 Volts minimum sound reasonable ? The maximum load current would be around 20 Amps at 12 Volts as a final voltage output.

I'm aware of the not so perfect performance of rewound MOT transformers, but I'm hopeful I might be able to get 15 Volts RMS from two MOT transformers wired in series for both primary and secondary windings. Thoughts and any suggestions appreciated, thanks for reading.

[Quarlo Klobrigney]

Firstly, a MOT is not a continuous duty device. Expect fire or worse from trying to use it in that manor. Hams use them as an intermittent power supply as most transmit duty cycles are very low.
As use as a spot welder, the duty cycle is even lower.
Your voltage sags because it is not a regulated voltage. Think about its intended use. Make a raw DC voltage with a very low capacitance to make microwave energy. It doesn't need to be good, just effective. I suggest finding a transformer that is rated for the purpose.

[davelectronic]

I do have transformers, both chassis mount and toroidal at low voltages and large current ratings. My intended use would be RF HF Radio related. The use is not for continuous, and about a 50% duty cycle would be typical. I'm just trying to get 15Volts RMS from two transformers secondaries, I know the AC output won't be stable with out a regulated output. That is what the plan is, but I'm unsure how the secondary windings will perform with both primarys in series receiving half the mains voltage each. So each primary is running on half its rated voltage, I'm trying to work out if it's a viable option. I only require 15 Volts RMS from the two secondaries in series. There won't be any fires, the input will be fused, and I would add an over temperature thermal cut off, way before they ever reached there full rated temperature. And with both primary transformers windings in series, they run quite cool. Hopefully I can find out if 15 Volts RMS is obtainable from this configuration. Thanks for replying.

[Quarlo Klobrigney]

What kind of radios and what's your current requirements? I'm not a transformer guru, but for 40 years I've learned a few things in using them. One of the things was surplus lighting transformers don't work backwards. Here in the US we have 208V & 277V lighting. The transformers work fine going from HV to LV but not the other way round. I don't know why they just don't. They exhibit poor regulation and low voltage. The same for the 440V ones. I was hoping to make a nice step up PS for tube gear. The result was returning them (different types) to the supplier, live and learn....

[davelectronic]

In terms of current, about as much as a HF linear amplifier. So about 25 Amps to 30 Amps at the highest, that would be at 50% duty cycle. I can get this from a single MOT but on its own it runs to hot. I've measured up to 90°C with limited cooling. A hurricane fan would cool it, but that's just a racket of noise of fan on full blast. A pair of MOT's in series seem to run quite cool from the primary side. But the series secondary windings need to be much higher number of turns as the primary is running at half the voltage. It's not a critical thing, some kind of obsession with microwave oven transformers lol. It would be cool if it worked well enough to get a 12 Volt regulated output from a pair of these transformers as the backbone of a psu.

[bob91343]

If you use two transformers with primaries in series, the output from two secondaries in series should be the same as with a single transformer.  You can probably get twice the load current as a single transformer.

Since the excitation voltage is low, the losses will be lower and it will be more efficient until you raise the load current.

The important parameter is temperature.  If the transformer runs cool, it's probably okay.

[davelectronic]

I'm using two 240 Volt MOT transformers in series, and measuring about 122 Volts on each transformers primary. My secondary winding is earthing cable that is 6mm 2 in area. I think i'm going to rewind these transformers with 12 AWG wire, the cable is silicone covered, but i add an additional layer of heatshrink tubing, just to be 100% sure no shorts accrue. The voltage is only 8.4 Volts with the secondaries in series, the cable is just to thick to get any more turns on. Iv'e got to 14.70 Volts before on a single MOT, and used enough capacitance and an LDR 12 Volt regulator. I manage a comfortable 12 Amps from that project MOT power supply. It didn't drop hardly any voltage under load. But i measured a running temperature of 95 C from the transformer, way to hot really. I know he insulation is class H but i wasn't comfortable leaving that unattended. But two MOT transformers in series with the primary's looks promising. If i can get the secodary voltage up to around 15 Volts or there abouts.   

[Quarlo Klobrigney]

The only point I was trying to make to persuade you not to use the MOT was the possibility of the insulation breaking down with the heat generated from extended use. I understand what you are trying to do just not why ... I'm sure the power supply will be on even in receive/standby mode, just not not loaded down.
For me #2 AWG around the core is all that should be replacing the HV winding.
My opinion, I don't trust MOT's (China what could go wrong) that much. But, it's about the experiment after all.

[Doctorandus_P]

Voltage sagging under load is a normal side effect of transformers.
Normallly the specified output voltage is meausered under full load.

When idling, the voltage can be 30% higher with very small transformer, and "a few percent" for big transformers (500W or so is considerd big).

Using the primaries of 2 MOT's in series seems a bit wasteful, and if you're rewinding anyway, have you thought about adding about 10% to 20% extra windings to the primary?
I believe those things already have an auxillary 6V winding, which you can put in series with the primary. That may be just enough to run it a bit cooler.

[David Hess]

I like the idea of using two MOTs with the primaries in series to reduce the magnetizing losses.  I have used the same trick in the past with common power transformers to reduce losses.

I would wire the secondaries in parallel to force the same volt-second product across both transformers which doubles the available current.  But as you noticed, MOTs do not have a large winding window so you cannot wind enough secondary turns to take full advantage of them.

[davelectronic]

Experiments, yes that's what it's all about really. And a ending up with a power supply, where by the transformers are free, other than the replacement winding cable and heat shrink tube. I know these this are poor performers in the run lean and clean arena. I think that's what makes it a task to try and achieve. Sure I will only do it once, just to say I did it. Retail transformers more so toroidal are not to expensive, but above 300 VA the current drops off, and the voltage goes up. And I would like to keep the input output voltage / margin as small as possible. There was a 1000 VA 25 Volts toroidal transformer from a UK supplier, but it's would make a great lab high current PSU. But just to much voltage for a 12V to 13.80V Linear amplifier power source. So it's just for experimentation. And the hobby.