# 3 x MOTs in series - combined output voltage issue

I had 2 x MOTs with both primaries and secondaries connected in series.

This was giving me about 86V AC / 116V DC

I added another MOT to boost the voltage a bit and increase the saturation safety margin.

Except that no matter which way I flipped both the primary and secondary coils on the third MOT I was only getting about 40V AC total.

It was only when I reversed the connection on the third primary that I got 100V AC / 140V DC

I can't get my head around why this would work with AC current going into the primaries.

What’s an MOT?

And circuit diagrams would be helpful…

Allan

allanhurst: What's an MOT?

And circuit diagrams would be helpful.....

Allan

Microwave Oven Transformer

This is the actual device. It is just 3 transformers with both primaries and secondaries connected in series.

Except that the third one (with the weight) has the connections swapped relative to the other 2.

When wiriing transformer secondaries in series together, they must all be in phase for the voltages to add - think of wiring dc cells in series.

Any one of them reversed will subtract it's voltage, not add.

Note if wiring them in parallel, if you get it wrong there will be a big bang!

Allan.

allanhurst: When wiriing transformer secondaries in series together, they must all be in phase for the voltages to add - think of wiring dc cells in series.

Any one of them reversed will subtract it's voltage, not add.

Note if wiring them in parallel, if you get it wrong there will be a big bang!

Allan.

I get that but only of I flip the secondaries or primaries so that the direction (clockwise or anticlockwise) of the wire changes. I tried this.

But that is not how I managed to get 100V AC out.

Out of desperation I swapped the connectors on the third MOT primary coil and got my 100V AC.

I don't understand how swapping the connectors changes the direction of the wire in the third primary coil.

swapping primary connections is the same as swapping the secondary - it inverts the phase.

Allan

I still don't get it Alan. It's alternating current and has no polarity so how is the phase inverted by inverting the connections? Are you able to explain it to me via some sort of diagram?

See the drawing - The upper sinewave ( say) is the input - and the phase of the output from your transformer will be the same if wired in one way ( though the voltage will be different) . Reversing either the input or output will give an inverted sinewave - the lower trace.

Allan

allanhurst:
See the drawing - The upper sinewave ( say) is the input - and the phase of the output from your transformer will be the same if wired in one way ( though the voltage will be different) . Reversing either the input or output will give an inverted sinewave - the lower trace.

Allan

Well since both connectors experience the full range of voltages from -250V to 250V, what you must be saying is that there is a time difference between when one connector experiences 250V and then they other connectors experiences 250V.

I presume due to the impedance in the coil and the fact that one connector comes from the inside of the coil and the other from the outside.

Is this correct?

Well, sort of. An AC signal’s instantaneous voltage varies with time as shown above.

Say at time = 0, the voltage of the mains L is at 0V with respect to the N. Since mains frequency is ( in the UK) 50Hz, after 1/50th second it will get back to 0V - 1 complete cycle as shown above.

At any point in between it varies as shown in the diagram. For example, after 1/4 of a cycle it’s at it’s highest +ve value. At 1/2 cycle it’s back to zero. At 3/4 cycle it’s at it’s highest -ve value. And so on.

If you swapped the L and N, you’d get the reverse of those readings - as in the lower diagram. Just as if you’d reversed the connections to a battery.

Any help?

Allan

Am I just being fussy, or is this lack of understanding of the basics and high voltage experimentation a little bit terrifying?

GypsumFantastic: Am I just being fussy, or is this lack of understanding of the basics and high voltage experimentation a little bit terrifying?

I hole heartily agree, what are you trying to achieve?

• What is the application?
• Are you aware of the output voltage of these transformers?
• Do you know how a transformer works and especially a Microwave Transformer?
• The two transformers on the left have labels on them, what do they show?
• How much current and voltage do you need?
• What is the circuit board on the right?

You need to answer these questions before applying power. Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom... :)

GypsumFantastic: Am I just being fussy, or is this lack of understanding of the basics and high voltage experimentation a little bit terrifying?

It is very terrifying.

One should NOT mess around with MOT's if you have absolutely NO understanding on how transformers work and should be phased.

// Per.

GypsumFantastic: Am I just being fussy, or is this lack of understanding of the basics and high voltage experimentation a little bit terrifying?

Well I aint go to learn anything practical if I don't have a crack.

And I have a healthy fear of 250V AC so that generally keeps one fairly safe.

allanhurst: Well, sort of. An AC signal's instantaneous voltage varies with time as shown above.

Say at time = 0, the voltage of the mains L is at 0V with respect to the N. Since mains frequency is ( in the UK) 50Hz, after 1/50th second it will get back to 0V - 1 complete cycle as shown above.

At any point in between it varies as shown in the diagram. For example, after 1/4 of a cycle it's at it's highest +ve value. At 1/2 cycle it's back to zero. At 3/4 cycle it's at it's highest -ve value. And so on.

If you swapped the L and N, you'd get the reverse of those readings - as in the lower diagram. Just as if you'd reversed the connections to a battery.

Any help?

Allan

If you just had two simple resistors connected it to the mains, with opposite connections, and you compared the voltage at the left hand end of both resistors millisecond by millisecond then surely the phase of the AC voltage would be identical? Or would it?

TomGeorge: I hole heartily agree, what are you trying to achieve?

• What is the application?
• Are you aware of the output voltage of these transformers?
• Do you know how a transformer works and especially a Microwave Transformer?
• The two transformers on the left have labels on them, what do they show?
• How much current and voltage do you need?
• What is the circuit board on the right?

You need to answer these questions before applying power. Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom... :)

They are modified MOTs - not in original condition.

The 2000V secondaries are gone and have been replaced by lower voltage (than 250V that is) secondaries made by removing loops from the primary coils from 3 other MOTs.

The setup actually required 6 MOTs in total.

The circuit board consists of a bridge rectifier and 400V electrolytic caps that I salvaged from cathode ray tv circuit boards.

The output voltage from the setup is about 120V DC.

I have a 10A fuse on the FET half bridge driver circuit that consumes the 120V DC. It is driving a Tesla coil primary. The bridge rectifier is rated for 15A and 250V AC.

And I have a 60% duty cycle limit on the square wave generator that ultimately drives the FET half bridge.

I'm of the view of other contributors... The primary of your MOT might only have 250vac on it, but the secondary will have 2000v+ capable of at least 1/3 ampere.

Very Dangerous.

If you reference one side of your voltmeter to N, then the two measurements will give you the upper of my 2 drawings ( about 250vac ) , and 0v respectively.

Allan

Hi,

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

How did you change the windings, that is open the laminate core and replace.

The minute you compromise the physical structure and windings of a mains transformer, you need to have its insulation tested and PLEASE power it through a RCD switch. (I don't mean DMM resistance measurement, I mean 500Vac resistance check)

This also worries me;

[u]both[/u] primaries and secondaries connected in series.

How will that give you more voltage or current or power.

Tom.. :)

allanhurst: I'm of the view of other contributors... The primary of your MOT might only have 250vac on it, but the secondary will have 2000v+ capable of at least 1/3 ampere.

Very Dangerous.

If you reference one side of your voltmeter to N, then the two measurements will give you the upper of my 2 drawings ( about 250vac ) , and 0v respectively.

Allan

Allan they DO NOT generate 2000V because they do not have their original secondaries.

I know because I used an angle grinder to remove the weld lines, opened up the iron cores, removed the secondary coils, added new secondary coils and glued the cores back together.

And I have measured the secondary voltage with my multimeter and it was about 90V AC.

TomGeorge: Hi,

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

How did you change the windings, that is open the laminate core and replace.

The minute you compromise the physical structure and windings of a mains transformer, you need to have its insulation tested and PLEASE power it through a RCD switch. (I don't mean DMM resistance measurement, I mean 500Vac resistance check)

This also worries me;How will that give you more voltage or current or power.

Tom.. :)

The circuit board contains this circuit:

Except that the diodes are all contained in a bridge rectifier IC from a tv circuit board.

The bridge rectifier is rated fro 250V AC because I got the datasheet from the part number.

The way they weld the laminated iron core together, on the outside faces, already compromises the iron core.

But apparently on the outside faces of the iron core the electrical paths do not cause significant problems with eddy currents.

So applying an angle grinder along those welds does not change the state of the iron core significantly.

Look its going to power a Tesla coil occasionally and I may muck around with it for an hour or two and then not use it for days.

I am not using this to power my power tools etc for 8 hours straight.