Isolation Transformer.

[Jerry]

Well I did as sugjested. I took 2 matching microwave oven transformers apart. I removed the primary and secondary coils from each. There is a small amount of loose laminations between these 2 coils, I took that out to.

Next I reinstalled the 2 primary coils back into one of the laminations. I placed these 2 coils next to each other. I then filled in the gap with the loose laminations at one end.

This single transformer creation works much better then the 2 transformers wired back to back.

With the 2 back to back, 120 volts in for 100 volts out. With the modified single transformer I get 120 volts in with 115 volts out. Only a 5 volt drop rather then a 20 volt drop.

Its much smaller and lighter this way. I placed a 1200 watt load on the new 115 volt secondary with only a 10 volt drop.

These things make for very cheap but hi powered isolation transformers. Also the moded unit is very quiet compaired to the original.

I'll try to get some pictures.

                          JK TAS Jerry

[Flux]

You have gained in many ways by doing this. you have halved the resistance and by moving those magnetic shunts from between the coils you have reduced the leakage reactance that was intentionally built into them.

Flux

[RogerAS]

Flux,

Could you please clarify exactly what leakage reactance is and how it happens?

TIA,

[Flux]

In a perfect transformer, all the flux produced in the core by one winding will link completely with the other coil. The only thing that would cause a drop in secondary volts would be resistance of the windings.

In the real case not all of the flux links and there is an extra volt drop due to this lack of coupling. The effect is the same as if a reactor was connected in series with one winding. The output volt drop is now due to an impedance, a combination of resistance and reactance.

With conventional transformers with the windings close together on the core the leakage reactance is small. When it is required to be as small as possible the primary and secondary are split and the parts of both windings are interleaved.

In other cases the reactance is a design feature and is intended to limit secondary current, this is used for welders, neon light transformers, boiler ignition transformers and others. This reactance can be increased by leaving a gap between primary and secondary and it can be further increased by adding a bunch of laminations between the primary and secondary to deliberately cause a leakage flux path. Some welders have this shunt on a screw to make the effect adjustable.

The leakage reactance also occurs in alternators, where the whole of the magnet flux fails to link with the windings. It is particularly bad with deep slots in iron cores and strange constructions like the smartdrive motor.

Flux

[Ungrounded Lightning Rod]

Leakage reactances are also used, along with core saturation, in the "regulating transformers" of the '60s era, used to produce a clean sine wave at a regulated voltage.  These were built both as standalone power cleaners and into the power supplies themselves in early computers (through the start of the "minicomputer" period).

In these the secondary side has an extra winding with a capacitor hung across it to resonate at the line frequency.  The flux for the resonance goes through the leak, but the primary and secondary are coupled well enough to "pump up" the resonance (and the field through the leak and secondary) until the secondary core segment saturates, regulating the voltage (and very slightly clipping the waveform).

As long as you don't pull the power out faster than it leaks in (or certain other bad things don't happen) the resonance stays pumped up and the voltage regulated.  Short dips and outages are covered by power from the ringing resonant circuit, while spikes get bypassed through the leak and don't have a chance to pump further.

These had downsides:  A phase hit could make them go overvoltage for a short time, making things worse than if they weren't there.  They were VERY large and heavy, much bigger than a non-regulating transformer for the same power, plus the space for the cap.  They burned some power all the time.  They had dangerous voltages across the cap.

They were replaced, once high-frequency semiconductors were up to it, by switching regulators, which are tiny, light, and do a MUCH better job of providing clean DC voltages from a range of line voltages, frequencies, and conditions.

[SparWeb]

I was playing around with this not long ago.  I rigged up a "high-tension line" across the garage with my axial alternator running off the drill press, bumping the voltage up with the microwave transformer, then stepping it back down with an identical one before rectifying into the battery.  I can say it worked; comparable to Jerry's initial tests.  The fact that my alternator has 12 coils, more than the typical 6 or 9, helps raise the frequency.  I will try removing those extra bits in the transformer and see if it improves anything.

(No I'm not expecting a breakthrough - this isn't likely to be worthwhile, I admit, but fun to try and doesn't cost anything because I have several of these things lying around).

Thanks for the ideas.

[jimovonz]

I have been meaning to give this a go too. I have two turbines now with all coil connections exosed to allow me to rewire for high/low voltage. Rewiring for high voltage drectly from the turbine will allow you to use only one transformer (per phase) though you will obviously have to rewind your microwave transformer to a more appropriate step down ratio (I have had no trouble rewinding many microwave transformers for various projects). I would also like to investigate if it is posible to benefit from winding dual isolated secondares so as to remove one diode from the conduction path (still use 2 diodes per phase/transformer but only one in the conduction path at any one time). Dual windings obviously take up double the space and are only 50% utilised so the trade off would be winding resistance and the associated losses. I'm picking you'd loose as much/more than you'd gain but I'd like to try it I still have a moutain of transformers in the shed so it would be good to do something useful with some!

[SparWeb]

"I still have a moutain of transformers"

Yet we continue to collect all this stuff...  :^)

[Ungrounded Lightning Rod]

Dual windings obviously take up double the space and are only 50% utilised so the trade off would be winding resistance and the associated losses.

That's not a big issue for a transformer with a core - the added core flux-path length to support the extra winding is minor and the losses in a non-conducting winding (eddy-currents in the tiny area of the wires) ditto.

The resistance tradeoff occurs in air-core alternators, where increasing the thickness of coils requires increasing the gap and thus reducing the flux (unless you also increase the amount of expensive magnetic material.)  So you want the coils to work on both half-cycles.

If you're already accepting the losses of putting transformers in, by all means do a dual (or center-tapped) secondary and get rid of a diode drop.

[jimovonz]

"...the added core flux-path..." I was considering the limited space in a single transformer, but you're right, its just as easy to put 2 x E's together and leave out the I's and have all the space I need for two low resistance secondaries. After some thought on the matter you are also right about using a standard centre taped secondary (doesn't have to be isolated) - just place the diodes on the outside legs. It's easy to get stuck in a thought pattern and overlook the obvious

[WXYZCIENCE]

Steven, I stopped counting numbers years ago when I ran out of fingers.


Joseph.