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Making a transformer
5

Making a transformer

Making a transformer

(OP)
I need to make a air-core type transformer that can produce 6 volts DC (rectified) and at least 600 ma's. What I am doing is winding the secondary coil with 27 awg magnet wire with a diameter of 1" about 150 turns. The primary will be same wire but 100 turns and about 1 1/4" diameter. The secondary coil will need to be able to spin freely in the middle of the primary for my project. I plan on sending 12 vdc at 200Khz through the primary coil to exite it. Am I on the right track or would I need more/less windings on either coil? What about heat buildup? I am not that experienced with transformers to figure it out and was hoping someone here had some knowledge? Thanks!

RE: Making a transformer

Are you using DC ('12vdc') or are you using some form of ac (200kHz)?  

RE: Making a transformer

(OP)
I am using DC 12v.

RE: Making a transformer

transformers cannot operate on DC!

RE: Making a transformer

(OP)
If you pulse DC into a coil you can make it produce electricity on the secondary wire. The trick is pulsing it, like 200Khz wave. I've seen it done, I just need to know how many turns for both sides would produce the output I need.

RE: Making a transformer

Is this a motor or a transformer?  We generally do not like our transformer windings to spin around!

RE: Making a transformer

May be more effective to use an iron core stator with the outside wound rotor spinning around it.

RE: Making a transformer

Not to mention at that frequency the interwinding capacitance would probably shunt out most of the primary.

RE: Making a transformer

2
What is going on in this thread?

A guy asks a simple and completely valid question.

He mentions chopped DC, which is DC - even if the "ripple contents" is high.

He also says that he wants the two halves to move (rotate) and that is being done all the time in static exciters etcetera. Rotating transformers are common.

We do not need to poke fun at him for asking these questions. Do we?

RE: Making a transformer

Ok guys, I think I have the picture. He wants to transfer some power through a rotary transformer.

Air cored transformers do not work very well because the magnetic field does not know where to go. As soon as you try to load the secondary, the magnetic field will try to go around it instead of through it. You absolutely must have a proper magnetic path if you want to transfer any power through a transformer.

The best solution is to use two ferrite pot core halves and place a coil in each half. One side can rotate freely and the other side can be fixed. These pot cores can be purchased with a central hole, so an axle or bearing is pretty easy to make. It will behave just like any other transformer, except the two halves can rotate freely with respect to each other. Ferrite is as hard as glass, so the two halves will run for a very long time without wearing out.

This idea is not new, it is often used to couple signals and power into and out of rotating machinery where for various reasons slip rings are not appropriate.

RE: Making a transformer

(OP)
Thank you for your input. I am posting a link where someone already has this idea working, they just don't give enough information as to the windings etc..and I need a little more power than he is producing:

example

RE: Making a transformer

Bob, why not just use a pair of slip rings and some spring loaded contacts to transfer dc power direct ??

RE: Making a transformer

(OP)
I was thinking of that but it would create alot of friction and noise.

RE: Making a transformer

It still might be a lot simpler. Just power the slip rings direct from an ac plugpack, and fit the rectifier, filter, and voltage regulator to the rotating part.


Friction will be low if the slip rings are very small diameter. Or how about using the bearings themselves at each end of the shaft. Lots of ways to do it.

RE: Making a transformer

There are also a lot of commercially made brushless selsyns a.k.a. shaft position resolvers that use a rotary transformer instead of brushes.

Most big city libraries in the U.S. have a a complete copy of the U.S. Patent Office files. Look up things like rotary transformers, brushless devices, wound rotor motors, and so forth and you will find a mountain of ideas and applications.

Mike Cole, mc5w@earthlink.net

RE: Making a transformer

(OP)
I was kind of looking for some technical help and not "go to the library" type help. Appreciate your time.

RE: Making a transformer

Hi Bob,

The biggest problem, other than the mechanics of making one part of the transformer rotate, may well be the size of the air gap across which you are trying to transfer power. Practically you will need some sort of transformer core, and at 200kHz it will effectively be limited to ferrite.

If you use a core, keep the airgaps as small as possible. This minimises flux leakage, making your transformer behave more like an 'ideal' transformer. The ratio of primary to secondary turns then can be closer to the theoretical V1/V2 = N1/N2. The additional turns you are considering are to compensate for the high leakage inductance of an air-cored coil.

I think Warpspeed's idea to use a pair of shell-type pot cores rotating on a common axis has some merit. If the mechanical axis is parallel to the flux lines, the windings will have no relative movement as far as the transformer is concerned, so it will behave more-or-less as it would when it was stationary.

Use a symmetrical waveform to drive the primary, rather than pulses. This will make better use of the core, maximising energy throughput.

Work on a maximum flux density of 0.3T for a general purpose ferrite like 3C8, and calculate your volts/turn for the primary winding on your chosen core. Calculate the secondary turns from the number of primary turns. You my need a few extra turns of the secondary to compensate for the effects leakage reactance.

Heating at 200kHz with tiny wire such as you are considering is pretty much according to Ohm's Law. Skin effect shouldn't be too significant, so use the DC resistance for your power calcs.

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

RE: Making a transformer

(OP)
Thank you for the detailed suggestions. I am starting to mess around with it now and working on finding some sort of core to use on the secondary. I figured on using more turns also on the secondary to comepensate. I did a small test of 24 guage wire and used equally lengths and sent 12v through the primary and it produced about 3v on the secondary. Thats a start..


RE: Making a transformer

Hi Bob,

When you are doing you tests for secondary voltage, are you loading the secondary winding? If you are testing with an air core, you will have poor mutual coupling of the coils, giving the low voltage, and the voltage will drop very quickly as you load it because the leakage impedance is very high giving poor regulation. Improving the mutual coupling and reducing the leakage are the two key elements for success in this task.

If you have any high-school physics books to hand, these often have a reasonably easy to follow study of magnetically coupled air-cored coils.

Good luck!

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

RE: Making a transformer

Excellent information Scotty.

It is very important to keep the air-gap as small as possible, better still run no air-gap. Ferrite is extremely hard and smooth and should run o/k as a dry bearing.

Scotty is also correct in pointing out that the drive waveform must be symmetrical. The best way to do this is to drive a flip flop that divides the frequency by two. That way, the positive and negative half cycles will always be exactly the same time period, even if the frequency changes. Some of the switchmode power supply control chips have an oscillator, a flip flop, and an output stage that might have enough grunt without using anything else.

It may be advisable to connect a large non polarised capacitor in series with the primary. That will prevent the driver stage from blowing up if the oscillator stops.

To get suitable bobbins to wind your primary and secondary, three segment plastic bobbins should available to suit the particular pot core. Just cut out the centre section leaving two narrow bobbin halves that will each occupy around one third of the internal space. It is about the only way you can do it to get mechanical clearance between the rotating and non rotating parts.

I still believe a pair of slip rings would be easier, smaller, cheaper, more efficient, and have less rotating friction.  

RE: Making a transformer

(OP)
Would the output performance be any better if I was to just put 16v AC through the primary instead of pulsing DC? What kind of power output could I expect using that? I know its hard to measure but a range would be good enough for me at this point. Would increasing the current on the primary side also help?

Thanks!

RE: Making a transformer

If you use a ferrite pot core, it must run at relatively high frequency to keep the flux density well below the saturation knee. As Scotty says, 300mT is a good safe maximum design figure to aim for.

Very high frequency sinewaves are much more inefficient to produce than squarewaves, so why would you want to do that ???

RE: Making a transformer

If you still prefer an air-core transformer, you might try the following:

Instead of a “plain” transformer, use the primary coil together with a capacitor over it as a parallel resonant circuit. The pulsating DC cannot be connected directly over the primary. Instead, add a tap (wire out) at about 1/10 of the total number of turns, and connect your pulsed source between this tap and the lower end of the coil. No "freewheeling" diode needed! A first guess for the number of turns in the secondary might be about half of the number of turns in the primary. Adjust the frequency of your source so that the circuit is in resonance.

The idea behind this suggestion is the following: The coupling between the primary and the secondary is weak in a coreless transformer. This means that you need a large primary current in order to get the desired secondary voltage (and current). The current in a good resonance circuit builds up to a value that is much larger than the current it takes from the source.

I made an experimental transformer of this type. I used the components I found hanging around, without any optimisation. The main parameters are the following:

Primary winding has about 230 turns (in four layers)  of 0.6mm enamelled wire on a core with a diameter of 53mm. The length of  the winding is about  50mm. The inductance of the primary winding is about 2.6 mH. The tap for the pulsed source is at about  30 turns from the lower end. The secondary has 100 turns (in one layer) of 0.25 mm wire on a core with a diameter of  about 45 mm. The capacitor over the primary winding is 47 nF, so that the resonance frequency is about  15 kHz.

I connected a 20 ohm resistor directly over the secondary, without any rectification. With a DC source voltage of  9V, the AC peak to peak voltage over the resistor was about 15V, so that the transmitted power is about 1.4W.  It was difficult to estimate the efficiency, but it was rather low, perhaps only about 30%. It should be possible to increase the efficiency by a careful design and/or experimentation, but you have to experiment a lot. In any case, this resonant circuit-transformer worked much better than a plain transformer made of the same coils.

Finally, to your question of the use of 60Hz (or 50Hz) AC. I do not think that it is a good idea. Because the inductance of an air-core transformer is small, the idle current will be large at this low frequency.

RE: Making a transformer

BobLWeiss
One more thing. I checked your link (the "example") first now. The transformer is defined as "20:25" in the link. My understanding is that this means that the primary has 20 turns, and the secondary 25 turns. This is the information you are after, isn't it !

The thickness of the wire is not very important. Something around 0.2...0.5 mm should be fine, I guess.

RE: Making a transformer

(OP)
Yes I saw that and wasn't sure if he meant 20 turns or a ratio or 20 to 25. Like 100 turns to 125 turns. Also he lists how much current he is able to get from it and thats not enough for what I need. I need something like 600 ma on the secondary. I saw someone else make one and they did it a little different. They made the primary and secondary both the same size (2" diameter) and put the secondary on top of the primary seperated by a piece of plastic that was spinning. The distance between the 2 coils was about 1/8". Do you think that way is better or is it better to put the secondary in the middle of the primary coil?

Thanks!

RE: Making a transformer

(OP)
I built a circuit to power the primary and it is driven by a 200Khz oscillator and then fed into a pair of transistors (PNP/NPN) and then fed into a 8A MOSFET. One end of the coil is wired directly to a 12vdc source and the other end is hooked to the MOSFET. The drain of the MOSFET is hooked to ground. The problem I am having is that the MOSFET is getting real hot right away. Should I use a resistor to limit the current, or properly heatsink the MOSFET as this is normal? The circuit that I am building is:

Circuit

He does not use a resistor and mentions that nothing runs hot...so I am not sure what I am doing wrong. I am not a beginner to electronics and I know the circuit is wired correcly.

RE: Making a transformer

1. One coil inside the other or on top of the other?
Well, I have no direct experience here. My _guess_ is that putting a well prepared flat coil on top of another flat coil might be better, because the gap can be smaller.

2. Running hot?
Heat sink yes, resistor no. Have you a load (a resistor, about 10 ohms, 10W) in the secondary circuit? You should. Is the FET switching properly on and off? (just guessing). This might boil down to the often mentioned weak coupling between the primary and secondary.

RE: Making a transformer

(OP)
1) The coil is on top of the other seperated by a 1/8" pc. of plastic. They are not necessarily flat but more like a thick rope shaped in a circle.

2) I didn't have a resistor on the secondary and I didn't think that putting a load on it would do anything towards the MOSFET getting hot but now I can see why.

RE: Making a transformer

While we are talking high freq's and narrow gaps, why not just use a air dielectric capacitve feed through. Much simpler in principle.

RE: Making a transformer

A capacitor, or actually two, why not. But a high frequency and a narrow gap would still be needed. And a high voltage, because the capacitance would be small, of the order of 100 pF, depending on the physical size and construction. A capacitance of 100pF and a frequency of 1 MHz, for example equals to 1.6 kOhm. 600 mA at 6V are needed. Thus, a step-up transformer to around 600V and a step-down transformer back might do the trick.

RE: Making a transformer

Headlines: electrocution by toy clock!

RE: Making a transformer

Yes, that is "why not".

RE: Making a transformer

Totally different idea: reduce the load power consumption. 3.6W is a fair bit with modern low power IC's and high efficiency LEDs. What is the load comprised of?

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

RE: Making a transformer

(OP)
I am using 2 pics (low power versions), 1 Real time chip, 43 High Brightness LEDs, 3 mosfets and 1 IR module. I figured it to be around 600 ma consumption but thats on paper. The LED's are only on (at not all at one time) for 1/2500 RPM. (estimate). Some are on (about 5) are all time.

RE: Making a transformer

If you have a 10MHz crystal source driving a 5W narrowband amplifier, into ijl's 100pF feed-through (rotory) cap, and on the rotor a series 2.533uH inductor for series "warpspeed" resonance (The L-C disappears!!). Then into a small bridge and cap for your source. Plus no need for rubber gloves (ie. 600V) when trouble shooting, the rotor does not even need to be turning to probe other problems!

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